A. Arterial Supply and Anastomoses
The brain receives its blood supply from paired vertebral and internal carotid arteries as well as from anastomotic channels derived from extracranial vessels. You should be familiar with the pattern of arterial supply in the brain.

1. Anastomotic Channels

a. Circle of Willis (via anterior communicating and posterior communicating arteries)

b. Channels between external and internal carotid arteries, especially via the ophthalmic artery.

c. Leptomeningeal anastomoses between peripheral branches of the anterior, middle and posterior cerebral arteries.

B. Venous Drainage
The veins of the brain drain into the dural venous sinuses and through them to the internal jugular veins. Since the surface veins of the brain have abundant anastomoses of large caliber, localized venous obstructions may produce few effects. Because the veins of the CNS are valveless, blood may flow in or out of the skull through these connections, providing a potential route for infection.
C. Factors Affecting CNS Blood Flow
Normally, autoregulation in the brain keeps cerebral blood flow constant over a wide range of perfusion pressures. The mechanism by which arterioles dilate when perfusion pressure falls and constrict when perfusion pressure rises is poorly understood, but several factors are known to regulate these phenomena. The most potent effect is exerted by carbon dioxide, i.e., increased carbon dioxide causes increased cerebral blood flow.

D. Factors Operating to Ensure Normal Substrate Requirements
1. Increased extraction of glucose and oxygen from the perfusing blood

2. Autoregulation of blood flow in response to changes in perfusion pressure

3. Anastomoses
4. Increased percentage of systemic blood flow to the CNS

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