Title : Cerebral blood flow during exercise ; mechanisms of regulation 1

21 The response of cerebral vasculature to exercise is different from other peripheral 22 vasculature; it has a small vascular bed, and is strongly regulated by cerebral 23 autoregulation and the partial pressure of arterial carbon dioxide (PaCO2). In contrast to 24 other organs, the traditional thinking is that total cerebral blood flow (CBF) remains 25 relatively constant and is largely unaffected by a variety of conditions, including those 26 imposed during exercise. Recent research, however, indicates that cerebral neuronal 27 activity and metabolism drives an increase in CBF during exercise. Increases in 28 exercise intensity up to approximately 60% of maximal oxygen uptake produce 29 elevations in CBF, after which CBF decreases towards baseline values because of lower 30 PaCO2 via hyperventilation-induced cerebral vasoconstriction. This finding indicates that 31 during heavy exercise CBF decreases despite the cerebral metabolic demand. In 32 contrast, this reduced CBF during heavy exercise lowers cerebral oxygenation and 33 therefore may act as an independent influence on central fatigue. In this review, we 34 highlight methodological considerations relevant for the assessment of CBF, and then 35 summarize the integrative mechanisms underlying the regulation of CBF at rest and 36 during exercise. In addition, we examine how CBF regulation during exercise is altered 37 by exercise training, hypoxia and aging, and suggest avenues for future research. 38 39