The cerebral circulation is protected from systemic blood pressure surges by a complex branching system and two resistance elements: the first of these lies in the large cerebral arteries, and the second in vessels of diameter <100 μm. Endothelial cells in cerebral capillaries contain few pinocytic vesicles and are sealed with tight junctions, without any anatomical gap. Several endogenous substances including catecholamines and vascular growth enhancing factor can dynamically modulate blood-brain barrier (BBB) permeability. Classical cerebral autoregulation assessment does not consider the latency of autoregulatory mechanisms, focusing instead on the maintenance of cerebral blood flow (CBF) at different steady state levels of cerebral perfusion pressure (CPP). Methods of measuring CBF may be regional or global, and may be applicable either to humans or primarily to experimental animals. Severe head injury is accompanied by both direct and indirect effects on CBF and metabolism, which show both temporal and spatial variations.