ABSTRACT
The brain, blood, and CSF are all contained within a closed cranial cavity. The whole of the intracranial contents adhere to the Monro-Kellie doctrine-i.e. the total volume of the skull must remain constant at any one time. As brain tissue is virtually incompressible, the only volumes that can change readily are those of the CSF and blood (CBV). CSF can be diverted from the intracranial to the intraspinal compartment. The CBV can be reduced by cerebral vasoconstriction of the arterioles. The veins and capillaries do not take part in active vasoconstriction, although their
