Response of rainbow trout to constant-pressure and constant-volume hemorrhage

Plasma oncotic pressure (OC), protein concentration (P), osmolality (Osm), and hematocrit (Hct) were measured in conscious rainbow trout subjected to 120 min of constant-pressure (CPH) or constant-volume (CVH) hemorrhage. Blood volume, tissue red cell, and plasma space were determined in control and CPH trout with 51Cr-labeled red cells and 125I-albumin. At the onset of CPH, blood loss exceeded 1.5 ml.min-1.kg-1 but quickly dropped to a constant rate of 0.1 ml.min-1.kg-1 by 15 min. After 2 h, 73% of the initial blood volume was lost, and Hct, OC, and P were reduced by 55 +/- 3, 39 +/- 4, and 19 +/- 9%, respectively; Osm did not change. Calculated blood volume was reduced by 39% (51Cr method) or by 56% (125I method). Red cell space was reduced in all tissues except brain. Plasma space decreased in stomach, increased in liver and kidney, and was unchanged in other tissues. CVH had similar effects on Hct, OC, and P, while Osm and plasma sodium increased. The time required for arterial pressure to recover to within 90% of control increased with successive bleeding from 39 to greater than 160 min. Twenty-four hours after CVH, Hct remained depressed, while all other parameters were returning toward control levels. Trout incorporate a variety of mechanisms to respond to hemorrhage, including mobilization of stored blood and reabsorption of fluid from interstitial and probably intracellular sources. High permeability of trout capillaries to protein may facilitate the restoration of protein and fluid and allow fish to survive extensive hypovolemia.