Cerebral metabolism.
The O2 supply to the brain depends on the arterial O2 content and the CBF. Cerebral O2 consumption in normal, conscious, young humans is ∼3.5 ml·100 g brain−1·min−1 (172). The rate of O2 consumption of the entire brain of average weight (1,400 g) is therefore ∼49 ml O2/min. The magnitude of this rate can be appreciated more fully when it is compared with the average metabolic rate of the whole body, ∼250 ml O2/min in the basal state for a 70-kg man. Therefore, the brain, which represents only ∼2% of total body weight, accounts for ∼20% of the resting total body O2 consumption. Previous studies in humans have shown that cerebral O2 consumption was unchanged with alterations in PaCO2 in the range comparable with those measured during cerebrovascular reactivity testing (97, 114, 212). The unchanged cerebral metabolic O2 consumption can be maintained via the increases in O2 extraction.