At rest, the human brain uses about 20% of the body’s oxygen.
The brain is a small fraction of total body mass, yet its neurons and supporting cells require a continuous supply of oxygen to maintain ion gradients, transmit signals, and support cellular metabolism. Most of this energy is generated through aerobic metabolism of glucose and other fuels.
Brain oxygen use is not uniform. Activity in local networks can alter regional blood flow and oxygen delivery, which is the physiological basis for techniques such as functional magnetic resonance imaging. The brain also has limited energy reserves, so interruptions in blood flow can cause damage quickly.
The figure is an approximate adult resting value, not a fixed percentage in every situation. Exercise, illness, age, body size, metabolic state, and measurement method can change the proportion. Oxygen consumption is also different from blood flow: blood delivers oxygen, while metabolism determines how much is used.