Introduction
Blood Flow Blockage. The brain receives about 25% of the body's oxygen, but it cannot store it. Brain cells require a constant supply of oxygen to stay healthy and function properly. Therefore, blood needs to be supplied continuously to the brain through two main arterial systems:
- The carotid arteries come up through either side of the front of the neck. (To feel the pulse of a carotid artery, place your fingertips gently against either side of your neck, right under the jaw.)
- The basilar artery forms at the base of the skull from the vertebral arteries, which run up along the spine, join, and come up through the rear of the neck.
The Circle of Willis is the joining area of several arteries at the bottom (inferior) side of the brain. At the Circle of Willis, the internal carotid arteries branch into smaller arteries that supply oxygenated blood to over 80% of the cerebrum.
A reduction of, or disruption in, blood flow to the brain is the primary cause of a stroke. Blockage for even a short period of time can be disastrous and cause brain damage or even death.
Click the icon to see an image of the brain.
A stroke is usually defined as two types:
- Ischemic (caused by a blockage in an artery)
- Hemorrhagic (caused by a tear in the artery's wall that produces bleeding in the brain)
The consequences of a stroke, the type of functions affected, and the severity, depend on where in the brain it has occurred and the extent of the damage.
Ischemic Stroke
Ischemic strokes are by far the more common type, causing over 80% of all strokes. Ischemia means the deficiency of oxygen in vital tissues. Ischemic strokes are caused by blood clots that are usually one of three types:
- Thrombotic stroke
- Embolic stroke
- Lacunar stroke
Thrombotic or Large-Artery Stroke and Atherosclerosis. The thrombotic stroke accounts for about 60% of all strokes. It usually occurs when an artery to the brain is blocked by a thrombus (blood clot) that forms as the result of atherosclerosis (commonly known as hardening of the arteries). These strokes are also sometimes referred to as large-artery strokes. The process leading to thrombotic stroke is complex and occurs over time:
- The arterial walls slowly thicken, harden, and narrow until blood flow is reduced, a condition known as stenosis.
- As these processes continue, blood flow slows. In addition, other events contribute to the coming stroke:
- The arteries become calcified, lose elasticity, and become susceptible to tearing. In this event, the thrombus (blood clot) forms.
- The blood clot then blocks the already narrowed artery and shuts off oxygen to part of the brain. A stroke occurs.
Embolic Strokes and Atrial Fibrillation. An embolic stroke is usually caused by a dislodged blood clot that has traveled through the blood vessels (an embolus) until it becomes wedged in an artery. Embolic strokes account for about 25% of all strokes and may be due to various conditions:
