Diagram of blood supply to the brain. Blood supply to the brain
CEREBRAL CIRCULATION- blood circulation through the cerebral vascular system. The blood supply to the brain is more intense than to any other organs: approx. 15% of the blood entering the systemic circulation during cardiac output flows through the blood vessels of the brain (its weight is only 2% of the body weight of an adult). Extremely high cerebral blood flow ensures the greatest intensity of metabolic processes in brain tissue. This blood supply to the brain is also maintained during sleep. The intensity of metabolism in the brain is also evidenced by the fact that 20% of the oxygen absorbed from the environment is consumed by the brain and used for oxidative processes occurring in it.
PHYSIOLOGY
The circulatory system of the brain provides perfect regulation of the blood supply to its tissue elements, as well as compensation for disturbances in cerebral blood flow. The human brain (see) is supplied with blood simultaneously by four main arteries - paired internal carotid and vertebral arteries, which are interconnected by wide anastomoses in the area of the arterial (Willisian) circle of the cerebrum (color. Fig. 4). Under normal conditions, the blood does not mix here, flowing ipsilaterally from each internal carotid artery (see) into the cerebral hemispheres, and from vertebrates - mainly into the parts of the brain located in the posterior cranial fossa.
Cerebral arteries are not elastic, but muscular type vessels with abundant adrenergic and cholinergic innervation, therefore, changing their lumen within a wide range, they can participate in regulating blood supply to the brain.
Paired anterior, middle and posterior cerebral arteries, extending from the arterial circle, branching and anastomosing among themselves, form a complex system of arteries of the pia mater (pial arteries), which has a number of features: branching of these arteries (down to the smallest, diameter 50 microns or less ) are located on the surface of the brain and regulate blood supply to extremely small areas; each artery lies in a relatively wide canal of the subarachnoid space (see Meninges), and therefore its diameter can vary within wide limits; the arteries of the pia mater lie on top of the anastomosing veins. From the smallest arteries of the pia mater radial arteries branch off in the thickness of the brain; they do not have free space around the walls and, according to experimental data, are the least active in terms of changes in diameter when regulating the muscle. There are no interarterial anastomoses in the thickness of the brain.
The capillary network in the thickness of the brain is continuous. The more intense the metabolism in the tissues, the greater its density, so it is much thicker in gray matter than in white matter. In each part of the brain, the capillary network is characterized by specific architecture.
Venous blood flows from the capillaries of the brain into the widely anastomosing venous system of both the pia mater (pial veins) and the great cerebral vein (vein of Galen). Unlike other parts of the body, the venous system of the brain does not perform a capacitive function.
For more details on the anatomy and histology of the blood vessels of the brain, see Brain.
Regulation of cerebral circulation is carried out by a perfect physiological system. The effectors of regulation are the main, intracerebral arteries and arteries of the pia mater, which are characterized by specific functions. features.
Four types of regulation of M. to. are shown in the diagram.
When the level of total blood pressure changes within certain limits, the intensity of cerebral blood flow remains constant. Regulation of constant blood flow in the brain during fluctuations in total blood pressure is carried out due to changes in resistance in the arteries of the brain (cerebrovascular resistance), which narrow when total blood pressure increases and expand when it decreases. Initially, it was assumed that vascular shifts were caused by the reactions of the smooth muscles of the arteries to varying degrees of stretching of their walls by intravascular pressure. This type of regulation is called autoregulation or self-regulation. The level of increased or decreased blood pressure, at which cerebral blood flow ceases to be constant, is called the upper or lower limit of autoregulation of cerebral blood flow, respectively. Experimental and wedge studies have shown that autoregulation of cerebral blood flow is in close relationship with neurogenic influences, which can shift the upper and lower boundaries of its autoregulation. The effectors of this type of regulation in the arterial system of the brain are the main arteries and arteries of the pia mater, active reactions of which maintain constant blood flow in the brain when the total blood pressure changes.
Regulation of M. to. with a change in the gas composition of the blood is that cerebral blood flow increases with an increase in the CO 2 content and with a decrease in the O 2 content in arterial blood and decreases when their ratio is inverse. The influence of blood gases on the tone of the arteries of the brain, according to a number of authors, can be carried out humorally: with hypercapnia (see) and hypoxia (see), the concentration of H + in the brain tissue increases, the ratio between HCO 3 - and CO 2 changes, which together with other biochemicals, shifts in the extracellular fluid directly affect the metabolism of smooth muscles, causing dilatation) of the arteries. The neurogenic mechanism also plays an important role in the action of these gases on the vessels of the brain, in which chemoreceptors of the carotid sinus and, apparently, other cerebral vessels participate.
Elimination of excess blood volume in the vessels of the brain is necessary, since the brain is located in a hermetically sealed skull and its excessive blood supply leads to increased intracranial pressure (see) and to compression of the brain. Excessive blood volume can occur when there is difficulty in the outflow of blood from the veins of the brain and when there is excessive blood flow due to dilation of the arteries of the pia mater, for example, with asphyxia (see) and with post-ischemic hyperemia (see Hyperemia). There is evidence that the effectors of regulation in this case are the main arteries of the brain, which narrow reflexively due to irritation of the baroreceptors of the cerebral veins or arteries of the pia mater and limit blood flow to the brain.
Regulation of adequate blood supply to brain tissue ensures correspondence between the intensity of blood flow in the microcirculation system (see) and the intensity of metabolism in brain tissue. This regulation occurs when there is a change in the intensity of metabolism in the brain tissue, for example, a sharp increase in its activity, and when there is a primary change in blood flow into the brain tissue. Regulation is carried out locally, and its effector is the small arteries of the pia mater, which control blood flow in negligibly small areas of the brain; the role of smaller arteries and arterioles in the thickness of the brain has not been established. Control of the lumen of effector arteries when regulating cerebral blood flow, according to most authors, is carried out humorally, i.e., under the direct action of metabolic factors accumulating in brain tissue (hydrogen ions, potassium, adenosine). Some experimental data indicate a neurogenic mechanism (local) of vasodilation in the brain.
Types of regulation of cerebral circulation. Regulation of cerebral blood flow when the level of total blood pressure changes (III) and when there is excessive blood supply to the cerebral vessels (IV) is carried out by the main arteries of the brain. When the content of oxygen and carbon dioxide in the blood changes (II) and when the adequacy of the blood supply to brain tissue is impaired (I) small arteries of the pia mater are included in the regulation.
METHODS FOR STUDYING CEREBRAL BLOOD FLOW
The Kathy-Schmidt method allows you to determine blood flow in the entire human brain by measuring the rate of saturation (saturation) of brain tissue with an inert gas (usually after inhaling small amounts of nitrous oxide). Saturation of brain tissue is determined by determining the gas concentration in venous blood samples taken from the jugular vein bulb. This method (quantitative) allows one to determine the average blood flow of the whole brain only discretely. It was found that the intensity of cerebral blood flow in a healthy person is approximately 50 ml of blood per 100 g of brain tissue per minute.
The clinic uses a direct method to obtain quantitative data on cerebral blood flow in small areas of the brain using the clearance (clearance rate) of radioactive xenon (133 Xe) or hydrogen gas. The principle of the method is that the brain tissue is saturated with easily diffusible gases (133 Xe solution is usually injected into the internal carotid artery, and hydrogen is inhaled). Using appropriate detectors (for 133Xe they are installed above the surface of the intact skull; for hydrogen, platinum or gold electrodes are inserted into any area of the brain) the rate at which brain tissue is cleared of gas is determined, which is proportional to the intensity of blood flow.
Direct (but not quantitative) methods include the method of determining changes in blood volume in superficially located vessels of the brain using radionuclides, which mark blood plasma proteins; in this case, radionuclides do not diffuse through the walls of the capillaries into the tissue. Blood albumins labeled with radioactive iodine have become especially widespread.
The reason for the decrease in the intensity of cerebral blood flow is a decrease in the arteriovenous pressure difference due to a decrease in total blood pressure or an increase in total venous pressure (see), with the main role played by arterial hypotension (see Arterial hypotension). Total blood pressure may drop sharply, and total venous pressure increases less frequently and less significantly. A decrease in the intensity of cerebral blood flow may also be due to an increase in resistance in the vessels of the brain, which may depend on reasons such as atherosclerosis (see), thrombosis (see) or vasospasm (see) of certain arteries of the brain. A decrease in the intensity of cerebral blood flow may depend on the intravascular aggregation of blood cells (see Red blood cell aggregation). Arterial hypotension, weakening blood flow throughout the brain, causes the greatest decrease in its intensity in the so-called. areas of adjacent blood supply, where intravascular pressure drops the most. When certain arteries of the brain are narrowed or occluded, pronounced changes in blood flow are observed in the center of the basins of the corresponding arteries. Of great importance are secondary patol, changes in the vascular system of the brain, for example, changes in the reactivity of the cerebral arteries during ischemia (constrictor reactions in response to vasodilator effects), unrestored blood flow in the brain tissue after ischemia or spasm of the arteries in the area of blood extravasation, in particular subarachnoid hemorrhages. An increase in venous pressure in the brain, which plays a less significant role in weakening the intensity of cerebral blood flow, may have independent significance when it is caused, in addition to an increase in general venous pressure, by local causes that lead to difficulty in the outflow of venous blood from the skull (thrombosis or tumor). In this case, phenomena of venous stagnation of blood in the brain occur, which lead to an increase in blood supply to the brain, which contributes to an increase in intracranial pressure (see Hypertensive syndrome) and the development of cerebral edema (see Edema and swelling of the brain).
Patol, increased intensity of cerebral blood flow may depend on an increase in total blood pressure (see Arterial hypertension) and may be due to primary dilatation (patol, vasodilation) of the arteries; then it occurs only in those areas of the brain where the arteries are dilated. Patol, an increase in the intensity of cerebral blood flow can lead to an increase in intravascular pressure. If the walls of the vessels are pathologically changed (see Arteriosclerosis) or there are arterial aneurysms, then a sudden and sharp increase in total blood pressure (see Crises) can lead to hemorrhage. Patol, an increase in the intensity of cerebral blood flow may be accompanied by a regulatory reaction of the arteries - their constriction, and with a sharp increase in total blood pressure it can be very significant. If the functional state of the smooth muscles of the arteries is changed in such a way that the contraction process is enhanced, and the relaxation process, on the contrary, is reduced, then in response to an increase in total blood pressure, vasoconstriction occurs patol, such as vasospasm (see). These phenomena are most pronounced with a short-term increase in total blood pressure. When the blood-brain barrier is disrupted and there is a tendency to cerebral edema, an increase in pressure in the capillaries causes a sharp increase in the filtration of water from the blood into the brain tissue, where it is retained, resulting in the development of cerebral edema. An increase in the intensity of cerebral blood flow is especially dangerous under the influence of additional factors (traumatic brain injury, severe hypoxia) that contribute to the development of edema.
Compensatory mechanisms are an obligatory component of the symptom complex, which characterizes every violation of M. k. In this case, compensation is carried out by the same regulatory mechanisms, which function under normal conditions, but they are more intense.
When total blood pressure increases or decreases, compensation is carried out by changing the resistance in the vascular system of the brain, with the main role played by the large cerebral arteries (internal carotid and vertebral arteries). If they do not provide compensation, then microcirculation ceases to be adequate and the arteries of the pia mater are involved in regulation. With a rapid increase in total blood pressure, these compensation mechanisms may not work immediately, and then the intensity of cerebral blood flow sharply increases with all possible consequences. In some cases, compensatory mechanisms can work very well and even with chronic hypertension, when general blood pressure is sharply increased (280-300 mm Hg) for a significant time; the intensity of cerebral blood flow remains normal and neurol, disturbances do not occur.
When total blood pressure decreases, compensatory mechanisms can also maintain the normal intensity of cerebral blood flow, and depending on the degree of perfection of their work, the limits of compensation may vary from person to person. With perfect compensation, normal intensity of cerebral blood flow is observed when total blood pressure decreases even to 30 mm Hg. Art., while usually the lower limit of autoregulation of cerebral blood flow is considered to be blood pressure not lower than 55-60 mm Hg. Art.
When resistance increases in certain arteries of the brain (during embolism, thrombosis, vasospasm), compensation is carried out due to collateral blood flow. In this case, compensation is provided by the following factors:
1. The presence of arterial vessels through which collateral blood flow can occur. The arterial system of the brain contains a large number of collateral pathways in the form of wide anastomoses of the arterial circle, as well as numerous interarterial macro- and microanastomoses in the system of arteries of the pia mater. However, the structure of the arterial system is individual, and developmental anomalies are not uncommon, especially in the area of the arterial (circle of Willis) area. Small arteries located deep in the brain tissue do not have arterial-type anastomoses, and although the capillary network throughout the brain is continuous, it cannot provide collateral blood flow to neighboring tissue areas if the blood flow into them from the arteries is disrupted.
2. An increase in the pressure drop in the collateral arterial pathways when there are obstacles to blood flow in one or another cerebral artery (hemodynamic factor).
3. Active expansion of collateral arteries and small arterial branches to the periphery from the site of closure of the artery lumen. This vasodilation is, apparently, a manifestation of the regulation of adequate blood supply to brain tissue: as soon as a deficiency of blood flow into the tissue occurs, a physiological mechanism begins to work, causing dilatation) of those arterial branches that lead to this microcirculatory system. As a result, the resistance to blood flow in the collateral pathways is reduced, which promotes blood flow to the area with reduced blood supply.
The effectiveness of collateral blood flow to the area of reduced blood supply varies from person to person. The mechanisms that ensure collateral blood flow may be disrupted depending on specific conditions (as well as other regulatory and compensation mechanisms). Thus, the ability of collateral arteries to expand during sclerotic processes in their walls decreases, which prevents collateral blood flow to the area of impaired blood supply.
Compensation mechanisms are characterized by duality, i.e. compensation for some disorders causes other circulatory disorders. For example, when blood flow in brain tissue that has experienced a deficiency of blood supply is restored, post-ischemic hyperemia may occur, in which the intensity of microcirculation can be significantly higher than the level necessary to ensure metabolic processes in the tissue, i.e., excessive blood perfusion occurs, promoting, in particular, the development of post-ischemic cerebral edema.
On adequate and pharmacological influences, a perverted reactivity of the arteries of the brain can be observed. Thus, the basis of the “intracerebral steal” syndrome is the normal vasodilator reaction of healthy vessels surrounding the focus of ischemia of brain tissue, and the absence of such in the affected arteries in the focus of ischemia, as a result of which blood is redistributed from the focus of ischemia to healthy vessels, and ischemia is aggravated.
PATHOLOGICAL ANATOMY OF CEREBRAL CIRCULATION DISORDERS
Morphol. signs of disturbance of M. to. are revealed in the form of focal and diffuse changes, the severity and localization of which are different and largely depend on the underlying disease and the immediate mechanisms of development of circulatory disorders. There are three main forms of violation
M. to.: hemorrhages (hemorrhagic stroke), cerebral infarctions (ischemic stroke) and multiple different types of small focal changes in the brain substance (vascular encephalopathy).
Wedge, manifestations of occlusive lesions of the extracranial part of the internal carotid artery in the initial period occur more often in the form of transient disorders of M. K. Nevrol, the symptoms are varied. In approximately 1/3 of cases, there is an alternating optic-pyramidal syndrome - blindness or decreased vision, sometimes with atrophy of the optic nerve on the side of the affected artery (due to discirculation in the ophthalmic artery), and pyramidal disorders on the side opposite to the lesion. Sometimes these symptoms occur simultaneously, sometimes dissociated. The most common signs of occlusion of the internal carotid artery are signs of discirculation in the middle cerebral artery basin: paresis of the limbs of the side opposite to the lesion, usually of the cortical type with a more pronounced hand defect. With infarctions in the left internal carotid artery, aphasia often develops, usually motor. Sensory disturbances and hemianopsia may occur. Occasionally, epileptiform seizures are observed.
In heart attacks caused by intracranial thrombosis of the internal carotid artery, which occurs with disconnection of the arterial circle, along with hemiplegia and hemihypesthesia, pronounced cerebral symptoms are observed: headache, vomiting, impaired consciousness, psychomotor agitation; secondary stem syndrome appears.
The syndrome of occlusive lesions of the internal carotid artery, in addition to the intermittent course of the disease and the indicated neurol manifestations, is characterized by a weakening or disappearance of the pulsation of the affected carotid artery, often the presence of a vascular noise above it and a decrease in retinal pressure on the same side. Compression of the unaffected carotid artery causes dizziness, sometimes fainting, and convulsions in healthy limbs.
An occlusive lesion of the extracranial section of the vertebral artery is characterized by “spotty” lesions of various parts of the spinobasilar system: vestibular disorders (dizziness, nystagmus), disorders of statics and coordination of movements, visual and oculomotor disturbances, dysarthria often occur; motor and sensory disorders are less frequently detected. Some patients experience attacks of sudden falling due to loss of postural tone, adynamia, and hypersomnia. Quite often there are memory disorders for current events such as Korsakov's syndrome (see).
When the intracranial part of the vertebral artery is blocked, persistent alternating syndromes of damage to the medulla oblongata are combined with transient symptoms of ischemia of the oral parts of the brain stem, occipital and temporal lobes. In approximately 75% of cases, Wallenberg-Zakharchenko, Babinsky-Nageotte syndromes and other syndromes of unilateral damage to the lower parts of the brain stem develop. With bilateral thrombosis of the vertebral artery, severe swallowing and phonation disorders occur, breathing and cardiac activity are impaired.
Acute blockage of the basilar artery is accompanied by symptoms of predominant damage to the pons with a disorder of consciousness up to coma, rapid development of lesions of the cranial nerves (III, IV, V, VI, VII pairs), pseudobulbar syndrome, paralysis of the limbs with the presence of bilateral patols. reflexes. Autonomic-visceral crises, hyperthermia, and disturbance of vital functions are observed.
Diagnosis of cerebrovascular disorders
The basis for the diagnosis of the initial manifestation of M.'s inferiority is: the presence of two or more subjective signs, often repeated; absence during normal neurol examination of symptoms of organic damage to c. n. With. and detection of signs of general vascular disease (atherosclerosis, hypertension, vasculitis, vascular dystonia, etc.), which is especially important, since the patient’s subjective complaints are not pathognomonic for the initial manifestations of vascular inferiority of the brain and can also be observed in other conditions (neurasthenia , asthenic syndromes of various origins). In order to establish a general vascular disease in a patient, it is necessary to conduct a comprehensive wedge and examination.
The basis for the diagnosis of an acute disorder of M. to. is the sudden appearance of symptoms of organic brain damage against the background of a general vascular disease with significant dynamics of cerebral and local symptoms. If these symptoms disappear in less than 24 hours. a transient disorder of M. is diagnosed; in the presence of more persistent symptoms, a cerebral stroke is diagnosed. In determining the nature of a stroke, it is not the individual signs, but their combination that is of key importance. There are no pathognomonic signs for one type of stroke or another. For the diagnosis of hemorrhagic stroke, high blood pressure and a history of cerebral hypertensive crises, sudden onset of the disease, rapid progressive deterioration of the condition, significant severity of not only focal but also general cerebral symptoms, distinct autonomic disorders, early onset of symptoms caused by displacement and compression of the brain stem, are important. rapidly occurring changes in the blood (leukocytosis, neutrophilia with a shift to the left in the leukocyte formula, an increase in the Krebs index to 6 or higher), the presence of blood in the cerebrospinal fluid.
Cerebral infarction is evidenced by the development of a stroke during sleep or against the background of weakened cardiovascular activity, the absence of arterial hypertension, the presence of cardiosclerosis, a history of myocardial infarction, the relative stability of vital functions, the preservation of consciousness with massive neurol, symptoms, the absence or mild severity of secondary stem syndrome, relatively slow development of the disease, no changes in the blood in the first day after the stroke.
Echoencephalography data (see) help in diagnosis - the shift of the M-echo towards the contralateral hemisphere is more likely to speak in favor of intracerebral hemorrhage. X-ray, examination of cerebral vessels after administration of contrast agents (see Vertebral angiography, Carotid angiography) for intrahemispheric hematomas reveals an avascular zone and displacement of the arterial trunks; In case of cerebral infarction, an occlusive process is often detected in the main or intracerebral vessels; dislocation of the arterial trunks is uncharacteristic. Computed tomography of the head provides valuable information when diagnosing a stroke (see Computer tomography).
Basic principles of therapy for cerebrovascular accidents
With initial manifestations of M.'s inferiority, therapy should be aimed at treating the underlying vascular disease, normalizing the work and rest regime, and using agents that improve the metabolism of brain tissue and hemodynamics.
In case of acute violations of M. to., urgent measures are required, since it is not always clear whether the violation of M. to. will be transient or persistent, therefore, in any case, complete mental and physical rest is necessary. A cerebral vascular attack should be stopped at the earliest stages of its development. Treatment of transient disorders of M. to. (vascular cerebral crises) should primarily involve the normalization of blood pressure, cardiac activity and cerebral hemodynamics with the inclusion, if necessary, of antihypoxic, decongestant and various symptomatic drugs, including sedatives, in some cases they are used anticoagulants and antiplatelet agents. Treatment for cerebral hemorrhage is aimed at stopping the bleeding and preventing its resumption, combating cerebral edema and impairment of vital functions. When treating a heart attack
brain carry out measures aimed at improving blood supply to the brain: normalizing cardiac activity and blood pressure, increasing blood flow to the brain by expanding regional cerebral vessels, reducing vascular spasm and improving microcirculation, as well as normalizing physical-chemical. properties of blood, in particular to restore balance in the blood coagulation system to prevent thromboembolism and to dissolve already formed blood clots.
Bibliography: Akimov G. A. Transient disorders of cerebral circulation, L., 1974, bibliogr.; Antonov I.P. and Gitkina L.S. Vertebro-basilar strokes, Minsk, 1977; B e to about in D. B. and Mikhailov S. S. Atlas of arteries and veins of the human brain, M., 1979, bibliogr.; Bogolepov N.K. Comatose states, p. 92, M., 1962; about n e, Cerebral crises and stroke, M., 1971; Gannushkina I.V. Collateral circulation in the brain, M., 1973; K Dosovsky B. N. Blood circulation in the brain, M., 1951, bibliogr.; K o l t o-vera. N.idr. Pathological anatomy of cerebral circulation disorders, M., 1975; Mints A. Ya. Atherosclerosis of cerebral vessels, Kyiv, 1970; Moskalenko Yu.E. and others. Intracranial hemodynamics, Biophysical aspects, L., 1975; Mchedlishvili G. I. Function of vascular mechanisms of the brain, L., 1968; o n, Spasm of the cerebral arteries, Tbilisi, 1977; Vascular diseases of the nervous system, ed. E. V. Schmidt, p. 632, M., 1975; Sh m and d t E. V. Stenosis and thrombosis of the carotid arteries and cerebrovascular accidents, M., 1963; Schmidt E. V., Lunev D. K. and Vereshchagin N. V. Vascular diseases of the brain and spinal cord, M., 1976; Cerebral circulation and stroke, ed. by K. J. Ztilch, B. u. a., 1971; Fisher S. M. The arterial lesions underlying lacunes, Acta neuropath. (Berl.), v. 12, p. 1, 1969; Handbook of clinical neurology, ed. by P. J. Vinken a. G. W. Bruyn, v. 11 -12, Amsterdam, 1975; Jorgensen L. a. Torvik A. Ischemic cerebrovascular diseases in an autopsy series, J. Neurol. Sci., v. 9, p. 285, 1969; Olesen J. Cerebral blood flow, Copenhagen, 1974; P u r v e s M. J. The physiology of the cerebral circulation, Cambridge, 1972.
D. K. Lunev; A. N. Koltover, R. P. Tchaikovskaya (pat. an.), G. I. Mchedlishvili (physics., path. physics.).
Under physiological conditions, every 100 g of brain tissue at rest receives 55–58 ml of blood in 1 min and consumes 3–5 ml of oxygen. That is, the brain, the mass of which in an adult is only 2% of body weight, receives 750 - 850 ml of blood, almost 20% of all oxygen and approximately the same amount of glucose in 1 minute. A constant supply of oxygen and glucose is necessary to preserve the energy substrate of the brain, the normal functioning of neurons, and maintain their integrative function.
The brain is supplied with blood by two paired main arteries of the head - the internal carotid and vertebral. Two-thirds of the blood is supplied to the brain by the internal carotid arteries and one-third by the vertebral arteries. The former form the carotid system, the latter the vertebrobasilar system. The internal carotid arteries are branches of the common carotid artery. They enter the cranial cavity through the internal opening of the carotid canal of the temporal bone, enter the cavernous sinus (sinus cavemosus), where they form an S-shaped bend. This part of the internal carotid artery is called the siphon, or cavernous part. Then it “pierces” the dura mater, after which the first branch departs from it - the ophthalmic artery, which, together with the optic nerve, penetrates into the cavity of the orbit through the optic canal. The posterior communicating and anterior villous arteries also depart from the internal carotid artery. Lateral to the optic chiasm, the internal carotid artery divides into two terminal branches: the anterior and middle cerebral arteries. The anterior cerebral artery supplies blood to the anterior part of the frontal lobe and the inner surface of the hemisphere, the middle cerebral artery supplies a significant part of the cortex of the frontal, parietal and temporal lobes, the subcortical nuclei and most of the internal capsule.
Figure 26.
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The cerebral vascular system with the most important anastomoses: |
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I - aorta; 2 - brachiocephalic trunk;
II - posterior cerebral artery;
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The vertebral arteries arise from the subclavian artery. They enter the skull through openings in the transverse processes of the CI-CVI vertebrae and enter its cavity through the foramen magnum. In the area of the brain stem (pons), both vertebral arteries merge into one spinal trunk - the basilar artery, which divides into two posterior cerebral arteries. They supply blood to the midbrain, pons, cerebellum and occipital lobes of the cerebral hemispheres. In addition, two spinal arteries (anterior and posterior), as well as the posterior inferior cerebellar artery, depart from the vertebral artery. The anterior communicating artery connects the anterior cerebral arteries, and the middle and posterior cerebral arteries are connected by the posterior communicating artery. As a result of the connection of the vessels of the carotid and vertebral-basilar basins, a closed system is formed on the lower surface of the cerebral hemispheres - the arterial (Williziev) circle of the cerebrum (Fig. 27).
Fig.27.
The vessels of the brain, depending on their functions, are divided into several groups.
The main, or regional, vessels are the internal carotid and vertebral arteries in the extracranial section, as well as the vessels of the arterial circle. Their main purpose is to regulate cerebral circulation in the presence of changes in systemic blood pressure (BP).
The arteries of the pia mater (stray) are vessels with a clearly expressed nutritional function. The size of their lumen depends on the metabolic needs of the brain tissue. The main regulator of the tone of these vessels are the metabolic products of brain tissue, especially carbon monoxide, under the influence of which brain vessels dilate.
Intracerebral arteries and capillaries, which directly provide one of the main functions of the cardiovascular system, the exchange between blood and brain tissue, are “exchange vessels”.
The venous system primarily performs a drainage function. It is characterized by a significantly larger capacity compared to the arterial system. Therefore, the veins of the brain are also called “capacitive vessels”. They do not remain a passive element of the vascular system of the brain, but take part in the regulation of cerebral circulation. Through the superficial and deep veins of the brain, from the choroid plexuses and deep parts of the brain, venous blood flows into the direct (through the great cerebral vein) and other venous sinuses of the dura mater. From the sinuses, blood flows into the internal jugular veins, then into the brachiocephalic and superior vena cava.
The brain regulates all structures of the body, allowing the stable functioning of physiological functions. As a result, intensive nutrition of nervous tissue plays a huge role in the life of the body. The blood supply to the brain is provided by two internal carotid and two vertebral arteries.
Arterial blood supply system
The physiology of the human body has not yet been fully studied, but the greatest mystery for scientists remains the brain, which is always active, even if a person is at rest and sleep. Blood supply to the brain is provided by two systems:
- The vertebral arteries, which begin in the subclavian, pass into the transverse processes of the cervical vertebrae and, in the area of the first of them, leave this canal, entering the foramen magnum in the skull. Here the PAs are located at the base of the medulla oblongata. At the border of the latter and the pons of the brain, the arteries listed above merge into one trunk of the basilar artery. At the border of the pons, it divides into a pair of posterior cerebral arteries.
If there are pathologies in the cervical spine, compression of the artery is often observed, which sometimes leads to irreversible consequences.
- The internal carotid artery is separated from the common carotid artery, which in turn is separated from the aorta and subclavian artery. Due to this, normal conditions for blood flow are created in the left artery system.
When a blood clot breaks off from the left region of the heart, it more often passes into the left carotid artery than into the right, since there is a direct connection with the aorta. The ICA enters the skull using the canal of the same name.
A diagram of the blood supply to the brain can be seen below.
The connection between both systems is due to the arterial circle of the cerebrum, which is otherwise called the circle of Willis and is formed due to the following blood supply elements:
- posterior brain (vertebrates);
- connecting posterior (internal carotid arteries);
- middle cerebral (internal carotid arteries);
- cerebral anterior (internal carotid arteries);
- connecting anterior (internal carotid arteries).
The purpose of the arterial circle of the cerebrum is to support proper blood flow to the brain, which is necessary if there is a violation in one of the arteries.
The system for transporting substances from the capillary to the nervous tissue is called the “blood-brain barrier,” which protects against the penetration of pathogenic factors (toxins, microbes, etc.) into the brain.
In the normal state of the barrier, substances such as:
- iodine compounds;
- immune bodies;
- salt;
- antibiotics.
Thus, medications containing the substances listed above cannot affect the nervous system.
At the same time, the following are able to overcome the blood-brain barrier:
- morphine;
- alcohol;
- tetanus toxin;
- chloroform.
In order for drugs used to treat infectious diseases of the brain to easily overcome this barrier, they must be injected into the fluid that surrounds the brain. This process is carried out through a puncture in the lumbar region of the spinal column or in the area under the back of the head.
The outflow of blood is carried out through veins that flow into the sinuses of the dura mater. They are slit-like canals in the connective tissue of the brain. Their peculiarity is that their lumen is always open in any conditions. This ensures a stable outflow of blood and prevents it from stagnating. Through the sinuses, venous blood enters the jugular foramen, located in the cranial base, where the jugular vein begins. Through it, blood flows into the superior vena cava.
Functionality of the arteries that make up the circle of Willis
The anterior cerebral artery supplies blood to the following areas:
- superior part of the postcentral and precentral gyri;
- cerebral cortex;
- olfactory tract;
- basal and internal part of the frontal lobe;
- white matter of the parietal and frontal lobes;
- head and outer part of the caudate nucleus;
- part of the corpus callosum;
- portion of the pedicle of the internal capsule;
- part of the lenticular nucleus.
The middle cerebral artery is responsible for supplying blood to the following areas:
- cerebral cortex;
- part of the lenticular and caudate nuclei;
- white matter of the surface of the cerebral hemispheres;
- in the temporal lobe of Wernicke's center;
- visual radiance;
- parietal lobe;
- part of the frontal gyri and lobes.
The posterior cerebral artery supplies the following areas:
- cerebral cortex;
- white matter;
- hypothalamus;
- cerebral peduncle;
- part of the optic thalamus;
- caudate nucleus;
- corpus callosum;
- Graziole bun;
- quadrigeminal.
The vertebral arteries supply the following brain areas:
- parts of the cerebellum;
- medulla oblongata;
- spinal cord.
The posterior inferior cerebellar artery provides blood supply to the following sections:
- posterior inferior cerebellum;
- part of the medulla oblongata.
An interesting fact is that there is no portal system in the blood supply to the brain. That is, the branches of the circle of Willis do not penetrate the medulla, as is usually the case in the vital organs of the body. They spread along the brain surface, branching off into thin branches at right angles. This fact determines the uniform distribution of blood supply. Therefore, there are no large vessels in the brain, but only capillaries and small arteries.
Still, there are large arteries in the head, which are located on the surface of the brain in the arachnoid membrane. Their location is fixed, since the vessels are not only suspended on trabeculae, but also supported at a specific distance relative to the brain.
Peculiarities
An interesting fact is that hemodynamics and changes in it do not affect blood circulation, since it contains self-regulation mechanisms.
The blood circulation of the gray matter has greater intensity compared to the white matter. The most saturated blood flow occurs in babies whose age has not yet reached one year. A newborn baby has a greater blood supply than an adult. As for the elderly, in this category of people it decreases by twenty percent, and sometimes more.
Control of this process occurs in the nervous tissue, and it is determined by metabolism. Centers for regulating nervous activity operate throughout life, without ceasing their functioning even during sleep.
The intracerebral structure of capillaries has some features, namely:
- A thin elastic membrane surrounds the capillaries, as a result of which they cannot stretch.
- Capillaries do not contain Roger cells, which can contract.
- Transudation and absorption are carried out due to precapillaries and postcapillaries.
Different blood flow and pressure in the vessels cause fluid transudation in the precapillary and absorption in the postcapillary.
This whole complex process makes it possible for there to be a balance between absorption and transudation without the participation of the system that the lymph forms.
Pregnancy has a particular impact on the blood supply to the entire body and the brain in particular, during which most medications are contraindicated, otherwise the fetus may have pathologies.
Impaired blood supply
A person can independently check the blood supply to the brain - normally, the scalp should move freely in all directions.
Temporary disturbances in blood flow can occur under the influence of various factors. For example, with osteochondrosis, the cervical vertebra presses on the blood vessels, and this is the cause of migraines. Increased blood pressure, tension and anxiety can also slow down blood flow. In such a situation, symptoms are often supplemented by loss of consciousness, vomiting and sensation. Most often, it is the asymmetry of blood flow through the arteries of the spine that provokes a violation of the blood supply.
If the blood supply is insufficient, then a low percentage of nutrients and oxygen in neurons is observed, which leads to brain damage and the development of pathological processes. An electroencephalographic study can reveal such conditions occurring in the brain.
Focal signs of pathological disorders imply the development of the following conditions:
- hemorrhagic stroke;
- cerebral infarction;
- hemorrhages in the intrathecal area.
Such conditions manifest themselves in the form of the following clinical picture:
- epilepsy;
- decreased sensitivity;
- intellectual disabilities;
- problems with coordination of movements.
When the blood supply to the brain is disrupted, a person feels such conditions subjectively, but they are also accompanied by objective neurological symptoms, which include:
- headache;
- paresthesia;
- dizziness;
- problems with the functioning of organs responsible for sensitivity.
Circulatory disorders are divided into three stages:
- Initial.
- Spicy.
- Chronic.
Acute disruption of blood circulation manifests itself in the form of strokes, hemorrhages and other disorders. Chronic conditions include encephalopathy and discirculatory myelopathy.
The clinical picture of circulatory disorders in the brain is as follows:
- headache;
- dizziness;
- red face;
- pain in the eye area;
- a frequently occurring symptom is tinnitus;
- nausea;
- convulsions;
- turning the head in the direction of the lesion worsens the condition;
- confusion.
An interesting fact is that pain tends to increase.
Often these conditions are accompanied by the following symptoms: chills, elevated body temperature and high blood pressure.
Reasons
The following pathologies can affect poor blood circulation in the brain:
- Atherosclerosis, which occurs more often in older people and those who suffer from impaired functionality of the cardiovascular system. During this process, sclerotic plaques accumulate in the arteries, which significantly impede blood circulation.
- Curvature of the spine, as well as a muscle pinched as a result, can also interfere with blood circulation.
- Hypertension.
- Stressful situations can also reduce blood flow.
- The cerebrospinal fluid also has a significant influence on the blood supply.
- Surgery or trauma to the skull.
- Injured spine.
- Improper venous outflow of blood from brain tissue.
Regardless of the reasons that lead to the difficulty of microcirculation, the consequences affect not only the brain, but also the functioning of internal organs.
Elimination of blood circulation disorders in the brain
Circulation can improve during deep breathing, due to which much more oxygen enters the tissues. To achieve a significant effect, you should use simple physical exercises, after consulting with your doctor.
Stable blood supply to the brain and spinal cord can be achieved exclusively through healthy blood vessels.
Thus, to achieve what you want, you need to do something that feeds the brain. For this purpose, those products that help eliminate cholesterol should be used.
More often, in order to normalize the condition, it is necessary to take appropriate medications, but they are prescribed exclusively by a doctor. It should be borne in mind that there is no drug that could cope with the problem alone. Treatment includes a complex of drugs of various types:
- Vasodilators, which act on smooth muscles, relaxing them, due to which the lumen of blood vessels expands, which can increase blood flow (Nimodipine or Cinnarizine).
- Nootropics that exert their effect due to their ability to improve metabolism. They stimulate blood flow and create resistance to existing hypoxia.
- Antithrombic, which are necessary in case of detection of plaques or atherosclerosis. They are able to seal the thin walls of blood vessels and at the same time eliminate plaque.
According to neurology, the use of sedatives is sometimes required.
Based on the diagnostic results, fibrinolytics, anticoagulants and antiplatelet agents may be prescribed.
You can also improve blood supply to the head through Ayurvedic remedies, dietary supplements and homeopathic medicines. At the initial stage, folk remedies, which are tinctures and decoctions of medicinal herbs, as well as massage, also help.
The famous homeopath Valery Sinelnikov writes in his works that pain in the head is a sign that a person is doing something wrong in his life, and in order to get rid of such unpleasant symptoms, one should reconsider his outlook on life, stop being a hypocrite and start treating many situations are easier.
Cerebral circulation is an independent functional system, with its own characteristics of morphological structure and multi-level regulatory mechanisms. In the process of phylogenesis, specific unequal conditions for blood supply to the brain were formed: direct and fast carotid (from the Greek karoo - “put me to sleep”) blood flow and slower vertebral blood flow provided by the vertebral arteries. The volume of circulatory deficit is determined by the degree of development of the collateral network, with the most discriminated subcortical areas and cortical fields of the cerebrum lying at the junction of the blood supply basins.
The arterial system of cerebral blood supply is formed from two main vascular territories: carotid and vertebrobasilar.
The carotid basin is formed by the carotid arteries. The common carotid artery on the right side begins at the level of the sternoclavicular joint from the brachiocephalic trunk, and on the left it departs from the aortic arch. Next, both carotid arteries go up parallel to each other. In most cases, the common carotid artery at the level of the upper edge of the thyroid cartilage (III cervical vertebra) or the hyoid bone expands, forming the carotid sinus (sinus caroticus, carotid sinus), and is divided into the external and internal carotid arteries. The external carotid artery has branches - the facial and superficial temporal arteries, which in the orbital area form an anastomosis with the system of internal carotid arteries, as well as the maxillary and occipital arteries. The internal carotid artery is the largest branch of the common carotid artery. When entering the skull through the carotid canal (canalis caroticus), the internal carotid artery makes a characteristic bend with its convexity upward, and then, passing into the cavernous sinus, it forms an S-shaped bend (siphon) with its convexity forward. The permanent branches of the internal carotid artery are the supraorbital, anterior cerebral and middle cerebral arteries, the posterior communicating and anterior villous arteries. These arteries provide blood supply to the frontal, parietal and temporal lobes and participate in the formation of the arterial circle of the cerebrum (Circle of Willis).
There are anastomoses between them - the anterior communicating artery and cortical anastomoses between the branches of the arteries on the surface of the hemispheres. The anterior communicating artery is an important collector connecting the anterior cerebral arteries, and therefore the internal carotid artery system. The anterior communicating artery is extremely variable - from aplasia (“disconnection of the circle of Willis”) to a plexiform structure. In some cases, there is no special vessel - both anterior cerebral arteries simply merge in a limited area. The anterior and middle cerebral arteries have significantly less variability (less than 30%). More often this is a doubling of the number of arteries, anterior trifurcation (joint formation of both anterior cerebral arteries and the middle cerebral artery from one internal carotid artery), hypo- or aplasia, and sometimes islet division of arterial trunks. The supraorbital artery arises from the medial side of the anterior convexity of the carotid siphon, enters the orbit through the optic nerve canal, and on the medial side of the orbit divides into its terminal branches.
Vertebro-basilar basin. Its bed is formed from two vertebral arteries and the basilar (main) artery (a. basilaris) formed as a result of their fusion, which then divides into two posterior cerebral arteries. The vertebral arteries, being branches of the subclavian arteries, are located behind the scalene and sternocleidomastoid muscles, rising to the transverse process of the VII cervical vertebra, bend around the latter in front and enter the canal of the transverse processes formed by the openings in the transverse processes of the VI–II cervical vertebrae, then go horizontally backwards, bending around the back of the atlas, forming an S-shaped bend with the convexity backwards and entering the foramen magnum of the skull. The fusion of the vertebral arteries into the basilar artery occurs on the ventral surface of the medulla oblongata and the bridge above the clivus (clivus, Blumenbach clivus).
The main bed of the vertebral arteries often branches, forming paired arteries that supply blood to the trunk and cerebellum: posterior spinal artery (lower part of the trunk, nuclei of the thin and cuneate fasciculi (Gaull and Burdach)), anterior spinal artery (dorsal parts of the upper part of the spinal cord, ventral parts of the trunk , pyramids, olives), posterior inferior cerebellar artery (medulla oblongata, vermis and rope bodies of the cerebellum, lower poles of the cerebellar hemispheres). The branches of the basilar artery are the posteromedial central, short circumflex, long circumflex and posterior cerebral arteries. Paired long circumflex branches of the basilar artery: inferior anterior cerebellar artery (pons, upper parts of the medulla oblongata, area of the cerebellopontine angle, cerebellar peduncles), superior cerebellar artery (midbrain, quadrigeminal tubercles, base of the cerebral peduncles, area of the aqueduct), labyrinthine artery (area of the cerebellopontine angle, area of the inner ear).
Deviations from the typical variant of the structure of the arteries of the vertebral-basilar basin are common - in almost 50% of cases. Among them are aplasia or hypoplasia of one or both vertebral arteries, their non-fusion into the basilar artery, low connection of the vertebral arteries, the presence of transverse anastomoses between them, and asymmetry of diameter. Options for the development of the basilar artery: hypoplasia, hyperplasia, duplication, the presence of a longitudinal septum in the cavity of the basilar artery, plexiform basilar artery, insular division, shortening or lengthening of the basilar artery. For the posterior cerebral artery, aplasia, duplication when originating from the basilar artery and from the internal carotid artery, posterior trifurcation of the internal carotid artery, origin from the opposite posterior cerebral artery or internal carotid artery, and insular division are possible.
Deep subcortical formations and periventricular areas are supplied with blood by the anterior and posterior villous plexuses. The first is formed from short branches of the internal carotid artery, the latter - from short arterial trunks, perpendicularly extending from the posterior communicating arteries.
The arteries of the brain are significantly different from other arteries of the body - they are equipped with a powerful elastic membrane, and the muscle layer is developed heterogeneously - at the sites of vessel division, sphincter-like formations are naturally found, which are richly innervated and play an important role in the processes of regulating blood flow. As the diameter of the vessels decreases, the muscle layer gradually disappears, again giving way to elastic elements. The cerebral arteries are surrounded by nerve fibers coming from the superior, intermediate (or stellate) cervical sympathetic ganglia, branches from the C1-C7 nerves, which form plexuses in the medial and adventitial layers of the arterial walls.
The venous system of the brain is formed from the superficial, deep, internal cerebral veins, venous sinuses, emissary and diploic veins.
Venous sinuses are formed by splitting the dura mater, which has an endothelial lining. The most constant are the superior sagittal sinus, located along the upper edge of the falx cerebri; the inferior sagittal sinus, located in the lower edge of the falx cerebri; direct sine – continuation of the previous one; the straight and superior flow into paired transverse sinuses on the inner surface of the occipital bone, which continue into the sigmoid sinuses, ending at the jugular foramen and draining blood into the internal jugular veins. On both sides of the sella turcica there are paired cavernous sinuses, which communicate with each other through the intercavernous sinuses, and with the sigmoid sinuses through the petrosal sinuses.
The sinuses receive blood from the cerebral veins. The superficial superior veins from the frontal, parietal, and occipital lobes bring blood to the superior sagittal sinus. The superficial middle cerebral veins flow into the superior petrosal and cavernous sinuses, which lie in the lateral sulci of the hemispheres and carry blood from the parietal, occipital and temporal lobes. Blood enters the transverse sinus from the inferior cerebral veins. The deep cerebral veins collect blood from the choroid plexuses of the lateral and third ventricles of the brain, from the subcortical regions, the corpus callosum and flow into the internal cerebral veins behind the pineal gland, and then merge into the unpaired large cerebral vein. The straight sinus receives blood from the great cerebral vein.
The cavernous sinus receives blood from the superior and inferior ophthalmic veins, which anastomose in the periorbital space with tributaries of the facial vein and the pterygoid venous plexus. The labyrinthine veins carry blood to the inferior petrosal sinus.
Emissary veins (parietal, mastoid, condylar) and diploic veins have valves and are involved in providing transcranial blood outflow with increased intracranial pressure.
Syndromes of damage to the arteries and veins of the brain. Damage to individual arteries and veins does not always lead to pronounced neurological manifestations. It has been noted that for the occurrence of hemodynamic disorders, a narrowing of the large arterial trunk by more than 50% or multiple narrowing of the arteries within one or several basins is necessary. However, thrombosis or occlusion of some arteries and veins have distinct specific symptoms.
Disruption of blood flow in the anterior cerebral artery causes motor disorders of the central type contralaterally on the face and limbs (most pronounced in the leg and shallow in the arm), motor aphasia (with damage to the left anterior cerebral artery in right-handed people), gait disturbance, grasping phenomena, elements of “ frontal behavior."
Disruption of blood flow in the middle cerebral artery causes contralateral central paralysis, predominantly of the “brachiofacial” type, when motor disturbances are more severely expressed in the face and hand, and sensory disorders develop - contralateral hemihypesthesia. In right-handed people, when the left middle cerebral artery is damaged, mixed aphasia, apraxia, and agnosia occur.
When the trunk of the internal carotid artery is damaged, the above disorders manifest themselves more clearly and are combined with contralateral hemianopia, disturbances of memory, attention, emotions, and motor disorders, in addition to the pyramidal nature, can acquire extrapyramidal features.
Pathology in the posterior cerebral artery basin is associated with loss of visual fields (partial or complete hemianopia) and, to a lesser extent, with disorders of the motor and sensory spheres.
The most total disturbances are caused by occlusion of the lumen of the basilar artery, manifested by Filimonov's syndrome - the “locked man”. In this case, only the movements of the eyeballs are preserved.
Thrombosis and occlusion of the branches of the basilar and vertebral arteries are manifested, as a rule, by alternating stem syndromes of Wallenberg - Zakharchenko or Babinsky - Nageotte with damage to the posterior inferior cerebellar artery; Dezherina - for thrombosis of the medial branches of the basilar artery; Millard - Gubler, Brissot - Sicard, Fauville - long and short circumflex branches of the basilar artery; Jackson - anterior spinal artery; Benedict, Weber - posterior cerebral artery, posterior villous artery and interpeduncular branches of the basilar artery.
Manifestations of thrombosis of the cerebral venous system, with rare exceptions, do not have a clear topical relationship. If the venous outflow is blocked, then the capillaries and venules of the affected drainage zone swell, which leads to the occurrence of congestive hemorrhages, and then large hematomas in the white or gray matter. Clinical manifestations include general cerebral symptoms, focal or generalized seizures, papilledema, and focal symptoms indicating damage to the cerebral hemispheres, cerebellum, or compression of the cranial nerves and brain stem. Thrombosis of the cavernous sinus can manifest itself as damage to the oculomotor, abducens and trochlear nerves (outer wall of the cavernous sinus syndrome, Foix syndrome). The appearance of a carotid-cavernous anastomosis is accompanied by pulsating exophthalmos. Lesions of other sinuses are less obvious.
The vascular system provides blood saturated with nutrients and oxygen, the main condition for its normal functioning. No other cells stop functioning as quickly as nerve cells when there is a sharp decrease or cessation of blood supply. Even a short-term disruption of blood flow to the brain can lead to fainting. The reason for this sensitivity is the great need of nerve cells for oxygen and nutrients, mainly glucose.
The total cerebral blood flow in humans is about 50 ml of blood per minute per 100 g of brain tissue and is unchanged. In children, blood flow values are 50% higher than in adults; in old people, they are 20% lower. Under normal conditions, unchanged blood flow through the brain as a whole is observed when mean arterial pressure fluctuates from 80 to 160 mm Hg. Art. Very sharp changes in the tension of oxygen and carbon dioxide in arterial blood affect the total cerebral blood flow. The constancy of total cerebral blood flow is maintained by a complex regulatory mechanism.
The blood supply to various parts of the brain depends on the degree of their activity.
With increased activity of the cerebral cortex (for example, when reading, problem solving)
blood flow in certain zones increases by 20-60% due to expansion
cerebral vessels. With general excitement it increases 1.5-2 times,
and in a state of rage - 3 times. Under anesthesia or hypothermia
cortical blood flow is significantly reduced.
Blood supply system of the brain
Blood enters the brain through 4 large vessels: 2 internal carotid and 2 vertebral arteries. Blood flows from it through 2 internal jugular veins.
Internal carotid arteries
The internal carotid arteries are branches of the common carotid arteries, the left one departs from the aortic arch. The left and right common carotid arteries are located in the lateral areas of the neck. The pulse vibrations of their walls can be easily felt through the skin by placing your fingers on the neck. Severe compression of the carotid arteries disrupts the blood supply to the brain. At the level of the upper edge of the larynx, the common carotid artery divides into the external and internal carotid arteries. The internal carotid artery penetrates the cranial cavity, where it takes part in the blood supply to the brain and eyeball, the external carotid artery nourishes the organs of the neck, face, and scalp.
Vertebral arteries
The vertebral arteries arise from the subclavian arteries, are directed to the head through a chain of openings in the transverse processes of the cervical vertebrae, and enter the cranial cavity through the foramen magnum.
Since the vessels supplying the brain extend from the branches of the aortic arch, the speed and pressure of the blood in them are high and have pulse fluctuations. To smooth them, at the entrance to the skull, the internal carotid and vertebral arteries form double bends (siphons). Having entered the cranial cavity, the arteries connect with each other, forming on the lower surface of the brain the so-called circle of Willis, or arterial circle of the cerebrum. It allows, if there is difficulty in delivering blood through any vessel, to redistribute it from other sources and prevent disruption of the blood supply to a region of the brain. However, under normal conditions, blood brought through different arteries does not mix in the vessels of the circle of Willis.
Cerebral arteries
The anterior and middle cerebral arteries depart from the internal carotid artery, supplying the inner and outer surfaces of the cerebral hemispheres (frontal, parietal and temporal lobes) and the deep parts of the brain. The posterior cerebral arteries, which supply the occipital lobes of the hemispheres, and the arteries that supply blood to the brain stem and cerebellum, are branches of the vertebral arteries. The vessels supplying the spinal cord also depart from the vertebral arteries. Numerous thin arteries arise from the large cerebral arteries and plunge into the brain tissue. The diameter of these arteries varies widely; according to their length, they are divided into short ones - feeding the cerebral cortex, and long ones - feeding the white matter. The highest percentage of hemorrhages in the brain is observed with pathological changes in the walls of these particular arteries.
The branches of small arteries form a capillary network, unevenly distributed in the brain - the density of capillaries in the gray matter is 2-3 times higher than in the white matter. On average, there are 15´107 capillaries per 100 g of brain tissue, and their total cross-section is 20 square meters. cm.
The capillary wall does not come into contact with the surface of the nerve cells, and the transfer of oxygen and other substances from the blood to the nerve cell is carried out through the mediation of special cells - astrocytes.
Blood-brain barrier
The regulation of the transport of substances from the blood capillary to the nervous tissue is called the blood-brain barrier. Normally, iodine compounds, salicylic acid salts, antibiotics, and immune bodies do not pass from the blood to the brain (are retained by a barrier). This means that drugs containing these substances, when introduced into the blood, do not affect the nervous system. Conversely, alcohol, chloroform, strychnine, morphine, tetanus toxin, etc. easily pass through the blood-brain barrier. This explains the rapid effect of these substances on the nervous system.
To avoid the blood-brain barrier, antibiotics and other chemicals used to treat brain infections are injected directly into the fluid surrounding the brain, the cerebrospinal fluid (CSF). This is done through a puncture in the lumbar spine or in the suboccipital region.
Internal jugular veins
The outflow of blood from the brain occurs through veins that flow into the sinuses of the dura mater. They are slit-like channels in the dense connective tissue membrane of the brain, the lumen of which remains open under any conditions. Such a device ensures uninterrupted outflow of blood from the brain, which prevents stagnation. The sinuses leave a mark in the form of wide grooves on the inner surface of the skull. Through the sinus system, venous blood from the brain moves to the jugular foramen at the base of the skull, from where the internal jugular vein originates. Through the right and left internal jugular veins, blood from the brain flows into the superior vena cava system.
The sinuses of the dura mater communicate with the superficial (subcutaneous) veins of the head through special veins passing through the bones of the skull. This allows, under certain conditions, to “dump” part of the venous blood from the cranial cavity not into the internal jugular vein, but through the subcutaneous vessels into the external jugular vein.
The evolution of the brain brought man to the top of the pyramid
wildlife. The brain belongs to the central nervous system
and performs the functions of regulation and coordination of activity in the body
of all organs, communicates them with the environment
and adapts the body to the changes occurring.
Cerebrovascular disorders
Temporary disorders of cerebral circulation occur for various reasons. Due to osteochondrosis, the openings in the cervical vertebrae narrow, the vessels passing through them are compressed, and the blood supply to the brain becomes difficult - headaches, migraines, etc. appear. With increased blood pressure, severe anxiety or tension, headaches, dizziness, a feeling of heaviness in the head also appear, sometimes vomiting and short-term loss of consciousness.
