Maintenance of blood volume in snakes: transcapillary shifts of extravascular fluids during acute hemorrhage

1. Tracer dilution analysis (D₂O,⁵¹Cr, and NaS¹⁴CN) was used to investigate the steadystate compartmentation of body fluids and the extent of fluid transfer from extravascular to vascular spaces during hemorrhage-induced hypovolemia in two species of snakes, Elaphe obsoleta and Crotalus viridis. 2. Fluid spaces of the two species are not significantly different (means, blood volume 6.1, 5.4; extracellular fluid 42.2, 41.9; total body water 77.2, 77.2% body mass, respectively), but values for extracellular fluid exceed those reported for other reptiles. 3. Both species of snake withstand graded hemorrhage where 4% of the initial blood volume is withdrawn every 10 min until the cumulative deficit is 32%. Some snakes are able to maintain their initial blood volume throughout hemorrhage, while others restore 90% of deficits within 2 h after hemorrhage ceases. Typically, 50-60% of the hemorrhaged deficit is transferred from the interstitium to the circulation throughout hemorrhage (Fig. 2). The source of fluid entering the vascular space is entirely extracellular during hemorrhage, but fluid from the intracellular space may enter the blood within 2 h after hemorrhage ceases. Snakes are able to maintain arterial pressure during these experiments (Fig. 3). 4. The ability of snakes to maintain hemodynamic stability despite substantial losses of blood can be explained in terms of a large interstitial fluid volume that may shift rapidly to the vascula space. Shifts in the opposite direction also occur in response to hemodynamic factors, implying a low resistance to fluid movement across the capillary wall.