Neurons have a limited supply of energy reserves and require a continuous supply of glucose and oxygen. The brain normally receives about 20% of the cardiac output and is responsible for about 15% of oxygen consumption, although the brain is only 2% of the body weight. Grey matter is more vascular than white matter to cope with greater metabolic requirements. There may also be variations in local blood flow in relationship to metabolic demands.
Approximately 70% of cerebral blood flow is derived from the internal carotid arteries, the remaining 30% being derived from the vertebral arteries. The circle of Willis is composed of these two arterial systems connected by the anterior and posterior communicating arteries. There is also communication between the external and internal carotid arteries through the ophthalmic arteries. There is considerable variation in the diameter of the various arteries forming the circle of Willis within normal individuals. Adequate blood flow to the brain should be able to withstand occlusion of one of the four main arteries in a normal person, but in the presence of narrowing may result in infarction.
Circulation to the central nervous system is under that influence of autoregulatory control that maintains a fairly constant flow of 55-60 ml/min/100g between mean arterial pressures of 40-170 mm Hg. As long as the mean arterial pressure remains above about 40 mm Hg variations in vascular tone maintain a fairly constant cerebral blood flow. If the mean arterial pressure falls below this level there is a dramatic fall in blood flow, which may result in infarction.
Cerebral perfusion pressure is the difference between systemic arterial blood pressure and intracranial pressure. A rise in intracranial pressure may also result in decreased blood flow. Abrupt rises in intracranial pressure are particularly liable to arise after closed head injuries, or following vasospasm induced by subarachnoid haemorrhage.
Local cerebral ischaemia is usually due to arterial stenosis or occlusion. Global cerebral ischaemia results from a fall in cerebral perfusion pressure below the threshold for autoregulation.
Neurons are the most vulnerable cells in the central nervous system (CNS) to ischaemia and some neurons are more vulnerable than others, such as pyramidal neurons in the Sommer sector of the hippocampus, and Purkinje cells within the cerebellum (Figure 1).
| Figure 1. |
Section of cerebellum showing acute hypoxic changes in Purkinje cells - loss of cytoplasmic detail, deep eosinophilia of the cytoplasm, and variable degrees of nuclear disintegration.
Certain sites within the brain are more vulnerable to hypotension or cardiac arrest. This varies between individuals, but in general the areas in the most distal territories of supply of the main arteries (watershed areas) are the most vulnerable and neurons of the hippocampus, cerebellum, and within layers 3,4,and 5 of the cortical grey matter, are the most sensitive (Figure 2).
| Figure 2. |
Section of cerebral cortex showing marked vacuolation of neurons in the middle and deeper layers following hypoxia.