The influence of the nasal mucosa and the carotid rete upon hypothalamic temperature in sheep

Abstract

SUMMARY1. In chronically-prepared sheep, intracranial temperatures were measured in the cavernous sinus among the vessels of the carotid rete and at the circle of Willis extravascularly, and in the preoptic area and in other brain stem regions. Extracranial temperatures were measured intravascularly in the carotid or internal maxillary arteries and on the nasal mucosa and the skin of the ear.2. At 20°C ambient temperature, shifts in temperature of the hypothalamus and of other brain sites paralleled temperature shifts in the cerebral arterial blood which was cooler than central arterial blood. During periods of arousal and of paradoxical sleep, vasoconstriction of the nasal mucosa and the ear skin occurred and temperatures at the cerebral arteries and in the brain rose without a comparable rise in central arterial blood temperature.3. Anaesthetic doses of barbiturate abolished the temperature oscillations in the cerebral arterial blood and the brain. When air was blown rapidly over the nasal mucosa in anaesthetized animals, temperatures dropped precipitously in the cavernous sinus, at the cerebral arteries, and in the brain, while central arterial temperature fell only slightly. Injections of latex into the facial venous system demonstrated a venous pathway from the nasal mucosa to the cavernous sinus.4. When sheep were exposed to 45-50°C ambient temperature, respiratory rate increased 5-10 times and the temperature gradient between central and cerebral arterial blood widened.5. It is concluded that venous blood returning from the nasal mucosa and the skin of the head to the cavernous sinus cools the central arterial blood in the carotid rete. This is an important factor in the maintenance of hypothalamic temperature in the wool-covered, long-nosed, panting * National Institutes of Health trainee in anatomical sciences. 562 MARY ANN BAKER AND JAMES N. HAYWARD sheep and undoubtedly affects hypothalamic thermoreceptors and temperature regulation in artiodactyls.

Selective brain cooling in goats: effects of exercise and dehydration.

“…Although this conclusion has been inferred from a number of earlier studies in both resting and exercising panting animals (Baker & Hayward, 1968;Baker, Chapman & Nathanson, 1974;Jessen & Pongratz, 1979;Laburn, Mitchell, Mitchell & Saffy, 1988), this is the first analysis of the relationship over a wide range of body core temperatures and evaporative rates. Kuhnen & Jessen (1991) found a positive relationship between SBC and REHL in resting goats when heat exchangers were used to produce parallel increases in temperature of the blood perfusing the head 689 and the body core, i.e.…”

“…In these animals, arterial blood destined for the brain traverses the carotid rete where it loses heat to venous blood draining the nasal passages (Baker, 1982). The magnitude of this selective brain cooling (SBC) is usually seen to be related directly to the rate of respiratory evaporative heat loss (REHL) and to the rate of blood flow through the upper respiratory passages (Baker & Hayward, 1968;Jessen & Pongratz, 1979;Baker, 1982;Bamford & Eccles, 1983). Selective brain cooling may also be affected by muscular venous sphincters BS 2016 which can change the direction of flow of blood draining the nasal passages (Johnsen, Blix, Mercer & Bolz, 1987;Nijland, Mitchell & Mitchell, 1990).…”

Regulation of brain temperature in winter-acclimatized reindeer under heat stress

“…Since the discovery by Baker and Hayward (Baker and Hayward, 1968) of SBC in sheep some 40 years ago, and the finding that the small subtropical Thomson's gazelle (Gazella thomsonii) is able to maintain a brain temperature that is 3°C lower than carotid (body) temperature during very fast running (Taylor and Lyman, 1972), it has been assumed that this is the way by which heat-stressed ungulates regulate brain temperature.…”

“…Here, heat is exchanged with the arterial blood in a rete on its way to the brain for selective brain cooling (SBC) (Baker and Hayward, 1968). The reindeer (Rangifer tarandus) is an Arctic animal that has adapted to annual changes of 80°C in ambient temperature by growing a fur of excellent insulation value in the autumn to be shed in the following spring.…”

The effect of feeding on cerebral blood flow and oxygen consumption in the new‐born calf

“…Although no measurements have been made of cerebral blood flow and oxygen consumption during feeding in any mammalian species, alterations in brain temperature during and after feeding have been described in the sheep (Baker & Hayward, 1968) and new-born infant (Cross, Stothers, Warner & Woodrough, 1975). The apparent rise in brain temperature which occurs after a feed in the new-born infant was ascribed to a fall in cerebral blood flow.…”