II. NORMAL VASCULAR ANATOMY
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
- 1. Anastomotic Channels
- a. Circle of Willis (via anterior communicating and posterior
- 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
C. Factors Affecting CNS Blood Flow
- 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.
- 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
- 2. Autoregulation of blood flow in response to changes in perfusion
- 3. Anastomoses
4. Increased percentage of systemic blood flow to the