Blood supply to the brain: anatomy, phases and course

Our brain is a complex machine that needs to be irrigated to function, with a red fuel that we all know: blood. And for that, it uses a set of strategically placed arteries that will be responsible for keeping the brain and other organs well nourished.

In this article we will take a look at how blood flow to the brain occurs. and what are the main arterial systems that make this possible.

    How does blood circulate in the brain?

    The human brain is a complex organ and is responsible for many bodily functions that we need to survive. Every machine needs fuel to run, and our brains were not going to be different. Although it makes up just over 2% of body weight, it consumes one-sixth of cardiac output and 20% of the oxygen our body needs at rest.

    The brain is constantly active (even when we are sleeping), which is why vascularization or blood supply is such an important process for an organ like this. The whole brain is supplied by four large arteries, which emerge from the aortic artery and go up through the neck. until it enters the skull.

    Blood circulation to the brain is made by two symmetrical systems, on either side of the neck: on the anterior side, the common carotid arteries; and on the back, the vertebral arteries, which make up a large part of their way into the cervical vertebrae.

    The common carotid arteries are divided into two branches, the external carotid, which mainly nourishes the extracranial structures (tongue, pharynx, face, cervical muscles, etc.) and the internal carotid, which enters the skull and supplies most of the blood with blood. of the anterior part. of the brain, which we call the brain.

    The vertebral arteries enter the skull and join together to form a common trunk called the basilar artery., Which is responsible for the irrigation of the cerebellum and brainstem. The internal carotid arteries and the basilar artery in turn divide into smaller and smaller branches and are distributed over the entire surface of the brain.

    The carotid system

    The carotid system is responsible for the anterior flow of blood to the brain and contributes almost 80% of the blood flow received by the brain.

    It is made up of the common carotids (right and left) which, once they enter the skull, form the internal carotids, branching into the anterior and middle cerebral arteries. These reach the cerebral cortex, reaching the frontal, parietal and temporal lobes.

    1. Anterior cerebral artery

    The anterior cerebral artery begins in the internal carotid artery. The arteries on either side are connected by the anterior communicating artery and both are part of the cerebral arterial circle or polygon of Willis, which we will discuss later.

    This artery has cortical (orbital), frontal (parietal) and central (including the internal striatal artery) branches, as well as the anterior communicating artery of the brain, and can be classified into 5 segments. It irrigates a large part of the internal face of the cerebral hemisphere.

    2. Middle cerebral artery

    The middle cerebral artery or Silvic artery (so named because when it breaks away from the internal carotid artery is inserted into the Silvio cleft) is a branch of the internal carotid artery, and is responsible for the irrigation of the blood through two segments: a basal part and a cephalic part.

    This artery has the most complex branching pattern of all the cerebral arteries, And has cortical (orbital, frontal, parietal and temporal) and central (grooved) branches. It has two main branches: a basilar, which is responsible for the irrigation of part of the caudate nucleus and the thalamus; and the cortical part, which supplies much of the cerebral cortex.

    When the middle cerebral artery is blocked, hemiparesis (paralysis on one side of the body) occurs, affecting parts of the body such as the face, arm, or leg, and causing sensory and visual alterations.

    The vertebrobasilar system

    The vertebrobasilar system is responsible for the subsequent flow of blood to the brain. It consists of the vertebral arteries (right and left), which integrate into the basilar artery, once they have entered the brain, and branch out into the posterior cerebral arteries which supply the occipital lobes (located in the area posterior) of the brain).

    This system facilitates the irrigation of part of the spinal cord, brainstem, cerebellum and much of the occipital and temporal lobes of the brain. When blood flow is reduced or completely stopped by a stroke or sprain, the consequences for the individual are often dire, given the importance of the brain structures involved in subsequent irrigation.

    1. Posterior cerebral artery

    The posterior cerebral arteries originate from the interpeduncular cistern and originate from the bifurcation of the basilar artery. Among its main branches are the posterior communicating artery, the posterior medial and lateral choroidal arteries, as well as the calcareous artery.

    On reaching the cortex, this artery divides into parietoccipital and calcareous arteries. The parietooccipital artery continues through the parietooccipital groove and is responsible for the supply of blood to the medial part of the parietal and occipital lobes; and, in turn, the calcareous artery runs its course through the calcareous cleft, performing irrigation functions from the visual cortex into the occipital lobe.

    2. The Willis polygon

    The cerebral arterial circle or polygon of Willis (named in honor of the English physician Thomas Willis, a pioneer in the study of comparative anatomy), is a heptagon-shaped structure located at the bottom of the brain, surrounding the stem of pituitary, optic chiasm and hypothalamus.

    This blood circulation system is formed by the union of the two systems that we have seen above: the carotid system and the vertebrobasilar system. This is called an anastomosis system, that is to say a network of interconnections of arteries in the form of an arterial polygon responsible for the supply of blood to the brain and the areas adjacent.

    Like other systems, the Willis polygon can be divided into its anterior and posterior part. The former is formed by the internal carotid artery and supplies blood to the anterior part of the brain, supplying most of the cerebral hemispheres, as well as some deep areas such as the caudate nucleus and putamen. The posterior area of ​​the arterial polygon consists of the vertebral arteries and is mainly responsible for the supply of blood to the cerebellum, trunk and the posterior area of ​​the cerebral hemispheres.

    The main function of the Willis polygon is to allow an alternate route in the event of occlusion or stoppage of the blood supply on its usual route. In addition, it also allows the equalization of blood flow between the two sides of the brain (right and left hemisphere).

    In short, this network of arteries facilitates the correct distribution of blood flow in our brain, Especially in the event that we suffer from any type of injury or stroke involving a decrease or paralysis of irrigation and cerebral vascularity.

    Bibliographical references:

    • Hendrikse J, van Raamt AF, van der Graaf Y, et al. Distribution of cerebral blood flow in the circle of Willis. Radiology 2005; 235: 184–89
    • Kandel, ER; Schwartz, JH and Jessell, TM (2001). Principles of neuroscience. Fourth edition. McGraw-Hill Inter-American. Madrid
    • Scheel P, Ruge C, Petruch UR, Schoning M. Double-sided measurement of cerebral blood flow volume in healthy adults. Stroke. 2000; 31: 147-150

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