Countercurrent Heat Exchange and Vascular Bundles in Sloths

Publisher Website
Google Scholar
Abstract
In the sloth, a few other terrestrial mammals and several aquatic mammals, the limbs and tail are provided with arteriovenous bundles (retes) where 20–40, or sometimes several hundred, small arteries and veins run parallel and intermingled. Obviously, heat exchange must take place in such a structure. The basic temperature relations in a simple countercurrent heat exchange system have been described and verified on a physical model. A striking feature of such a system is the poor conduction of heat and the consequent steep temperature gradient along the line of flow. Temperature measurements in the rete of the sloth revealed gradients sometimes as large as 1 degree/cm, i.e. some 30 times steeper than in a human arm. The gradient immediately decreased when the venous return was reduced, proving the existence of A-V heat transfer. Legs and arms developed steep subcutaneous gradients upon cooling of the body. It took some 4 hours to rewarm a moderately chilled limb to normal temperature. Two other tropical animals (coati and white-faced monkey), without retes, rewarmed their arms after chilling 5 times faster. Hot wire measurements indicated that flow often decreased in the arm rete when a sloth was chilled. At the same time the rete gradient steepened. The sloth is a temperature-sensitive animal which just barely keeps warm in the tropics, and the retes in sloths and other mammals are interpreted as structures which conserve the body heat at the expense of cooling the limbs. Submitted on October 1, 1956