Exchange of Water Between Blood and Tissues

Abstract

D2O arterial disappearance curves in 5 humans, 11 dogs and 1 cat are presented. In all cases 2 exponential rates of distr. were adequate to descr. the decline in D2O concn. from 1 min. after injn. to equilibrium of distr. at 3 hrs. This decline in D2O concn. correlates in time with dilution by cell water. Simultaneous arterial-femoral venous, and arterial-antecubital venous curves, in 1 dog and 3 human subjects demonstrates large D2O concn. gradients in the early phase of D2O distr. The time-course of appearance of D2O in tissue water relative to arterial blood water in brain, liver, gastric juice, cisternal cerebrospinal fluid, bone and urine is descr. From these curves it is inferred that there is a spectrum of water exchange rates, with brain, liver and stomach exchanging water more rapidly than skeletal muscle, cerebrospinal fluid and bone. These data indicate that each tissue exchanges water at an independent and characteristic rate. The distr. of D2O in the perfused hindlimb of the cat can be descr. as a single rate process. By appln. of the Fick principle it was concluded that equilibration of D2O with cell water accounts for virtually all of the distr. time. In vitro dialysis of D2O-enriched skeletal muscle in Ringer"s soln. is interpreted in terms of cell hindrance to the diffusion of water. A critique of calculations of rates of cell water exchanges from the constants of D2O arterial disappearance curves is given.