Species: rat (Ratus norvegicus; mammal).
Technique: paraffin embedding, staining with Masson trichome.
Arteries, the blood vessels conducting the blood from the heart to the organs, show different diameters. The larger arteries are known as elastic arteries, while the middle-sized arteries are referred to as muscular arteries. Arterioles have a diameter smaller than 100 µm, progressively decrease in size until they transition to capillaries.
Like other arteries with larger diameters, arterioles are made up of three layers, or tunica. The innermost layer, which is in contact with the blood, is the tunica intima, comprising an endothelium and a layer of connective tissue (Figure 1). The tunica media is located more externally and is mainly formed of smooth muscle cells aligned parallel to the longitudinal axis of the arteriole, along with fibroelastic connective tissue. Unlike larger arteries, the arteriole tunica media is poorly developed and has fewer than three layers of smooth muscle cells. By definition, when more than 2 layers of smooth muscle cells are found in the tunica media, the blood vessel is classified as small artery and as muscular artery if it is more than 8-10 layers. The tunica adventitia is the outermost layer, which mostly consists of fibroelastic connective tissue. However, the tunica adventitia is may be difficult to distinguish.
AArterioles distribute the blood to the capillary nets and play a crucial role in regulating blood pressure (arterial tension). By cell contraction, the smooth muscle cells of the tunica media decrease the blood flow to capillaries. At the junction between arteriole and capillary, there is a slight increase in the amount of smooth muscle cells forming the so-called precapillary sphincter. Arterioles are able to distribute the blood where it is needed by regulating the blood flow. For example, they can supply blood to the intestine during digestion, to muscles during exercise, or to the brain for higher neuronal activity.
In the pathology of hypertension, the diameter of the arterioles and small arteries is reduced because the vessel walls increase in size. This may be a consequence of the deposition of lipids among the smooth muscle cells or the increase in the number of muscle cells. Both of these pathological processes can occur simultaneously.