Cerebral circulation involves a most remarkable system of supply and demand. The brain, having a cell metabolism utterly dependent on immediate availability of oxygen, and having by far the highest metabolic rate of any organ in the body, requires excellent circulatory flow in order to function.
The brain is the most active metabolic organ in the body, and also one of the most vulnerable to metabolic upset. Without significant variation between wakefulness or sleep or levels of physical/mental activity, the central nervous system uses some 15-20% of one's oxygen intake and only a slightly lesser percentage of the heart's output. Circulating blood volume within the brain at any instant is about 750ml and remains essentially constant, although regional variations occur within the brain with change in mental activity.
Virtually all of this oxygen use is for conversion of glucose to CO2. Since neural tissue has no mechanism for storage of oxygen, there is an oxygen metabolic reserve of only about 8-10 seconds.
When brain cells are deprived of their blood supply, a stroke occurs. Without access to vital nutrients and oxygen, brain cells die. The effects of a stroke can vary widely depending on where it occurs in the brain, the severity of the attack, and the general health of the person. A minor stroke may not even be noticed, while a major one can cause crippling mental and physical disabilities or even death.
The brain automatically regulates the blood pressure between a range of about 50 to 140mm Hg. If pressure falls below 50mm Hg, adjustments to the vessel system cannot compensate, brain perfusion pressure also falls, and the result may be hypoxia and circulatory blockage. Pressure elevated above 140mm Hg results in increased resistance to flow in the cerebral arterial tree. Excessive pressure can overwhelm resistance, leading to elevated capillary pressure, loss of fluid to the meager tissue compartment, and brain swelling.
Disruptions to the brain's blood circulation include cerebral hemorrhage, thrombosis, embolism, or subarachnoid hemorrhage. These usually result from pre-existing vascular disease or congenital weakness and may be precipitated by trauma. Most commonly these problems occur in older persons.
Chronic hypertension may result in vascular sclerotic lesions and lead to headache, dizziness, digestive symptoms, and even seizures. The final outcomes may include any of the problems described above.
Treatment involves weight loss (if this is part of the problem), sodium restriction, and drugs to reduce pressure, in addition to symptomatic treatments if these do not subside.
Part of the clinical problem is that by the time these symptoms manifest themselves, there may have been extensive damage.
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