Blood supply to the brain
The blood supply to the brain is the part of the bloodstream that supplies the brain with oxygen , glucose and other nutrients and removes metabolic products and carbon dioxide . It is subject to some anatomical and physiological peculiarities. The reason for this is that the brain as an organ has a very high basal metabolism - the human brain already uses a fifth of the body's total oxygen requirement at rest. In addition, unlike other body cells , nerve cells are not able to meet their energy requirements to a sufficient extent without oxygen, i.e. anaerobically . There are therefore several safety systems to ensure the continuous supply of oxygen and substrate.
Four large arteries supply the brains of humans and most mammals with oxygen-rich blood (old name: "arterial blood"). There are two on each side of the neck , the internal carotid arteries (Arteriae carotides internae) in front and the vertebral arteries (Arteriae vertebrales) behind . After passing through the brain, the blood flows off via special venous cerebral blood conductors (sinus durae matris) , which have some peculiarities compared to the veins .
The blood volume per 100 ml of brain matter is just under 4 ml at rest. The normal blood flow in human brain tissue is between 40 and 50 ml of blood per 100 g of tissue per minute. It is significantly higher in the gray matter (90 ml / 100 g / min) than in the white matter (25 ml / 100 g / min). A drop in the blood flow rate by half can easily be compensated for (among other things through higher oxygen exhaustion). However, a drop below 20 ml / 100 g / min initially leads to reversible failure phenomena. When the blood flow rate drops to less than 15 ml / 100 g / min, the cells gradually die within a few minutes to a few hours . Less than 10 ml / 100 g / min are not tolerated by the nerve cells - the final cell death occurs within eight to ten minutes.
The anatomical situation in humans is described below - if not explicitly stated otherwise.
It is common to distinguish between an anterior and a posterior circuit in the tributaries, even if there are connections between the two, so-called anastomoses .
The main contribution to the arterial inflow is made by the right and left internal carotid arteries ( arteria carotis interna dextra et sinistra ), which arise from the common carotid artery ( arteria carotis communis dextra et sinistra ) on each side of the neck. The carotid artery, in turn, is one of the major outlets from the aortic arch . Your pulse can easily be felt in front of the head turner muscle ( Musculus sternocleidomastoideus ).
After entering the skull through the carotid canal , a vessel branches off from the internal carotid artery on each side to the eye ( ophthalmic artery ). After releasing further smaller branches, it divides into the main trunks of the anterior circulation, the middle cerebral artery ( Arteria cerebri media ) and the anterior cerebral artery ( Arteria cerebri anterior , in animals called Arteria cerebri rostralis ). The former supplies the side ( lateral ), the latter facing towards the middle ( medial ) part of each cerebral hemisphere with the exception of parts of the temporal lobe and the entire occipital lobe , which are fed from the rear circuit. The deep core areas ( basal ganglia , thalamus ) have a mixed supply. The two anterior cerebral arteries are connected by the very short anterior communicating artery .
The situation is different with ruminants : Here, the section of the internal carotid artery that lies outside the cranial cavity closes after birth and only the part that lies inside the skull remains open. This then gets its blood supply secondarily from the maxillary artery ( maxillary artery ). A fine, widely branched network of smaller vessels forms in the area of the mouth of these supplying branches, which anatomists call a miracle network ( rete mirabile ). Even in adult cats, the part of the internal carotid artery that is outside the cranial cavity closes . Here the maxillary artery itself forms a miracle network ( rete mirabile arteriae maxillaris ) from which several branches ( rami retis ) run through the orbital fissure into the cranial cavity and take over the blood supply to the anterior circulation.
The right and left vertebral arteries ( arteria vertebralis dextra et sinistra ), which arise from the clavicle arteries ( arteria subclavia ) and run along the cervical spine , have a smaller diameter than the carotid arteries. They pass through openings in the transverse processes of the upper six cervical vertebrae . The two vertebral arteries enter the cranial cavity through the foramen magnum and unite at the level of the caudal bridge to form the unpaired basilar artery .
The vertebral arteries in their terminal segments and the basilar artery send branches to the brain stem and cerebellum ( A. cerebelli inferior posterior , A. cerebelli inferior anterior , A. cerebelli superior ). Above the bridge, the arteria basilaris divides again and becomes the two posterior cerebral arteries , resulting in the arteries occipitales medial or the lateral parts and the rear regions of the cerebrum , as well as portions of diencephalon supply. A posterior communicating artery , which varies in strength, connects the posterior cerebral artery on each side with the internal carotid artery.
Supply areas of the cerebral cortex :
About a third of the normal population shows deviations in the individual course of the described vessels from this “textbook case”: One or more Aa are very common . communicantes hypoplastic . The trunk of the A. cerebri anterior can also be hypoplastic, in which case the vessel on the opposite side takes over the supply via the A. communicans anterior . The embryonic supply type is the unilateral or bilateral exit of the posterior cerebral artery from the carotid area, the posterior communicating artery then forming its first stretch and the vascular bed of the cerebrum in the latter case being completely supplied by the anterior circulation. Many people have a unilateral vertebral artery that is weakly or not at all.
These variants are per se without disease value and are fully compensated for in healthy people, but in individual cases they can be a risk factor for a stroke. Additional vessels, such as an accessory cerebral artery or remnants of embryonic anastomoses, such as the trigeminal artery, are also rare .
Connections between the anterior and posterior circulation
The anterior communicating artery , the first section of the anterior cerebral artery , a short section of the internal carotid artery , the posterior communicating artery and the first section of the posterior cerebral artery , when viewed both sides together, form a ring-shaped connection under the base of the brain ( circulus arteriosus cerebri Willisi ). This vascular ring represents an anastomotic system that anatomically, but not always functionally, connects the flow areas of the arteriae carotides internae and the arteria basilaris sufficiently. Basically (that is, with sufficient adaptation time), however, it can enable a single main vessel to maintain the entire blood flow to the brain.
The capillaries of the brain form the fact that the endothelial cells with tight junctions are fixedly connected together, an impermeable for larger molecules (impermeable) barrier, the blood-brain barrier . To a lesser extent, the basement membrane and the uninterrupted population of the capillaries with astrocyte protrusions also contribute to this. The blood-brain barrier protects the brain from potentially harmful substances circulating in the blood.
The capillary density is different in the individual regions of the brain and usually corresponds fairly precisely to the mean metabolic activity of the respective area. In contrast to the rest of the body, the hair vessels of the brain are always completely flooded; there is no reserve capacity.
The brain has small venules and veins like other organs, but they run independently of the arteries. They are divided into a deep ( Venae profundae cerebri ) and a superficial ( Venae superficiales cerebri ) group. The largest cerebral vein is the magna cerebri ( Galen ) vein, which is only about 1 cm long, under the splenium of the bar . The oxygen-poor blood is collected in anatomically specially constructed cerebral blood conductors , the sinus durae matris : These are duplicates of the hard meninges , which are lined on the inside with endothelium . The sinuses form an interconnected system and finally flow into the internal jugular veins .
The first embryonic development is the paired dorsal aorta and the also paired ventral aorta , which are connected to each other by six branchial arch arteries . In the further development, individual vascular sections are receded, while others become significantly stronger. The left fourth branchial arch artery forms the aortic arch , the ventral aorta between the third and fourth branchial arch arteries forms the common carotid artery, and the external carotid artery and its branches in the rostral area . The third branchial arch artery becomes the first section of the internal carotid artery , which continues into the dorsal aorta. The section of the aorta dorsalis between the third and fourth branchial arch arteries, on the other hand, is receded (see figure opposite). The primitive internal carotid artery divides into a cranial and a caudal branch at the base of the future brain. The former initially forms the anterior cerebral artery and the anterior choroidal artery around the 4th embryonic week; the later much stronger media cerebral artery does not develop from one of several side branches until around the 9th week . The caudal branch sends segmental arteries to the neural tube from which, among other things, the cerebellar arteries derive. Around the 7th week of the embryo, this branch also gives rise to the posterior cerebral artery . Around the same time, the two sections of the caudal branch merge in front of the brain stem and form the unpaired basilar artery . The vertebral arteries are formed from the fusion of smaller longitudinal anastomoses (lying in the longitudinal direction of the body) between the primitive segmental arteries of the neck region. They establish a connection to the early basilar artery . Around the 9th week, the direction of flow in the basilar artery is reversed up to the level of the posterior cerebral artery , so that it now hemodynamically belongs to the posterior vascular system with all its daughter vessels. Finally, the anterior communicating artery is formed by partial fusion of the anterior cerebral arteries .
One of the "safety systems" to protect against too little, but also too high perfusion, is the autoregulation of cerebral blood flow. The resistance vessels keep the effective blood pressure in the brain (the so-called perfusion pressure, which results from the difference between the systemic blood pressure and the intracranial pressure ) almost constant through various complex interrelated control mechanisms, while the systemic blood pressure can fluctuate between 50 and 170 mmHg. These include the Bayliss effect , the regulation by the sympathetic and parasympathetic innervation of the larger vessels and directly to the myocytes of the smooth muscle acting endocrine and chemical factors ( pH , adenosine , potassium and others). The limits of this adaptation shift upwards with persistent high blood pressure ; Long-term, poorly controlled diabetes mellitus can affect the overall autoregulatory ability.
Brain areas with increased neuronal activity are supplied with more blood. The mechanisms of this phenomenon, known as reactive hyperemia or neurovascular coupling , include the reaction of the resistance vessels to the local carbon dioxide partial pressure , other vasoactive factors and the neurogenic control of the vasotonus, but are not fully understood in detail.
Measurement and display methods
The vessels supplying the brain can be visualized with imaging methods , in particular with angiography . A radiopaque contrast medium is applied for digital subtraction angiography ; during the screening with X-rays , the skeleton portions are eliminated. This means that only the vessels through which the contrast medium flows are shown.
A newer method is the three-dimensional reconstruction of magnetic resonance tomography images after administration of contrast medium ( MR angiography ). This is increasingly displacing invasive angiography. There are also qualitatively inferior MR sequences for vascular imaging without contrast media (time-of-flight magnetic resonance angiography). Vascular imaging is also possible with computed tomography after administration of contrast agent. Circumscribed changes in the microcirculation can be visualized with positron emission tomography , SPECT and with a special (oxygen saturation weighted ) MR signal ( BOLD contrast ). Optical methods rely on measuring changes in the concentration of hemoglobin . They can only be used to measure changes in blood flow close to the surface.
The extracranial Doppler and duplex sonography allows the assessment of vessel cross-sections, wall changes and flow properties in the large extracranial (outside of the skull) vessels. Using transcranial Doppler and duplex sonography , it is possible to measure flow velocities and profiles of selected intracranial vessels through the skullcap or the foramen magnum in adults on the temporal "bone window" as well as transorbitally (through the eye socket) and transnuchal (over the neck) . This is much easier in infancy and the blood flow parameters can be examined through the fontanel and into the arteriae cerebri anteriores without any problems.
A sudden closure of one of the vessels described above usually leads to a stroke, the rapid death of brain tissue in the relevant area. The respective failures (neurological deficits) can turn out very differently, from discrete, almost completely unnoticed failures to unconsciousness and death. Depending on the duration of the interruption in the blood supply and the reversibility of the symptoms, the transient ischemic attack (TIA) is differentiated from a complete infarct. In the case of occlusions in the anterior circulation, hemiplegia , aphasia (speech disorders) and sensitivity disorders dominate , in the posterior circulation, however, visual field deficits , dizziness , ataxia (coordination disorder) and impaired consciousness. Ischemic infarcts are usually caused either by arteriosclerotic constrictions of the large supplying vessels with subsequent plaque rupture and thrombosis, or by the inundation of blood clots ( embolism ), which can occur especially with atrial fibrillation .
Another problem arises when blood vessels rupture and bleeding occurs. Here, too, a wide range of symptoms is possible , depending on the location and extent of the bleeding . Extremely high blood pressure can also lead to bleeding into the brain tissue, especially if the blood vessels are damaged.
Bleeding caused by trauma usually affects the subdural or epidural space . Many people carry small aneurysms on the vessels of the base of the brain without even realizing them. The sudden rupture leads to the highly acute picture of subarachnoid hemorrhage .
The outflow of blood can also be disturbed. The main symptoms of these more chronic diseases are headaches , lack of drive , seizures and visual disturbances . This group of disorders includes sinus thrombosis , cerebral vein thrombosis and, according to some authors, also the pseudotumor cerebri .
If the entire blood supply fails (for example in the event of cardiac arrest ), a general oxygen deficiency, known as global hypoxia, occurs in the brain. So after about ten seconds it comes to unconsciousness . Brain tissue begins to die off after just two to three minutes of failure, and brain death occurs after around ten minutes . If the metabolic processes are greatly slowed down (hypothermia, certain types of poisoning), the brain can possibly survive significantly longer ischemia times.
Malformations of cerebral vessels are mostly congenital. They occur in different places and sometimes reach extreme proportions. Accordingly, very different symptoms are also possible. In addition to arteriovenous shunts , cavernomas , hemangiomas and fistulas with the sinus system are known. Vascular malformations often occur in phacomatoses .
The first written assumptions about the blood supply to the brain with a description of the main vessels go back to the Greek doctor and anatomist Galenus of Pergamon (1st century AD). However, he mainly drew his findings from the dissection of animals and often transferred the anatomical relationships to humans without being examined. This is how he wrongly described a rete mirabile in humans. In late antiquity and the Middle Ages, Galen was considered an incontestable authority, so most of his errors could only be corrected in the early modern period, when dissection of human corpses was carried out in universities. While Niccolò Massa - probably out of respect for Galen - claimed to have also observed the miracle net in humans, his contemporaries Jacopo Berengario da Carpi and Andreas Vesalius contradicted this . However, the basic anatomical knowledge is due to two English doctors. William Harvey recognized in 1628 the true nature of the blood flow as a circuit. The first detailed and accurate description of the vessels of the human brain and the circulus arteriosus was provided by Thomas Willis a little later.
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