Exam 4: Chapter 26: The Urinary System Flashcards
1
Q
Kidneys
Ureters
Urinary bladder
Urethra
A
Components of the urinary system
2
Q
oPrimary excretory organs
A
Kidneys
3
Q
oEach kidney’s excretory products are carried by a ______ to a single urinary bladder
A
Ureters
4
Q
oEach kidney’s excretory products are carried by a ureter to a single _______
A
Urinary Bladder
5
Q
oThe urinary bladder is emptied of the waste liquid by the _______
A
Urethra
6
Q
*1.excess water; 2.excess ions; 3:metabolic wastes, including the protein by-product, urea; and 4.toxic substances
A
Urine Consists of
7
Q
Excretion
Regulation of blood volume and pressure
Regulation of blood solute concentration
Regulation of extracellular fluid pH
Regulation of RBC synthesis
Regulation of vitamin D synthesis
A
Functions of the Urinary System
8
Q
*The kidneys filter waste products from the blood
*Nearly 21% of cardiac output is filtered by the kidneys each minute
*Fluid and waste are captured by an extensive network of tubes found throughout the kidney
*Large molecules, such as proteins, remain in the blood, whereas smaller molecules and ions enter the filtered fluid
*As the fluid flows through the kidneys, it is slowly modified until it is converted into urine
A
Excretion
9
Q
*The kidneys play a major role in controlling the extracellular fluid volume in the body
*The kidneys can produce either a large volume of dilute urine or a small volume of concentrated urine, depending on the hydration level of the body
*Through urine production, the kidneys regulate blood volume and hence blood pressure
A
Regulation of Blood Volume and Pressure
10
Q
*The kidneys help regulate the concentration of the major ions, such as Na+, Cl-, K+, Ca^2+, HCO3-, and HPO4^2-
*The kidneys also regulate other solute concentrations, such as urea
A
Regulation of Blood Solute Concentration
11
Q
*The kidneys secrete variable amounts of H+ to help regulate the extracellular fluid pH
A
Regulation of Extracellular Fluid pH
12
Q
*The kidneys secrete a hormone, erythropoietin, that stimulates the synthesis of red blood cells in red bone marrow
A
Regulation of RBC Synthesis
13
Q
*The kidneys play an important role in controlling blood levels of Ca^2+ by regulating the synthesis of vitamin D
A
Regulation of Vitamin D Synthesis
14
Q
*The kidneys are bean-shaped organs
*They are located in the nonmesenteric region of the abdomen, or are retroperitoneal
*This means they lie behind the peritoneum
*Additionally, the kidneys are located on each side of the vertebral column near the psoas major muscles
*They are each about the size of a tightly clenched fist
*The kidneys extend from the lower portion of the rib cage at the level of the last thoracic (T12) vertebra to the third lumbar (L3) vertebra
*The liver is superior to the right kidney, causing the right kidney to be slightly lower than the left
*Each kidney measures about 11cm long, 5cm wide, and 3cm thick, and each weighs about 130g, which is approximately the weight of 1 cup of flour
A
Location and External Anatomy of the Kidneys
15
Q
*The kidneys are each surrounded by an outer layer of connected tissue called the renal capsule
*Surrounding the outside of the capsule is a thick layer of adipose tissue, which cushions and protects the kidneys
*A thin layer of connective tissue, the renal fascia, surrounds the adipose tissue and helps anchor the kidneys to the abdominal wall
*More adipose tissue surrounds the renal fascia
*Located inside the abdominal cavity but between the body wall and peritoneum
*Supported by fascia and adipose tissue that hold it in place
*The renal vein, ureter, and lymphatic vessels also pass through the renal sinus before they exit the kidney at the hilum
A
Location and External Anatomy of the Kidneys
16
Q
*Is a small area on the concave, medial side of the kidney that is continuous with an adipose and connective tissue-filled cavity of the kidney, called the renal sinus
A
Hilum
17
Q
*The helium is a small area on the concave, medial side of the kidney that is continuous with an adipose and connective tissue-filled cavity of the kidney, called the ________
A
Renal Sinus
18
Q
Renal columns
A
Renal Cortex
19
Q
*Between the renal pyramids and their medullary rays, there are extensions of cortical tissue toward the medulla, called _______
A
Renal Columns
20
Q
*Renal pyramid
*Urine from several minor calyces are emptied into a larger, funnel-shaped chamber called a major calyx
*In each kidney, there are between 8 and 20 minor calyces converging to form about 2 or 3 major calyces
*From the major calyces, urine empties into a single, enlarged, funnel-shaped chamber called the renal pelvis
*The renal pelvis is embedded in and surrounded by the renal sinus
*At the helium, it narrows significantly, forming the small-diameter tube called the ureter
*Urine moves from the renal pelvis into the ureter for transport to the urinary bladder
A
Renal Medulla
21
Q
*The medulla is composed of many cone-shaped structures called _____, whose bases project into the cortex
*Are a collection of tubes and ducts that transport fluid throughout the kidney and modify it into urine
*Once urine is formed, ducts in the ______ transport it toward the renal sinus
*Renal papillae
A
Renal Pyramid
22
Q
*The tips of the pyramids, the _______, point toward the renal sinus
*When urine leaves a ______, it empties into a small, funnel-shaped chamber surrounding the tip of the papilla called a minor calyx
A
Renal Papillae
23
Q
*When urine leaves a renal papilla, it empties into a small, funnel-shaped chamber surrounding the tip of the papilla called a minor calyx
*Urine from several minor calyces are emptied into a larger, funnel-shaped chamber called a major calyx
*In each kidney, there are between 8 and 20 minor calyces converging to form about 2 or 3 major calyces
*From the major calyces, urine empties into a single, enlarged, funnel-shaped chamber called the renal pelvis
*The renal pelvis is embedded in and surrounded by the renal sinus
*At the helium, it narrows significantly, forming the small-diameter tube called the ureter
*Urine moves from the renal pelvis into the ureter for transport to the urinary bladder
A
Fluid Movement in Kidney
24
Q
*Urine drains from the renal pyramid through the renal papillae
*Renal papillae empty into minor calyx
*Minor calyx empties into major calyx
*Major calyx empties into the renal pelvis
*Renal pelvis is drained by the ureter
A
Fluid Movement in Kidney
25
*Is the histological and functional unit of the kidney
*There are approximately 1.3 million ______ distributed throughout the cortex and medulla of each kidney
*Usually measure about 50-55mm in length
*There are four separate regions of a _______
*The four regions of a _____ include 1.a renal corpuscle, 2.a proximal convoluted tubule, 3.a nephron loop, and 4.a distal convoluted tubule
*The distal convoluted tubules connect to collecting ducts, forming a series of arched tubes that drain multiple ______
*Each portion of a _____ plays a different role in urine production
*There are two types of _____ in the kidney: 1.juxtamedullary ______ and 2.cortical _______
*Juxtamedullary _______ have renal corpuscles that are found deep in the cortex near the medulla
*They have long _____ loops, which extend deep into the medulla
*Longer ______ loops are well adapted for water conservation
*Only about 15% of _______ are juxtamedullary ______
*Cortical _____ have renal corpuscles that are distributed throughout the cortex
*Their ______ loops are shorter than those of juxtamedullary ______ and are closer to the outer edge of the cortex
Structure of a Nephron
26
Renal corpuscle
Renal Tubule
Two Major Components
27
*Filters the blood, the proximal convoluted tubule returns filtered substances to the blood, the nephron loop helps conserve water and solutes, and the distal convoluted tubule rids the blood of additional wastes
*The filtration portion of the nephron is housed in the ____
*Glomerular capsule
*Glomerulus
*Has several unique characteristics that make it particularly efficient at filtration-the main function of the kidneys
*Together, the structures in the corpuscle make up the filtration membrane
*The filtration membrane consists of capillary endothelium, the basement membrane, and the podocytes of the glomerular capsule
*The filtration membrane performs the first major step in urine production
*Urine production begins when the filtration membrane filters the blood
*The filtered fluid, called filtrate, then enters the lumen, or capsular space, inside the glomerular capsule
Renal Corpuscle
28
*Is an indented, double-walled chamber surrounding the glomerulus
*From the _____, the filtered fluid flows into the proximal convoluted tubule region of the renal tubule
*Parietal layer
*Capsular space
*Visceral layer
Glomerular Capsule
29
*The outer layer is the _____
*It is constructed of simple squamous epithelial cells
*The epithelial cells become cube-shaped at the beginning of the proximal convoluted tubule
Parietal Layer
30
*The inner layer is the _____
*It is constructed of specialized cells called podocytes, which wrap around the glomerular capillaries
*Podocytes
Visceral Layer
31
*Is a network of capillaries twisted around each other like a ball of yarn
*Fluid filtered from the glomerular capillaries is called the filtrate
Glomerulus
32
The glomerular capillaries are highly permeable due to the presence of fenestrae
*Capillaries have different levels of permeability, depending on the size of their pores
*In the case of fenestrae, neither large proteins nor blood cells can fit through them
Porous Capillaries
33
Gaps, called filtration slits, are between the cell processes of the podocytes of the visceral layer
*A basement membrane lies sandwiched between the endothelial cells of glomerular capillaries and the podocytes of the glomerular capsule
Porous Visceral Layer
34
An afferent arteriole supplies blood to the glomerulus for filtration
*An efferent arteriole transports the filtered blood away from the glomerulus
*The glomerular capillaries have much higher pressure than other capillaries due to the smaller diameter of the efferent arteriole compared to the afferent arteriole
High Pressure
35
Proximal convoluted tubule (PCT)
Nephron loop
Distal convoluted tubule (DCT)
Juxtaglomerular apparatus
Renal Tubule
36
*The first section is the ______
*It is approximately 14mm long and 60 in diameter
*The wall of the_____ cells rest on a basement membrane, which forms the outer surface of the tubule
*These cells have many microvilli projecting from the luminal surface of the cells
*The microvilli substantially increase the surface area of the epithelial cells
Proximal Convoluted Tubule (PCT)
37
*As the proximal convoluted tubule continues descending toward the medulla, the cell type begins to change
*At this point, the renal tubule is called the ______
*Descending limb
*Ascending limb
Nephron Loop
38
*The first part of the _____ is similar in structure to the proximal convoluted tubule
*The portion of the nephron loop that extends into the medulla becomes very thin near the bend of the loop
*The lumen in the thin part narrows, and an abrupt transition occurs from simple cuboidal epithelium to simple squamous epithelium
Descending Limb
39
*Like the descending limb, the first part of the _____ is thin and made of simple squamous epithelium
*Soon, however, it becomes thicker, and simple cuboidal epithelium replaces the simple squamous epithelium
*The thick part of the _____ returns toward the renal corpuscle and ends by transitioning to the distal convoluted tubule near the macula densa
Ascending Limb
40
*Is shorter than the proximal convoluted tubule
*Its epithelium is simple cuboidal
*These cells are smaller and have fewer microvilli
Distal Convoluted Tubule (DCT)
41
*An important regulatory structure, called the _______, is located next to the glomerulus
*Consists of a unique set of afferent arteriole cells and specialized cells in the distal convoluted tubule that are in close contact with each other
*Secretion of the enzyme renin by the _____ plays an important role in the regulation of filtrate formation and blood pressure
Juxtaglomerular Apparatus (JGA)
42
*Several distal convoluted tubules connect to a single ______, which is composed of simple cuboidal epithelium
*The _______, which is larger in diameter than the segments of the nephron, form much of the medullar rays and extend through the medulla toward the tips of the renal pyramids
*DCT of several nephrons will empty into a common _______
*Drains into the papillary duct
*Site of final adjustments to urine concentration
Collecting Duct
43
*A system of blood vessels allows the exchange of materials that occurs in the kidneys
*The renal arteries branch off the abdominal aorta and enter the kidneys
*As the renal artery extends deep into the kidney, it branches into smaller and smaller blood vessels
*Blood from the peritubular capillaries, including the vasa recta, will return to the general circulation through the veins of the kidneys
*25% of total cardiac output is directed toward the kidneys
*Each kidney receives blood via the renal artery and is drained by the renal vein
*The glomerulus is supplied by the afferent arteriole and drained by the efferent arteriole
Arteries and Veins of the Kidneys
44
*The renal artery delivers approximately 21% of cardiac output per minute
*The segmental arteries branch from the renal artery to each portion of the kidney
*The interlobar arteries pass between the renal pyramids
*The arcuate arteries branch from the interlobar arteries
*They arch between the cortex and the medulla
*Cortical radiate arteries branch off the arcuate arteries and project into the cortex
*The afferent arterioles arise from branches of the cortical radiate arteries
*The afferent arterioles carry blood into the glomerular capillaries
*The glomerular capillaries are the locations of filtration
*Efferent arterioles carry blood away from the glomerular capillaries
*The peritubular (around the tubes) capillaries branch from the efferent arterioles
*They surround the proximal convoluted tubules, the distal convoluted tubules, and the nephron loops
*The vasa recta are specialized portions of the peritubular capillaries that extend deep into the medulla of the kidney and surround the nephron loops and collecting ducts
List of Arteries, in Order, Starting With the Renal Arteries and Progressing to the Blood Vessels that Surround Each Nephron
45
Glomerular filtration
Tubular reabsorption
Tubular secretion
*Urine consists of substances filtered directly from the blood and those that are secreted into the renal tubule, minus any reabsorbed substances
3 Basic Processes
46
*Blood pressure in the glomerular capillaries forces fluid and small molecules out of the blood
*The filtered fluid is now called filtrate
*Filtration is nonselective and separates based only on size or charge of molecules
*Filtration does not remove everything in the blood-it removes only those substances small enough to fit through the filtration membrane
Glomerular Filtration
47
*Cells in the renal tubules contain many transport proteins
*These transport proteins move water and some filtered molecules from the filtrate back into the blood in the peritubular capillaries
*This prevents them from being lost from the body as components of urine
*Most of the filtered water and useful solutes have been returned to the blood by the time filtrate has been modified to urine, whereas the remaining waste of excess substances and a small amount of water from urine
Tubular Reabsorption
48
*Certain tubule cells transport additional solutes from the blood into the filtrate
*Some of these solutes may not have been filtered by the filtration membrane
Tubular Secretion
49
Plasma: 180
Filtrate: 180
Net movement of solute: 178.6
Urine:1.4
Urine concentration/plasma concentration: N/A
Water (L)
50
Organic molecules (mg/100mL)
Plasma: 3900-5000
Filtrate: 6-11
Net movement of solute: -100.0
Urine: 0+
Urine concentration/plasma concentration: 0
Protein
51
Organic molecules (mg/100mL)
Plasma: 100
Filtrate: 100
Net movement of solute: -100.0
Urine: 0
Urine concentration/plasma concentration: 0
Glucose
52
Organic molecules (mg/100mL)
Plasma: 26
Filtrate: 26
Net movement of solute: -11.4
Urine: 1820
Urine concentration/plasma concentration: 70
Urea
53
Organic molecules (mg/100mL)
Plasma: 3
Filtrate: 3
Net movement of solute: -2.7
Urine: 42
Urine concentration/plasma concentration: 14
Uric Acid
54
Organic molecules (mg/100mL)
Plasma: 1.1
Filtrate: 1.1
Net movement of solute: 0.5
Urine: 196
Urine concentration/plasma concentration: 180
Creatine
55
Ions (mEq/L)
Plasma: 142
Filtrate: 142
Net movement of solute: -141.0
Urine: 128
Urine concentration/plasma concentration: 0.9
Na+
56
Ions (mEq/L)
Plasma: 5
Filtrate: 5
Net movement of solute: -4.5
Urine: 60:
Urine concentration/plasma concentration: 12.0
K+
57
Ions (mEq/L)
Plasma: 103
Filtrate: 103
Net movement of solute: -101.9
Urine: 134
Urine concentration/plasma concentration: 1.3
Cl-
58
Ions (mEq/L)
Plasma: 28
Filtrate: 28
Net movement of solute: -27.9
Urine: 14
Urine concentration/plasma concentration: 0.5
HCO3-
59
*Is a nonspecific process whereby materials are separated based on size or charge
*The kidneys also demonstrate size ______ by filtering the blood, but here, the driving force of ______ is blood pressure
*Is the first step in urine production
*All blood components except blood cells and most proteins can leave the glomerular capillaries and enter the glomerular capsule as filtrate
*It is the filtrate that will be modified into urine
*The importance of ______ is indicated by the large percentage of cardiac output, or blood, sent through the kidneys each minute
*This percentage of cardiac output that flows through the kidneys is called the renal fraction
*It varies from 12% to 30% of the cardiac output in healthy, resting adults, but it averages 21%
*There are several measurements of ______ that can be calculated as an indication of proper kidney function
*These calculations can measure either 1.rate of whole blood flow or 2.rate of plasma flow
Filtration
60
*When calculated over the entire day, there are about 180,000mL, or 180L, of filtrate produced daily
*This enormous volume is equal to about ninety 2-liter soft drink bottles per day
*Because a healthy person produces only 1000-2000 milliliters (1-2 liters) of urine each day, the equivalent of one 2-liter soft drink bottle, it is readily apparent that not all of the filtrate becomes urine
*About 99% of the filtrate volume is reabsorbed into the blood as it travels through the renal tubule, and less than 1% becomes urine
*Glomerular filtrate is the liquid entering the capsular space
*Water and small ions pass from blood to capsular space thru filtration membrane
Filtration
61
* To calculate the _____ through the kidneys, called renal blood flow rate, two pieces of information are necessary:1.the renal fraction, or 21%, and 2.the cardiac output, or 5600mL?min
*The following equation is used:
*Renal blood flow rate= Cardiac output x Renal fraction
*1176mL/min= 5600mL/min x 0.21
Rate of Blood Flow
62
*To calculate the ____ through the kidneys, called the renal plasma flow rate, we must account for the percentage of whole blood made up by plasma, which is approximately 55%
*The equation is the following:
*Renal plasma flow rate= Renal blood flow rate x % of whole blood that is plasma
*650mL/min x 0.55
Rate of Plasma Flow
63
* To calculate the rate of filtrate formed per minute, called the ______ (GRF), you first need to know what percentage of plasma is filtered from the blood
*This is called filtration fraction
*The filtration fraction is approximately 19%
*To calculate the GFR, the following equation is used:
*GFR= Renal plasma flow rate x Filtration fraction
*123.5mL plasma/min= 650mL/min x 0.19
*In other words, the GFR indicated that the kidneys form approximately 125mL/min of filtrate
Glomerular Filtration Rate
64
*It separates materials on the basis of size and charge of the blood components
*Allows water and small molecules to leave the blood while preventing blood cells and most proteins from leaving the blood
*Fenestrated glomerular capillaries
*Basement membrane
*Visceral layer of glomerular capsule
*Together, these components prevent molecules larger than 7nm in diameter or those having a molecular mass equal to or greater than 40,000 daltons from passing through
*The exclusion of molecules larger than 7nm is partially due to the fact that the fenestrae are about 7nm in size
*Most plasma proteins are slightly larger than 7nm in diameter and are retained in the glomerular capillaries
*Albumin, which has a diameter just slightly less than 7nm, enters the filtrate only in small amounts
*The filtrate is not protein-free but, rather, contains about 0.03% protein
Filtration Membrane
65
*Some protein hormones, such as thyrotropin-releasing hormone, oxytocin, and antidiuretic hormone, are small enough to pass through the filtration membrane
*Any protein that is filtered is actively reabsorbed by endocytosis and metabolized by the cells in the proximal convoluted tubule
*The basement membrane and the podocytes further contribute to filtration through charge exclusions
*They contain negatively charged glycoproteins, which repel negatively charged plasma proteins and prevent them from exiting the blood
*The combined effect of the filtration membrane components prevents most proteins from exiting the blood on the basis of size and charge, and only a small amount of protein is found in the urine of healthy people
Filtration Membrane
66
*Between the capillary wall and the visceral layer of the glomerular capsule
Basement Membrane
67
Amount per minute (mL): 1176
Calculation: Amount of blood flowing through the kidneys per minute= cardiac output x the percentage of cardiac output that enters the kidneys
5600mL blood/min x0.21 = 1176mL blood/min
Renal Blood Flow
68
Amount per minute (mL): 650
Calculation: Amount of plasma flowing through the kidneys per minute= renal blood flow x % of the blood that is plasma
1176mL blood/min x 0.55 ≈ 650mL filtrate/min
Renal Plasma Flow
69
Amount per minute (mL): 125
Calculation: Amount of plasma (filtrate) that enters the glomerular capsule per minute = renal plasma flow x % of the plasma that enters the renal capsule
650mL plasma/min x 0.19 ≈ 125mL filtrate/min
Glomerular Filtration Rate (GFR)
70
Amount per minute (mL): 1
Calculation: Nonreabsorbed filtrate that leaves the kidneys per minute= glomerular filtration rate x % of the filtrate that is not reabsorbed into the blood
125mL filtrate/min x 0.008 =1mL urine/min
Milliliters of urine per minute can be converted to liters of urine per day by multiplying by 1.44
1mL urine/min x 1.44= 1.4L/day
Urine
71
*No matter the type of size-based filter, there is one commonality: they all require a force to cause movement through it
*For the filtration membrane, three forces, or pressures, determine the amount of filtrate formed
*The combination of these three pressures is called ______
*To calculate ______, all three filtration pressures are summed
*In a normal kidney, GCP is greater than the combination of CHP and BCOP
*Is a net outward pressure of approximately 10mm Hg:
o________ = Glomerular capillary pressure – Capsular hydrostatic pressure -Blood colloid osmotic pressure
o10mm Hg = 50mm Hg – 10mm Hg – 30mm Hg
Filtration Pressure
72
*Normally, the filtrate does not exert an osmotic force on fluid movement out of the glomerular capillaries because the solute concentration of the filtrate is very low
*This is because few proteins cross the filtration membrane
*In a disease such as glomerular nephritis, the filtration membrane becomes more permeable, allowing more protein than normal to enter the filtrate
*The elevated protein in the filtrate increases the colloid osmotic pressure of the filtrate
*This results in elevated filtration pressure, thereby increasing the filtrate volume
Filtration Pressure
73
*Glomerular capillary pressure (GCP)
Capsular hydrostatic pressure (CHP)
Blood colloid osmotic pressure (BCOP)
Filtration pressure is dependent on 3 pressures associated with the renal corpuscle
74
*The GCP is essentially the blood pressure inside the glomerular capillaries
*It is an outward pressure from blood pressing on the fenestrated capillary walls
*The GCP forces fluid and solutes out of the blood into the glomerular capsule
*This GCP is higher than that in other capillaries of the body
*The higher GCP is due to the smaller diameter of the efferent arteriole compared to that of the afferent arteriole and glomerular capillaries
*As the blood flows from the larger-diameter afferent arteriole through the glomerular capillaries to the smaller-diameter efferent arteriole, the blood pressure increases in the glomerular capillaries
*Consequently, filtrate is forced across the filtration membrane into the lumen of the glomerular capsule
*The GCP is approximately 50mm Hg compared with approximately 30mm Hg at the arterial end of other capillary networks
Glomerular Capillary Pressure (GCP)
75
*The CHP is an inward pressure that opposes filtration
*CHP is due to pressure from the filtrate fluid in the capsular space
*The CHP is about 10mm Hg
Capsular Hydrostatic Pressure (CHP)
76
*The BCOP is also an inward pressure that opposes filtration
*It is due to the osmotic pressure of plasma proteins in the glomerular capillaries
*Through osmosis, these proteins draw fluid back into the glomerular capillary from the glomerular capsule
*The BCOP is greater at the end of the glomerular capillary than at its beginning because there is a higher protein concentration at the end of the glomerulus
*The average BCOP is approximately 30mm Hg
Blood Colloid Osmotic Pressure (BCOP)
77
*The glomerular filtration rate (GFR) is very stable
*It does not significantly change even if systemic blood pressure drops as low as 90 or raises as high as 180mm Hg
Regulation of Glomerular Filtration Rate
78
Renal autoregulation: kidneys maintain constant renal blood flow
*The _____ mechanisms are collectively referred to as autoregulation and are due to properties inherent to structures of the renal corpuscle
*Autoregulation is achieved through two processes: 1.the myogenic mechanism and 2.tubuloglomerular feedback
*The myogenic mechanism is associated with intrinsic properties of smooth muscle cells
*In the afferent and efferent arterioles, smooth muscle cells act as stretch receptors
*These receptors detect changes in blood pressure
*Elevated blood pressure causes increased stretch of the smooth muscle cells in the wall of the afferent arteriole
*The smooth muscle cells contract in direct response to stretch
*This causes vasoconstriction of the afferent arteriole
Intrinsic
79
*On the other hand, smooth muscle cells in the wall of the afferent arteriole relax when blood pressure decreases
*Relaxation of the smooth muscle cells causes vasodilation of the afferent arteriole
*In this way, blood supply to the glomerulus, and thus GFR, fluctuates very little, even when the mean arterial pressure changes
*Tubuloglomerular feedback matches filtrate flow past the macula densa cells of the juxtaglomerular apparatus to GFR
*When the macula densa cells detect an increased flow rate, they send a signal to the juxtaglomerular cells of the afferent arteriole to constrict
*Glomerular filtration rate decreases due to a decreased glomerular capillary pressure
Intrinsic
80
Neural regulation
*The _____ mechanisms are governed by the autonomic nervous system and particular hormones
*Autoregulation maintains renal blood flow and filtrate formation at a relatively constant rate unless sympathetic stimulation is intense
*In severe conditions such as hemorrhage or dehydration, the mean arterial pressure can drop below 90mm Hg, and the sympathetic nervous system causes a dramatic decrease in renal blood flow and GFR to maintain homeostatic blood pressure
*Because norepinephrine-secreting sympathetic neurons innervate the blood vessels of the kidneys, sympathetic stimulation constricts the small arteries and afferent arterioles, thereby decreasing renal blood flow and filtrate formation
*Intense sympathetic stimulation, as may occur during shock or intense exercise, decreases the rate of filtrate formation to only a few milliliters per minute; however, small changes in sympathetic stimulation have a minimal effect on renal blood flow and filtrate formation
Extrinsic
81
*The process of______ entails nearly every type of cell transport mechanism
*Is the transport of water and solutes from the filtrate into the blood
*Principally, water movement in the kidney is governed by osmosis
*Osmosis is the movement of water toward solutions with higher solute concentrations, or high osmotic pressure
*Solute movement is quite often through diffusion across the renal tubule cells
*Diffusion is the net movement of solutes down their concentration gradient
*Some molecules are too large to pass through the plasma membrane, or the cell may need to move a molecule against its concentration gradient
*In these instances, cells use transport proteins
*There are several types of transport proteins used by renal tubule cells
Tubular Reabsorption
81
*Severe stress or circulatory shock causes dramatic vasoconstriction of the afferent arterioles
*This lowers renal blood flow so severely that the blood supply to the kidney is inadequate to maintain normal kidney metabolism
*As a consequence, kidney tissues can be damaged and thus unable to perform their normal functions if blood flow is not reestablished
*Therefore, shock should be treated quickly
*On the other hand, reduced blood flow to the kidneys during stress or shock is consistent with homeostasis
*Intense vasoconstriction maintains blood pressure at levels adequate to sustain blood flow to organs such as the heart and brain
*A reduction in blood flow to organs such as the kidneys is only harmful if the lack of blood flow is prolonged
*Under low blood pressure conditions, the juxtaglomerular cells in the juxtamerular apparatus secrete the enzyme renin
*Secretion of this enzyme results in the activation of the potent vasoconstrictor angiotensin II
*By stimulating vasoconstriction, angiotensin II helps maintain GFR at normal levels
Extrinsic
82
*For example, symporters move two molecules or ions in the same direction
*Occasionally, the energy from Na+ diffusion drives the movement of a second ion or molecules
*This process is secondary active transport
*Proper______ is critical in preventing the body from becoming overly dehydrated and deficient in important materials
*Nearly all (99%) of the water and solutes are rapidly returned to the blood via the renal tubules, and because of this, toxins are quickly removed from the blood
*The filtrate leaves the lumen of the glomerular capsule and flows first through the proximal convoluted tubule, into the nephron loop and the distal convoluted tubule, and then finally into the collecting ducts
Tubular Reabsorption
83
*As the filtrate passes through these structures, many of the substances in the filtrate are removed
*Inorganic salts, organic molecules, and about 99% of the filtrate volume leave the renal tubule and enter the interstitial fluid
*Because the pressure is low in the peritubular capillaries, these substances enter the peritubular capillaries and flow through the renal veins to enter the general circulation
*Solutes reabsorbed from the lumen of the renal tubule to the interstitial fluid include amino acids, glucose, and fructose, as well as Na+, K+, Ca^2+, HCO3-, and Cl-
*The small volume of the filtrate (approximately 1%) that forms urine contains urea, uric acid, creatinine, K+, and other substances
*The regulation of solute reabsorption and the permeability characteristics of portions of the nephron allow for the production of a small volume of very concentrated urine or a large volume of very dilute urine
Returning most of filtered water and solutes to bloodstream
Reabsorption can occur paracellularly or transcellularly.
Tubular Reabsoprtion
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*Is the site of the majority of reabsorption
*The mechanisms underlying reabsorption can be better understood by considering the cells found there
*The cells of the _____ have numerous microvilli, which dramatically increase the surface area available for reabsorption
*To establish the portion of each _____ cell where a particular process for reabsorption occurs, these portions are named
*Majority of solutes and water reabsorbed
*100% of most organic solutes
*80-90% of carbonate ions
*65% of water, Na+ and K+
*50% Cl-
*Reabsorption of solutes promotes reabsorption of water via osmosis
Reabsorption in the Proximal Convoluted Tubule (PCT)
85
Basal membrane
Apical membrane
Lateral surface
The portions of each cell are the following
86
Form the outer wall of the renal tubules
Basal Membrane
87
Make up the inside surface of the renal tubule wall
It is the ______ of the proximal convoluted tubule that houses the microvilli
Apical membrane
88
Bind the renal tubule cells to adjacent cells in the renal tubule
Lateral surface
89
oReabsorption of most solutes is linked to the diffusion of Na+ into the cells of the proximal convoluted tubule
There is a steep concentration gradient for Na+ from the filtrate into the cytoplasm of the cells of the proximal convoluted tubule
This concentration gradient is established by active transport of Na+ across the basal membrane of the cells of the proximal convoluted tubule
The Na+-K+ pump actively transports NA+ out of these cells and into the interstitial fluid, which keeps the concentration of Na+ low in their cytoplasm
Thus, Na+ moves by facilitated diffusion through a symporter from the filtrate into the cytoplasm of the cells of the proximal convoluted tubule
This movement of Na+ into these cells is responsible for the secondary active transport of many other solutes from the lumen of the proximal convoluted tubule into the cytoplasm of the tubule cells
oCarrier proteins that transport amino acids, glucose, and other solutes are located within the apical membrane, which separates the lumen of the proximal convoluted tubule from the cytoplasm of the cells of the proximal convoluted tubule
Each of these carrier proteins binds specifically to one of those substances to be transported and to Na+
The concentration gradient for Na+ provides the energy that moves both the Na+ and the other molecules or ions from the lumen into the tubule cell
Once the symported molecules are inside the cell, they cross the basal membrane of the cell by facilitated diffusion or symport
The number of carrier proteins limits the rate at which a substance can be transported
For example, the high blood glucose in someone with untreated diabetes mellitus can lead to such high glucose levels in the filtrate that not all of it can be removed by the glucose transport proteins
The excess glucose remains in the filtrate and becomes part of the urine
Process of Reabsorption in the Proximal Convoluted Tubule
90
oSome solutes also diffuse from the lumen of the proximal convoluted tubule into the interstitial fluid by moving between the cells across their lateral surfaces
As other solutes are transported out of the lumen, through the proximal convoluted tubule cells, and into the interstitial fluid, water follows by osmosis
The reabsorption of water causes the concentration of solutes that remain in the lumen to increase
When the concentration of these solutes in the lumen becomes higher than in the interstitial fluid, these solutes will diffuse between the tubule cells into the interstitial fluid
Examples of solutes that diffuse between the tubule cells of the proximal convoluted tubule include K+, Ca^2+, and Mg^2+
These solutes are reabsorbed by diffusion, even though the same ions are also sometimes reabsorbed by symport processes
oReabsorption of both solutes and water in the proximal convoluted tubule is extensive
oAs solute molecules are transported out the filtrate, water also moves by osmosis out of the filtrate
oBy the time filtrate has reached the end of the proximal convoluted tubule, its volume has been reduced by approximately 65%
oBecause the proximal convoluted tubule is permeable to water, the concentration of the filtrate there remains about the same as that of the interstitial fluid
Process of Reabsorption in the Proximal Convoluted Tubule
91
Reabsorption in the descending limb of the nephron loop
Reabsorption in the ascending limb of the nephron loop
Reabsorption in each limb of the nephron loop
91
*The ______ becomes very concentrated toward the bend of the ________, but the concentration of the filtrate is reduced to about 100mOsm/kg by the time the fluid reaches the distal convoluted tubule
*The concentration of the interstitial fluid in the cortex is about 300mOsm/kg
*The filtrate entering the distal convoluted tubule is much more dilute (hypotonic) than the interstitial fluid surrounding it
*Water reabsorption occurs in the descending limb
*Ions reabsorbed in the ascending limb
Reabsorption in the Nephron Loop (Loop of Henle)
92
The epithelial tissue in the majority of the _______, in particular the thin segment, is simple squamous epithelial tissue
Is highly permeable to water
Is moderately permeable to ions such as Na+ and Cl-, as well as molecules such as urea
Water moves by osmosis out of the ________, while some solutes move by diffusion into the _______
Ultimately, by the time the filtrate has reached the end of the thin segment, the volume of the filtrate has been reduced by another 15% and its concentration has significantly increased to 1200mOsm/L
Reabsorption in the Descending Limb of the Nephron Loop
93
As the nephron loop makes it hairpin turn to the _______, the simple squamous epithelium persists, but is has become impermeable to water
It is still permeable to solutes, which exit the ascending limb, thereby again reducing the concentration of the filtrate
oAs the _____ continues, the epithelial tissue transitions to become simple cuboidal
oThis portion of the ______ is now called the thick segment
oThe thick segment of the _____is impermeable to both water and solutes
oInstead, the cells of the thick segment house multiple types of transport proteins, including ATP-powered pumps and symporters
oThese transport proteins remove a significant portion of the solutes from the filtrate, which then enters the interstitial fluid
oIt is this active transport of solutes that contributes to the kidney’s ability to conserve water
Reabsorption in the Ascending Limb of the Nephron Loop
94
oAs the ______ continues, it is now called the thick segment
The thick segment of the _____ is impermeable to both water and solutes
Instead, the cells of the thick segment house multiple types of transport proteins including ATP-powered pumps and carrier molecules
oThese carrier molecules remove a significant portion of the solutes from the filtrate, which then enters the interstitial fluid
It is this active transport of solutes that contributes to the kidney’s ability to conserve water
Cotransport is responsible for moving K+ and Cl- with Na+ across the membrane of the ascending limb of the nephron loop
Once inside the cells of the ______, Cl- and K+ exit the cells of the ascending limb via facilitated diffusion
Reabsorption in the ascending nephron loop
95
*Some solutes (K+ and H+) are not reabsorbed until farther along the renal tubule in the ______ or collecting duct
*The reabsorption of these solutes is generally under hormonal control and depends on the current conditions of the body
*The ______ and the collecting duct are not always permeable to water; however, hormone regulation can change their permeability to water
*Reabsorption of water occurs through osmosis across the wall of the ______ and the collecting duct when the hormone ADH is present
*The interstitial fluid surrounding the _____ and collecting duct is more concentrated than the filtrate, so the water moves toward the high solute concentration area
*In these conditions, a small volume of concentrated urine is produced
*ADH causes the tubule wall to become more permeable to water
*When ADH is absent, the _____ and collecting duct are not permeable to water and water stays in the filtrate
*In this case, a large volume of dilute urine is produced
*Also plays a major role in secretion
*Final adjustments in solute concentration and volume of tubular fluid are made
*Active transport of Na+ and Cl-
*Primary site of Ca2+ reabsorption
Reabsorption in the Distal Convoluted Tubule (DCT)
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*Is the movement of nonfiltered substances from the blood into the filtrate
*These substances include toxic by-products of metabolism and drugs or molecules not normally produced by the body
*As with tubular reabsorption, _____ can be either active or passive
*For example, ammonia is a toxic by-product of protein metabolism
*It is produced when the epithelial cells of the renal tubule remove amino groups from amino acids, which diffuse into the lumen of the renal tubule
*On the other hand, H+, K+, penicillin and para-aminohippuric acid (PAH) are actively secreted by either active transport or antiport processes into the renal tubule
*An example of an antiport process in the kidney is the secretion of H+, which plays a major role in regulating body fluid pH
*If blood pH is too acidic, the kidney secreted H+
*The secreted H+ is produced when CO2 and water react to form H+ and HCO3-
*A Na+/H+ antiporter in the tubule cells will move Na+ into the proximal and distal convoluted tubule cells and move H+ out of the proximal and distal convoluted tubule cells
*More specifically, Na+ and HCO3- are symported across the basal membrane of the tubule cells and enter the peritubular capillaries
*Hydrogen ions are secreted into the lumens of the proximal and distal convoluted tubules
*By secreting H+, the blood pH stays in its normal range and does not become too acidic
Transfer of materials from bloodstream and tubular cells to tubular fluid
Tubular Secretion
97
*Ammonia, ammonia and urea
Secretion in Proximal Convoluted Tubule (PCT)
98
*K+, H+, and ammonium ions
Secretion in the Distal Convoluted Tubule (DCT)
99
The _______ of the nephron loop is important for establishing a concentration gradient in the renal medulla
*The kidneys utilize a _______
*A ________ is one where fluid in separate structures flows in opposite directions relative to each other
*As the fluids pass by each other, materials can be exchanged between the fluids
*There are two types of ________ in the kidney that are critical for either conserving or eliminating water in the body
*These two mechanisms are: 1.a countercurrent multiplier and 2.a countercurrent exchanger
*The ______ in the nephron loop is responsible for a large percentage of a very high concentration of solutes that is found in the interstitial fluid within the medulla of the kidney
*The permeability changes in the nephron loop are responsible for its role as the countercurrent multiplier
*The countercurrent exchanger in the vasa recta maintains the high solute concentration in the interstitial fluid
*Because the flow rate and blood pressure in the vasa recta are so slow and very low, the blood and the interstitial fluid are always in equilibrium and solutes are not carried away from the interstitial fluid by the blood in the vasa recta
Urine Concentration Mechanism
100
Medullary Concentration Gradient
*The interstitial fluid in the medulla of the kidney has a very high solute concentration compared with that of the cortex
*This is called the medullary concentration gradient
*The high solute concentration of the interstitial fluid develops from: 1.the actions of the two ________ and 2.the recycling of the protein breakdown product, urea
*The concentration of solutes in the medulla increases from 300mOsm/kg to 1200mOsm/kg deep in the medulla at the tip of the renal pyramid
Countercurrent Mechanisms
101
Countercurrent multiplier
Countercurrent exchanger
Urea cycling
To create and maintain the medullary concentration gradient, the following processes occur
102
As filtrate travels through the descending limb of the nephron loop, water moves across its simple squamous epithelium by osmosis toward the higher solute concentration in the interstitial fluid
This osmosis of water out of the filtrate causes the filtrate to become highly concentrated
Then, as the filtrate begins to move through the ascending limb, the permeability of the epithelium shifts such that solutes diffuse out of the filtrate
This increases the concentration of the interstitial fluid even more
In addition, the cells of the thick segment of the ascending limb actively pump solutes into the interstitial fluid, causing a very concentrated solution to form
The movement of solutes out of the nephron loop dramatically increases the solute concentration of the interstitial fluid
Only the juxtamedullary nephrons have nephron loops that descend deep into the medulla, but enough of them exist to maintain the high concentration of solutes in the interstitial fluid of the medulla
Not all of the nephrons need to have nephron loops that descend into the medulla to concentrate urine effectively
The cortical nephrons have the same function as the juxtamedullary nephrons, but their nephron loops are not as efficient at concentrating urine
However, because the filtrate from the cortical nephrons passes through the collecting ducts, water can diffuse out of the collecting ducts into the interstitial fluid, thus concentrating the filtrate
Countercurrent Multiplier
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The vasa recta supply blood to the kidney medulla, and they remove excess water and solutes from the medulla without changing the high concentration of solutes in the medullary interstitial fluid
The vasa recta have a countercurrent mechanism because blood flows through them to the kidney medulla, and after the vessels turn near the tip of the renal pyramid, the blood flows the opposite direction, back toward the cortex
The walls of the vasa recta are permeable to both water and solutes
As blood flows toward the medulla, water moves out of the vasa recta, and some solutes diffuse into them
As blood flows back toward the cortex, water moves into the vasa recta, and some solutes diffuse out of them
The directions of diffusion are such that the vasa recta carry slightly more water and solute from the medulla than to it
The composition of the blood at both ends of the vasa recta is nearly the same, with the volume and osmolality slightly greater as the blood once again reaches the cortex
Blood pressure in the vasa recta is very low and blood flow rate is extremely slow, even sluggish
This encourages ready diffusion of solutes into and back out of the vasa recta, ensuring the maintenance of the high medullary concentration gradient
Countercurrent Exchanger
104
Urea is responsible for a substantial part of the high osmolality in the kidney medulla
Due to their histology, the walls of the descending limbs of the nephron loops are permeable to urea; thus, urea diffuses into the descending limbs from the interstitial fluid
Due to their histology, the ascending limbs of the nephron loops and the distal convoluted tubules are impermeable to urea, so the urea remains in the nephron loops until it reaches the collecting ducts, which are permeable to urea
Some urea then diffuses out of the collecting ducts into the interstitial fluid of the medulla
Urea is recycled form the interstitial fluid into the descending limbs of the nephron loops, through the ascending limbs, through the distal convoluted tubules, and into the collecting ducts
Most urea then diffuses from the collecting ducts back into the interstitial fluid of the medulla
Consequently, a high urea concentration is maintained in the medulla of the kidney
Urea Cycling
105
oSodium ions and other solutes are actively transported into the interstitial fluid of the medulla, maintaining a high medullary osmolarity
oBecause blood flows sluggishly and there is low blood pressure in the vasa recta, solutes are not washed away from the medulla
oMuch urea returns to the medulla from the collecting duct, rather than exiting with the urine
Several Key Events Occur in the Renal Tubule to Establish and Maintain a High Medullary Solute Concentration
106
Exchange between fluids flowing in two different directions
Counter Current
107
*Descending limb: fluid flows towards the medulla, this area of the loop is permeable to water but not ions
*Ascending limb: fluid flows towards cortex, this area is impermeable to water but has transport mechanisms for some ions
Na+ and Cl- pumped out of the ascending limb into peritubular fluid increases the tonicity of the peritubular fluid
Water flows out of the descending limb via osmosis
Result: hypertonic peritubular solution in the medulla
In the Nephron Loop
108
*Is more sensitive to changes in blood pressure
*Is initiated under low blood pressure conditions
*When blood pressure decreases, cells of the juxtaglomerular apparatuses in the kidneys secrete the enzyme renin
oThe kidneys detect the low blood pressure when juxtaglomerular cells detect reduced stretch of the afferent arteriole
oIn addition, the macula densa cells signal the juxtaglomerular cells to secrete renin when the Na+ concentration of the filtrate drops
*Upon secretion, renin enters the blood and converts angiotensinogen, a plasma protein produced by the liver, to angiotensin I
*Angiotensin-converting enzyme (ACE) is a proteolytic enzyme produced by capillaries of organs such as the lungs
oACE converts angiotensin I to angiotensin II
oAngiotensin II is a potent vasoconstricting hormone that increases peripheral resistance, causing blood pressure to increase
oHowever, angiotensin II is rapidly broken down, so its effect lasts for only a short time
oAngiotensin II also increases the rate of aldosterone secretion, the sensation of thirst, salt appetite, and ADH secretion
oThe rate of renin secretion decreases if blood pressure in the afferent arteriole increases, or if the Na+ concentration of the filtrate increases as it passes by the macula densa of the juxtaglomerular apparatuses
Renin-Angiotensin-Aldosterone Hormone Mechanism
109
*A large decrease in the concentration of Na+ in the interstitial fluid acts directly on the aldosterone-secreting cells of the adrenal cortex to increase the rate of aldosterone secretion
oAngiotensin II is much more important than the blood level of Na+ for regulating aldosterone secretion
oIn addition, angiotensin II is critical for returning GFR to normal levels
*Aldosterone is a steroid hormone secreted by the cortex of the adrenal glands
*Aldosterone binds to its receptor in both the distal convoluted tubules and the collecting ducts
oAldosterone molecules diffuse through the plasma membranes and bind to their nuclear receptors
*Binding of aldosterone to its receptor increases synthesis of the Na+ -K+ pump and other Na+ transport proteins
oThe Na+-K+ pump increases the reabsorption of Na+ and the secretion of K+ across the basal membrane of tubule cells.
oAs a result, the rate of Na+ reabsorption increases
oSimultaneously, because of the action of the Na+-K+ pump, K+ secretion increases, rather than its reabsorption
Renin-Angiotensin-Aldosterone Hormone Mechanism
110
*Reduced secretion of aldosterone decreases the rate of Na+ reabsorption
*Reduced Na+ reabsorption keeps the concentration of Na+ in the distal convoluted tubules and the collecting ducts elevated
*Because the concentration of filtrate passing through the distal convoluted tubules and the collecting ducts has a greater-than-normal concentration of solutes, water’s capacity to move by osmosis from the distal convoluted tubules and the collecting ducts is diminished, urine volume increases, and the urine has a greater concentration of Na+
*Because increases in the number of Na+-K+ pumps increases the rate of K+ secretion, increases in blood K+ levels stimulate aldosterone secretion
*Conversely, decreases in blood K+ levels decrease aldosterone secretion
Renin- Angiotensin-Aldosterone Hormone Mechanism
111
*Is more sensitive to changes in blood osmolality
*Neurons of the supraoptic nucleus of the hypothalamus produce ______ (ADH), also known as vasopressin, which is stored in the posterior pituitary gland
*ADH is released into the blood from the posterior pituitary
*Cells called osmoreceptor cells in the supraoptic nucleus are very sensitive to even slight changes in the osmolality of the interstitial fluid
*If the osmolality of the blood and interstitial fluid increases, these cells stimulate the ADH-secreting neurons
*Action potentials are then propagated along the axons of the ADH-secreting neurons to the posterior pituitary gland, where the axons release ADH from their ends
Antidiuretic Hormone Mechanism
112
*Reduces osmolality of the interstitial fluid within the supraoptic nucleus inhibits ADH secretion from the posterior pituitary gland
*Baroreceptors that monitor blood pressure in the atria of the heart, large veins, carotid sinuses, and aortic arch also influence ADH secretion when the blood pressure changes by more than 5-10%
*Decreases in blood pressure are detected by baroreceptors when there is reduced stretch of the blood vessel wall
*This reduced stretch of the baroreceptors causes them to send a lower frequency of action potentials of the hypothalamus along afferent pathways
*These pathways terminate in the supraoptic nucleus of the hypothalamus
*As a result, the hypothalamus triggers secretion of more ADH
Antidiuretic Hormone Mechanism
113
*The distal convoluted tubules and collecting ducts remain relatively impermeable to water in the absence of ADH
*More urine is produced when little ADH is secreted
*A large part of the 19% of the filtrate that is normally reabsorbed in the distal convoluted tubules and the collecting ducts becomes part of the urine
*Insufficient ADH secretion results in a condition called diabetes insipidus; the word diabetes refers to the production of a large volume of urine, and the word insipidus means the urine is clear, tasteless, and dilute
*People who secrete insufficient ADH often produce 10-20L of urine per day and develop major problems, such as dehydration and ion imbalances
*In contrast to diabetes insipidus, diabetes mellitus refers to the production of a large volume of urine that contains a high concentration of glucose
Antidiuretic Hormone Mechanism
114
*ADH secretion promotes increased water reabsorption by the distal convoluted tubule when blood osmolality increases or when blood pressure declines significantly
*Water reabsorption lowers blood osmolality
*It also increases blood volume, which elevates blood pressure
*Conversely, when blood osmolality decreases or when blood pressure goes up, ADH secretion declines
*The reduced ADH levels cause the kidneys to reabsorb less water and to produce a larger volume of dilute urine
*The greater loss of water in the urine raises blood osmolality and lowers blood pressure
*ADH secretion occurs in response to small changes in osmolality, whereas a substantial change in blood pressure is required to alter ADH secretion
*Thus, ADH is more important in regulation blood osmolality than it is in regulating blood pressure
Antidiuretic Hormone Mechanism
115
oADH moves from the peritubular capillaries and binds to ADH receptors in the plasma membranes of the distal convoluted tubule cells and the collecting duct cells
oWhen ADH binds to its receptor, a G protein mechanism is activated, which in turn activates adenylate cyclase
oAdenylate cyclase increases the rate of cAMP synthesis
Cyclic AMP promotes the insertion of aquaporin-2-containing cytoplasmic vesicles into the apical membranes of the distal convoluted tubules and collecting ducts, thereby increasing their permeability to water
Water then moves by osmosis out of the distal convoluted tubules and collecting ducts into the tubule cells through the aquaporin-2 water channels
oWater exits the tubule cells and enters the interstitial fluid through aquaporin-3 and aquaporin-4 water channels in the basal membranes
The Mechanism of Antidiuretic Hormone (ADH) on Water Conservation
116
*Is a calculated value representing the volume of plasma that is cleared of a specific substance per each minute.
*Can be used to estimate GFR if the appropriate substance is monitored
*Such a substance must have the following characteristics: 1.it must pass through the filtration membrane of the renal corpuscle as freely as water or other small molecules, 2.it must not be reabsorbed, 3.it must not be secreted into the renal tubule, and 4.it must not be either metabolized or produced in the kidneys
*Creatinine is a naturally occurring metabolite that has these characteristics
*As filtrate forms, the filtrate has the same concentration of creatine as plasma; however, as the filtrate flows through the renal tubule, none of the creatine gets reabsorbed
*Thus, creatine gets steadily removed from the blood as it passes through the kidney
*The rate of creatine remove, called _____, is equal to the GFR
Plasma Clearance
117
*Can also be used to calculate renal plasma flow
*However, substances with the following characteristics must be used: 1.the substance must pass through the filtration membrane of the renal corpuscle, and 2.it must be secreted into the renal tubule at a sufficient rate that very little of it remains in the blood as the blood leaves the kidney
*Para-aminohippuric acid (PAH) meets these requirements
*As blood flows through the kidney, essentially all the PAH is either filtered or secreted into the renal tubule
*The clearance calculation for PAH is equal to the volume of plasma flowing through the kidney each minute
*Also, if the hematocrit is known, the total volume of blood flowing through the kidney each minute can be calculated easily
*The concept of _____can be used to help determine how drugs or other substances are excreted by the kidney
*A ______ value greater than the creatinine clearance value suggests that the substance is secreted by the tubule into the filtrate
Plasma Clearance
118
oPlasma clearance (mL/min) = Quantity of urine (mL/min) x Concentration of substance in urine/ Concentration of substance in plasma
The Plasma Clearance can be Calculated for any Substance that Enters the Blood According to the Following Formula
119
*Is the maximum plasma concentration of a substance before it begins to appear in the urine
*For example, glucose is usually almost completely reabsorbed from the tubule by active transport
*However, one of the first indications of diabetes mellitus is the presence of glucose in the urine, or glucosuria
*Glucosuria occurs when the renal threshold for glucose is surpassed and not all of the filtered glucose is reabsorbed, which leaves excess glucose to be excreted in the urine
Renal Threshold
120
*Is the maximum rate at which a substance can be actively reabsorbed from the filtrate
*Different substances have different transport maximum levels
*This is because the ____ is dependent on the number of transport proteins and their rate of transport for a particular substance
*For example, the ______ for glucose is actually higher than the renal threshold for glucose
*This is because the transport proteins in some nephrons become saturated before those in other nephrons
*Once all nephrons have reached their maximal capacity to reabsorb glucose, the transport maximum for the kidneys is reaches
*Thus, glucose will appear in the urine before the transport maximum is reached
*Urine volume in people with glucosuria is also greater than normal because the glucose molecules in the filtrate increase the osmolality of the filtrate in the tubule and reduce the effectiveness of water reabsorption by osmosis
Transport Maximum
121
*Are tubes through which urine flows from the kidneys to the urinary bladder
*Extend inferiorly and medially from the renal pelvis and exit the kidney at the renal hilum
*Descend through the abdominal cavity and enter the urinary bladder
Ureters
122
*Is a hollow, muscular container that lies in the pelvic cavity just posterior to the symphysis pubis
*The ureters enter on its posterolateral surface
*In males, the ______ is just anterior to the rectum; in females, it is just anterior to the vagina and inferior and anterior to the uterus
*Its volume increases and decreases, depending on how much or how little urine is stored in it
Urinary Bladder
123
* The _______, which transports urine to the outside of the body, exits the urinary bladder inferiorly and anteriorly
*The triangular area of the urinary bladder’s posterior wall between the two ureters and the _____ on the urinary bladder’s anterior wall is called the trigone
*This region is histologically unique
*The trigone does not expand with the urinary bladder wall as it fills
*This causes the trigone to act as a funnel for emptying the urinary bladder
*Cystitis is an inflammation of the urinary bladder, which usually results from a bacterial infection
*Typically, bacteria from outside the body enter the bladder
*Infection by the bacterium E.coli is the most common cause of cystitis
*Transitional epithelium lines both the ureters and the urinary bladder
*Transitional epithelium is specialized so that the cells slide past one another, and the number of cell layers decreases as the volume of the ureters and urinary bladder increases
Urethra
124
*The rest of the walls of these structures consists of a lamina propria, a muscular coat, and a fibrous adventitia
*The wall of the urinary bladder is much thicker than the wall of a ureter because it consists of layers of primarily smooth muscle, sometimes called the detrusor muscle
*Contraction of this smooth muscle forces urine of the urinary bladder
*The epithelium itself ranges from four or five cells thick when the urinary bladder is empty to two or three cells thick when it is distended
*Is lined with stratified or pseudostratified columnar epithelium
*At the junction of the urinary bladder and the ____, smooth muscle forms an internal urethral sphincter that prevents urine leakage from the urinary bladder
*In males, the internal urethral sphincter contracts to keep semen from entering the urinary bladder during sexual intercourse
*Females have smooth muscle fibers within the wall of the urethra at the neck of the bladder
*These smooth muscle fibers assist with preventing urine leakage, but whether these fibers form a distinct sphincter is still under scrutiny
*Both males and females have a well-defined external urethral sphincter
*The external sphincter is formed of skeletal muscle that surrounds the _____ as the _____ extends through the pelvic floor
*The external urethral sphincter allows a person to voluntarily start or stop the flow of urine through the ________
*In males, the ______ extends to the end of the penis, where it opens to the outside
*The female _______ is much shorter (approximately 4cm) than the male _______ (approximately 20cm) and opens into the vestibule anterior to the vaginal opening
Urethra
125
*The flow of urine from the kidney to the urinary bladder through the ureter is relatively continuous
*The urinary bladder acts as a reservoir for urine until it can be eliminated relatively quickly at an appropriate time and place
*The urinary bladder can stretch to hold a large urine volume
*At its maximum volume, the urinary bladder can contain 1L (about 1 quart) of urine, but discomfort becomes noticeable when urine volume exceeds approximately 500mL
Micturition
126
*Urination is called ______
*The ______ reflex is activated when the urinary bladder wall is stretched as urine fills the urinary bladder
*Integration of the _______ reflex occurs in the sacral region of the spinal cord and is modified by centers in the pons and cerebrum
*The slow increase in internal pressure helps explain why there is little urge to urinate when the urinary bladder contains less than 300mL
*The pressure in the urinary bladder increases rapidly once its volume exceeds approximately 400mL
*The frequency of action potentials conducted by the ascending spinal pathways to the pons and cerebrum also increases, resulting in a stronger urge to urinate
Micturition
127
*Voluntary initiation of micturition requires an increase in action potentials sent from the cerebrum to facilitate the ____ reflex and to voluntarily relax the external urethral sphincter
*In addition, voluntary contraction of the abdominal muscles increases abdominal pressure and thereby enhances the ______ reflex by increasing pressure applied to the urinary bladder wall
*Normally, the urge to urinate results from stretch of the urinary bladder wall, but irritation of the urinary bladder or the urethra by a bacterial infection or some other condition can also initiate the urge to urinate, even if the urinary bladder is nearly empty
*If the spinal cord is damaged above the sacral region, no micturition reflex exists for a time; however, if the urinary bladder is emptied frequently, the _______ reflex eventually regains the ability to cause the urinary bladder to empty
*Although a typical _______ reflex may exist, the person has no conscious control over its onset or duration
*This condition is called automatic bladder
Micturition
128
*On the other hand, if the spinal cord is damaged in the sacral region of the spinal cord, it eliminates the _____ reflex altogether
*The urinary bladder is unable to contract even though the external urethral sphincter is relaxed
*The urinary bladder fills to capacity, and urine is forced in a slow dribble through the external urethral sphincter
*In older people and in patients with damage to the brainstem or spinal cord, there is a reduction in inhibitory action potentials to the sacral region of the spinal cord
*Without this inhabitation, the sacral centers are hyperexcitable, and even a small amount of urine in the urinary bladder can elicit an uncontrollable _______ reflex
Micturition
129
oThe wall of the urinary bladder contains large folds, similar to those of the stomach, which unfold to enlarge the lumen
oThe lining of the urinary bladder is transitional epithelium, which stretches
oThe smooth muscle wall of the urinary bladder, with the exception of the trigone, also stretches to accommodate fluid
As urine enters the urinary bladder, it lifts and expands superiorly to accommodate the fluid
The Urinary Bladder's Capacity to distend is Due to Three Factors
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oUrine filling the urinary bladder stimulates stretch receptors, which produce action potentials
oThe action potentials are carried by sensory neurons to the sacral segments of the spinal cord through the pelvic nerves
oIn response, action potentials travel to the urinary bladder through parasympathetic fibers in the pelvic nerves
The parasympathetic action potentials cause the smooth muscle of the urinary bladder (the detrusor muscle) to contract
In addition, decreased somatic motor action potentials cause the external urethral sphincter, which consists of skeletal muscle, to relax
Urine flows from the urinary bladder when the pressure there is great enough to force the urine through the urethra while the external urethral sphincter is relaxed
The micturition reflex produces a series of contractions of the urinary bladder
oAction potentials carried by sensory neurons from stretch receptors in the urinary bladder wall also ascend the spinal cord to a micturition center in the pons and to the cerebrum
Steps in the Micturition Reflex
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oThe micturition reflex integrated in the spinal cord is automatic, but it is either stimulated or inhibited by descending action potentials sent to the sacral region of the spinal cord
For example, higher brain centers prevent micturition by sending action potentials from the cerebrum and pons through spinal pathways to inhibit the spinal micturition reflex
Consequently, parasympathetic stimulation of the urinary bladder is inhibited, and somatic motor neurons that keep the external urethral sphincter contracted are stimulated
The micturition reflex, integrated in the spinal cord, predominates in infants
The ability to inhibit micturition voluntarily develops at the age of 2-3 years; subsequently, the influence of the pons and cerebrum of the spinal micturition reflex predominates
oThe brain voluntarily controls the external urethral sphincter through somatic motor nerves, causing the sphincter to relax or constrict
Steps in the Micturition Reflex
