The brain is metabolically very active and under normal condition, cerebral oxygen consumption is about 20% of total body oxygen consumption.
Glucose is the source of 90% of energy of brain in normal conditions. This energy is required for maintaining electrical gradient across cell membrane and transmission of electrical impulses. Rest of energy comes from ketone bodies and other small energy metabolites but very little from fatty acids.
Cerebral blood flow is closely autoregulated and kept constant for mean arterial pressures (MAP) between around 60-150mmHg. Typically, cerebral blood flow is about 750ml per minute or round 15% of cardiac output.
Cerebral autoregulation relies on
- Vasomotor reflex. Increased sympathetic tone moderates blood flow through cerebral arteries at higher blood pressures
- Local metabolites produced when a region of brain tissue becomes more active (e.g. CO2, ADP, K+, H+) cause vasodilatation of local arterioles, thereby increasing blood flow to that region. Occurring throughout the brain this mechanism matches cerebral blood flow to demand both at the local level and for the brain as a whole.
- While marked hypoxia (pO2 < 6-8KPa) will stimulate cerebral arteriolar vasodilatation to increase blood flow to the brain, throughout the physiological range of arterial pO2 (8-15Kpa) cerebral arterioles are largely insensitive to oxygen. Furthermore, supplemental oxygen above this level, if anything, tends to cause vasoconstriction of cerebral arterioles and reduced blood flow.