ArteryA artery is a blood vessel that carries blood away from the heart. In the body’s circulation, an artery always carries oxygen-rich blood, whereas in the pulmonary circulation it always carries oxygen-poor blood, since it transports the oxygen-poor blood from the heart to the lungs to enrich the oxygen. Arteries change their microscopic (histological) structure depending on their diameter and position in the body.
A further distinction is made between small arteries, so-called arterioles, and tiny hair vessels, so-called capillaries. In comparison to veins, arteries are thicker-walled, as there is a high internal pressure (blood pressure) in arteries, which is counteracted by this. Furthermore, arteries have a round inner shape (lumen).
The arteries are the high pressure system of the blood system. The internal arterial pressure varies between the ejection phase (systole), i.e. the maximum contraction of the heart, and the filling phase (diastole) of the heart. The largest artery in the human body is the main artery (aorta). It has a diameter of up to three centimeters, depending on the body stature.
Microscopic wall construction
An artery consists of three layers. The first and innermost layer, i.e. the layer that comes into contact with the blood flowing through it, consists of a single-layered cell layer, a so-called single-layer unkeratinized squamous epithelium. This innermost layer is also called endothelium or intima (tunica intima).
It is the decisive barrier between the inside of the blood vessel (intravascular space), i.e. the blood, and the area outside the blood vessel (extravascular space). The second, subsequent layer consists mainly of smooth, non-randomly controllable muscles that cannot be controlled arbitrarily. This layer is called media (tunica media).
In addition to the smooth musculature, the second layer contains additional elastic fibres, depending on the type in the body. This layer is mainly used to adjust the wall tension of an artery and the width of the vessels. If the smooth musculature contracts, the wall tension increases and the artery becomes narrower.
The third and outermost layer of an artery is called adventitia (tunica adventitia). The adventitia consists mainly of connective tissue, which anchors the artery with the surrounding tissue in the body. Furthermore, depending on its composition, the adventitia determines the mechanical properties of an artery in addition to the middle layer. Furthermore, the adventitia of larger arteries contains small blood vessels (Vasa vasorum), which supply the wall of the arteries with blood.
Arterioles are smallest arteries with a diameter of about 20 micrometers. They are defined as vessels with only one closed muscle layer. They are very densely innervated and play a decisive role in the body’s own regulation of blood pressure, as they provide the greatest resistance due to their small diameter and are therefore also called resistance vessels.