Effects of heat stress on blood flow in respiratory and non-respiratory muscles in the sheep.

Abstract

Conscious sheep were subjected to mild and severe heat stress, and capillary blood flow in some respiratory and non-respiratory skeletal muscles was measured using radioactive microspheres. In non-respiratory muscles, blood flow decreased during mild heat stress and increased during advanced stages of severe heat stress. This was probably part of an overall redistribution of cardiac output required to combat the heat stress. Increased blood flow in the diaphragm, crus and intercostal muscles provides supporting evidence for a previous hypothesis that panting during mild heat stress and initial stages of severe heat stress is primarily diaphragmatic whereas there is a significant thoracic component during advanced stages of severe heat stress. Assuming that capillary blood flow parallels metabolic rate, the present studies indicate that while the metabolic rate of respiratory muscles increases to perform the work of panting, the effect on total body metabolic rate is offset by a decrease in metabolic rate of other tissues. Thus, estimates of the energy cost and efficiency of panting based on measurements of whole body metabolic rate may be considered misleading in having indicated that panting requires little or no energy expenditure.