Blood flow through the ophthalmic veins during exercise in humans.

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 degrees 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 msw 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.(ABSTRACT TRUNCATED AT 250 WORDS)