8-4 Body Fluid Compartments

Question 1

What are the body fluid compartments?

Answer 1

TBW - Total Body Water = 0.6 X body weight, ~42 L

ICF - Intracellular fluid = 0.4 x body weight, ~28 L

ECF - Extracellular fluid = 0.2 x body weight, ~14 L

IF - Interstitial fluid = 3/4 of ECF, ~10.5 L

Plasma = 1/4 of ECF, ~3.5 L

Question 2

What are the components of ECF? What separates them?

Answer 2

Interstitial fluid

Plasma

Transcellular fluid

Separated by capillary wall

Question 3

What makes up transcellular fluid? What space does it occupy?

Answer 3

Small amt of water - epithelial secretions, synovial fluid, CSF

• o ccupies third space

Question 4

What are the different constituents of intracellular versus extracellular body fluids?

Answer 4

Extracellular

High in Na+, Cl-, HCO3-

low in K+

Intracellular

High in K+, Mg++, PO4-

Low in Na+

Question 5

The membrane between the ICF and the ECF compartments is what? What is this permeable to? What is it not permeable to?

Answer 5

Cell membrane

Permeable to water, impermeable to charge/electrolytes

Question 6

Fluid distribution between ECF and ICF is dependent on what? What drives this fluid distribution?

Answer 6

Osmotic effects - mostly Na+ and Cl-

Distribution of ions determined by ATPase activity

Question 7

The membrane between ECF compartments separates what? What is it permeable to?

Answer 7

Plasma and interstitial fluid makes up the ECF compartment, and is separated by the capillary wall

Permeable to small ions

Question 8

What affects fluid distribution in the ECF? What are these a balance of?

Answer 8

Fluid distribution in plasma and interstitial fluid that makes up the ECF is determined by Starling forces

• balance between hydrostatic pressure and colloid osmotic pressure
• these are determined by fluid volume and protein concentration in the vasculature and interstitial spaces

Question 9

An imbalance of fluids between compartments frequently results in what? What are some causes?

Answer 9

Edema - non-pitting or pitting

Causes: loss of plasma proteins, ATPase activity

Question 10

What is non-pitting edema?

Answer 10

swollen cells due to increased ICF volume; non-responsive to diuretic action

• difficult to treat

Question 11

What is pitting edema? What are some causes of it, and what is this treated with?

Answer 11

increased interstitial fluid volume

–Nephrotic syndrome, CHF, pregnancy, cirrhosis

• can be treated with diuretics, usually less severe than non-pitting

Question 12

Osmotic equilibriation is established between ECF and ICF. Why?

Answer 12

Cell membranes separate these 2, and are highly permeable to water

Question 13

Infusion of hypertonic solution into ECF will result in what changes in ICF? What is this called?

Answer 13

Hyperosmotic volume expansion

Initially, ECF will have an increased mOsm/L or concentration, and an increased volume due to saline infusion

Then, water will rush into ECF, leaving ICF with a decreased volume but an increased mOsm/L or concentration

Water will rush into ECF until ICF and ECF are roughly equilibrated as far as mOsm/L or concentration

Results in larger volume for ECF, and a lower volume for ICF, and a higher mOsm/L for both

Question 14

What would happen if you were to infuse a solution of isotonic saline into ECF? What is this called?

Answer 14

Isosmotic volume expansion

ECF volume increases

No change in osmolality between ICF and ECF

But! Plasma protein concentration/cell count goes down due to dilution (extra fluid)

Arterial BP goes up, expanded ECF (plasma + third space) happened

Question 15

What happens to osmotic equilibration with diarrhea? What is this called?

Answer 15

Loss of isotonic fluid - isosmotic volume contraction

ECF volume decreases

No change in osmolality - no water shift between ICF and ECF

Plasma protein concentration, hematocrit increases

Loss of arterial blood pressure

Decrease in ECF volume, no change in osmolality for ICF or ECF

Question 16

If you were to wander in the desert for 40 days, what would happen to ICF and ECF? What is this called?

Answer 16

Hyperosmotic volume contraction

Sweat is hyposmotic

ECF osmolality is increased

ECF volume is decreased - loss of volume via sweat

Water leaves ICF

ICF osmolality increases until ECF is equivalent

ICF volume decreases

Question 17

What would happen to ECF and ICF if you had SIADH? What is this called? What about hematocrit and blood protein levels?

Answer 17

Gain of water - hyposmotic volume expansion

Excess water is retained

ECF has more water, increased volume

ECF osmolality decreases

Water goes into cells

ICF osmolality decreases until its equivalent to ECF

ICF volume increases

Plasma protein conc drops, RBCs become bigger so hematocrit stays the same

Question 18

What would happen to your ECF and ICF if you had adrenal insufficiency? What is this called?

Answer 18

Adrenocortical insufficiency = loss of NaCl, hyposmotic volume contraction

Osmolality of ECF decreases - decreased Na+ reabsorption in kidneys, excrete more NaCl than water

ECF volume decreases

Water shifts into cells

ICF osmolarity decreases until it equilizes with ECF

ICF volume increases

Plasma protein conc increases due to decreased ECF volume

Hematocrit increases - reduced ECF volume, RBCs swollen due to more water

Arterial blood pressure decreases due to decreased ECF volume

Question 19

What is the response to volume expansion?

Answer 19

Volume expansion

• decreased SNS activity, decreased renin, decreased aldosterone
• heart secretes ANP and BNP, causing dilation of afferent arteriole
• ANP and BNP work on brain, reducing ADH output
• urodilantin is released, causing increased dilation and volume in collecting ducts
• Na+, water excretion increased

Question 20

What is the response to volume contraction?

Answer 20

Volume Contraction

• increased SNS, constriction of afferent arteriole, increased renin
• decreased pressure on heart, increased ANP and BNP synthesis
• less ANP and BNP works on brain, increased ADH secreted
• increased renin causes increased aldosterone
• increased aldosterone favors reabsorption of water, Na+
• less water, Na+ excretion
• less urodilantin, contracted collecting tubule

Question 21

What does tonicity relate to?

Answer 21

Tonicity of a solution relates to the effect of that extracellular solution on cell volume

Implies 2 entities: cell and the solution its sitting in

Question 22

What does tonicity rely on?

Answer 22

Depends on the concentration of impermeant solutes in the extracellular vs. intracellular fluid

Question 23

In regards to tonicity, what is the difference between isotonic, hypotonic, and hypertonic?

Answer 23

–No change in cellular volume = isotonic solution
–Cellular volume increase (swelling) = hypotonic solution
–Cellular volume decrease (shrinking) = hypertonic solution