Human circulatory and thermoregulatory adaptations with heat acclimation and exercise in a hot, dry environment.

1. Heat acclimation was induced in eight subjects by asking them to exercise until exhaustion at 60% of maximum oxygen consumption rate (VO2) for 9-12 consecutive days at an ambient temperature of 40 degrees C, with 10% relative humidity (RH). Five control subjects exercised similarly in a cool environment, 20 degrees C, for 90 min for 9-12 days; of these, three were exposed to exercise at 40 degrees C on the first and last day. 2. Acclimation had occurred as seen by the increased average endurance from 48 min to 80 min, the lower rate of rise in the heart rate (HR) and core temperature and the increased sweating. 3. Cardiac output increased significantly from the first to the final heat exposure from 19.6 to 21.4 l min-1; this was possibly due to an increased plasma volume and stroke volume. 4. The mechanism for the increased plasma volume may be an isosmotic volume expansion caused by influx of protein to the vascular compartment, and a sodium retention induced by a significant increase in aldosterone. 5. The exhaustion coincided with, or was elicited when, core temperature reached 39.7 +/- 0.15 degrees C; with progressing acclimation processes it took progressively longer to reach this level. However, at this point we found no reduction in cardiac output, muscle (leg) blood flow, no changes in substrate utilization or availability, and no recognized accumulated 'fatigue' substances. 6. It is concluded that the high core temperature per se, and not circulatory failure, is the critical factor for the exhaustion during exercise in heat stress

Análise de algumas variáveis fisiológicas para avaliação do grau de adaptação de ovinos submetidos ao estresse por calor Analysis of some physiological variables for the evaluation of the degree of adaptation in sheep submitted to heat stress

Investigaram-se a temperatura retal, a freqüência respiratória e a taxa de evaporação total de ovinos Corriedale sob três temperaturas ambientes, visando uma melhor compreensão dos mecanismos de termorregulação desses animais. Inicialmente, 21 animais adultos foram alojados em câmara climática à temperatura de 45ºC, e pressão parcial de vapor (PV) variável, registrando-se a freqüência respiratória (FR) e a temperatura retal (TR). Baseando-se na FR e TR, foram selecionados 10 animais, cinco com os valores mais baixos, assumindo-os como mais adaptados ao calor (grupo 1) e cinco com valores mais altos, assumindo-os como menos adaptados (grupo 2). Os animais selecionados foram mantidos em câmara climática, onde mediram-se novamente TR, FR e taxa de evaporacão total (TET), sob 20, 30 e 40&ordm;C de temperatura do ar e PV variável. Não houve diferença estatística entre os grupos classificados, para todas as variáveis medidas. Concluiu-se que a utilização das variáveis fisiológicas TR e FR como parâmetros únicos para a seleção destes animais não é suficiente para avaliar o grau de adaptação a temperaturas elevadas.<br>It was investigated the rectal temperature, respiratory frequency and total evaporative heat loss rate in Corriedale sheep under three air temperatures, aiming a better comprehension of thermoregulation mechanisms of these animals. Initially, 21 adult animals were housed in climatic chamber under 45&ordm;C and variable air humidity (PV), recording the respiratory frequency (FR) and rectal temperature (TR). Basing on the FR and TR, it was selected 10 animals, five of the lowest values, assuming as being the best heat adapted (group 1) and five of the highest values, assuming as the worst heat adapted (group 2). The selected animals were maintained in climatic chamber, where it was measured again TR, FR and total evaporation rate (TET), under 20, 30 and 40&ordm;C of air temperature and variable PV. There was no statistical difference between the classified groups, for all the measured variables. In conclusion, the use of the physiological variables TR and FR as mainly parameters for these animals selection, is not enough for evaluate the level of adaptation under high temperatures