Significance of cranial circulation for the brain homeothermia in rabbits. II. The role of the cranial venous lakes in the defence against hyperthermia.

Chronic experiments were conducted on five freely moving rabbits at ambient temperatures of 0-42 degrees Celsius. The influence of nasal mucosal thermal changes on the venous blood temperature inside the pterygoid plexus and on the temperatures at three intracerebral sites were investigated against the background of the carotid arterial blood temperature shifts. A correlation was found between: (i) the fluctuations in the nasal mucosal temperature reflecting its vasomotor responses, (ii) temperature shifts of the pterygoid plexus venous blood, and (iii) of the ventral brain. Mucosal vasodilatation caused parallel drops in both the plexal blood and brain temperatures. However, mucosal vasoconstriction was accompanied by increases in temperatures at those sites. Intracranial thermal shifts were independent of the arterial blood temperature changes. During motor activity in normothermia nasal mucosal vasoconstriction was present, and in that case brain temperatures exceeded arterial blood temperature. During rest, mucosal vasodilatation appeared and brain base cooled below the arterial blood temperature. During panting in dry heat, the brain base was cooler than the arterial blood by as much as 0.5 degree Celsius. The intensity of the selective brain cooling was directly proportional to deep body temperature. The blockade of the respiratory evaporation in heat elicited an increase of the plexal venous blood as well as brain temperatures above the arterial blood temperature. We conclude that the venous blood outflowing from the nasal mucosa exerts a cooling influence on the brain through the pterygoid plexus.