The blood from the face flows into the intracranium through the ophthalmic veins when human subjects become hyperthermic. To investigate a possible mechanism underlying this change in direction of flow, five young men were subjected to either passive body warming or exercise on a cycle ergometer, in a climatic chamber whose air temperature and relative humidity were 28°C and 40%. Tympanic (Tty) and oesophageal temperatures, forehead sweat rate (msw), skin blood flow. (Qsk) and blood flow through the ophthalmic vein (Qov) were measured, and the mean skin (Tsk) and mean body (Tb) temperatures were computed. Passive body warming was induced by a box-shaped body warming unit enclosing all but the subject's head. Exercise was performed either at an intensity of 60% maximal oxygen consumption or with the intensity increasing in increments. During both tests, msw and Qsk started to increase shortly after the imposition of the heat load. The Qov began to change with the venous blood flowing from the face into the intracranium and a complete reversal in the direction of Qov (from the face to the intracranium). came significantly later than the increases in Qsw and Qsk. The Tty at the time of flow reversal was the same in both tests. The Tsk (and hence Tb) at flow reversal was, however, significantly higher during passive body warming than during exercise. The mechanism for switching the direction of Qov appeared to have been triggered by a high temperature in the brain, and not by thermal input from the periphery of the body. In a febrile subject who volunteered for this study, the direction of Qov was consistently inwards even when sitting quietly. From these results, we suggest that there are elements within the brain that control the mechanisms for switching the direction of venous flow through the emissary veins to keep the brain cool during hyperthermia.
Passive body warming Cycle exercise Emissary veins Flow reversal Tympanic temperature Oesophageal temperature