Makindo Medical Notes

Physiology

• The brain weighs around 1.4 kg, which is ~2% of total body weight in a 70 kg adult. Despite its relatively small mass, it receives ~15% of the cardiac output (~750 ml/min of blood flow).
• Perfusion is supplied via two internal carotid arteries and two vertebral arteries. About two-thirds of the blood supply is through the anterior (carotid) circulation, while the vertebral arteries contribute to the posterior circulation (primarily brainstem and cerebellum).
• The Circle of Willis provides an anatomical connection between the carotid and vertebrobasilar systems. However, under normal circumstances, there is minimal physiological mixing of blood, because no significant pressure gradient exists to drive cross-circulation.
  • In the event of an arterial occlusion, collateral flow may improve perfusion, but often not sufficiently to prevent infarction (stroke).
• Grey matter receives substantially more blood flow than white matter (by a ratio of around 6:1), reflecting higher metabolic demands. Cortical neurons are more vulnerable to hypoxia than neurons in lower brain regions, and irreversible neuronal injury can occur within 5 minutes of interrupted oxygen supply.
• Overall, the brain consumes about 20% of the body’s oxygen uptake. It primarily relies on glucose metabolism, making it sensitive to any interruption in blood flow, oxygen delivery, or glucose availability.

Cerebral Blood Flow

• Cerebral blood flow (CBF) is tightly regulated by chemical and autoregulatory mechanisms. It rises with increased arterial PCO₂ or [H⁺]. A 70% rise in arterial pCO₂ can approximately double CBF, as hydrogen ions cause vasodilation to wash out excess CO₂.
• Local blood flow adjustments can occur within seconds in areas of heightened neuronal activity. For instance, instructing a subject to clench a fist can cause a rapid (100–150%) increase in flow to the contralateral motor cortex.
• Sudden increases in systemic blood pressure are countered by sympathetic-mediated vasoconstriction of large and medium-sized cerebral arteries, protecting the fragile distal microcirculation.
  • This process is part of autoregulation, which typically maintains relatively constant CBF over a mean arterial pressure (MAP) range of about 50–150 mmHg in healthy adults.
• Hypocapnia (low arterial CO₂), typically caused by hyperventilation, leads to cerebral vasoconstriction. This reduces both CBF and intracranial pressure (ICP), which is sometimes used therapeutically (e.g., in ITU settings) to manage elevated ICP.
  • In mechanically ventilated patients, an arterial paCO₂ of around 35–40 mmHg is often targeted.

Strokes and Perfusion

Stroke (cerebrovascular accident) commonly results from occlusion (thrombosis or embolism) of an artery within the cerebral circulation. Reduced or absent blood flow causes hypoxic injury and infarction of the affected brain region, leading to neurological deficits. In other cases, haemorrhagic stroke (ruptured cerebral artery) can also disrupt local perfusion and increase intracranial pressure, causing tissue damage.