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Physiology

• The typical mass of the brain is 1.4 kg which is 2% of the weight of a 70Kg person. The brain can receive about 15% of cardiac output. Cerebral Blood flow is approximately 750 ml/minute. Perfusion is from the two carotid arteries and the two vertebral arteries. About 2/3rd of the blood supply is through the anterior carotid circulation.
• The circle of Willis does connect the branches of the internal carotid artery but there is very little physiological mixing and blood from the carotid supplies the ipsilateral cortex. There is no pressure gradient that would rive mixing. Occlusion of a carotid artery may change this flow pattern but is often associated with stroke. The grey matter is better perfused than the white matter in a ratio of 6:1
• The grey matter has a much higher metabolic demand. Neuronal cell death occurs within 5 minutes of hypoxia. Cortical neurons are more susceptible to hypoxia than brainstem neurons.

Cerebral blood flow

• Cerebral blood flow increases with an increase in PCO2 or an increase in plasma [H+] concentration. Cerebral blood flow can increase two-fold with a 70% rise in pCO2. Hydrogen ions appear to vasodilate cerebral vessels.
• This makes sense to clear unwanted CO2 and H+ from the cerebral circulation which can significantly affect neuronal function. Studies looking at local flow have shown that changes of 100 to 150% of flow can occur within seconds of increased neuronal activity for instance in the contralateral primary motor cortex on asking a subject to clench a fist.
• It has also been seen that sudden rises in systemic BP are compensated for by increased sympathetic tone constricting the large and medium-sized arteries to protect the distal circulation.
• Hypocapnia constricts normal vessels and reduces cerebral blood flow and intracranial pressure. It is used therapeutically in the ITU. Patients are intubation and hyperventilation to maintain a paCO2 of 35-40 mmHg.

Brain perfusion

Brain perfusion