The blood supply to the brain: anatomy, phases and pathway

Our brain needs large amounts of Blood to function.

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

In this article we will see how the blood circulation in the brain occurs and which are the main artery systems that make it possible.

• Recommended article: "The lobes of the brain and their different functions". .

How does blood circulate in the brain?

The human brain is a complex organ and is responsible for a multitude of bodily functions that we need to survive. All machinery needs fuel to function, and our brain is no different. Although it represents little more than 2% of body weight, it consumes one-sixth of the cardiac output and 20% of the oxygen our body needs at rest.

The brain is constantly active (even when we sleep), which is why vascularization or blood supply is such an important process for such an organ. The entire brain is irrigated by four large arteries, which emerge from the aorta artery and ascend through the neck to penetrate the skull. penetrate the skull.

Blood circulation in the brain occurs through two symmetrical systems, on both sides of the neck: on the anterior side, the common carotid arteries; and on the posterior side, the vertebral arteries, which make a large part of their route inside the cervical vertebrae.

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

The vertebral arteries penetrate the skull and join together to form a common trunk known as the basilar artery, which is responsible for supplying blood to most of the anterior part of the brain, which we call the brain.which is in charge of irrigating the cerebellum and the brain stem. The internal carotid arteries and the basilar artery divide, in turn, into smaller and smaller branches, and are distributed throughout the cerebral surface.

The carotid system

The carotid system is responsible for the anterior circulation of blood in the brain and provides almost 80% of the blood flow in the brain. and provides almost 80% of the blood flow to the brain.

It is formed by the common carotids (right and left) which, once they have penetrated 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 originates from the internal carotid artery.. The arteries on each side are connected by the anterior communicating artery and both are part of the cerebral arterial circle or polygon of Willis, which will be discussed later.

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

2. Middle cerebral artery

The middle cerebral artery or sylvian artery (so called because as soon as it is detached from the internal carotid artery it enters the fissure of Sylvius) is a branch of the internal carotid artery, and is responsible for supplying blood through two segments: a basal and a cephalic portion.

This artery has the most complex branching pattern of all cerebral arteries.It has cortical (orbital, frontal, parietal and temporal) and central (striated) branches. It has two main branches: a basilar branch, which irrigates part of the caudate nucleus and the thalamus; and the cortical portion, which irrigates a large part of the cerebral cortex.

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

The vertebrobasilar system

The vertebrobasilar system is responsible for the posterior circulation of blood in the brain.. It is constituted by the vertebral arteries (right and left), which integrate into the basilar artery, once they have penetrated the brain, and branch into the posterior cerebral arteries that supply the occipital lobes (located in the posterior area of the brain).

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

1. Posterior cerebral artery

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

Upon reaching the cortex, this artery divides into the parietoccipital and calcarine arteries. The parietooccipital artery continues through the parietooccipital sulcus and is responsible for supplying blood to the medial portion of the parietal and occipital lobes; and the calcarine artery continues its course through the calcarine fissure, carrying out irrigation functions of the visual cortex in the occipital lobe.

2. The polygon of Willis

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

This blood circulation system is formed by the union of the two systems we have seen above: the carotid system and the vertebrobasilar system.. It is what is called an anastomosis system, that is, a network of interconnecting arteries in the form of an arterial polygon in charge of supplying blood to the brain and adjacent areas.

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

The main function of the polygon of Willis is to allow an alternative pathway to exist in case an occlusion occurs or the blood supply stops in its usual route. In addition, it also allows blood flow to be equalized 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 case we suffer any type of damage or cerebrovascular accident that involves a decrease or paralysis of cerebral irrigation and vascularization.

Bibliographic 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, E.R.; Schwartz, J.H. & Jessell, T.M. (2001). Principles of neuroscience. Fourth edition. McGraw-Hill Interamericana. Madrid
• Scheel P, Ruge C, Petruch UR, Schoning M. Color duplex measurement of cerebral blood flow volume in healthy adults. Stroke. 2000;31:147-150.