Unit 1: Homeostasis and Cellular response

Question 1

Intracellular Fluid Compartment

Answer 1

In the adult, 40% of total body weight is the water contained within the ICF compartment. Water can diffuse out of the ICF and cause cell shrinkage or cellular dehydration. Conversely, water can enter the ICF and cause cell swelling or cellular edema.

Question 2

Extracellular Fluid Compartment

Answer 2

In the adult, 20% of total body weight is the water contained within the ECF compartment. Most of the ECF is found within the intravascular compartment or blood vessels. The ECF contains electrolytes, oxygen, glucose, and other nutrients to be delivered to cells, as well as cellular waste products designated for excretion.

Question 3

Interstitial Fluid Compartment

Answer 3

ISF, which is a filtrate of the blood, is located between the cells and between the cells and capillaries. Like blood, it contains water and electrolytes, mainly sodium (Na+). ISF lacks proteins because they are too large to diffuse out of the blood vessels into the interstitial spaces. However, during inflammation, capillary membranes become extrapermeable; the pores enlarge, allowing proteins such as white blood cells out to the tissues.

Question 4

Hydrostatic Pressure

Answer 4

is the pushing force exerted by water in the bloodstream. The heart’s pulsatile pumping action is the source of hydrostatic pressure, which exerts an outward force that pushes water through the capillary membrane pores into the ISF and ICF compartments

Question 5

Transport Mechanisms

Answer 5

Diffusion
Osmosis
Facilitated transport
Active transport

Question 6

Diffusion

Answer 6

The process by which molecules passively spread from areas of high concentration to areas of low concentration. Water and electrolytes diffuse from high concentration to lower concentration until an equilibrium is reached.

Question 7

Osmosis

Answer 7

The tendency of molecules of a solvent to pass through a semipermeable membrane from a less concentrated solution into a more concentrated one, equalizing the concentrations on each side of the membrane. Electrolytes and water move through the cell’s semi-permeable plasma membrane, but large proteins such as albumin cannot pass through the membrane. A semipermeable membrane selectively allows some molecules through its pores and obstructs others according to size.

Question 8

Facilitated transport

Answer 8

The passing of certain molecules through the plasma membrane with assistance from carrier proteins. Glucose undergoes facilitated transport into the cell by the carrier protein insulin.

Question 9

Active transport

Answer 9

Occurs when a substance requires energy to pass through a membrane against a concentration gradient. Sodium and potassium require active transport using the N+/K+ pump, which is within the plasma membrane to retain potassium as the major intracellular ion and sodium as the major extracellular ion. Sodium is a solute that draws water with it.

Question 10

Starling’s Law of Capillary Forces

Answer 10

Starling’s Law of Capillary Forces explains the movement of fluid that occurs at every capillary bed in the body. There are two major opposing forces at every capillary membrane:

1. Hydrostatic pressure
2. Osmotic pressure (includes oncotic pressure)

Within every capillary, electrolytes and proteins within the blood exert osmotic pressure. The fluid within the capillary exerts hydrostatic pressure. These pressure forces oppose each other and attempt to balance each other out at every capillary membrane, thereby creating a state of homeostasis

hydrostatic pressure pushes water outward from the ECF to the ICF at the capillary–cell interface.

osmotic pressure pulls water from the ICF into the ECF at every cell–capillary interface. The osmotic pressure opposes the hydrostatic pressure; in healthy conditions, each force balances out the other.

when osmotic pressure is lower than hydrostatic pressure, osmotic pressure is overwhelmed and hydrostatic pressure is an unopposed force, causing water to flow from the ECF to the ICF.

Question 11

Osmotic Pressure

Answer 11

is the pressure exerted by the solutes in solution. In the bloodstream, osmotic pressure is exerted by electrolytes, mainly sodium ions and plasma proteins. Osmotic pressure is a force that pulls water into the bloodstream from the ICF and ISF and opposes hydrostatic pressure at all capillary membranes (see Fig. 7-3). Osmotic pressure is determined by the number of particles or their concentration within the solution. A solution with a greater number of particles has a higher osmotic pressure.

When a membrane such as a cell membrane separates two solutions with different osmotic pressures, fluid will move from the solution with lower osmotic pressure into the solution with the higher osmotic pressure, which is why a high osmotic pressure in the bloodstream favors fluid movement from the ICF and ISF into the bloodstream. Conversely, when the osmotic pressure is reduced, fluid moves out of the bloodstream and into interstitial and intracellular spaces (see Fig. 7-4).

Question 12

Osmolality

Answer 12

is a measurement of the concentration of solutes per kg of solvent. It is based on 1 mole (or gram molecular weight equivalent) of a substance dissolved in 1 kilogram of water. In clinical practice, osmolality can be used to evaluate the body’s hydration status based on the concentration of fluid and particles in solution. Normal plasma osmolality is 282 to 295 milliosmoles per kilogram of water. Low osmolality indicates a lesser amount of solutes in solution, whereas high osmolality indicates a greater amount of solutes. If the bloodstream is well hydrated, serum osmolality is 282 milliosmoles per kg of water or less. If the bloodstream is concentrated and has low water, the serum osmolality will be 295 milliosmoles per kg of water or greater. Serum osmolality can be calculated using the following mathematical formula: milliosmoles of solute /kg of water = 2 × serum sodium + serum glucose /18 + BUN / 2.4.

Question 13

Osmolarity

Answer 13

is the number of osmoles of solute per liter of solution; it is dependent on the number of particles suspended in a solution. In the body, the major solutes are albumin, sodium (Na+), potassium (K+), phosphate (PO4−), magnesium (Mg++), calcium (Ca++), bicarbonate (HCO3−), and glucose. The major protein within the bloodstream is albumin, which is the solute in the ECF that exerts the most osmotic pressure. Sodium, the main determinant of osmolarity, is a positive ion, also called a cation; it is found mostly in the ECF and assists in the maintenance of fluid balance and osmotic pressure. Potassium is the main intracellular cation; it assists in the maintenance of neuromuscular excitability and acid–base balance. Both sodium and potassium require the cell’s Na+/K+ pump to maintain Na+ as the extracellular ion and K+ as the intracellular ion. Phosphate is an intracellular negative ion, also called an anion. Magnesium plays an important role in enzymatic systems within the body. Calcium plays an important role in neuromuscular irritability, blood clotting, and bone structure. Bicarbonate is responsible for acid–base balance.

Question 14

Tonicity

Answer 14

refers to the concentration of solutes in solution compared with the bloodstream. The term is also used to describe the various intravenous (IV) solutions used in the clinical setting. There are three types of IV solutions:

Question 15

Isotonic solution:

Answer 15

This has the same tonicity as blood; when infused as an IV solution, it does not cause fluid shifts or alter body cell size. It has a concentration of particles and fluid that is similar to blood and body fluids. A standard isotonic IV solution is 0.9% NaCl solution, also called normal saline. It is used frequently as a bloodstream volume expander. Often an isotonic solution is used to keep an open connection to the IV route for medication administration or a blood transfusion.

Question 16

Hypotonic solution:

Answer 16

This has fewer particles and more water than blood and body fluids. When a hypotonic solution is infused, water is added to the bloodstream and causes a fluid shift from ECF to ICF to deliver water to the body, as in dehydration treatment. A standard hypotonic solution is 0.45% NaCl and is also referred to as half normal saline.

Question 17

Hypertonic solution

Answer 17

This contains more particles and less water than blood and body fluids. When a hypertonic solution is infused into the bloodstream, solutes are added to the bloodstream and cause fluids to shift from ICF to ECF, causing body cells to shrink. A commonly used hypertonic IV solution is mannitol. It can be used to diminish cell swelling, particularly in cerebral edema. Another hypertonic solution that is used less often is 3.0% NaCl.

Question 18

Fluid Homeostasis

Answer 18

Various physiologic mechanisms work together in order to maintain fluid homeostasis. In terms of fluid volume, both fluid intake and output must be regulated to prevent fluid volume overload, also known as edema, and fluid volume deficit, also known as dehydration. However, in addition to fluid volume status, the relative composition of body fluids, including electrolyte and acid or base concentrations, needs to be consistent. The kidney, rennin–angiotensin–aldosterone system (RAAS), osmoreceptors, thirst sensation, antidiuretic hormone (ADH), and natriuretic peptides work together to maintain fluid homeostasis in the body.

Question 19

Osmoreceptors, ADH, and thirst.
-Arginine Vasopressin

Answer 19

Changes in plasma osmolarity are responsible for both the sensation of thirst and the release of ADH, also called arginine vasopressin. High plasma osmolarity stimulates osmoreceptors in the hypothalamus. This stimulates the hypothalamic thirst center of the brain, as well as promoting the release of ADH from the posterior pituitary.

Question 20

Osmoreceptors, ADH, and thirst.
-Thirst

Answer 20

Thirst is a conscious desire to drink fluids. It is triggered by a response in the thirst center, which is located in the anterior hypothalamus. The osmoreceptors respond to changes in both blood osmolarity and blood fluid volume. When there is an increase in blood osmolarity, ICF shifts into ECF and the cells shrink, stimulating the thirst center. This center transmits signals to the cerebral cortex, promoting the sensation of thirst. Thirst causes a conscious desire to drink fluids, which brings water into the body’s bloodstream to reduce osmolarity. Massive loss of blood and fluid volume, as is seen in severe trauma, will trigger the sense of thirst as well.

Question 21

Osmoreceptors, ADH, and thirst.
-Healthy Person

Answer 21

In a healthy person, osmoreceptors, ADH, and thirst responses work together. ADH is produced by the hypothalamus. Once the ADH is synthesized, it travels by an axonal transport mechanism to the posterior pituitary gland. When the bloodstream lacks sufficient water, plasma osmolarity is increased and the osmoreceptors shrink. This stimulates the ADH neurons to depolarize, releasing ADH from the posterior pituitary. In addition to changes in osmolarity, other factors such as pain, trauma, and medications stimulate the release of ADH.

After release into the bloodstream, ADH stimulates water reabsorption from the nephron tubule fluid at the collecting duct into the bloodstream. This raises the blood’s water content and decreases the water in the tubule fluid, which eventually becomes concentrated urine. When there is enough water in the bloodstream, plasma osmolarity decreases, and ADH secretion is inhibited.

Question 22

RAAS

Answer 22

Hypotension, hypovolemia, dehydration, and low cardiac output cause low circulation throughout the body. Reduced circulation causes low renal perfusion, which stimulates renin secretion by the kidney’s juxtaglomerular apparatus. Renin initiates the RAAS, a compensatory mechanism used to replenish blood volume and raise blood pressure

Question 23

RAAS
-Renin

Answer 23

is an enzyme released from the kidney in response to decreased renal perfusion. Renin cleaves angiotensinogen, which is a large protein produced by the liver, to produce angiotensin I. In the lungs, angiotensin-converting enzyme (ACE) changes angiotensin I into angiotensin II, a powerful vasoconstrictor. Angiotensin II binds to receptors in the adrenal cortex, stimulating the synthesis and secretion of aldosterone, a mineralocorticoid that increases sodium and water reabsorption into the bloodstream at the distal tubule of the nephrons. Aldosterone also stimulates the excretion of potassium into the nephron tubules, which eventually becomes urine. When blood volume decreases, aldosterone begins the reabsorption of sodium from the distal tubules into the bloodstream, bringing sodium back into the bloodstream. This causes more absorption of water and increased blood volume. When the blood volume returns to normal, aldosterone secretion is reduced.

Question 24

Atrial Natriuretic Peptide (ANP)

Answer 24

ANP is produced by the heart’s atria and is secreted in response to excess ECF volume that stretches the heart’s atrial chambers.

Question 25

C-type natriuretic peptide (CNP)

Answer 25

CNP is produced by endothelial cells of the arteries and ventricular cells of the heart.

Question 26

B-type natriuretic peptide) (BNP)

Answer 26

BNP is produced in the heart’s ventricles and, to a lesser extent, in the brain. It is excreted in response to fluid volume overload stretching the heart’s ventricles—the more the ventricle is stretched by blood volume, the more BNP is secreted. Both ANP and BNP promote natriuresis at the glomerulus by increasing glomerular filtration rate. CNP has limited diuretic and natriuretic effects compared with ANP and BNP, but it has only been recently identified and is not completely understood.

Question 27

Edema

Answer 27

Edema occurs when there is an excess of fluid in the ISF and ICF compartments. It can occur because of elevated hydrostatic pressure created by excess water in the bloodstream or diminished osmotic force created by a low amount of solutes in the bloodstream. Edema can also occur because of inflammation, which causes increased capillary permeability; the capillary pores enlarge to allow fluid and cells out of the bloodstream to reach the site of injury. The fluid that moves into the ISF and ICF causes the edema.

When edema occurs because of high hydrostatic pressure in the bloodstream, the osmotic pressure force is overwhelmed and does not balance out the hydrostatic force. Consequently, according to Starling’s Law of Capillary Forces, hydrostatic pressure pushes fluid out of the capillary membrane pores into the ISF and ICF. An example of this occurs in left-sided heart failure, where high hydrostatic pressure develops in the pulmonary bloodstream. The high hydrostatic pressure forces fluid out of the pulmonary blood vessels and into the alveolar spaces and the interstitial tissue. This is known as pulmonary edema. Edema can also occur in the peritoneal cavity as ascites, the pleural cavity as pleural effusion, and the lower extremities as ankle edema.

Edema can also occur because of a low amount of solute in the bloodstream. Low albumin in the blood, or hypoalbuminemia, causes an imbalance in capillary forces. Because albumin is the major source of oncotic pressure, hypoalbuminemia will cause low oncotic pressure in the bloodstream. According to Starling’s Law of Capillary Forces, for homeostasis to occur, oncotic pressure must equal hydrostatic pressure. When oncotic pressure is low, hydrostatic pressure will be the overriding force and push fluid out of the capillary into the ISF and ICF compartments, thereby creating an edematous state.

An example of edema caused by hypoalbuminemia occurs in severe protein starvation. Without sufficient nutritional protein, blood albumin levels become extremely low and, consequently, oncotic pressure is diminished. An imbalance between oncotic pressure and hydrostatic pressure occurs at every capillary–cell interface. Hydrostatic pressure overwhelms oncotic pressure, and water is pushed out of the capillary into the ISF and ICF. Edema occurs throughout the body at every capillary–cell interface, and this is often most apparent in the peritoneal cavity as a swollen abdomen. In persons who are starving the disorder is known as kwashiorkor.

A specific kind of edema, called dependent edema, often forms in the lower extremities. Under healthy conditions, venous return to the heart from the lower extremities is assisted by venous valves and muscle contractions. A weakened venous valve system, lack of muscle contractions, and gravitational forces can allow venous blood to collect in the lower extremities. When an individual stands or sits in one position for an extended period, venous blood can pool in the lower extremities. Increased hydrostatic pressure in the veins allows fluid to flow out of the capillary into interstitial tissues. Fluid accumulates in the ankles and feet, which are the dependent parts of the body. To avoid dependent edema, brisk venous circulation back to the heart and vigorous muscle activity must be maintained in the lower extremities.