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.
Causes and Development
Disruptions to the brain's blood circulation include cerebral hemorrhage
, 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.
- Hemorrhage usually occurs in atherosclerotic vessels. The resulting blood clot destroys brain tissue, and the neural tissue remaining next to the clot may be softened, leading to later complications. The clot and dead tissue are removed by macrophages, and the damaged area is invaded by connective tissue and glia, often producing a fluid-filled cyst.
- Thrombosis most commonly involves formation of a clot at a site of vessel lumen constriction due to growth of atherosclerotic plates. Blockage of circulation leads to tissue softening and death, and to congestion of flow and edema in adjacent areas.
- Embolism is the blockage of a cerebral vessel by a physical object, such as a dislodged clot, air, tumor cells, infectous mass. Often the situation involves multiple embolisms, complicating the clinical picture.
- Aneurisms are the expansion of a vessel, usually an artery, and these balloons may reach a diameter of several inches. If this occurs within the cranial cavity, the displacement of neural tissue and the compression of other vessels and of cranial nerves can obviously lead to severe problems. Most arise from the middle cerebral artery or the internal carotid, and if expansion is slow enough, there may be extensive erosion of bone. Clinically there is the added danger that the weakened vessel will rupture, giving the added problems of subarachnoid hemorrhage. Treatment usually involves surgery, if the vessel is accessible. Methods include removal of the sac or reinforcing the vessel wall with muscle or connective tissue, or ensheathing or replacing the weakened vessel with plastic.
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 and Prevention
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.