Red blood cell distribution in the hypothermic ground squirrel

¹ Department of Physiology, School of Medicine and Dentistry, University of Rochester, Rochester, New York

The quantity and distribution of the circulating red blood cell mass were studied in the ground squirrel during immersion hypothermia of 2°–4°C esophageal temperature. Mixing of isotopically tagged erythrocytes within the total red cell mass, as determined by comparison with euthermic values, was essentially complete within 10 minutes, so that little, if any, sequestration of blood occurred. The hematocrit ratio of blood within the heart lumina was maintained throughout this cold stress. Complete mixing of Cr⁵¹-tagged red blood cells took place within 1 hour in the erythrocytes of the kidney, cardiac muscle, diaphragm and leg muscles. The liver of the cold animal picked up radioactivity, perhaps due to damage of erythrocytes in the labeling process. Splenic contraction within 2 hours of acute hypothermia was indicated by a sharp decrease in its circulating red blood cell mass and by visual observation. The total erythrocyte mass in the cheek pouch of the hypothermic ground squirrel required about 3 hours to equilibrate with labeled cells, so that a small region of very slowly flowing blood did exist in this tissue. Maintenance of red blood cell flow to tissues indicated that some vasomotor control was intact during this acute cold stress.