Meninges:The brain is covered by 3 meninges: dura mater, arachnoid mater, and the pia mater. The arachnoid membrane, the middle meningeal layer, is an extremely delicate layer that loosely encloses the brain.

The subarachnoid space, located between the arachnoid membrane and the pia mater, contains the cerebrospinal fluid (CSF) which cushions and supports the brain. It also contains a network of arteries and veins which support the metabolic needs of the brain. In the event of an arterial subarachnoid hemorrhage, bleeding occurs into the subarachnoid space. While bleeding continues intracranial pressure (ICP) will increase as a result of the intraarterial pressure exerted within a closed system. When ICP increases, veins are compressed, venous outflow is reduced and perfusion therefore decreases.

Arterial circulation: The brain is only about 2% of the body's total weight, yet it receives approximately 20% of the heart's resting cardiac output. Obviously, significant blood volume courses through the cerebral circulation every minute.

The primary source of blood flow to the brain is carried by three arteries: two internal carotids and the basilar artery. Together the basilar artery, the two internal carotids and their branches form the Circle of Willis. While the Circle of Willis appears to provide redundant circulation in the event of an occlusion, angiographic studies demonstrate that the flow of each carotid is distributed almost exclusively to the corresponding hemisphere.



Venous circulation: Venous blood is drained from the brain via deep veins and dural sinuses. The veins and sinuses connect to the juglar veins or paravertebral veins.

Cerebral spinal fluid: CSF fills the subarachnoid space, ventricles and cisterns. It is produced by choroid plexuses and absorbed by the arachnoid villi at about 500mL/day. That volume is cycled about 3.7 times per day.

In the event of a subarachnoid hemorrhage, blood mixes with the CSF. Clots can then form and collect in the ventricles, cisterns and foramen. If the clots block the normal flow of CSF hydrocephalus can result. Hydrocephalus can result in decreased perfusion as a result of increased ICP.