The structure of brain arteries:

Brain arteries can be likened to steel cylindrical pipes, each consisting of a wall enclosing a hollow space (the "lumen"). Of course, blood (comprised of liquid serum and blood cells) normally flows in the lumen (it can spurt out of the lumen following rupture of a brain aneurysm; take me straight to Brain Aneurysms). The larger brain arteries run in a space on the surface of the brain known as the subarachnoid space. From these vessels, very small branches penetrate the substance of the brain.

The wall of a brain artery is comprised of three major layers and a total of six main components (see Figure 1, below). The three main layers of an artery are the intima (the innermost thin layer closest to the lumen), the media (the relatively thick middle layer of the wall), and the adventitia (the outermost layer of the wall). Between the intima and the media is a thin layer of elastic tissue. This layer is referred to as the elastic lamina, and it is the only elastic layer occurring in the wall of a brain artery (in arteries elsewhere in the body, there are two layers of elastic tissue, an inner one between the intima and media, and an outer one between the media and adventitia). The elastic layer has many naturally occurring openings (perforations) in it.

The six main components of a blood vessel wall are endothelial cells, collagen fibers, elastic fibers, smooth muscle cells, fibroblasts, and nerve fibers. In the smallest of brain vessels, known as brain capillaries, there are also two other cell types, namely, astrocytes (actually supporting cells in the brain which send their "foot processes" out and over the capillary wall) and pericytes (cells scattered along the capillary wall whose precise function is not known). The intima is made up of a single layer of cells, referred to as the endothelium. This layer rests on a protein-rich layer called the basal lamina. Then, moving in an outwards direction across the blood vessel wall, we come across the elastic lamina (rich in elastin protein), the media (comprised of smooth muscle cells), and the adventitia (comprised of cells called fibroblasts which produce fibrous collagen protein). In the adventitia, we also find nerve fibers which supply (i.e., innervate) the smooth muscle cells.

Structure of a Normal Brain Artery:

Figure 1 shows a section across the wall of a brain artery. It has been cut to show the various layers which were described above. The innermost part is a hollow space (the lumen) containing serum and blood cells. From the lumen outwards across the wall, note the following layers: the intima [which contains endothelial cells (EC; light blue cells in figure) lying on their basal lamina (inner part of the black circle in the figure)]; the elastic lamina (lies just under the basal lamina, and is represented in the figure as the outer part of the black circle); the media [comprised of many smooth muscle cells (SMC) shown as large red cells in the figure); and the outermost layer called the adventitia [made up of fibroblasts (FB; thin green cells in the figure) and their collagen fibers, and nerve fibers (NV; yellow-orange fibers in the figure) on their way to the smooth muscle cells]. Also shown is part of an astrocyte (AC; dark blue cell in the figure). These supporting cells give off long thin "foot process" which surround the endothelial cells, but only at the level of the smallest of brain vessels, known as brain capillaries.

The organization of brain arteries:

Brain arteries are organized as follows: from the main pipes ("trunks") that enter into the brain (the two internal carotid arteries at the front under surface of the brain and the two vertebral arteries at the back under surface of the brain), a ring of arteries arises that encircles the under surface of the brain. This ring (or "anastomotic" vessel network) is known as the "Circle of Willis". In approximately 20-25% of persons, this ring is not complete (i.e., not a complete circle), a normal variation of brain vessel anatomy.

The Circle of Willis:

Figure 2 shows the under-surface of the brain. The major arteries in this region are shown in red. Together, they form a ring-like structure called the "Circle of Willis", a critical point of communication between the main arteries supplying the substance of the brain. The front part of this group of arteries (in the top part of the figure) is referred to as the "anterior circulation", the back part (in the bottom part of this figure) is referred to as the "posterior circulation". All of these arteries lie in the "subarachnoid space" (SAS), a space normally filled with circulating cerebrospinal fluid.

Emanating from or part of the circle of Willis are vessels known as the anterior cerebral arteries, anterior communicating artery, middle cerebral arteries, posterior communicating arteries, carotid termini, posterior cerebral arteries, and basilar artery apex (or basilar "caput"), and all their absolutely critical tiny branches (also known as "perforators") which supply vital deep structures of the brain and brainstem. From the big arteries at the base of the brain arise smaller (pial) arteries that course over the surface of the brain and dip into the valleys (sulci, plural of sulcus) or grooves between the cliff edges (gyri, plural of gyrus) or folds that make up the outer brain surface (cortex, or bark as in tree bark). From the pial arteries, many smaller arterioles take off (usually at right angles) and perforate into the brain substance. These end in capillaries, which then drain into venules and then larger veins, which then make their way into very high-volume, low-pressure venous systems known as venous (or dural) sinuses. These high "throughput" channels will eventually empty into the internal jugular veins on their way back to the heart's right atrium.

How brain arteries function:

The job of a brain artery is to allow nutrients in blood to reach ("perfuse") the target tissue, the brain. In fact, at any given time, one-fifth of the heart's output is directed towards the brain. Veins complete this circuit by draining metabolic waste products from the brain. The job of carrying blood to the brain from the heart to, and through, the brain is not a simple or passive process, i.e., blood vessels are not a set of rigid, inert tubes connecting two organs. Blood flow to the brain is a complex and active process subject to stringent regulation. The normal regulation of brain blood flow (cerebral blood flow, CBF) is an integrated process involving all layers of the blood vessel wall, i.e., endothelium, smooth muscle and adventitial nerve fibers, in addition to brain (parenchymal) neurons and supporting (glial) cells such as astrocytes, and also intracranial blood, cerebrospinal fluid (CSF) and extracellular/"interstitial" fluid compartments. An adequate description of CBF regulation therefore must include endothelial, myogenic, neurogenic-neuroglial and metabolic mechanisms, and is a rather technical subject discussed elsewhere ( see Key References).

At present, there is no single hypothesis that decisively unifies and explains the biomechanics of CBF regulation. Compartmentalization of the cerebrovascular microenvironment into vascular endothelium, smooth muscle cells, perivascular nerves and extracellular and subarachnoid spaces aids in classifying vasoactive mediators according to their putative sites of action, however does not shed any light upon their mechanisms of action and interaction. Although it is beyond the scope of this Site to discuss every known mediator, it is of some benefit to consider those mediators of proposed primary importance ( see Key References). As measured by its pivotal vascular actions and extensive cross-talk with a variety of other vasoactive systems, it is becoming clear that the nitric oxide (NO) signaling system is a major, if not the principal candidate in this arena, possibly representing a putative “final common pathway” of vascular modulation. Other important vasomodulators considered are carbon monoxide (CO), eicosanoids (arachidonic acid metabolites), oxygen-derived free radicals, and endothelins. Myogenic, endothelial, and neurogenic-neuroglial aspects of CBF regulation are discussed elsewhere, as are the vasomodulatory effects of arterial oxygen (O₂) and carbon dioxide (CO₂) and hydrogen ion (H⁺) ( see Key References).