Effects of acute cold exposure on the distribution of cardiac output in the sheep

Conscious adult Merino sheep were studied in a thermoneutral environment and then during cold exposure. Microspheres labelled with¹⁴¹Ce,⁵¹Cr,⁸⁵Sr, and⁴⁶Sc were used to measure blood flow ( \(\dot Q\) ) in most tissues of the body. (Microspheres labelled with¹²⁵I were found to be unsatisfactory.) Cold exposure which caused a marked decrease in skin surface temperature and mild, continuous shivering but no change in deep body temperature, was associated with mean increases in oxygen consumption, heart rate, and cardiac output (C.O.) of 100%, 62%, and 48%, respectively; blood pressure and total peripheral resistance were unchanged. There was approximately a 6-fold increase in \(\dot Q\) in perirenal white adipose tissue, and a 3- to 4-fold increase in muscles of the upper foreleg and hindleg, and the intercostals; myocardial \(\dot Q\) also increased significantly. There was a marked decrease in \(\dot Q\) in skin of the legs and ears, in the maxillo turbinals and in the nasal mucosa, and a decrease in the total proportion of C.O. passing through arteriovenous anastomoses. Thus, not only did C.O. increase, but there was a redistribution appropriate to meet the challenge with which the animal was confronted.