Blood supply to the brain: anatomy, phases and course

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

In this article we will see how blood circulation occurs in the brain and what 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 was not going to be different. Although it represents just over 2% of body weight, it consumes one sixth of cardiac output and 20% of the oxygen that 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 an organ like this.

The entire brain is supplied by four large arteries, which arise from the aorta and ascend through the neck. until it penetrates the skull.

Blood circulation in the brain occurs through two symmetrical systems, on both sides of the neck: on the front, the common carotid arteries; and on the back, the vertebral arteries, which make a large part of their journey within the cervical vertebrae.

The common carotid arteries divide into two branches, the external carotid, which mainly supplies 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 brain.

The vertebral arteries penetrate the skull and unite to form a common trunk known as the basilar artery., which is responsible for 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 entire brain surface.

the carotid system

The carotid system is responsible for the anterior circulation of blood in the brain. and contributes practically 80% of the blood flow received by 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 form part of the cerebral arterial circle or circle of Willis, which will be discussed later.

This artery has cortical (orbital), frontal (parietal), and central (including the artery internal 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 inner face 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 sylvian fissure) is a branch of the internal carotid artery, and is responsible for supplying blood through two segments: a basal portion and a cephalic portion.

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

When the middle cerebral artery becomes blocked, hemiparesis (paralysis of one side of the body), affecting parts of the body such as the face, arm or leg, and generating sensory and visual disturbances.

The vertebrobasilar system

The vertebrobasilar system is responsible for the posterior circulation of blood in the brain.. It is made up of the vertebral arteries (right and left), which are integrated 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 part of the brain). brain).

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

1. posterior cerebral artery

The posterior cerebral arteries arise from the interpeduncular cistern and arise 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, for its part, the calcarine artery follows its course through the calcarine fissure, performing irrigation functions of the visual cortex in the occipital lobe.

2. Willis' polygon

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

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

Like the rest of the systems, the Willis polygon can be divided into its front and back parts. The anterior is formed by the internal carotid artery and supplies blood to the front 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 zone of the arterial polygon is made up of the vertebral arteries and is mainly responsible for supplying blood to the cerebellum, the trunk, and the posterior zone of the arteries. brain hemispheres.

The main function of the circle of Willis is to allow an alternative route to exist in case an occlusion occurs or the blood flow stops in its usual route. In addition, it also allows the blood flow between the two sides of the brain (right and left hemisphere) to be equalized.

Definitely, this network of arteries facilitates a correct distribution of blood flow in our brain, especially in the event that we suffer any type of damage or stroke that implies a decrease or paralysis of the irrigation and cerebral 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