Effects of exposure to hot environments on the regional distribution of blood flow and on cardiorespiratory function in sheep.

Abstract

Sheep were exposed to thermoneutral conditions and then to either mild heat stress (40° C dry bulb/26° C wet bulb temperature) or to severe heat stress (42/39° C). The following measurements were made: regional distribution of cardiac output, deep body and skin surface temperatures, respiratory frequency, heart rate, blood pressure, cardiac output, oxygen consumption, haemoglobin concentration, thePCO2,PO2, pH,SO2 andCO2 of arterial, mixed venous, dorsal sagittal sinus and transverse sinus blood. Body temperature and heart rate increased, and panting resulted in a marked respiratory alkalosis during severe heat stress, but otherwise, changes in gross cardiorespiratory functions were small. During both mild and severe heat stress approximately 11% of the cardiac output passed through arteriovenous anastomoses, compared with approximately 1.5% under thermoneutral conditions. Changes in blood flow to the skin, nasal mucosa and turbinates, tongue, heart, thyroids, adrenals, gut, spleen, kidneys, skeletal muscle, brain and spinal cord, illustrate a redistribution of cardiac output appropriate to combat the heat stress, but with probable influences of the respiratory alkalosis and alterations in local tissue metabolic rate.