The rate of metabolism of the brain is amongst the highest of all organs in the body. Aerobic metabolism provides for all the essential brain functions, and a good supply of oxygen is essential for normal brain function. The arterial blood supply to the brain is the source of oxygen, and if the blood supply is compromised (ischaemia), then brain tissue can be destroyed.
This tutorial considers the arterial supply and venous drainage of the brain. In addition factors that complicate the issue such as raised intracranial pressure and haemorrhage are considered briefly.
Key Words: Cerebral arteries and veins. Intracranial haematomas and haemorrhages.
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The Circle of Willis.
Arterial Supply to the Brain
The brain receives its blood supply via the internal carotid artery and the vertebral arteries on both sides, and these anastomose with each other in the circle of Willis.
The arterial supply to the forebrain arises from the circle of Willis.
The Anterior Cerebral Artery supplies the medial side of the frontal lobe.
The middle cerebral artery that supplies the thalamus, internal capsule and the somatosensory and motor cortex.
This artery is most commonly affected in stroke.
The posterior cerebral artery suppplies the occipital lobe, an area concerned with vision.
The basilar artery, formed by the confluence of two vertebral arteries supplies the brainstem and has branches supplying the cerebellum.
The diagrams indicate the areas of distribution of these arteries.
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Venous Drainage of the Brain
The Cerebral Veins carry blood from the forebrain to Venous sinuses that drain into the Internal Jugular Vein
The major venous sinuses are the Superior and Inferior Sagittal Sinuses, and the Sigmoid Sinus.
The venous sinuses are also involved in the reabsorption of cerebrospinal fluid. Arachnoid granulations occur at sites of reabsorption of CSF.
Venous pressure in the sinuses is negative when in the erect posture; consequently if the adjacent skull is fractures there can be a risk of air embolism if the sinus is exposed to the air.
Image source: medscape.com
In this scan a subdural haematoma is pressing on the brain and displacing it towards the right of the image.
Head Trauma and Consciousness
The delicate structure of the brain is protected from our external environment by the skull. This is a hard and bony structure which cannot expand, and offers protection for the delicate structure of the brain.
The Monro-Kellie doctrine states that the volume of the skull is constant, and any new mass developing within the skull displaces CSF and venous blood, which compromise the oxygenation of the tissues.
If the brain becomes swollen (e.g. because of tumours or haemorrhage), pressure rises within the skull because it has a fixed volume. CSF pressure rises and the cerebral vessels, especially the veins, are compressed, and blood flow becomes compromised.
The reduction in cerebral blood flow is highly significant because the rate of metabolism of the brain is high, and the brain's metabolism requires oxygen. If blood flow is compromised the metabolic needs of the tissue are not satisfied and brain swelling (oedema and other types of damage) can occur.
This can easily lead to a vicious circle: increased swelling -> increased pressure -> reduced blood flow -> more ischemia (=lack of blood) -> more damage to the tissue and the blood capillaries -> more swelling - > etc -> etc. THis vicious circle leads to unconsciousness and death because the brain becomes starved of oxygen.
Monitoring the level of consciousness is vital in this situation and a medically induced coma may be used to reduce brain activity and metabolism. Drugs such as anaesthetics and sedatives can be used to induce unconsciousness and reduce cerebral metabolism. This also reduces the accumulation of interstitial fluid within the brain - and therefore the cause of tissue swelling. It enables the doctors to break the vicious circle.
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The middle meningeal artery can be ruptured by head injuries, and can bleed into the space outside the dura mater; this is an extra-dural haematoma, and can put pressure on the brain, thereby restricting capillary blood flow.
Types of Brain Haemorrhage
Image source: Medscape.com
Types of brain haemorrhage caused by trauma. Epidural = extradural.