Cerebral circulation in animals
Cerebral circulation constituents about 14% of the cardiac output. The arteries that supply the brain are the anterior, middle and posterior arteries of both sides through a common arterial pathway, the circle of Willis.
In most of the species this circle is supplied by the internal carotid arteries and vertebral arteries, whereas in cat, dog, sheep and goat the circle is supplied by a vascular net, the rete mirable. This carotid rete aids in thermoregulatory mechanism of the brain.
The circle of the Willis freely communicates with the contralateral side of the brain from both the carotid and vertebral arteries. There are no arteriovenous anastomoses in the brain.
The walls of the capillaries are non porous and are separated by the nerve cells by neuroglia which prevents diffusion of high molecular weight substances thereby forming a blood brain barrier.
The blood is drained through the internal jugular vein or through the vertebral venous plexus and external jugular vein.
The cerebral blood vessels receive both sympathetic and parasympathetic nervous system.
Regulation of cerebral circulation
Cerebral circulation is controlled primarily by auto regulatory mechanisms mediated by local pH changes.
An increase is arterial PCO2 or a decrease in PO2 causes vasodilatation. Potassium ions produce vasodilatation whereas an increase in bicarbonate concentration causes vasoconstriction.
The cerebral blood flow is decreased in chronic alkalosis and increased in chronic acidosis. Neural regulation of blood flow plays only a secondary role.