ECF Volume Contraction & Expansion

Water moves along osmotic gradients. Osmolarity is the concentration of solute particles within a solution (there are intertextual discrepancies regarding osmolarity vs. osmolality). A change in the amount of solute and/or water will cause water to shift between body fluid compartments. Isosmotic = no change in extracellular fluid osmolarity. Hyperosmotic = increase in extracellular fluid osmolarity. Hyposmotic = decrease in extracellular fluid osmolarity. To predict how changes in water volume or solutes affect water distribution, we'll ask ourselves the following three questions: First, how did the extracellular fluid change? Was water volume or solute concentration changed? Was it an increase or a decrease? Second, does the change produce an increase, decrease, or no change in the osmolarity of the extracellular fluid? Third, if extracellular osmolarity changes, will water shift into or out of the intracellular compartment? About two-thirds of total body water is in the intracellular compartment; this is the water within all the body's cells. The remaining body water is in the extracellular compartment (1/3). The osmolarity of the extracellular and intracellular compartments is equal, despite differences in specific solute concentrations (this is addressed in detail, elsewhere). Furthermore, recognize that "body water" is not synonymous with "pure" water. Occurs when isosmotic fluid is lost from diarrhea. Because the fluid lost in diarrhea has roughly the same osmolarity as that of the ECF, the volume, but not the osmolarity, of extracellular fluid decreases. And, because osmolarity remained stable, there is no water shift from the intracellular compartment, which remains unchanged. Occurs when hyposmotic fluid is lost from the extracellular compartment. For example, imagine an individual running in the desert without drinking water; he will experience excessive sweating without fluid replenishment. Be aware that sweat is hyposmotic, that is, its water:solute ratio is higher than that of the blood. Excessive sweating leads to contraction of the extracellular compartment volume. Because sweat is hyposmotic, its loss from the extracellular compartment will increase the osmolarity of the remaining fluid. Then, because the osmolarity of the extracellular compartment is higher than that of the intracellular compartment, water shifts from the ICF to the ECF. Notice that the water shift only partially compensates for volume contraction. Now, the remaining fluid in the intracellular compartment is also hyperosmotic, because only water, not solutes, moved to the ECF. Occurs when hyperosmotic fluid is lost from the ECF in individuals with adrenal insufficiency. Recall that sodium is a key extracellular solute, and that aldosterone promotes its reabsorption in the distal nephron; thus, aldosterone is essential for maintaining ECF osmolarity. However, individuals with primary adrenal insufficiency do not produce adequate aldosterone, so excess salt is excreted in the urine. Extracellular osmolarity decreases due to salt loss. At this point, extracellular osmolarity is lower than intracellular osmolarity. To correct this imbalance, water shifts from the ECF to the ICF until the osmolarity of the two compartments is reduced to the same level. Intracellular fluid volume is increased by the water shift. Caused by an infusion of isotonic saline. Extracellular fluid volume increases, but does not change its osmolarity. Because there is no change in extracellular osmolarity, there is no water shift. The volume and osmolarity of the intracellular fluid compartment are unchanged. Caused by the addition of dry solute to the ECF, for example, upon ingestion of salty foods. The addition of solute to the extracellular fluid increases its osmolarity. In response, water shifts from the intracellular compartment until the two compartments have the same osmolarity. Thus, extracellular volume increases, only partially compensating for the increased osmolarity. And, Intracellular volume decreases, leaving the remaining intracellular fluid hyperosmotic. Recall that antidiuretic hormone (ADH) promotes water reabsorption in the collecting ducts; In individuals with Syndrome of Inappropriate Antidiuretic Hormone (SIADH), excessive AHD results in too much water reabsorption. This water is added to both the extracellular and intracellular compartments. And, because water was added, osmolarity decreases in both compartments.