Body Fluid Physiology

Question 1

describe the relative size of each body fluid compartment

Answer 1

60% of body weight is water
1. intracellular compartment: 40% of body weight
2. extracellular compartment: 20% of body weight
2a. interstitial compartment: 3/4 (75%) of extracellular fluid
2b. intravascular compartment: 1/4 (25%) of extracellular fluid

there is also a transcellular fluid compartment but it makes up such a small percent of body water it is almost insignificant

Question 2

describe the mechanisms that affect the movement of water between the intracellular and extracellular fluid compartments

Answer 2

osmosis: movement of water down its concentration gradient through aquaporins (from low solute concentration to high solute concentration)

Question 3

describe the mechanisms that affect the movement of water between the intravascular and the interstitial fluid compartments

Answer 3

starling forces; four primary forces that determine whether fluid will move out of the capillary lumen into the interstitial fluid or in the opposite direction (blood/capillary hydrostatic pressure, blood/capillary oncotic pressure, interstitial fluid hydrostatic pressure, interstitial fluid oncotic pressure)

Question 4

define each of the starling forces (4)

Answer 4

1. capillary hydrostatic pressure: the pressure exerted by blood against the wall of a capillary
2. capillary oncotic pressure: osmotic pressure generated by plasma proteins (particularly albumin)
3. interstitial fluid hydrostatic pressure: the pressure exerted by interstitial fluid against the wall of a capillary
4. interstitial fluid oncotic pressure: the pressure generated by proteins in interstitial fluid

Question 5

identify the major cations and anions that contribute to the osmolarity of the intracellular and extracellular fluids; describe healthy osmolarity

Answer 5

in health, osmolarity is approx 300 mOsm/L in each compartment, but the composition of osmoles varies between them

intracellular cations: mainly potassium plus magnesium
intracellular anions: proteins, bicarb, phosphates
extracellular cations: sodium
extracellular anions: chloride and bicarbonate

Question 6

compare the relative concentration of protein in each of the intracellular, interstitial, and intravascular fluid compartments

Answer 6

extracellularly:
protein relatively low in interstitial fluid; higher in plasma/intravascular compartment

is highest in intracellular compartment, but its relatively high concentration in plasma is essential for starling forces

Question 7

predict how changes in starling forces affect the movement of fluid across the capillary wall and the size of intravascular and interstitial fluid compartments

Answer 7

capillary hydrostatic pressure:
increase: fluid moves from capillary lumen to interstitium
decrease: fluid moves from interstitium into capillary lumen

capillary oncotic presure:
increase: fluid moves from interstitium to capillary lumen
decrease: fluid moves from capillary lumen to interstitium

interstitial fluid hydrostatic pressure:
increase: fluid moves from interstitium to capillary lumen
decrease: fluid moves from capillary lumen to interstitium

interstitial fluid oncotic pressure:
increase: fluid moves from capillary lumen to interstitium
decrease: fluid moves from interstitium to capillary lumen

Question 8

what component of body fluid makes the greatest contribution to maintain extracellular fluid in the intravascular space and why?

Answer 8

large proteins such as albumins; do not move easily between compartments like smaller molecules, so can contribute their own oncotic force

Question 9

what is the main force driving fluid movement from the capillary to the interstitium? what is this movement called?

Answer 9

capillary hydrostatic pressure; this movement from capillary lumen to interstitium is called filtration

Question 10

what is the main force favoring fluid movement from interstitium to capillary lumen? what is this movement called?

Answer 10

capillary oncotic pressure; this movement is called reabsorption

Question 11

describe the normal balance of filtration and reabsorption

Answer 11

normally filtration exceeds reabsorption, promoting entry of some interstitial fluid into lymphatic vessels

Question 12

list 5 factors that can contribute to peripheral edema, or excess of interstitial fluid

Answer 12

1. decrease in plasma oncotic pressure
2. increase in interstitial colloid osmotic pressure
3. increase in capillary hydrostatic pressure
4. lymphatic obstruction
5. increased capillary wall permeability to albumin (pulls water out of capillaries into interstitial fluid)

Question 13

define tonicity versus osmolarity and classify fluids as hypertonic, isotonic, or hypotonic based on their osmolarity

Answer 13

osmolarity decribes the total concentration of fully dissociated solutes in a solution, while tonicity describes the ability of an extracellular solution to make water move into or out of a cells via osmosis and its effect on cell volume

hypertonic: solution is hyperosmotic (higher osmolarity than ICF)
hypotonic: solution is hypoosmotic (lower osmolarity than ICF)
isotonic: solution is isoosmotic (same osmolarity as ICF)

Question 14

predict the transcellular fluid exchange that would be caused by placing cells in isotonic, hypotonic, or hypertonic solutions

Answer 14

isotonic solution: no change in ICF volume or osmolarity

hypotonic solution: increase in ICF volume as water rushes into the cell, decrease in ICF osmolarity due to increased fluid

hypertonic solution: decrease in ICF volume as water rushes out of cell; increase in ICF osmolarity due to decreased fluid

Question 15

predict the changes in the volume and osmolarity of the intracellular and extracellular fluid (including intravascular and interstitial fluid) compartments caused by extracellular fluid loss or gain of hypotonic, isotonic, or hypertonic fluids

Question 16

predict how changes in total body water affect each starling force

Question 17

provided with information on changes that affect body fluid distribution, predict their implications for the animal

Question 18

when an animal is dehydrated (loss of total body water, produces hypertonicity)