An approximately 20% increase in cerebral blood flow (CBF) and the
cerebral metabolic rate for oxygen manifest during whole body exercise with determination of brain tissue flow and arterial inflow to the brain. Yet, during intense exercise CBF approaches the resting level in response to the decrease in arterial carbon dioxide tension with the exponential increase in ventilation. Here it is illustrated that the increase in CBF during exercise appears to depend to the ability to raise cardiac output and it is speculated whether there is a sympathetic restrain on CBF when the increase in cardiac output is small. Furthermore, it is considered whether the restrain in CBF during intense exercise affects cerebral oxygenation to an extent that it provokes so-called central fatigue. Finally it is highlighted that the cerebral activation associated with exercise provokes uptake of carbohydrate, notably of lactate, that cannot be accounted for by the concomitant increase in the cerebral metabolic rate for oxygen and it is pointed out that it remains
unknown why that apparently surplus carbohydrate uptake by the brain is in need
