The brain consumes about one-fifth of total body oxygen utilization
The brain is metabolically one of the most active of all organs in the body. This consumption of O2 provides the energy required for its intense physicochemical activity. The most reliable data on cerebral metabolic rate have been obtained in humans. Cerebral O2 consumption in normal, conscious, young men is approximately 3.5 ml/100 g brain/min (Table 31-1); the rate is similar in young women. The rate of O2 consumption by an entire brain of average weight (1,400 g) is then about 49 ml O2/min. The magnitude of this rate can be appreciated more fully when it is compared with the metabolic rate of the whole body. An average man weighs 70 kg and consumes about 250 ml O2/min in the basal state. Therefore, the brain, which represents only about 2% of total body weight, accounts for 20% of the resting total body O2 consumption. In children, the brain takes up an even larger fraction, as much as 50% in the middle of the first decade of life [15].