Thermal Homoiostasis Under Hypoxia In Man
The complex regulatory mechanisms involved in maintaining optimal thermal conditions for the vital functions in the homoiothermic organism provide functional integrity over a limited range of variation in the temperature of the environment. These adaptations to thermal stress are mediated by humoral and neural pathways which are known to be susceptible to oxygen deprivation. Moreover, certain physiological responses elicited by heat or cold may be in conflict with others engaged to counteract hypoxia and vice versa. Human subjects were exposed to cold (4°C RH 30 %), warm (40. 5°C RH 80 %) and neutral (27°C RH 30 %) environmental conditions for two hours while breathing gas mixtures simulating an altitude of 6000 m (inspired PO : 65 mm Hg and for a control period of the same duration breathing air. In the cold, no difference was observed in the course of skin temperature between the hypoxic and eupoxic tests. Core temperatures were maintained constant in the presence of vigorous shivering whereby metabolic rate was increased 2 to 3 fold. In the warm environment, the core temperatures (rectal and gastric) were consistently higher with oxygen lack than in the controls, but the rate of increase in temperature was the same. At the end of the tests rectal temperature was an average 39°C. The effects of combined thermal and hypoxic stress on cardiovascular and respiratory activity appeared to be additive. Subsequently, similar experiments were performed on lightly anesthetized dogs where hypoxia of a more severe degree (inspired PO : 52, 41 and 29 mm Hg) was employed. In these animals hypoxia invariably inhibited or entirely suppressed shivering and in the cold they suffered a more rapid fall of mean body temperature under hypoxia than on air. Experiments in which a normal partial pressure of CO was maintained by partial rebreathing suggest that hypocapnia may contribute to the suppression of shivering in the cold. During the exposure to heat there was a marked facilitation of panting under hypoxia, giving rise to extreme hyperpnea with hypocapnia. The animals were sacrificed in hypoxia by progressive rebreathing to determine the critical oxygen tension. Under heat stress the animals succumbed at significantly higher oxygen tensions than in the cold or neutral environment. This may be due to the compound stress of heat, hypoxia and hypocapnia.
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