Osmotic "control" of transmembrane fluid flux
Intravascular, interstitial, and intracellular fluid compartments
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There are three major body fluid compartments in the body: intravascular, interstitial, and intracellular. These three compartments
are separated by semipermeable membranes which are freely permeable to water. The distribution of water across these membranes
is determined by the osmotic gradients of solutes that are not permeable to the respective membrane. In order for a solute
to be osmotically effective, it must be maintained in higher concentrations on one side of the membrane.
Oncotic Pressure
Colloids are large molecules that are not freely permeable across the vascular membrane, are present in the vascular fluid
compartment in larger concentrations than in the interstitium, and therefore are osmotically responsible for retaining crystalloids
within the vascular fluid compartment according to the Starling equilibrium of transvascular fluid flux, assuming normal vascular
permeability.
Albumin is the prominent intravascular colloid. Hypoproteinemia may be associated with simultaneous hypovolemia, and subcutaneous
edema and ascites. This is not a straight-line relationship since decreases in plasma albumin concentration are initially
offset by a dilutional decrease in perivascular albumin concentration. An increased capillary permeability to the extent
that albumin and other colloids are freely permeable would also result in hypovolemia and interstitial edema. Crystalloids
are freely permeable across the vascular membrane. Changes in sodium concentration have no effect on transvascular fluid
flux. Intravenously administered crystalloid fluids are rapidly redistributed to the interstitial fluid compartment in an
amount proportional to the relative size of the interstitial fluid compartment.
Measuring oncotic pressure
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Colloid osmotic pressure (or oncotic pressure) is measured with a colloid osmometer which employs a semipermeable membrane
and measures the change in pressure in the reference chamber when an unknown solution is placed in the test chamber. The
membrane pore size in these instruments is 20 or 30 kilodaltons and therefore are freely permeable to the small electrolytes.
This analyzer is "blind" to the sodium concentration and the osmolality of the solution.
Normal colloid osmotic pressure is 20 to 25 mm Hg. Values in the high "teens" are common in critically ill patients but are
not considered to warrant treatment, per se. Values in the low "teens" are considered to be too low, and warrant treatment
with an artificial colloid or plasma. Values in the single digits (commonly seen in patients with portocaval shunts) also
need to be treated but there is a need to do it slowly. Rapid administration of colloids to these patients has caused edema,
presumably by increasing capillary hydrostatic pressure ahead of increases in colloid osmotic pressure and upsetting the precariously
balanced starling forces.
Osmolality
Sodium is pumped out of the cell by the sodium-potassium-ATPase pump in the cellular membrane. As sodium and its related
anion (predominantly chloride and bicarbonate) are maintained in the extravascular fluid compartment, they are primarily responsible
for the osmotic attraction and retention of water in the extracellular fluid compartment.
Acute changes in extracellular sodium concentration result in transcellular fluid fluxes (hyponatremia causes cellular edema;
hypernatremia causes cellular dehydration). Diseases associated with inadequate cellular energy production and ATP depletion
are associated with the influx of sodium (and water) into the cell, cellular edema, and, eventually, cellular disruption.
The endothelial membrane is freely permeable to these crystalloids and sodium concentration has no impact upon the transvascular
fluid flux.
It is actually extracellular osmolality, rather than sodium concentration, that is important to transcellular fluid flux.
Sodium (and its related anion) are by far the largest component of measured osmolality. Osmolality is normally calculated
as 2 x [Na+] + 10 (which accounts for the normal contributions of glucose and blood urea nitrogen [BUN]). If the glucose
and BUN are known, their contributions can be calculated as glucose (mg/dl)/18 + BUN (mg/dl)/2.8. While hyponatremia is the
only cause of hypo-osmolality, there are many causes of hyperosmolality. The measured osmolality is normally about 10 to 15
mOsm/Kg higher than the calculated value. A higher osmolar gap is indicative of unmeasured osmols.
