Hypertonic extracellular expansion in anuria.

To test the osmotic function of body tissues in hypertonic extracellular (ECF) expansion in vivo, we studied 20 anuric dogs either infused (infusion solution was 0.85 M saline for group A, 1.6 M mannitol for group B and isotonic saline for group C), or not infused with any solution (group D). We compared the observed ECF volume increase (radiosulfate, or cold sodium and chloride space changes), final osmolality, and body water (tritiated water space or apparent sodium volume of distribution) to the values predicted from perfect osmometric function. Observed ECF volume expansion was not different from that predicted (groups A-C). Observed final osmolality and body water were significantly greater than predicted final osmolality (groups A-D) and osmotic volume of distribution (groups A and B), respectively. We then computed 'new' solute generated from anuria (groups A-D) and from dilution of ECF solute controlled by hormonal feedback (groups A-C); this solute was comparable for all four groups. Corrected for this solute observed final osmolality and osmotic volume of distribution were not different from the predicted final osmolality (groups A-D) and measured body water (groups A and B), respectively. It is concluded that in hypertonic ECF expansion, perfect osmometric function predicts accurately the degree of ECF expansion and, if appropriate corrections are made for experimental conditions other than hypertonicity, final osmolality and body water.