Within the skull there is a fixed volume and clearly what goes in must come out (e.g., blood, cerebrospinal fluid). If there is a space-occupying lesion, such as an extradural hematoma, there is no space into which it can decompress. As the lesion expands, the brain becomes compressed and the intracranial pressure increases. This pressure compresses vessels, so lowering the cerebral perfusion pressure. To combat this the homeostatic mechanisms of the body increase the blood pressure to overcome the increase in intracerebral pressure. Unfortunately, the increase in intracranial pressure is compounded by the cerebral edema that occurs at and after the initial insult.