Cerebral autoregulation

A healthy brain holds cerebral blood flow roughly constant across a wide range of perfusion pressures — conventionally quoted as about 50 to 150 mmHg — by constricting and dilating its arterioles. Outside that range, flow follows pressure passively: too little and the tissue is ischaemic, too much and you get hyperaemia, oedema and haemorrhage.

Two important qualifications:

The curve shifts. Chronic hypertension moves it to the right, so a “normal” pressure may be inadequate for that patient.
Injury impairs it. After traumatic brain injury, subarachnoid haemorrhage or hypoxic injury, autoregulation may be regionally or globally lost — leaving flow directly pressure-dependent in exactly the patients in whom that is most dangerous.