The relationship between intracranial pressure and volume is explained by the pressure-volume curve. The Monro-Kellie hypothesis states that in the rigid container of the skull, the volume of the intracranial constituents is constant (i.e. brain, blood and CSF). An increase in any component, or the addition of a component (i.e. a brain tumour), must be offset by a decrease in one or more of the others if intracranial pressure is to remain constant. Once this compensatory reserve is exhausted, ICP rises precipitously.