Exchange of Water Between Blood and Tissues

N UMEROUS studies concerned with estimates of transcapillary rates of exchange of water and ions based on ‘isotopic arterial disappearance curves’ have appeared in recent years (1-9). For the most part these studies have been unsupported by examinations of the simultaneous events in tissues. It is our purpose here a) to describe water exchange and distribution rates in various tissues; b) to define the rate limiting barriers to the free diffusion of water; and c) to examine the implications of these data on the calculation of the rates of exchange of water between body fluids. The tracer employed was D20 (heavy water). In a previous paper (IO) the evidence that heavy water is a suitable tracer for body water was summarized. In 1940, Hevesy and Jacobsen (II) reported on the heavy water arterial dilution curve in rabbits. They found that for the time interval from 30 seconds after injection to 30 minutes, the arterial curve reflected dilution of the tracer by cell water; capillary penetration being evident only in the first 30 seconds. Flexner and his co-workers (I, 4) studied the disappearance of heavy water from arterial blood in guinea pigs and humans. In the guinea pig, they found that a single exponential rate of distribution occurred (I) while in three human subjects (pregnant women) the arterial curve could be resolved into two exponential rates plus a constant term by successive subtractions on a semilogarithmic plot (4). Assuming that hindrance to capillary penetration accounted for both rates of distribution, they calculated rates of water exchange across the capillary from these curves (4). This paper presents evidence placing both rate limiting processes for water distribution as being between cell water and extracellular fluid. A critique of calculations of cell water exchange from the D20 arterial disappearance curve is offered. The observations to be presented consist of arterial disappearance curves, arterialvenous difference curves, tissue uptake curves, perfusion through the isolated hind limb of the cat and in vifro dialyses of skeletal muscle.