Vascular diseases of the nervous system are one of the most common causes of mortality and disability.
Features of blood supply to the brain
Brain activity as a whole and all the processes specific to the nervous tissue (conducting a nerve impulse, synaptogenesis, storing and processing incoming information, maintaining the spatial and functional architectonics of the brain, the formation of functional ensembles of neurons, etc.) are in close dependence on the level of energy metabolism, determined primarily by the supply of oxygen and glucose into the nervous tissue with the blood flow. Making up about 2% of the human body weight, the brain consumes 20–25% of the oxygen entering the body and up to 70% of the free glucose formed in the liver. However, the nervous tissue has limited energy resources. The own reserves of glucose in the brain tissue are extremely small compared to the intensity of its consumption and can be used up only for oxidation processes in 3–6 minutes. Thus, the functioning of the brain is completely dependent on the constant supply of glucose. With a critical decrease in its concentration in the blood, the brain tissue begins to use the free glycogen fraction to maintain oxidative processes, but due to its small size, the complete oxidation of brain glycogen runs out after 5–7 minutes. Complete cessation of blood flow in the brain for 5–8 minutes leads to irreversible changes. In this regard, a consistently high perfusion of the brain is an essential condition for its functional activity and viability.
An important feature of the blood supply to the brain is an effective system of anastomoses (see Section 2.6) capable of ensuring the stability of cerebral blood flow in conditions of stenosis or even complete occlusion of one of the four main arteries supplying the brain.
A certain autonomy of the cerebral blood supply is provided by the mechanism of autoregulation. Its physiological essence is the narrowing or dilation of the cerebral arteries in response to an increase or decrease in the systemic BP, respectively. Normally, with fluctuations of systolic BP from 60 to 180 mm Hg, the cerebral blood flow remains relatively stable. In hypertension, the upper limit of autoregulation of cerebral blood flow can be shifted to systolic BP of 200–220 mm Hg. In various pathological conditions (hypertension, acute disorders of cerebral circulation, traumatic brain injury, brain tumors), autoregulation mechanisms are ineffective, and with fluctuations in the systemic BP changes in cerebral blood flow passively follow BP level.
Classification
There are the following types of cerebral circulation disorders:
- chronic (chronic cerebral circulation insufficiency);
- acute (acute cerebrovascular accident).