Pathophysiology of Fluid Shifts

Question 1

detail the relative size of each body fluid compartment

Answer 1

60% of body weight is water (higher in puppies/kittens; lower in obese animals)

40% of body weight is intracellular fluid (2/3 of total body water)

20% of body weight is extracellular fluid (1/3 of total body water) and consists of 75% interstitial fluid and 25% plasma, so plasma is 5% of body weight

Question 2

describe total blood volume in relation to body weight

Answer 2

total blood volume (plasma and RBCs) is 9% of body weight, or 90mL/kg (also the calculation for the full dose of shock fluids)

Question 3

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

Answer 3

between intracellular and extracellular: osmosis

between intravascular and interstitial: starling’s forces

Question 4

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

Answer 4

most important intracellular cation: potassium

most important extracellular cation: sodium

difference established by sodium-potassium-ATPase pump

chloride, proteins, and bicarb are the anions that also contribute, but not as majorly as Na+/K+

Question 5

in most disease states, where is fluid solutes initially lost from? describe the 3 basic types of fluid and solute losses that may occur

Answer 5

lost from the extracellular fluid first!

1. loss of more solute than water: loss of hypertonic fluids, will cause ECF to become hypotonic to ICF, so water will move into ICF, decreasing ECF fluid volume even more, leading to hypovolemia and shock (example is serum loss from oozing wounds like burns)
2. loss of solute and water at equal in equal amounts: isotonic loss; will not result in any shift of water, but if enough isotonic fluid is lost can lead to hypovolemia and shock (example is vomiting and diarrhea)
3. loss of more water than solutes: loss of hypotonic fluids; ECF tonicity increases, causing water to shift out of cells and into ECF, preserving the effective circulating blood volume and actually protecting against shock (example is water deprivation)

Question 6

define each of starling’s forces

Answer 6

capillary oncotic pressure: osmotic pressure generated by plasma proteins (especially albumin! but also globulins)

interstitial fluid oncotic pressure: osmotic pressure generated by proteins in interstitial fluid

capillary hydrostatic pressure: pressure exerted by blood against the capillary wall (high in arteries (favor filtration), no net movement in capillaries, and low in veins (favor reabsorption))

interstitial fluid hydrostatic pressure: the pressure exerted by interstitial fluid against the wall of a capillary

the sum of these forces equals the net filtration pressure;
if NFP is positive: net filtration across capillaries into interstitial spaces
if NFP is negative: net absorption from interstitial spaces into capillaries

water flux can also be affected by area, permeability of a vessel to water, and permeability of a vessel to protein!

Question 7

describe the mechanisms of edema formation (4)

Answer 7

1. increased hydrostatic pressure: can be due to increased plasma volume (fluid overload), right-sided heart failure, venous obstruction (thrombus), or a cranial mediastinal mass
2. decreased plasma oncotic pressure (albumin <1.5-2.0g/dL: can be due to protein loss (protein-losing enteropathy or nephropathy), reduced albumin synthesis (indicating liver disease), or malnutrition (but must be SEVERELY malnourished before see a decreased albumin)
3. increased capillary permeability: can be due to allergic reactions, septicemia, burns, inflammation
4. lymphatic dysfunction: usually due to neoplasia where the cancer obstructs lymphatics in some way

Question 8

describe the mechanisms of edema protection (3)

Answer 8

1. increased interstitial pressure: opposes microvascular filtration and promotes lymph flow
2. increased lymph flow: can increase 10-50 fold when fluid begins to accumulate, allowing the lymphatics to carry away large amounts of fluid and proteins and return them back to circulation
3. decreased interstitial colloid osmotic pressure: dilution of proteins in interstitial space allows proteins to be carried away by lymphatic flow

Question 9

list the functions of albumin (5) where is it synthesized?

Answer 9

1. maintain colloid osmotic pressure
2. carrier function: carries drugs, hormones, bilirubin, calcium, zinc
3. free radical scavenger
4. necessary for coagulation and platelet function
5. necessary for normal healing

albumin is synthesized in the liver!

Question 10

what would cause albumin to be low?

Answer 10

1. decreased production by liver: can be due to liver failure, maldigestion/malabsorption, malnutrition, or inflammation (negative acute phase protein)
2. increased loss of albumin: can be due to hemorrhage, PLE or PLN, wounds, inflammation