Thermoregulatory Demands Research Articles

Human cardiovascular adjustments to rapid changes in skin temperature during exercise.

In 11 normal men, central circulatory responses were measured while skin temperature was changed in a square-wave pattern during uninterrupted exercise (26% to 64% maximal oxygen consumption). Skin temperature was changed at 30-minute intervals, beginning at 32°C. On raising it to 38.2°C at low oxygen consumption (V · o 2 ), cardiac output increased 2.5 liters/min, and central blood volume, aortic mean pressure, and stroke volume fell (7%, 7%, and 11%, respectively). Right atrial mean pressure fell 2.2 and 2.3 mm Hg during control and heating periods, respectively. All variables returned to control levels when skin temperature was reduced toward 26.9°C. Raising it to 40°C reproduced these changes with a more clear-cut drop in right atrial mean pressure. Results indicated reduced peripheral venous tone and cutaneous pooling of blood during heating and rapid reversal on cooling. On raising skin temperature to 38.7°C at high V · o 2 , cardiac output increased 19% (3.1 liters/min), stroke volume decreased 14%, and central blood volume rose slightly. Aortic mean pressure fell during the control period and was maintained or rose during heating periods. On cooling, central blood volume and stroke volume rose, cardiac output remained elevated, and aortic mean pressure fell. Increases in cardiac output during heating were related to skin temperature and not to V · o 2 or body temperature. At high V · o 2 , circulatory adjustments favor metabolic rather than thermoregulatory demands.

The Adaptive Role of Winter Fattening in the White-Crowned Sparrow with Comments on its Regulation

In White-crowned Sparrows wintering in southeastern Washington, there is an inverse correlation between the quantity of body fat and mean air temperature below about 3 C. Between 3 C and 9 C the fat reserves are independent of air temperature. Because of seasonal variation in fat-free body weight in this species, variation in total body weight is not a highly reliable index of variation in lipid reserves. A review of published data indicates that winter fattening is a common phenomenon in small birds which winter in cold habitats or high latitudes, although exceptions can be found in species that evidently in part substitute behavioral adaptations for winter fattening. It may be concluded provisionally that the magnitude of winter fattening tends to be less inwarmer habitats or at lower latitudes.It is absent in a subequatorialwintering population of Yellow Wagtails. Although it is reasonable to assume that winter fattening is an adaptation to the thermoregulatory demands of low ambient temperature, the tacit assumption that cold is the proximate factor that induces it is not fully substantiated by the evidence now available. Rather, it appears at least equally plausible that the low environmental temperature of winter must be regarded as the primary ultimate factor. The proximate factor, or battery of proximate factors, which could include reduced environmental temperature, involved in the physiological regulation of fat storage in winter remain an open question.