Haemodynamic responses to graded haemorrhage were studied in unanaesthetized black ratsnakes (Elaphe o. obsoleta) wherein fixed percentages of the blood volume were removed at regular intervals over periods which varied from one to several hours. Initially measured whole blood volumes averaged about 6% of the body mass and were determined from plasma dilutions of Evans blue dye or the bleeding and washing out of red cells. In response to haemorrhage, heart rate increased, haematocrit decreased, plasma protein concentration decreased, and arterial pressure remained relatively constant until a critical blood loss at which pressure dropped precipitously. Critical volume deficits occurred at the withdrawal of 63–122% of the initially measured blood volume. It appears that cardiovascular reflexes assist the regulation of arterial pressure during haemorrhage; additionally, there occurs a progressive and significant restitution of plasma volume owing to the absorption of extravascular fluid. Fluid volumes absorbed by the circulation during haemorrhage replaced 20–71% of the haemorrhaged blood loss and were correlated with the magnitude of the critical volume deficit. Body movements increased arterial pressure by improving venous cardiac return.

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