Lecture exam #3 Flashcards

Question

Identify and describe the three layers that make up the glomerular filtration membrane

Answer 1

The filtration membrane is a porous, thin, and negatively charged structure that is formed by the glomerulus and visceral layer of the glomerular capsule. It is composed of three sandwiched layers. For a substance in the blood to become part of the filtrate, it must be able to pass through these three layers of the “filter,” from innermost (closest to the lumen of the glomerulus) to outermost: 1. Endothelium of glomerulus.The endothelium of the glomerulus is fenestrated. It allows plasma and its dissolved substances to be filtered while restricting the passage of large structures, such as the formed elements (erythrocytes, leukocytes, and platelets). 2. Basement membrane of glomerulus.The porous basement membrane is composed of glycoprotein and proteoglycan molecules. It restricts the passage of large plasma proteins, such as albumin, while allowing smaller structures to pass through. 3. Visceral layer of glomerular capsule. The visceral layer of the glomerular capsule is composed of specialized cells called podocytes. Podocytes are octopus-like cells that have long, “footlike” processes called pedicels that wrap around the glomerular capillaries to support the capillary wall but do not completely ensheathe it. The pedicels are separated by thin spaces called filtration slits, which are covered with membrane. Pedicels of one podocyte interlock with pedicels of a different podocyte. The membrane-covered filtration slits restrict the passage of most small proteins.

Answer 2

∙ Freely filtered. Small substances such as water, glucose, amino acids, ions, urea, some hormones, water-soluble vitamins (i.e., vitamins B and C), and ketones can pass easily through the filtration membrane and become part of the filtrate. These substances have the same concentration of ions, molecules, and wastes in filtrate as in the plasma. ∙ Not filtered. Formed elements of blood and large proteins are structures that cannot normally pass through the filtration membrane. These substances are usually restricted from becoming part of the filtrate. ∙ Limited filtration. Proteins that are of intermediate size are generally not filtered. They are blocked from filtration either because their size prevents movement through the openings of the filtration membrane or because they are negatively charged and repelled by the membrane’s negative charge. Only limited amounts of these substances become part of the filtrate.

Answer 3

Filtrate is characterized as filtered plasma with certain solutes and minimal amounts of protein. Filtrate is caught within the capsular space and then funneled into the proximal convoluted tubule. Components of blood that are not filtered exit the renal corpuscle through the efferent arteriole and then continue through either the peritubular or vasa recta capillaries. Some of the material being filtered becomes trapped within the basement membrane. One of the functions of the mesangial cells is to phagocytize macromolecules (e.g., antigen-antibody complexes) that become caught within the basement membrane, thus helping to keep the filtration membrane clean.

Answer 4

Blood pressure in the glomerulus is called the glomerular hydrostatic (blood) pressure (HPg). It is the driving force that “pushes” water and some dissolved solutes out of the glomerulus and into the capsular space of the renal corpuscle. It is the HPg that promotes filtration. HPg has a higher value than the blood pressure of other systemic capillaries. This higher pressure is required for filtration to occur, and it is due to the relative diameter size difference in the afferent and efferent arterioles. afferent diameter >> efferent diameter

Answer 5

They are the blood colloid osmotic pressure and capsular hydrostatic pressure. Blood colloid osmotic pressure (OPg) is the osmotic pressure exerted by the blood due to the unfiltered dissolved solutes it contains. The most important of these solutes are the plasma proteins (colloid). Blood colloid osmotic pressure opposes filtration because it tends to pull, or draw fluids into the glomerulus. Capsular hydrostatic pressure (HPc) is the pressure in the glomerular capsule due to the amount of filtrate already within the capsular space. The presence of this filtrate impedes the movement of additional fluid from the blood into the capsular space, and thus it also opposes filtration.

Answer 6

Filtration occurs if the pressure that promotes filtration, HPg, is greater than the sum of the pressures that oppose filtration (OPg and HPc). The difference in these pressures is the net filtration pressure (NFP). HPg−(OPg +HPc)=NFP

Answer 7

It is defined as the rate at which the volume of filtrate is formed, and it is expressed as volume per unit time (usually 1 minute). The net filtration pressure (NFP) directly influences the GFR. As NFP increases, usually as the consequence of HPg, the GFR also increases. Likewise, as NFP decreases, GFR decreases. As the NFP increases and GFR increases, more filtrate is produced. The increase in amount of filtrate results in increased fluid volume mov- ing more rapidly through the tubules, so there is less time to reabsorb substances from the tubular fluid. Consequently, more substances remain in the tubular fluid and are excreted in the urine.

Answer 8

GFR is primarily influenced both by changing the luminal diameter of the afferent arteriole (“the inflow pipe”) and by altering the surface area of the filtration membrane. The processes involved include (1) intrinsic control (within the kidney), which consists of renal autoregulation that maintains GFR at a normal level, and (2) extrinsic controls (external to the kidney) that involve nervous system or hormonal regulation, which can decrease or increase GFR, respectively. Renal autoregulation is the intrinsic ability of the kidney to maintain a constant blood pressure and glomerular filtration rate despite changes in systemic arterial pressure. Neural and Hormonal Control: Extrinsic Controls In comparison, extrinsic controls, which are described here, involve physiologic processes to change GFR to adjust urine output based on physiologic need. GFR can be decreased when the kidney is stimulated by the sympathetic division, and it can be increased with atrial natriuretic peptide stimulation.

Answer 9

The myogenic response involves contraction and relaxation of smooth muscle in the wall of the afferent arteriole in response to changes in stretch. A decrease in systemic blood pressure (as occurs when you are taking a nap) results in a lower volume of blood entering the afferent arteriole, reducing the stretch of the smooth muscle in the arteriole wall. The smooth muscle cells in the vessel relax, resulting in vasodilation of the afferent arteriole. The wider vessel lumen of the afferent arteriole allows more blood into the glomerulus, which compensates for the lower systemic blood pressure. The juxtaglomerular apparatus also helps maintain a normal glomerular blood pressure through the tubuloglomerular feedback mechanism, which is based on detection of NaCl levels in the tubular fluid. If the myogenic response is not sufficient to maintain normal glomerular blood pressure in response to an increase in systemic blood pressure, then glomerular blood pressure increases and the amount of NaCl remaining in the tubular fluid increases. Ultimately, an increase in tubular fluid NaCl concentration is detected by macula densa cells in the juxtaglomerular apparatus. The macula densa cells respond by releasing a signaling molecule (most likely, ATP) that binds to and stimulates contraction of smooth muscle cells in the afferent arteriole wall. This paracrine stimulation results in further vasoconstriction of the afferent arteriole and a decreased volume of blood entering the glomerulus.

Answer 10

Activation of the sympathetic division as part of the fight-or-flight response results in a decrease in GFR through both vasoconstriction of the afferent arteriole and decreased surface area of the filtration membrane. The sympathetic division sends increased nerve signals to the kidneys during exercise or in an emergency. Both afferent and efferent arterioles vasoconstrict as a result. Severe vasoconstriction of the afferent arteriole greatly reduces blood flow into the glomerulus. Glomerular blood pressure and GFR decrease.

Answer 11

Atrial natriuretic peptide (ANP) increases GFR through (1) vasodilation of the afferent arteriole to increase blood flow into the glomerulus and (2) inhibition of renin release and the subsequent relaxation of mesangial cells, which increase the surface area of the glomerulus. Both GFR and urine production are increased

Answer 12

The barrier that a substance must cross is the simple epithelium of the tubule wall. 2. Substances can either pass between the epithelial cells of the tubular wall by paracellular transport or, more commonly, move through the epithelial cells by transcellular transport. 3. During transcellular transport, a substance must cross two plasma membranes: the luminal membrane that is in contact with tubular fluid and the basolateral membrane that rests on the basement membrane. The order in which the substance crosses these membranes depends upon whether it is being reabsorbed or secreted. Different transport proteins are embedded within the two membranes. They control the movement of various substances using membrane transport processes that include simple or facilitated diffusion, osmosis, primary and secondary active transport, and vesicular transport . 5. Peritubular capillaries have both low hydrostatic pressure, because of the loss of fluid during filtration in the glomerulus, and high colloid osmotic (oncotic) pressure exerted by protein, because most proteins remain in the blood during filtration.

Answer 13

The transport maximum (Tm) is the maximum amount of a substance that can be reabsorbed (or secreted) across the tubule epithelium in a given period of time (its rate of movement). This maximum is dependent upon the number of the transport proteins in the epithelial cell membrane specific for the substance. The maximum plasma concentration of a substance that can be transported in the blood without eventually appearing in the urine is called the renal threshold.

Answer 14

Nutrients are normally reabsorbed completely in the proximal convoluted tubule where each nutrient has its own specific transport proteins. Glucose concentration is relatively high inside the tubule cell and relatively low within both the tubular fluid and interstitial fluid. Glucose is first transported into the tubule cell across the luminal membrane by Na+/glucose symporter proteins. Kinetic energy from Na+ moving down its concentration gradient into the tubule cell is used to move glucose up its concentration gradient into the tubule cell by secondary active transport. Glucose is then moved by glucose uniporters (carriers) out of the tubule cell down its concentration gradient via facilitated diffusion across the basolateral membrane. Glucose ultimately is returned to the blood in the peritubular capillaries. As with many other substances that rely on membrane transport proteins, there is a maximum amount of glucose that can be reabsorbed per unit time. 1 Glucose is transported up its concentration gradient by secondary active transport. 2 Glucose diffuses down its concentration gradient by facilitated diffusion.

Answer 15

Although most proteins are not freely filtered in the glomerulus because of their size and negative charge, some small and medium-sized peptides, such as insulin and angiotensin, and limited amounts of large proteins may appear in the filtrate. Protein is transported from the tubular fluid in the proximal convoluted tubule back into the blood so as not to be excreted in the urine. We use the general term transported here (instead of reabsorbed) because the proteins actually undergo structural changes while being reabsorbed. Protein is moved across the luminal membrane by pinocytosis or receptor-mediated endocytosis. Lysosomes in these tubule cells then digest the proteins into their amino acid building blocks. These amino acids are moved by facilitated diffusion across the basolateral membrane back into the blood. Very small peptides, such as angiotensin II, are degraded by peptidases within the luminal membrane and the amino acids are absorbed directly into the tubule cell. Thus, proteins and small peptides are first degraded into amino acids, which are then absorbed into the blood.

Answer 16

A number of substances fall into the category of undergoing regulated reabsorption, including Na+, water, K+, HCO3−, and Ca2+.

Answer 17

Na resorbption occurs along most of the length of the renal tubule. The majority of Na+ is reabsorbed in the proximal convoluted tubule. Na+ is transported across the luminal membrane down its concentration gradient by facilitated diffusion via various types of Na+ transport proteins, and across the basolateral membrane against its concentration gradient by Na+/K+ pumps. (c) The amount of Na+ excreted in the urine is regulated in the distal convoluted tubule, collecting tubules, and collecting ducts by hormones. Aldosterone binds to receptors within principal cells, increasing the number of both Na+ channels and Na+/K+ pumps. The net effect is that additional Na+ is reabsorbed, water follows by osmosis, and additional K+ is secreted. Hormones That Influence Na+ Reabsorption: Increase: Aldosterone Decrease: ANP

Answer 18

Obligatory water reabsorption occurs in the proximal convoluted tubule (PCT): about 65%. In the nephron loop, approximately 10% of the water is reabsorbed. The amount excreted in the urine is regulated in the collecting tubules (CTs) and collecting ducts (CDs) in response to binding of antidiuretic hormone (ADH). (b) ADH binds to principal cells to cause vesicles containing aquaporin proteins (produced within tubular cells) to migrate to the luminal membrane. Additional water is “pulled” out of tubules by osmosis through the greater number of aquaporins. The exact amount of water reabsorbed depends upon both fluid intake and fluid excreted through other routes (e.g., sweating). aldosterone increases the number of Na+/K+ pumps and Na+ channels in principal cells, thus increasing both Na+ and water reabsorption. Consequently, the concentration of tubular fluid is maintained. In contrast, antidiuretic hormone (ADH) that is released from the posterior pituitary gland when we are dehydrated binds to receptors of principal cells to increase the migration of vesicles containing aquaporins to the luminal membrane. This action provides additional channels for water reabsorption. The osmotic force caused by the concentration gradient within the interstitial fluid pulls water from the tubule. Thus, water reabsorption regulated by ADH near the end of the tubule is independent of Na+ reabsorption, and as a result solute concentration of the tubular fluid increases. Tubular reabsorption of water in response to ADH is referred to as facultative water reabsorption.

Answer 19

The pH of urine, and consequently the pH of blood, is regulated by intercalated cells. Exactly how this occurs depends upon the blood concentration of H+, which is expressed as [H+]. Increased [H+] typically occurs, for example, in individuals consuming a more acidic diet, which is a diet that includes animal protein and wheat. As a result, newly synthesized HCO3− is reabsorbed into the blood, and H+ is secreted into the tubular fluid, by type A intercalated cells. The result is an increase in blood pH (more alkaline) and a decrease in urine pH (more acidic), which averages a pH of about 6.0. Decreased blood [H+] is more typical of individuals consuming a more alkaline diet, which is a diet high in fruits and vegetables and little or no animal protein. In this case, type B intercalated cells are active. The action of type B cells is the reverse of that of type A cells. Think of type B intercalated cells as “flipped” type A cells: Type B cells reabsorb H+ and secrete HCO3− to lower blood pH and increase urine pH

Answer 20

Potassium is unlike other substances previously covered because it is both reabsorbed and secreted in the tubular fluid. The result may be a net reabsorption of K+, with little being lost in urine, or a net secretion, with larger losses in the urine. In the proximal convoluted tubule, 60% to 80% of the K+ in the tubular fluid is reabsorbed by paracellular transport; it is dependent upon the movement of Na+, as follows: 1. Sodium is reabsorbed across the luminal membrane. 2. Water follows the Na+. 3. The concentration of the remaining solutes in the tubular fluid increases as water follows the movement of Na+. 4. Consequently, the solute concentration of tubular fluid is greater than in the interstitial fluid, creating a gradient between the tubular fluid and interstitial fluid. 5. Potassium moves down its concentration gradient from the tubular fluid by the paracellular route. 6. These conditions also allow the passive reabsorption of other solutes, including other cations (Mg2+, Ca2+), phosphate ion (PO43−), fatty acids, and urea.

Answer 21

Calcium and phosphate are two substances generally considered together because 99% of the body calcium is stored in bone, and the majority is stored as calcium phosphate. Approximately 60% of the Ca2+ in blood becomes part of the filtrate and then the tubular fluid. The remainder of the Ca2+ is bound to protein in the blood and is prevented from being filtered. In comparison, 90% to 95% of the PO43− is filtered as blood passes through glomerular capillaries. The amount of Ca and PO4 excreted in the urine is regulated by parathyroid hormone (PTH), and thus it influences blood levels of both Ca2+ and PO43− PTH inhibits PO43− reabsorption in the proximal convoluted tubule, and it stimulates Ca2+ reabsorption in the distal convoluted tubule. As additional PO43− is eliminated via the urine, less PO43− is available to form calcium phosphate, the major calcium salt deposited in bone. Thus, Ca2+ redeposited in the bone decreases, and blood Ca2+ increases.

Answer 22

The body’s main nitrogenous waste products are (1) urea, a small, water-soluble molecule produced from protein break- down in the liver; (2) uric acid, produced from nucleic acid breakdown primarily in the liver; and (3) creatinine, produced from metabolism of creatine in muscle tissue. Urea and uric acid are both reabsorbed and secreted, whereas creatinine is only secreted.

Answer 23

Certain drugs. Antibiotics (including penicillin and sulfonamides), aspirin (salicylate), morphine, chemotherapy drugs, saccharin, and chemicals in marijuana are just a few examples of the drugs that are eliminated in the urine. Other metabolic wastes. Urobilin and hormone metabolites (e.g., intermediates of hormone breakdown) are examples of metabolic wastes that are eliminated in urine. Some hormones. Human chorionic gonadotropin (hCG) is a hormone produced in abundance during the early months of a woman’s pregnancy. It is an example of a hormone that is eliminated in the urine. Other examples of excreted hormones include epinephrine and prostaglandins.

Answer 24

A positive feedback mechanism called the countercurrent multiplier involves the nephron loop and is partially responsible for establishing the salt concentration gradient within the interstitial fluid. Countercurrent refers to the tubular fluid’s “reversing” its relative direction as it moves first through the descending limb and then through the ascending limb of the loop. Multiplier refers to the positive feedback loop that increases the concentration of salts (e.g., Na+, Cl–) within the interstitial fluid. The juxtamedullary nephrons with their long nephron loops are primarily important in this process.

Answer 25

The countercurrent exchange process occurs as follows: ∙ As the blood flows through the vasa recta deep into the renal medulla alongside the ascending limb, water moves by osmosis out of these capillaries into the more concentrated interstitial fluid. At the same time, salts in the interstitial fluid enter the vasa recta by diffusion down their concentration gradients. Thus, the blood in the vasa recta is losing water and gaining salts, and the concentration of total salt in the blood increases. Thus, as blood within the vasa recta moves into the deepest part of the renal medulla, it becomes more and more concentrated. ∙ If the vasa recta were to continue deep into the renal medulla, these salts would be transported away in the blood. However, the path of the blood flow in the vasa recta makes a 180-degree turn and is positioned alongside the descending limb of the nephron loop toward the cortex. Salts are now transported into a region in which osmotic and solute gradients reverse. Here, the salts diffuse back out of the blood into the interstitial fluid, while water moves into the vasa recta. Thus, as blood within the vasa recta flows toward the renal cortex, it becomes less and less concentrated. In fact, the blood within the vasa recta returning to the renal cortex has approximately the same concentration (or is slightly less concentrated) as when it first left the cortex.

Answer 26

One way to assess kidney function is to measure the rate filtrate is formed per unit time—that is, the glomerular filtration rate (GFR). To conduct this test, an individual is injected with inulin, a polysaccharide derived from plants that is freely filtered and neither reabsorbed nor secreted in the kidney, so the amount in the urine is equal to the amount that is filtered. (Inulin should not be confused with the hormone insulin, which regulates blood glucose levels.) Enough inulin is injected into a subject to achieve a blood plasma concentration of 1 mg/mL. Urine is collected and measured for volume and concentration of the inulin. Additionally, blood is drawn and the plasma concentration of inulin is measured at given time intervals. Glomerular filtration rate is determined by the following formula: GFR = UV/P where U = concentration of inulin in urine, V = volume of urine produced per minute, and P = concentration of inulin in plasma.

Answer 27

The renal plasma clearance test measures the volume of plasma that can be completely cleared of a substance in a given period of time—usually in 1 minute. We may infer from this test whether a substance is reabsorbed or secreted. If a substance (like inulin) is neither reabsorbed nor filtered, its renal plasma clearance is equal to the GFR (125 mL/min). However, if a substance is reabsorbed, its renal plasma clearance is lower than GFR because less of the substance is excreted, or “cleared,” in the urine. For example, the renal plasma clearance of urea is 70 mL/min. If urea is filtered at a rate of 125 mL/min (the normal GFR), and only 70 mL/min is cleared, the rest (55 mL/min) is reabsorbed. In contrast, the renal plasma clearance of glucose is normally 0 mL/min because in a healthy individual 100% of the glucose is reabsorbed and none is excreted in the urine.

Answer 28

Substances that are filtered and secreted have renal plasma clearance values higher than the GFR. This occurs because additional amounts of the substance are secreted into the tubular fluid and excreted in the urine. For example, creatinine has a renal plasma clearance of 140 mL/min, indicating that the substance is both filtered and secreted. Note that in clinical practice, renal plasma clearance of creatinine can be used to approximate glomerular filtration rate because its clearance is only slightly higher than GFR. Measuring creatinine clearance avoids the need to inject inulin into the patient’s blood.

Answer 29

Urine is the product of filtered and processed blood plasma. It typically is a sterile excretion unless contaminated with microbes in the kidney or urinary tract. Urine characteristics include its composition, volume, pH, specific gravity, color and turbidity, and smell. The normal pH for urine ranges between 4.5 and 8.0; the average value is 6.0, which is slightly acidic. Specific gravity is the density (g/mL) of a substance compared to the density of water (1 g/mL). For example, if your urine were com- posed only of pure water, it would have a specific gravity of 1.000. The average specific gravity of urine is slightly higher, with levels ranging from 1.003 to 1.035 because solutes are normal components of urine. he color of urine ranges from almost clear to dark yellow, depending upon the concentration of pigment from urobilin. Normal vaginal secre- tions, excessive substances in urine (cellular material, protein), crystallization or precipitation of salts if collected and left standing, and bacteria will increase the turbidity (cloudiness) of the urine. Urinoid is the term used for the normal smell of fresh urine. Urine may develop an ammonia smell if allowed to stand because bacteria convert the nitrogen in urea into ammonia (NH3). Typical urine composition is approximately 95% water and 5% solutes. These solutes include salts (e.g., Na+, Cl−, K+, Mg2+, Ca2+, SO42−, H2PO4−, NH4+), nitrogenous wastes (e.g., urea, uric acid, creatinine), some hormones, drugs, and small amounts of ketone bodies (a waste product of digesting fatty acids)

Answer 30

The ureters are long, epithelial-lined, fibromuscular tubes that transport urine from the kidneys to the urinary bladder. The urinary bladder is an expandable, muscular organ that serves as a reservoir for urine. The urethra is an epithelial-lined, fibromuscular tube that extends from the anteroinferior surface of the urinary bladder to the urethral opening. The urethra transports urine to the exterior of the body

Answer 31

The female urethra has a single function: to transport urine from the urinary bladder to exterior of the body. The lumen of the female urethra is primarily lined with a stratified squamous epithelium. The urethra is approximately 4 centimeters long, and it opens to the outside of the body at the external urethral orifice located in the female perineum. The male urethra has both urinary and reproductive functions because it serves as a passageway for both urine and semen (but not at the same time). It is approximately 19 centimeters long (7.5 inches) and is partitioned into three segments: the prostatic urethra, the membranous urethra, and the spongy urethra.

Answer 32

The expulsion of urine from the bladder is called micturition

Answer 33

The storage of urine in the urinary bladder is controlled by both the autonomic and somatic nervous systems. During the filling of the urinary bladder, urine moves through the ureters from the kidneys. Varying nerve signals conducted by sympathetic axons cause smooth muscle cells of (1) the detrusor muscle of the urinary bladder wall to relax, which allows the bladder to accommodate the urine, and (2) internal urethral sphincter to contract, so that urine is retained within the bladder. This process is referred to as the storage reflex. The skeletal muscle of the external urethral sphincter is also continuously stimulated by nerve signals along the pudendal nerve so it remains contracted. Micturition Reflex The process of micturition is also controlled by both the autonomic and somatic nervous systems in a toilet-trained individual, and it usually proceeds as follows: 1 When the volume of urine retained within the bladder reaches approximately 200 to 300 mL, the bladder becomes distended, and baroceptors in the bladder wall are activated. 2 These baroceptors send nerve signals through visceral sensory neurons to stimulate the micturition center within the pons. 3 The micturition center alters nerve signals propagated down the spinal cord and through the pelvic splanchnic nerves (which are parasympathetic nerves). 4 Parasympathetic stimulation causes the smooth muscle cells composing the detrusor muscle to contract and the internal urethral sphincter to relax. In infants, urination occurs at this point because they lack voluntary control of the external urethral sphincter.

Answer 34

An individual’s conscious decision to urinate is due to altering nerve signals relayed from the cerebral cortex through the spinal cord and along the pudendal nerve to cause relaxation of the external urethral sphincter. Expulsion of urine is facilitated by the voluntary contraction of muscles in the abdominal wall and expiratory muscles as part of the Valsalva maneuver

Answer 35

The specific percentage of body fluid depends upon two variables: the age of an individual and the relative amounts of adipose connective tissue and skeletal muscle tissue: ∙ Age. Infants have the highest percentage of fluid, at approximately 75% fluid by weight. In contrast, elderly individuals have the lowest percentage of fluid at 45%. Children, and both young and middle-aged adults, are usually somewhere in between these two extremes, with a general trend of decreasing percentage of body fluid seen with increasing age. Relative amounts of adipose connective tissue to skeletal muscle tissue. The percentage of fluid in the body at each age depends upon the ratio of adipose connective tissue to skeletal muscle tissue because of the difference in water content of these tissues. Adipose connective tissue is approximately 20% water, whereas skeletal muscle tissue is approximately 75% water. This accounts for the general differences in the percentage of body fluid that are observed between females and males of the same age after puberty. Lean adult females are, on average, typically composed of 55% body fluid, whereas lean adult males are, on average, typically composed of 60% body fluid. This difference reflects the relatively lower amounts of skeletal muscle and relatively higher amounts of adipose connective tissue in a lean adult female compared to a lean adult male.

Answer 36

The fluid in our body is partitioned into two fluid compartments: intracellular fluid and extracellular fluid. Intracellular fluid (ICF) is the fluid within our cells. A majority, or approximately two-thirds, of the total fluid is within our cells. The fluid outside of cells is referred to collectively as extracellular fluid (ECF). Intracellular fluid is the most distinct compartment; it contains more potassium (K+) and magnesium (Mg2+) cations, phosphate anion (PO43–), and negatively charged proteins than the extracellular fluidExtracellular fluid includes both interstitial fluid (IF), the fluid that surrounds and “bathes” the cells, and blood plasma, which is the fluid within the blood vessels Both interstitial fluid and blood plasma have a high concentration of these ions: sodium (Na+) cation, and chloride (Cl–) and bicarbonate (HCO3–) anions. Interstitial fluid and blood plasma exhibit one significant difference: protein is present in blood plasma, but there is very little protein within the interstitial fluid.

Answer 37

This happens when the fluid concentration in one fluid compartment becomes either hypotonic or hypertonic, with respect to another compartment; water immediately moves by osmosis between the two compartments until the water concentration is once again equal. water always moves by osmosis from the hypotonic solution to the hypertonic solution. When you drink water, it enters your blood from the gastrointestinal (GI) tract and becomes part of the blood plasma. Consequently, the plasma osmolarity decreases and blood plasma becomes hypotonic to both the interstitial fluid and the ICF. Thereafter, as blood moves through the capillaries, water first moves out of the blood plasma to become part of the interstitial fluid, then moves from the interstitial fluid into cells. This means there is a net movement of water from the blood plasma into the cells. In contrast, if water is lost from the body without being replaced in a timely manner, then dehydration occurs. Blood plasma osmolarity increases, and blood plasma becomes hypertonic to both the interstitial fluid and the cells. Consequently, a net movement of water occurs from the cells into the blood plasma. It moves first into the interstitial fluid and then into the blood plasma

Answer 38

Fluid balance exists when fluid intake is equal to fluid output, and a normal distribution of water and solutes is present in the two major fluid compartments.

Answer 39

Fluid intake is the addition of water to the body: ∙ Ingested (preformed) water includes the water absorbed from food and drink taken into the GI tract. On average, this is approximately 2300 milliliters (mL) of fluid intake per day. ∙ Metabolic water includes the water produced daily from aerobic cellular respiration and dehydration synthesis. It is approximately 200 mL of fluid per day. Fluid output must equal fluid intake to maintain fluid balance. Fluid is lost from the body through the normal mechanisms of ∙ Breathing ∙ Sweating ∙ Cutaneous transpiration (evaporation of water directly through the skin) ∙ Defecation ∙ Urination The amount of water lost through each of these processes depends upon physical activity, environmental conditions, and internal conditions of the body.

Answer 40

Sensible water loss is measurable, and it includes fluid lost through feces and urine. In contrast, insensible water loss is not measurable. It includes both fluid lost in expired air and fluid lost from the skin through sweat and cutaneous transpiration. Obligatory water loss is a loss of water that always occurs, regardless of the state of hydration of the body. It includes water lost through breathing and through the skin (insensible water loss), as well as fluid lost in the feces and in the minimal amount of urine produced to eliminate wastes from the body, approximately 0.5 L (500 mL) per day. Facultative water loss is controlled water loss through regulation of the amount of urine expelled from the body. It is dependent upon the degree of hydration of the body and is hormonally regulated in the distal convoluted tubule, collecting tubules, and collecting ducts in the nephrons of the kidney. Note that the only physiologic mechanism to control fluid output is the hormonal regulation of urine output. When the body is overhydrated, hormonally controlled facultative water loss normally plays a significant role in eliminating excess fluid. In comparison, when the body is dehydrated, obligatory fluid output always occurs, regardless of the hydrated state of the body. Therefore, hormonal regulation of urine output can only decrease fluid loss when the body is dehydrated, but not inhibit it completely.

Answer 41

Fluid intake is greater than fluid output. When fluid intake is greater than fluid output, the following occur: an increase in blood volume and an increase in blood pressure—if water gain exceeds solute gain, then there is also a decrease in blood osmolarity. Fluid intake > Fluid output: ↑ blood volume ↑ blood pressure (perhaps ↓ blood osmolarity) Stimuli to Turn off the Thirst Center Stimuli for inhibiting the thirst center are produced when fluid intake is greater than fluid output. All of these stimuli (except distension of the stomach) oppose stimuli that activate the thirst center. These include the following: ∙ Increased blood volume and blood pressure. Blood volume and blood pressure increase with the addition of fluid. This rise in blood pressure inhibits the kidney from releasing renin, and the subsequent production of angiotensin II decreases. A decrease in angiotensin II results in a reduced stimulation of the thirst center. ∙ Decreased blood osmolarity. Blood osmolarity decreases when additional fluid enters the blood. In response, the thirst center is no longer stimulated directly, and the hypothalamus decreases stimulation of ADH release from the posterior pituitary. ∙ Increased salivary secretions. When body fluid level is high, salivary secretions increase, and the mucous membranes of the mouth and throat become moist. Sensory input to the thirst center decreases. ∙ Distension of the stomach. Fluid entering the stomach causes it to stretch, and nerve signals are relayed to the hypothalamus to inhibit the thirst center. ∙ Fluid output is greater than fluid intake. When fluid output is greater than fluid intake, the following occur: a decrease in blood volume and a decrease in blood pressure—if more water is lost than solutes, there is also an increase in blood osmolarity. Fluid output > Fluid intake: ↓ blood volume ↓ blood pressure (perhaps ↑ blood osmolarity) Stimuli to Turn on the Thirst Center Stimuli for activating the thirst center, which occurs when fluid intake is less than fluid output, include the following: ∙ Decreased blood volume and blood pressure. When fluid intake is less than fluid output, blood volume decreases, with an accompanying decrease in blood pressure. Renin is released from the kidney in response to a lower blood pressure. Renin (and the ACE enzyme) initiates the conversion of angiotensinogen to angiotensin II. An increase of 10–15% in the concentration of angiotensin II within the blood stimulates the thirst center. ∙ Increased blood osmolarity. This occurs most commonly from insufficient water intake and dehydration. The increase in blood osmolarity directly stimulates sensory receptors in the thirst center within the hypothalamus and stimulates the hypothalamus to initiate nerve signals to the posterior pituitary to release antidiuretic hormone (ADH). ADH activates the thirst center. ∙ Decreased salivary secretions. A separate mechanism not related to blood volume, blood pressure, and blood osmolarity can also stimulate the thirst center. This additional stimulus is a relatively dry mouth. Mucous membranes are not as moist when less fluid is available and saliva production decreases. Sensory receptors in the mucous membranes of the mouth and throat relay sensory input to the thirst center.

Answer 42

Four major hormones are involved in regulating urine output: angio-tensin II, antidiuretic hormone (ADH), aldosterone, and atrial natriuretic peptide (ANP). Angiotensin II, ADH, and aldosterone help decrease urine output. These three hormones function to maintain both blood volume and blood pressure. In contrast, ANP increases urine output to decrease both blood volume and blood pressure. The specific mechanisms employed by each of these hormones in regulating fluid output in the kidneys also function in regulating some electrolytes

Answer 43

Molecules that do not dissociate (or come apart) in solution are called nonelectrolytes. Most of these substances are covalently bonded organic molecules (e.g., glucose). In contrast, an electrolyte is any substance that dissociates in solution to form cations and anions. The term electrolyte refers directly to the ability of these substances, when dissolved and dissociated in solution, to conduct an electric current. Electrolytes include salts, acids, bases, and some negatively charged proteins.

Answer 44

Because osmotic pressure is dependent upon the number of solutes, NaCl exerts twice the osmotic pressure of the same concentration of a nonelectrolyte, such as glucose, which does not dissociate into ionic forms. Further, CaCl2 in solution dissociates into three components, Ca2+ and two Cl–, and exerts three times the osmotic pressure compared to that of glucose.

Answer 45

The common electrolytes include Na+, K+, Cl–, Ca2+, PO43–, and Mg2+

Answer 46

Sodium is the principal cation within the extracellular fluid (ECF). (a) Normal blood plasma Na+ concentration is between 135 mEq/L and 145 mEq/L and is important in maintaining fluid balance within the ECF. Sodium level increases through the diet and decreases through urine, feces, and sweating. Sodium content and concentration are regulated by aldosterone, antidiuretic hormone (ADH), and atrial natriuretic peptide (ANP). (b) Changes in Na+ concentration cause movement of water between fluid compartments. Decreased H2O or increased Na+ concentration - hypertonic Increased H2O or decreased Na+ concentration - hypotonic Aldosterone - Retains Na+ and water Maintains Na+ blood plasma concentration ADH - Retains water Decreases Na+ blood plasma concentration ANP - Increases excretion of Na+ and H2O Decreases Na+ blood plasma concentration

Answer 47

Recall that angiotensinogen is an inactive hormone synthesized and released continuously from the liver. Its activation, which occurs within the blood, is initiated by the enzyme renin. Renin is released from the juxtaglomerular (JG) apparatus of the kidneys in response to either (1) low blood pressure or (2) stimulation by the sympathetic division. The sequential action of renin and angiotensin- converting enzyme (ACE) (which is bound to the endothelial lining of blood vessels) causes the formation of angiotensin II (the active form of the hormone). It causes an increase in resistance, a decrease in fluid output (which helps to maintain blood volume and blood pressure), and an increase in blood volume (if fluid intake occurs). Consequently, blood pressure increases. Increasing blood pressure is aided by the release of both ADH and aldosterone. As blood pressure returns to within normal homeostatic levels, both renin release and angiotensin II synthesis are decreased by negative feedback.

Answer 48

Blood vessels: Stimulates vasoconstriction of systemic blood vessels to increase total peripheral resistance, which increases systemic blood pressure ∙ Kidneys: Decreases urine output from the kidneys as a result of decreased glomerular filtration rate (GFR) in the nephrons by stimulating vasoconstriction of afferent arterioles and contraction of the mesangial cells within the glomerulus; this decreases urine output and helps to maintain systemic blood volume, and thus blood pressure ∙ Thirst center: Stimulates the thirst center in the hypothalamus; if fluid intake occurs, this increases blood volume, which increases systemic blood pressure ∙ Hypothalamus and adrenal cortex: Stimulates both the hypothalamus to activate the posterior pituitary to release ADH and the adrenal cortex to release aldosterone (both described shortly)

Answer 49

Three primary types of stimuli—low blood pressure, low blood volume, or an increase in blood osmolarity—signal the hypothalamus regarding the need to retain fluid: Angiotensin II (produced with a decrease in blood pressure) Low blood volume (detected by sensory input from baroreceptors in heart and vessels) Increased blood osmolarity (detected by chemoreceptors within hypothalamus)

Answer 50

Thirst center. ADH stimulates the thirst center in the hypothalamus. If fluid intake occurs, blood volume and blood pressure increase, and blood osmolarity decreases. ∙ Kidneys. ADH increases water reabsorption in the kidneys. ADH binds to principal cells of the collecting tubules and ducts, stimulating these cells to increase the number of aquaporins in the tubular membrane. Additional water is reabsorbed through these aquaporins. This helps to both maintain blood volume by decreasing fluid loss in urine and decrease blood osmolarity. ∙ Blood vessels. High doses of ADH (which occur, for example, with severe hemorrhage) cause vasoconstriction of systemic blood vessels, which increases peripheral resistance. This action is the reason that ADH is also referred to as vasopressin. Systemic blood pressure increases as a result.

Answer 51

Angiotensin II (produced with a decrease in blood pressure) Decreased Na+ blood plasma levels Increased K+ blood plasma levels

Answer 52

The effect of aldosterone is to decrease urine output to maintain blood volume and blood pressure, as well as to alter blood plasma concentration of Na+ and K+ to within normal homeostatic limits.

Answer 53

Increased blood volume (as detected by increased stretch of baroreceptors in atria) Blood vessels. ANP dilates systemic blood vessels, resulting in decreased total peripheral resistance. Systemic blood pressure decreases as a result. Kidneys. ANP causes vasodilation of the afferent arterioles in the kidneys and relaxation of mesangial cells; both increase the glomerular filtration rate. Additionally, ANP inhibits Na+ and water reabsorption by nephron tubules, resulting in additional loss of Na+ and water. These changes increase urine output. Blood volume and systemic blood pressure decrease.

Answer 54

Fixed acid (also called nonvolatile acid or metabolic acid) is the wastes produced from metabolic processes (other than from carbon dioxide). Examples of fixed acids include lactic acid from glycolysis, phosphoric acid from nucleic acid metabolism, and keto-acids from metabolism of fat. Note that the term fixed acids was given to these types of acids because they are not produced from carbon dioxide, which is volatile and expired by the lungs. Rather, they are nonvolatile acids that are “fixed” in the body. Volatile acid is carbonic acid produced when carbon dioxide combines with water, as shown: CO2 + H2O →← H2CO3 (carbonic acid) The level of each category of acid is altered by separate physio-logic buffering systems: ∙ Fixed acid is regulated by the kidney (type A and type B intercalated cells) through the absorption and elimination of HCO3– and H+. ∙ Volatile acid is eliminated by the respiratory system, and the amount expired is changed through the regulation of the respiratory rate and depth

Answer 55

Chemical buffers are substances that resist changes in pH by absorbing or releasing hydrogen ions. They include bicarbonate, phosphate, and proteins. These buffers act rapidly to prevent drastic changes in pH. bicarbonate buffer, phosphate buffer, protein buffer system bicarbonate buffer acts in the blood and interstitial fluid Phosphate buffer functions in the urine and Intracellular fluid Protein buffer operates in both blood and intracellular fluid Physiological buffers, on the other hand, involve the body's respiratory and renal systems. The respiratory system regulates pH by controlling the amount of carbon dioxide in the blood, while the renal system regulates pH by controlling the excretion of hydrogen ions and reabsorption of bicarbonate ions in the kidneys.

Answer 56

The input of acid into the blood, except that produced from CO2, occurs from two major sources: nutrients absorbed by the gastrointestinal (GI) tract and body cells Acid is normally added to the blood from the gastrointestinal (GI) tract as a result of diets rich in animal protein and wheat, as well as cellular metabolic wastes that include lactic acid from glycolysis, phosphoric acid from nucleic acid metabolism, and ketoacids from fat metabolism.

Answer 57

intercalated type A cells of the distal convoluted tubule and collecting tubules respond to increased blood H+ concentration, as follows: ∙ Secrete H+ into the filtrate (which is then excreted in the urine) ∙ Synthesize new HCO3– , which is then absorbed into the blood

Answer 58

Intercalated type B cells of the distal convoluted tubule and collect- ing tubules respond to alkaline conditions as follows: ∙ Synthesize new HCO – that is secreted into the filtrate (which 3 is then excreted in the urine) ∙ Absorb H+ into the blood

Answer 59

The respiratory system also serves as a physiologic buffering system to maintain acid-base balance, but it does so by regulating the level of the volatile carbonic acid (H2CO3), which is produced from CO2 During exercise, additional CO2 is produced by body cells and enters the blood, causing a temporary increase in Pco2. Chemoreceptors are stimulated—central chemoreceptors in the brainstem (by an increase in H+ produced from CO2 that diffuses from the blood into the cerebrospinal fluid) and peripheral chemoreceptors in the aorta and carotid blood vessels (primarily by changes in blood H+ and CO2). Sensory input is relayed to the respiratory center, which responds by increasing breathing depth (referred to as hyperpnea), and additional CO2 is expired. These adjustments occur relatively quickly, usually within several minutes. Because of these adjustments in breathing depth, blood levels of CO2 during exercise (like at rest) are normally maintained within homeostatic limits. Consequently, whether the body is at rest or engaging in exercise, blood Pco2 and H2CO3 do not usually have daily fluctuations and do not normally affect acid-base balance.

Answer 60

The protein buffering system is a chemical buffering system com- posed of proteins within cells and blood plasma. It accounts for about three-quarters of the chemical buffering in body fluids. The amine group (– NH2) of amino acids acts as a weak base to buffer acid, whereas the carboxylic acid (– COOH) of amino acids acts as a weak acid to buffer base. With the addition of strong acid, shown in equation (1), the weak base (—NH2) of the protein buffering system binds the H+ that was added to the solution. This weak base becomes a weak acid (NH3+) as a result. The net effect is the elimination of a strong acid (H+) and the production of a weak acid (NH3+). In comparison, the addition of strong base, as in equation (2), causes the weak acid (—COOH) of the protein buffering system– to release H+, and as it does so, it becomes a weak base (COO ). The net effect is the removal of a strong base (OH–) and the production of a weak base (COO–). Proteins are components within both cells (cellular proteins) and blood (both plasma proteins and hemoglobin within erythrocytes), and their buffering systems help minimize pH changes throughout the body. The most notable exception is in the cerebrospinal fluid (CSF), where there are no proteins.

Answer 61

The phosphate buffering system is found within intracellular fluid (ICF). It is especially effective in buffering metabolic acid produced by cells because phosphate (PO43–) is the most common anion within cells. The phosphate buffering system is also composed of both a weak base and a weak acid. Here hydrogen phosphate (HPO42–) is the weak base and dihydrogen phosphate (H2PO4–) is the weak acid. HPO 2– + H+ → H 2PO 4 H2PO4 + OH → HPO4 The addition of acid, shown in equation (3), is buffered by the weak base (HPO42–) that binds the H+ to become a weak acid (H2PO4–). In contrast, the addition of base, as in equation (4), causes the weak acid H2PO4– to release H+. As it does so, it becomes the weak base HPO42–, and water is formed. As with the protein buffering system, the net result is either a strong acid buffered to produce a weak acid or a strong base buffered to produce a weak base.

Answer 62

Bicarbonate (HCO3–) serves as a weak base, whereas carbonic acid (H2CO3) acts as a weak acid. Bicarbonate binds the excess H+ and becomes the weak acid H2CO3–. In comparison, with the addition of base, the weak acid H2CO3 releases H+. As it does so, it becomes a weak base (HCO3–), and water is formed. The net result again is that a strong acid is buffered to produce a weak acid, or a strong base is buffered to produce a weak base.

Answer 63

An acid-base disturbance occurs when the buffering capacity of chemical buffering systems is exceeded. Consequently, there is a transient, or temporary, change in blood H+ concentration, resulting in a change in blood pH beyond the normal range of 7.35 to 7.45. In response to the transient acid-base disturbance, the physiologic buffering system of the kidneys, the respiratory system, or both, attempts to offset the disturbance. Four major types of acid-base disturbances are distinguished based on two criteria: whether the primary disturbance is respiratory or metabolic in nature and whether the pH change is acidic or alkaline. The four cat- egories are respiratory acidosis, respiratory alkalosis, metabolic acidosis, and metabolic alkalosis.

Answer 64

Respiratory acidosis is the most common acid-base disturbance and is clinically recognized as occurring when the Pco2 in the arterial blood becomes elevated above 45 mm Hg. Respiratory acidosis is a consequence of impaired respiratory function and can have many different causes, including ∙ Hypoventilation, which is breathing that is too slow or too shallow as a result of (1) disorders of the nerves or muscles involved with breathing or (2) injury to the respiratory center that is perhaps caused by trauma, drug overdose, or poliovirus infection ∙ Decreased airflow caused by (1) an object lodged in the respiratory tract (choking), (2) severe bronchitis, or (3) severe asthma ∙ Impaired alveolar gas exchange due to (1) reduced respiratory membrane surface area or (2) thickened width of the respiratory membrane An individual with impaired respiratory function expires less CO2. Consequently, more CO2 remains within the blood. The additional CO2 drives the carbonic anhydrase reaction (CO2 + H2O ⇄ H2CO3 ⇄ HCO3− + H+) to the right, with the most significant change being increased formation of H+.

Answer 65

Respiratory alkalosis is clinically recognized as occurring when the Pco2 decreases to levels below 35 mm Hg. Respiratory alkalosis occurs due to hyperventilation, which is when the breathing rate or depth is increased to eliminate greater amounts of CO2 than are being produced by the body’s cells. Causes include the following: ∙ Severe anxiety ∙ Hypoxia, which is insufficient oxygen delivery to body cells (e.g., as might occur when climbing to a high altitude where there is a decrease in the partial pressure of oxygen [Po2]; during congestive heart failure; as a result of severe anemia) ∙ Aspirin overdose (a condition that stimulates the respiratory center) During hyperventilation, an individual expires more CO2 than is being produced by body cells. Consequently, less CO2 remains within the blood. The decrease in CO2 drives the carbonic anhydrase reaction (CO2 + H2O ⇄ H2CO3 ⇄ HCO3− + H+) to the left, with the most significant change being decreased formation of H+. If the buffering capacity of chemical buffers is exceeded and blood pH increases above 7.45, alkalosis results.

Answer 66

Metabolic acidosis ∙ Generally occurs when there is an accumulation of fixed acid from (1) increased production of metabolic acids or (2) decreased elimination of acid due to renal dysfunction ∙ May occur from an excessive loss of HCO3- (e.g., severe diarrhea because HCO3- is normally lost in the feces) The abnormal increase in blood H+ levels (from ingestion of fixed acid from the GI tract, or formation from cellular metabolism, or the kidneys failing to secrete H+) binds with HCO3- and lowers blood HCO3- levels. If the buffering capacity of chemical buffers is exceeded, blood pH decreases below 7.35, resulting in acidosis. Metabolic alkalosis This increase: ∙ Generally occurs due to loss of H+ from vomiting or prolonged nasogastric suction (due to loss of stomach secretions) May occur from increased loss of acids by the kidneys with overuse of diuretics (medications that increase urine output) ∙ Occasionally results from increased base input from consuming large amounts of antacids The abnormal decrease in H+ (from the stomach or by the kidney in response to certain diuretics) increases blood HCO3- levels. If the buffering capacity of chemical buffers is exceeded, blood pH increases above 7.45, resulting in alkalosis.

Answer 67

Renal compensation (physiologic adjustments of the kidney to chang- es in pH) occurs in response to elevated blood H+ concentration due to a cause other than a renal dysfunction. We know that the normal physiologic activities of type A intercalated cells result in excretion of H+ and reabsorption of HCO –. However, during renal compensation, this occurs to a greater degree than normal. Consequently, higher than normal levels of H+ are excreted, and higher than normal amounts of HCO3– are synthesized and reabsorbed into the blood. Renal compensation results in elevated values for blood HCO3–. As expected, urine pH is lower than normal. In comparison, renal compensation in response to a decrease in blood H+ concentration involves the normal response of type B intercalated cells to excrete HCO3– and reabsorb H+. During renal compensation, this activity occurs to a greater degree than normal; higher than normal levels of H+ are reabsorbed into the blood, and higher amounts of HCO3– are excreted. Renal compensation results in lower than normal for blood HCO3–. Urine pH in this case is higher than normal.

Answer 68

Respiratory compensation occurs in response to metabolic acidosis that is caused by abnormally low levels of HCO3- (base). Breathing rate increases, and higher than normal amounts of CO2 are expired and lower than normal levels of CO2 (volatile acid) remain in the blood. Respiratory compensation also occurs in response to metabolic alkalosis that is caused by increased levels of HCO3- (base). Breathing rate decreases, and lower than normal amounts of CO2 are expired and higher than normal levels of CO2 (volatile acid) remain in the blood. Generally, respiratory compensation is less effective in addressing metabolic acid-base disturbances than renal compensation. The ability to decrease respiratory rate in response to metabolic alkalosis is limited by the development of hypoxemia. As the respiratory rate decreases, blood Po2 levels also decrease. When blood Po2 levels decrease below critical values, the respiratory rate is stimulated to increase. This may prevent complete compensation for metabolic alkalosis.

Answer 69

Both females and males have paired primary reproductive organs called gonads: These are ovaries in females and testes in males. The gonads produce sex cells called gametes. Gametes are oocytes in females and sperm in males, which unite at fertilization to initiate the formation of a new individual. ∙ The gonads also produce relatively large amounts of sex hormones, which affect maturation, development, and changes in the activity of the reproductive system organs. ∙ Both sexes also have accessory reproductive organs, including ducts to carry gametes away from the gonads toward the site of fertilization (in females) or to the outside of the body (in males).

Answer 70

Puberty is initiated when the hypothalamus begins secreting gonadotropin-releasing hormone (GnRH). GnRH acts on specific endocrine cells in the anterior pituitary and stimulates them to release the gonadotropins follicle-stimulating hormone (FSH) and luteinizing hormone (LH). (Prior to puberty, FSH and LH are virtually nonexistent in girls and boys.) As levels of FSH and LH increase, the gonads produce significant levels of sex hormones and start the processes of both gamete and sexual maturation.

Answer 71

Autosomes contain genes that code for cellular functions. These genes also help determine most human characteristics, such as eye color, hair color, height, and skin pigmentation. A pair of matching autosomes is called homologous chromosomes. ∙ The pair of sex chromosomes consists of either two X chromosomes or an X and a Y chromosome. These chromosomes primarily determine whether an individual is female (two X chromosomes) or male (one X chromosome and one Y chromosome), although they also contain genes that code for cellular functions.

Answer 72

Somatic cell division * Produces 2 daughter cells genetically identical to parent cell * Diploid daughter cells * No crossing over * Sex cell division * 4 daughter cells genetically different from parent cell * Haploid daughter cells * Includes crossing over

Answer 73

Oogenesis is the maturation of a primary oocyte to a secondary oocyte. Follicular phase: As the ovarian follicles develop, their granulosa cells release the hormone inhibin. Inhibin (and the initial low levels of estrogen secreted by the primary follicles) helps inhibit FSH production, thus preventing excessive ovarian follicle development and allow- ing the current primary follicles to mature. Shortly thereafter, a few of these primary follicles mature and become secondary follicles. The primary follicles that do not mature undergo atresia. While the earlier stages of follicular development (primordial to primary to secondary) primarily are the result of molecular signals between the oocyte and follicle cells, the later stages of follicular development (secondary to antral to mature follicle) are more directly dependent on increased FSH and LH levels. Thus, FSH and LH now directly act on the secondary follicles. Typically, only one secondary follicle in an ovary develops into an antral follicle. Late in the follicular phase, this antral follicle develops into a mature follicle. Ovulation is induced only when there is a peak in LH secretion.

Answer 74

Oogonia are diploid cells (containing 23 pairs of chromosomes, or 46 total) that are the origin of oocytes. Mitotic divisions of oogonia produce primary oocytes, which are diploid cells. Primary oocytes start the process of meiosis but are arrested in prophase I. Primordial follicles develop before birth. Monthly, from puberty to menopause Up to about 20 primordial follicles are stimulated to mature into primary follicles every month. Some primary follicles mature into secondary follicles. Primary follicles that do not mature undergo atresia. Typically only one antral follicle develops into a mature follicle, where the primary oocyte completes the first meiotic division to produce a polar body and a secondary oocyte. The secondary oocyte is a haploid cell (containing 23 chromosomes only) that is arrested in the second meiotic metaphase. If the secondary oocyte is fertilized, it completes the second meiotic division and becomes an ovum. If the secondary oocyte is not fertilized, it degenerates.

Answer 75

The follicular phase occurs during days 1–13 of an approximate 28-day ovarian cycle. GnRH stimulates FSH and LH secretion: Some ovarian follicles develop and produce estrogen. One secondary follicle develops into an antral follicle. Functional layer of the endometrium is shed. Estrogen and inhibin inhibit the hypothalamus and anterior pituitary, causing a drop in FSH. One follicle continues to mature and produce estrogen. Functional layer of the endometrium is rebuilding. Days 13–14 Ovarian cycle: Follicular phase, ovulation The increase in estrogen above threshold stimulates the hypothalamus and anterior pituitary, causing an LH surge. The LH surge induces ovulation. Days 15–28 Ovarian cycle: Luteal phase Corpus luteum forms and secretes large amounts of progesterone, estrogen, and inhibin. Combined, these inhibit GnRH, FSH, and LH secretion. Progesterone stimulates uterine lining growth. If the oocyte is not fertilized, the corpus luteum regresses and hormone levels drop.

Answer 76

Days 1–5 Ovarian cycle: Follicular phase Uterine cycle: Menstrual phase GnRH stimulates FSH and LH secretion: Some ovarian follicles develop and produce estrogen. One secondary follicle develops into an antral follicle. Functional layer of the endometrium is shed. Days 6–12 Ovarian cycle: Follicular phase Uterine cycle: Proliferative phase Estrogen and inhibin inhibit the hypothalamus and anterior pituitary, causing a drop in FSH. One follicle continues to mature and produce estrogen. Functional layer of the endometrium is rebuilding. Days 13–14 Ovarian cycle: Follicular phase, ovulation Uterine cycle: Proliferative phase The increase in estrogen above threshold stimulates the hypothalamus and anterior pituitary, causing an LH surge. The LH surge induces ovulation. Days 15–28 Ovarian cycle: Luteal phase Uterine cycle: Secretory phase Corpus luteum forms and secretes large amounts of progesterone, estrogen, and inhibin. Combined, these inhibit GnRH, FSH, and LH secretion. Progesterone stimulates uterine lining growth. If the oocyte is not fertilized, the corpus luteum regresses and hormone levels drop.

Answer 77

the hormone prolactin is produced in the anterior pituitary and is responsible for milk production. Thus, when the amount of prolactin increases, the mammary gland grows and forms more expanded and numerous alveoli. The hormone oxytocin, pro- duced by the hypothalamus and released from the posterior pituitary, is responsible for milk ejection.

Answer 78

Most of follicular phase 1 Hypothalamus secretes GnRH, which stimulates anterior pituitary. 2 FSH and LH help stimulate follicular development. 3 Maturing ovarian follicles secrete inhibin (which inhibits FSH production) and low levels of estrogen (which initially inhibit both the hypothalamus and anterior pituitary). 4 Estrogen also assists with the development of the mature follicle. 5 Mature follicle produces a large threshold amount of estrogen, which stimulates the hypothalamus and anterior pituitary. Late follicular, ovulation, and luteal phases 6 An LH surge from the anterior pituitary induces ovulation. 7 The corpus luteum forms under the influence of LH. 8 The corpus luteum secretes large amounts of progesterone, estrogen, and inhibin, which inhibit the hypothalamus and anterior pituitary.

Answer 79

The response begins with the excitement phase, where parasympathetic innervation of reproductive structures such as the mammary glands, clitoris, vaginal wall, bulbs of the vestibule, and labia become engorged with blood. The nipples become erect as a result of the blood engorgement in the mammary glands. The vestibular glands and the glands within the vaginal wall both produce mucin for lubrication. The uterus shifts from an anteverted position to a more erect position within the pelvic cavity. As the excitement phase continues, the erectile tissue of the clitoris swells as it engorges with blood and becomes very sensitive to tactile stimuli. The inferior part of the vaginal wall constricts slightly. The woman’s heart rate, blood pressure, and respiratory rate increase during this time as orgasm nears. Both divisions of the autonomic nervous system control many of these physiologic responses, and the excitement phase also is facilitated by somatosensory signals, such as the sensation of the penis in the vagina or the caressing of parts of the body. Orgasm in the female refers to the time period where there are intense feelings of pleasure, a feeling of a release of tension, perhaps a feeling of warmth, and some pelvic throbbing. The vagina and uterus contract rhythmically for a period of many seconds. The resolution phase follows orgasm. The uterus returns to its original position and the vaginal wall relaxes at this time. The excess blood leaves the other reproductive organs.

Answer 80

Spermatogenesis is the process of sperm development that occurs within the seminiferous tubule of the testis. Spermatogenesis does not begin until puberty, when significant levels of FSH and LH stimulate the testis to begin gamete development. 1 Germ cells that are the origin of sperm are diploid cells (containing 46 chromosomes, or 23 pairs) called spermatogonia. Mitotic divisions of these cells produce a new germ cell and a committed cell. The committed cell is a primary spermatocyte. 2 Meiosis I begins with the diploid primary spermatocytes. The haploid cells (containing 23 chromosomes only) produced during meiosis I are called secondary spermatocytes. 3 Meiosis II originates with the secondary spermatocytes and produces spermatids. 4 The process of spermiogenesis begins with spermatids and results in morphologic changes needed to form sperm that will be motile.

Answer 81

Both female and male gametes undergo meiosis, but only a single viable secondary oocyte is produced, whereas four sperm are produced. All female oocytes have initiated (and then become arrested in) meiosis prior to the female being born. In contrast, male spermatogonia do not undergo spermatogenesis until puberty, but after this time they can divide and produce spermatocytes throughout a male’s adult lifetime.

Answer 82

Formed in testes. Mature in the epididymis. Then passes through the vas deferens, then through the ejaculatory duct into the prostatic urethra, then through the intermediate urethra, then through the spongy urethra and finally to the body exterior.

Answer 83

The erectile bodies of the penis are composed of a complex network of venous spaces surrounding a central artery. During sexual excitement, blood enters the erectile bodies and fills the venous spaces. As these venous spaces become engorged with blood, the erectile bodies become rigid, a process called erection. The rigid erectile bodies compress the veins that drain blood away from the venous spaces. Thus, the spaces fill with blood, but the blood cannot leave the erectile bodies until the sexual excitement ceases. Parasympathetic innervation is responsible for increased blood flow and thus the erection of the penis. The internal urethral sphincter of the urinary bladder contracts to ensure that no urine enters the urethra at this time. Ejaculation typically occurs at the ending stage of an orgasm and is the process by which semen is expelled from the penis with the help of rhythmic contractions of the smooth muscle in the wall of the urethra. Sympathetic innervation is responsible for ejaculation.

Answer 84

parasympathetic innervation is necessary to achieve an erection, while sympathetic innervation promotes ejaculation. Reduction of autonomic activity after sexual excitement reduces blood flow to the erectile bodies and shunts most of the blood to other veins, thereby returning the penis to its flaccid condition. Following an orgasm, there is a resolution phase, which is marked by feelings of intense relaxation. The sympathetic division is stimulated to contract the central artery of the penis and contract small muscles around the erectile tissue, which expels the engorged blood. Gradually, the penis becomes soft and flaccid again. Resolution in men is followed by a refractory period, during which the man cannot attain another erection.

Answer 85

Genetic sex is also called genotypic sex, and it refers to the sex of an individual based on the sex chromosomes inherited. An individual with two X chromosomes is a genetic female, whereas a person with one X and one Y chromosome is a genetic male. Genetic sex is determined at fertilization. In contrast, phenotypic sex refers to the appearance of an individual’s internal and external genitalia. A per- son with ovaries and female external genitalia (labia) is a phenotypic female, whereas a person with testes and male external genitalia (penis, scrotum) is a phenotypic male. Phenotypic sex starts to become apparent after the seventh week of development.

Answer 86

In males, the sex-determining region Y (SRY) gene is located within the larger testis-determining factor (TDF) region on the Y chromosome. If the Y chromosome is present and the SRY gene is appropriately expressed, this gene produces proteins to stimulate the production of androgens that initiate male phenotypic development. If a Y chromosome is absent, or if the Y chromosome is either lacking or has an abnormal SRY gene, a female phenotypic sex results.

Answer 87

Because no SRY proteins are produced in the developing female, the mesonephric ducts degenerate. Between weeks 8 and 20 of development, the paramesonephric ducts develop and differentiate. The caudal (inferior) ends of the paramesonephric ducts fuse, forming the uterus and the superior part of the vagina. The cranial (superior) parts of the paramesonephric ducts remain separate and form two uterine tubes. The remaining inferior part of the vagina is formed from the urogenital sinus, which also forms the urinary bladder and urethra.

Answer 88

The urogenital folds (or urethral folds) are paired, elevated structures on either side of the urogenital membrane, a thin partition separating the urogenital sinus from the outside of the body. The genital tubercle is a rounded structure anterior to the urogenital folds. The labioscrotal swellings (or genital swellings) are paired elevated structures lateral to the urethral folds. They do not become clearly differentiated until about week 20. In the absence of testosterone, female external genitalia develop. The genital tubercle becomes the clitoris. The urogenital folds do not fuse, but become the labia minora. Finally, the labioscrotal folds also remain unfused and become the labia majora. Production and circulation of testosterone in the male cause the primitive external structures to differentiate. The genital tubercle enlarges and elongates, forming the glans of the penis and part of the dorsal side of the penis. The urogenital folds grow and fuse around the developing urethra and form most of the body of the penis. Finally, the labioscrotal swellings fuse at the midline, forming the scrotum.

Answer 89

Puberty is a period in adolescence where the reproductive organs become fully functional and the external sex characteristics become more prominent, such as breast enlargement in females and pubic hair in both sexes. The timing of puberty may be affected by genetics, environmental factors, and overall health of the individual. Puberty typically begins between ages 8 and 12 for girls and between ages 9 and 14 for boys

Answer 90

The earliest signs of puberty are the development of breast buds in girls and the appearance of pubic and axillary hair in both boys and girls. Menarche is one of the later events of puberty and tends to occur about 2 years after the first signs appear. Boys experience growth in the testicles and penis, and they may begin to get erections at this time and experience ejaculations during the night. Male voices start to change and become lower in pitch, as the increase in testosterone causes rapid growth of the laryngeal structures. Prior to puberty, FSH and LH are virtually nonexistent in girls and boys. As levels of FSH and LH increase, the gonads produce significant levels of sex hormones and start the processes of gamete maturation and sexual maturation.

Answer 91

The prenatal period begins with fertilization, when a secondary oocyte and sperm combine, and it ends approximately 38 weeks later with birth. The pre-embryonic period is the first 2 weeks of development (the first 2 weeks after fertilization), when the single cell produced by fertilization (called the zygote) becomes a spherical, multicellular structure (called the blastocyst). This period ends when the blastocyst implants in the lining of the uterus. ∙ The embryonic period extends from the third through eighth weeks of development. It is a remarkably active time during which rudimentary versions of the major organ systems appear in the developing body, which is now called an embryo. ∙ The fetal period includes the remaining 30 weeks of development prior to birth, when the organism is called a fetus. During the fetal period, the fetus continues to grow, and its organs increase in complexity.

Answer 92

Fertilization is the process by which two haploid gametes (sex cells) fuse to form a new diploid cell containing genetic material derived from both parents. Besides combining the male and female genetic material, fertilization restores the diploid number of chromosomes, determines the sex of the organism, and initiates cleavage. Fertilization typically occurs in the widest part of the uterine tube, called the ampulla.

Answer 93

Fertilization Within 12–24 hours after ovulation Ampulla of uterine tube Sperm penetrates secondary oocyte; secondary oocyte completes meiosis II and becomes an ovum; ovum and sperm pronuclei fuse Zygote At the end of fertilization Ampulla of uterine tube Diploid cell produced when ovum and sperm pronuclei fuse Cleavage 30 hours to day 3 post-fertilization Uterine tube Zygote undergoes cell division by mitosis to increase cell number, but overall size of structure remains constant Morula Days 3–4 post-fertilization Uterine tube Structure formed resembles a solid ball of cells; 16 or more cells are present, but there is no change in diameter from original zygote Blastocyst Days 5–6 Uterus Hollow ball of cells; outer ring formed by trophoblast; inner cell mass (embryoblast) is a cell cluster inside blastocyst Implantation Begins late first week and is complete by end of second week Functional layer of endometrium of uterus Blastocyst adheres to functional layer of uterus; trophoblast cells and functional layer together begin to form the placenta

Answer 94

Implantation is the process by which the blastocyst burrows into and embeds within the endometrium. The blastocyst begins the implantation process by about day 7 (the end of the first week of development), when trophoblast cells begin to invade the functional layer of the endometrium. Simultaneously, the trophoblast subdivides into two layers: a cytotrophoblast, which is the inner cellular layer of the trophoblast, and an outer layer termed the syncytiotrophoblast

Answer 95

The syncytiotrophoblast is responsible for producing a hormone called human chorionic gonadotropin (hCG). hCG signals the corpus luteum that fertilization and implantation have occurred. Thus, the corpus luteum does not degenerate but rather persists for another 3 months, producing large amounts of progesterone and estrogen that thicken and maintain the uterine lining. By the end of the second week of development, sufficient quantities of hCG are produced to be detected in a woman’s urine. When hCG declines, the corpus luteum degenerates as well. However, by this time, the corpus luteum is no longer needed because the placenta is producing its own estrogen and progesterone to maintain the pregnancy.

Answer 96

During the second week of development, as the blastocyst is undergoing implantation, changes also occur to the embryoblast portion of the blastocyst. By day 8, the cells of the embryoblast begin to differentiate into two layers. A layer of small, cuboidal cells adjacent to the blastocyst cavity is termed the hypoblast layer, and a layer of columnar cells adjacent to the amniotic cavity is called the epiblast layer. Together, these layers form a flat disc termed a bilaminar germinal disc, or blastodisc.

Answer 97

The bilaminar germinal disc and trophoblast also produce extraembryonic membranes. These membranes first appear during the second week of development and continue to develop during the embryonic and fetal periods. They protect the embryo and assist in vital functions. These extraembryonic membranes are as follows: ∙ The yolk sac is the first extraembryonic membrane to develop. It is formed from and continuous with the hypoblast layer of the bilaminar germinal disc. In humans, it does not store yolk as it does in eggs of birds and reptiles, but it is an important site for early blood cell and blood vessel formation. ∙ The amnion is a thin membrane that is formed from and continuous with the epiblast layer of the bilaminar germinal disc. The amnion eventually encloses the entire embryo in a fluid-filled sac called the amniotic cavity to protect the embryo from desiccation (drying out). The amniotic membrane is specialized to secrete the amniotic fluid that bathes the embryo. ∙ The chorion is the outermost extraembryonic membrane and is formed from both the rapidly growing cytotrophoblast cells and syncytiotrophoblast (both were formed from the trophoblast). These cells implant within the functional layer of the endometrium and together these structures eventually form the placenta, which is the site for providing oxygen (O2) and nutrients to the embryo/fetus and removing carbon dioxide (CO2) and other wastes from the embryo/fetus.

Answer 98

The placenta begins to form during the second week of development. The fetal portion of the placenta develops from the chorion, whereas the maternal portion of the placenta forms from the functional layer of the uterus. The early organism is connected to the placenta via a structure called the connecting stalk. This connecting stalk eventually contains the umbilical arteries and umbilical vein that distribute blood to and from the embryo or fetus. The connecting stalk is the precursor to the future umbilical cord

Answer 99

The main functions of the placenta are as follows: ∙ Exchange of nutrients, respiratory gases, and waste products between the maternal and fetal blood Transmission of maternal antibodies to the developing embryo or fetus. ∙ Production of estrogen and progesterone to maintain and build the uterine lining Progesterone Supports the lining of the uterus, which provides an environment for the fetus and placenta to grow. Progesterone also prevents the lining from shedding, which could result in pregnancy loss. Estrogen Levels rise towards the end of pregnancy. Estrogens are essential for women's healthy sexual and reproductive development. Human placental lactogen (hPL) Promotes mammary gland growth in preparation for lactation. Vascular endothelial growth factor (VEGF) Assists in vasodilation, which is vital for the growth and development of the fetus.

Answer 100

Gastrulation occurs during the third week of development immediately after implantation, and it is one of the most critical periods in the development of the embryo. Gastrulation is a process by which the cells of the epiblast migrate and form the three primary germ layers, which are the cells from which all body tissues develop. The three primary germ layers are called ectoderm, mesoderm, and endoderm. Once these three layers have formed, the developing trilaminar (three-layered) structure may be called an embryo.

Answer 101

Cephalocaudal folding occurs in the cephalic (head) and caudal (tail) regions of the embryo. Essentially, the embryonic disc and amnion grow very rapidly, but the yolk sac does not grow at all. This differential growth causes the head and tail regions to fold on themselves. Transverse folding (or lateral folding) occurs when the left and right sides of the embryo curve and migrate toward the midline. As these sides come together, they restrict and start to pinch off the yolk sac. Eventually, the sides of the embryonic disc fuse in the midline and create a cylindrical embryo. Thus, the ectoderm is now solely along the entire exterior of the embryo, whereas the endoderm is confined to the internal region of the embryo. As this midline fusion occurs, the yolk sac pinches off from most of the endoderm (with the exception of one small region of communication called the vitelline duct). Thus, cephalocaudal folding helps form the future head and buttocks region of the embryo, whereas transverse folding creates a cylindrical trunk or torso region of the embryo.

Answer 102

The fetal period extends from the beginning of the third month of development (week 9) to birth. It is characterized by maturation of tissues and organs, and rapid growth of the developing fetus. Fetal length increases dramatically in months 3 to 5.

Answer 103

The first trimester encompasses the first 3 months of pregnancy (or the first 12 weeks of development of the embryo and fetus). During this time period, the zygote develops into an embryo and then into an early fetus. ∙ The second trimester includes months 4 to 6 of pregnancy and is marked by growth of the fetus and expansion of maternal tissues. ∙ The third trimester encompasses months 7 to 9 of pregnancy. During this time period, the fetus grows most rapidly and gains weight, and the mother’s body prepares for the eventual labor and delivery.

Answer 104

The high levels of progesterone and estrogen suppress FSH and LH secretion, so the ovarian cycle and additional follicular development are arrested during the pregnancy. They facilitate uterine enlargement, mammary gland enlargement, and fetal growth. Many pregnant women report faster-growing and stronger nails (likely due to increased levels of these hormones), and their hair tends to be fuller and thicker in response to these hormones. In addition, estrogen is primarily responsible for relaxation of many ligamentous joints, such as the sacroiliac joints and pubic symphysis, in preparation for labor. Progesterone is responsible for growth of functional layer of the uterus and the prevention of menstruation during pregnancy. Relaxin is another hormone that is secreted by the corpus luteum and placenta. Relaxin appears to promote blood vessel growth in the uterus. The placenta also becomes a major secretor of corticotropin-releasing hormone (CRH). Small amounts of CRH are produced and released by the hypothalamus and stimulate the anterior pituitary to secrete adrenocorticotropic hormone (ACTH), which acts on the adrenal cortex to release glucocorticoids (e.g., cortisol). The placenta also secretes CRH during pregnancy, but in much larger amounts than that of the hypothalamus. CRH is thought to play a role in regulating the length of pregnancy and the timing of childbirth. CRH also is responsible for the rise in aldosterone in the mother, which promotes fluid retention and an overall increase in blood volume The placenta secretes human chorionic thyrotropin (HCT), which is similar to thyroid-stimulating hormone and thus stimulates the thyroid gland. As a result, a pregnant woman’s metabolic rate increases, with an accompanying increase in body temperature Human placental lactogen (HPL)HPL does affect how the pregnant woman metabolizes certain nutrients—the mother metabolizes more fatty acids instead of glucose, leaving greater glucose reserves for the fetus. HPL also inhibits the effects of insulin so there are greater circulating levels of glucose in the blood (again, for use by the fetus). Prolactin is produced by the anterior pituitary and is responsible for milk production. Prolactin levels increase 10-fold during pregnancy to ensure that lactation occurs after giving birth. Oxytocin levels increase in the second and third trimesters, in response to rising estrogen levels, and peak during labor.

Answer 105

Prior to pregnancy, the uterus is approximately 8 cm by 5 cm (3 inches by 2 inches) and situated within the pelvic cavity. Once implantation occurs, the uterus begins to enlarge and expand as its muscle cells hypertrophy and undergo hyperplasia The mammary glands typically are tender and sore during the first trimester due to the increasing levels of estrogen and progesterone. The placenta secretes melanocyte-stimulating hormone (MSH), which is in part responsible for the darkening of the areolae and nipples during this time Human placental lactogen (HPL) affects how the mother metabolizes certain nutrients, so that she metabolizes more fatty acids instead of glucose, leaving the glucose for the developing fetus. The elevated levels of HPL, as well as higher levels of corticosteroids (e.g., cortisol), estrogen, and progesterone, also result in increased insulin resistance in the pregnant mother. The mother also may experience epistaxis (nosebleeds) due to the increased blood circulation Progesterone increases the sensitivity of central chemoreceptors to blood carbon dioxide (CO2) levels, ultimately functioning to lower the blood CO2 levels. mother’s oxygen consumption increases about 20–30% to meet the oxygen demands of both mother and fetus plasma volume increases by about 50% throughout pregnancy To increase cardiac output, the body increases both heart rate (on average an increase of 10 to 20 beats per minute) and stroke volume Up to 50% more plasma volume must be filtered by the kidneys. Glomerular filtration rate thus increases about 30–50% during pregnancy and urine output increases slightly. Compression by the expanding uterus on the urinary bladder can lead to frequent urination in both the first and third trimesters. compression of the ureter or kidney by the uterus can result in urine drainage issues. All of these factors put pregnant women at much greater risk for urinary tract infections (UTIs) Progesterone causes smooth muscle relaxation in the ureters, which may cause expansion of the ureters and renal pelvis of the kidneys. This dilation and the increased urine volume may result in urine stasis (slowing) from the kidneys to the urinary bladder.

Answer 106

In the later stages of pregnancy, the increasing levels of estrogen counteract the calming influence of progesterone on the uterine myo- metrium and increase the uterine myometrium sensitivity. In addition, the rising levels of estrogen stimulate the production of oxytocin receptors on the smooth muscle cells of the uterine myometrium, so as the levels of oxytocin also rise, more receptors are available on the uterus for binding this hormone. All of these factors result in the uterine myometrium becoming more sensitive and “irritable” in the later stages of pregnancy, and contractions begin to occur. These contractions typically are weak and irregular, but as levels of estrogen and oxytocin continue to rise in the later stages, they become more intense and frequent. Thus, weak contractions may occur and be noticed as soon as the second trimester of pregnancy.

Answer 107

True labor is defined as uterine contractions that increase in intensity and regularity, and that result in changes to the cervix. The mother and the fetus both have an active role in initiating true labor. As the pregnancy nears term, the mother’s hypothalamus triggers the posterior pituitary to release increasing levels of oxytocin. (This increase in oxytocin levels is in response to a cascade of changes in both fetal and maternal hormones responsible for maintaining pregnancy.) Near the beginning of true labor, the fetus's hypothalamus is also triggering release of oxytocin from the fetus's posterior pituitary. Oxytocin from both the mother and fetus stimulate the placenta to secrete prostaglandins. Prostaglandins are eicosanoids that act as local hormones to stimulate smooth muscle contraction, most notably uterine muscle contraction. Prostaglandins are also responsible for the softening and dilating of the cervix. The combined actions of maternal oxytocin, fetal oxytocin, and the rising levels of prostaglandins initiate the rhythmic contractions of true labor. Rising levels of maternal and fetal oxytocin together initiate the positive feedback mechanism of true labor. Oxytocin stimulates uterine contractions as well as the release of prostaglandins, which promote cervical stretching and facilitate uterine contractions. Sensory input relayed from the uterus to the hypothalmus stimulates it to release more oxytocin from the posterior pituitary. The process escalates until the fetus is expelled from the uterus.

Answer 108

Prior to birth, respiratory gases were exchanged between maternal and fetal circulation at the placenta. The fetal lungs are not fully inflated because they are not yet fully functional. However, within about 10 seconds after being born, the neonate typically takes its first breath. This first breath is thought to be caused by the central nervous system responding to the change in environment and temperature. This process may be facilitated by a general respiratory acidosis (caused by clamping of the umbilical vessels and constriction of the umbilical vessels prior to birth), but note that the first breath typically occurs regardless of whether the umbilical vessels have been clamped or not. Once this first breath is taken, the lungs become inflated and the surfactant that is present in the alveoli keeps the alveoli patent (open). Thus, every breath after the first is easier now that the alveoli remain patent. Premature infants born earlier than 28 weeks are not producing sufficient levels of surfactant to keep their alveoli patent, so these infants may need to placed on a ventilator until their lungs mature.

Answer 109

Given that the fetal lungs are not functional, other pathways (i.e., ductus arteriosus, foramen ovale) shunt blood away from the non- functional lungs and directly to the fetal circulation. As a result, the fetal cardiovascular system has some structures that are modified or that cease to function once the fetus is born. At birth, the fetal circulation begins to change into the postnatal pattern. When the neonate takes its first breath, pulmonary resistance drops, and the pulmonary arteries dilate. As a result, pressure on the right side of the heart decreases and the pressure is then greater on the left side of the heart, which handles the systemic circulation.

Answer 110

Within a few days after giving birth, estrogen and progesterone levels plummet, because the uterine lining no longer needs to be maintained for pregnancy. The decrease in progesterone also affects the respiratory system. Without the high levels of progesterone, the chemoreceptors are less sensitive to CO2 levels. As a result, respiratory rate, tidal volume, and pulmonary ventilation return to pre-pregnancy levels. the levels of corticotropin-releasing hormone (CRH) drop dramatically, now that there is no longer a placenta producing copious amounts of this hormone. Prolactin levels and oxytocin levels drop after birth as well. However, because both of these hormones are involved in lactation, periodic surges occur in these hormone levels each time a baby nurses.

Answer 111

As the blood volume decreases, the woman's cardiac output returns to pre-pregnancy levels. A portion of the blood volume, as well as mucus and hyper- trophied endometrial tissue, is released from the uterus as lochia. Lochia is similar to a menstrual period, in that blood and some endometrial tissue are expelled from the uterus via the vagina. However, lochia results in much heavier bleed- ing than a typical menstrual period, because the uterine lining buildup occurred over a 9-month period, instead of a typical 28-day cycle. Thus, the first 5 days of lochia typically result in very heavy bleeding, after which it lightens but progresses for at least 2 to 3 weeks. A woman also may expel excess fluids via increased urination. The decline in CRH after birth results in a decline in aldosterone, which precipitates the overall drop in blood volume and interstitial fluid levels. The lymphatic system cycles some of the excess interstitial fluid into the blood circulation, where it may be filtered by the kidneys and secreted as urine. Within about 24 hours after giving birth, most women experience copious, frequent urination, the result of the kidneys “working overtime,” in filtering out this excess fluid. High levels of estrogen positively influence the secretion of prolactin, so as estrogen levels rise during pregnancy, so do prolactin levels. Both estrogen and prolactin cause mammary gland acini proliferation and branching of the lactiferous ducts. Paradoxically, the high levels of estrogen and progesterone are also responsible for preventing breast milk secretion until after birth. It isn’t until levels of estrogen and progesterone drop that prolactin works unopposed to stimulate breast milk production.

Lecture exam #3 Flashcards

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