8-4 Body Fluid Compartments Flashcards
What are the body fluid compartments?
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
What are the components of ECF? What separates them?
Interstitial fluid
Plasma
Transcellular fluid
Separated by capillary wall
What makes up transcellular fluid? What space does it occupy?
Small amt of water - epithelial secretions, synovial fluid, CSF
- o ccupies third space
What are the different constituents of intracellular versus extracellular body fluids?
Extracellular
High in Na+, Cl-, HCO3-
low in K+
Intracellular
High in K+, Mg++, PO4-
Low in Na+
The membrane between the ICF and the ECF compartments is what? What is this permeable to? What is it not permeable to?
Cell membrane
Permeable to water, impermeable to charge/electrolytes
Fluid distribution between ECF and ICF is dependent on what? What drives this fluid distribution?
Osmotic effects - mostly Na+ and Cl-
Distribution of ions determined by ATPase activity
The membrane between ECF compartments separates what? What is it permeable to?
Plasma and interstitial fluid makes up the ECF compartment, and is separated by the capillary wall
Permeable to small ions
What affects fluid distribution in the ECF? What are these a balance of?
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
An imbalance of fluids between compartments frequently results in what? What are some causes?
Edema - non-pitting or pitting
Causes: loss of plasma proteins, ATPase activity
What is non-pitting edema?
swollen cells due to increased ICF volume; non-responsive to diuretic action
- difficult to treat
What is pitting edema? What are some causes of it, and what is this treated with?
increased interstitial fluid volume
–Nephrotic syndrome, CHF, pregnancy, cirrhosis
- can be treated with diuretics, usually less severe than non-pitting
Osmotic equilibriation is established between ECF and ICF. Why?
Cell membranes separate these 2, and are highly permeable to water
Infusion of hypertonic solution into ECF will result in what changes in ICF? What is this called?
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
What would happen if you were to infuse a solution of isotonic saline into ECF? What is this called?
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
What happens to osmotic equilibration with diarrhea? What is this called?
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
If you were to wander in the desert for 40 days, what would happen to ICF and ECF? What is this called?
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
What would happen to ECF and ICF if you had SIADH? What is this called? What about hematocrit and blood protein levels?
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
What would happen to your ECF and ICF if you had adrenal insufficiency? What is this called?
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
What is the response to volume expansion?
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
What is the response to volume contraction?
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
What does tonicity relate to?
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
What does tonicity rely on?
Depends on the concentration of impermeant solutes in the extracellular vs. intracellular fluid
In regards to tonicity, what is the difference between isotonic, hypotonic, and hypertonic?
–No change in cellular volume = isotonic solution
–Cellular volume increase (swelling) = hypotonic solution
–Cellular volume decrease (shrinking) = hypertonic solution
