Hemodilution during standardized hemorrhage in high-altitude acclimatized rats.

Ten control rats and sixteen high-altitude acclimatized rats were bled at sea level into a reservoir which maintained arterial pressure at 35 mm Hg. As soon as the animals had spontaneously taken back 30% of the maximum bleeding volume, all the shed blood remaining in the reservoir was reinfused. Hemodilution was studied during the first half phase of hypotension starting from the point of initial blood withdrawal and ending at the point of maximum blood loss. Changes in hematocrit, hemoglobin content, total plasma protein, and arterial plasma osmolality were measured. The initial and the maximum blood withdrawal, the oligemic time, and the survival time of the altitude-acclimatized rats were all greater than those for non-acclimatized rats. The higher tolerance to standardized hemorrhagic shock in altitude-exposed rats seemed to be due in part to their more marked hemodilution which allowed more efficient homeostatic regulation of vascular volume. The difference in rate of hemodilution between the two animal groups could not be attributed to arterial hyperosmolality.