What are the main blood vessels supplying the human brain? What are the collateral circulation pathways of cerebral blood flow? What are the stages of reserve function of the brain?
I. Brain tissue and blood supply vessels
1.Brain tissue
Brain tissue weighs only about 1400 grams, accounting for only two percent of the body weight, but the blood flow is about 700 to 1000 milliliters each time, accounting for 20 percent of the whole heart transfusion, and its oxygen consumption accounts for 25 percent of the whole body oxygen consumption, so the brain requires a very rich blood vessel, and there is a feature that brain tissue does not have energy reserves, completely rely on this brain The brain requires very rich blood vessels, and there is also a characteristic that the brain tissue has no energy reserves, and it completely relies on the input of cerebral blood to supply its energy.
2.Blood supply vessels
(1) Internal carotid artery system
There is a thick artery on the left and right side of the neck, called the common carotid artery. After entering the skull, it divides into five major branches, namely the ophthalmic artery, the posterior communicating artery, the anterior cerebral artery and the anterior choroidal artery, and the middle cerebral artery, which supply blood to the eye and the first 3/5 of the cerebral hemisphere, including the frontal lobe, temporal lobe, parietal lobe, and basal ganglia area.
(2) Vertebrobasilar artery system
(1) The vertebral artery emanates from the subclavian artery, one on each side, and passes through the five transverse foramina on both sides of the cervical vertebrae. After rising into the skull through the foramen magnum, the two vertebral arteries merge together at the lower edge of the cerebral bridge to form a thick basilar artery, which is commonly referred to as the vertebrobasilar artery system.
②The basilar artery to the midbrain is further divided into two posterior cerebral arteries, which supply blood to the posterior 2/5 of the brain, including the occipital lobe, the base of the temporal lobe, and part of the thalamus. The vertebrobasilar artery branches in the pontocerebellum and cerebellum, supplying blood to the pontocerebellum and cerebellum.
Second, the collateral circulation of cerebral blood flow compensates
1.The composition of the collateral circulation
(1) Willis loop: The collateral blood flow between the two sides of the internal carotid artery system is provided by the open anterior communicating artery, and the collateral blood flow between the internal carotid artery system and the vertebrobasilar artery system is provided by the posterior communicating artery. This compensatory capacity is often not evident or does not manifest under normal conditions.
(2) Ophthalmic artery: It is an important channel for collateral circulation between the internal and external carotid arteries. It is a secondary collateral pathway. In addition, the carotid artery can also anastomose with the facial, maxillary, and superficial temporal arteries through the ophthalmic artery.
(3) Soft meningeal anastomotic branch: It is a secondary collateral pathway and is a rich collateral anastomosis between the endings of cortical branches of the anterior, middle and posterior cerebral arteries.
(4) Anterior choroidal artery: it is a rare collateral pathway, when the internal carotid artery or anterior choroidal artery is obstructed proximally, its collateral blood flow originates from the reverse flow of the posterior circulation, mainly supplying the middle cerebral artery region.
(5) Branches of the external carotid artery and dural anastomosing branches: In severe occlusion of the internal carotid artery, the fundic arterial ring is often completely occluded, and the only source of collateral blood flow may be branches of the external carotid artery.
(6) Connections between the intracranial and extracranial arteries.
(i) Internal carotid artery-ocular artery-external carotid artery.
(ii) Internal carotid artery – ophthalmic artery – facial, maxillary and superficial temporal arteries.
(iii) internal carotid artery-pterygoid artery-external carotid artery.
(iv) vertebral artery-occipital artery-external carotid artery.
2.Manifestations of cerebral ischemia
(1) Foci of ischemic infarction (the most common).
(2) vascular stenosis and impaired collateral circulation.
(3) cerebral blood flow hypoperfusion state.
The three forms can be a single manifestation or can occur simultaneously.
3, factors affecting local cerebral blood flow
(1) whether the blood supply artery is stenosed or not
(2) Whether the collateral circulation is open or not.
(3) Cerebral blood flow velocity.
4.Factors affecting the function of brain cells
(1) whether the local cerebral blood flow is decreased
(2) the degree of tolerance of brain cells to ischemia
(3) The content of oxygen concentration in the blood.
5.Brain artery perfusion physiology
(1) The normal blood flow in brain tissue.
(2) Local cerebral blood flow is determined by local cerebral arterial perfusion pressure and local cerebrovascular circulatory resistance.
(3) When the local cerebral arterial pressure decreases, the cerebral vessels will compensatory dilate, the collateral circulation opens, and the vascular circulatory resistance decreases to facilitate the maintenance of normal cerebral blood flow.
Third, the reserve function of the brain
It is determined by several factors, then it will go through several stages: for example, cerebral blood flow reserve period, functional reserve period, structural reserve period, and another chemical reserve period, of course, the adjustment of each reserve period is different, for example, cerebral blood flow reserve period it should be vasodilatation, vasodilatation as the main mode of regulation; then the functional reserve period, it is through a variety of automatic regulation of the If it is further aggravated to the structural reserve period, it will have the problem of collateral circulation, which is to increase the reserve function of the brain through the collateral circulation; if it is further aggravated, it will reach the chemical reserve period, and the ischemic state will appear.
Fourth, the brain’s compensation for ischemia
The compensation of the brain to ischemia is also determined by several phases. The first one is the lateral circulation period, the cerebral blood flow reserve period, the cerebral metabolic reserve period, and finally the infarction period. At this time, the cerebral arterial pressure, cerebral blood flow and cerebral blood volume are all at a normal level and in a normal state, and the special condition is open to regulate the brain’s compensation for ischemia; but when the cerebral blood flow reserve period is reached, that is, if I open the circulation, then I cannot slow down the cerebral ischemia, then at this time, I will increase the blood vessels. If I reach the cerebral metabolic reserve period, then it is not compensated by increasing the cerebral blood volume, then I have to solve the ischemia through some metabolic pathways, and if it cannot be solved again, finally it will reach the infarction period, that is, the ischemia cannot be compensated, then the brain tissue will be necrosis.