Arteries

Arteries are circular membranous channels, used for transporting blood from the heart to all the tissues of the body; moving away from it they give rise to branches that diminish gradually in diameter, until they flow into the capillary nets. Nevertheless, the arteries are far from being simple inert conduits; we are in fact talking about dynamic structures, capable of contracting and expanding in response to the needs of the organism.

Classification of arteries

Contractility and elasticity are characteristics differently represented in the various types of arteries. Based on their structural particularities we distinguish:

large arteries or elastic arteries : they have a diameter that exceeds 7mm, with a large light and a particularly elastic wall, necessary to dampen the strong pressures given to the blood by the heart. Conduction arteries are also defined; examples are the aorta and its main branches, and the pulmonary arteries.

Arteries of medium caliber or muscular arteries : they have a diameter between 2.5 and 7 mm, with a large light and a strong but not too elastic wall; they also offer low resistance to blood flow. They are considered distribution arteries; examples are coronary and renal arteries.

Small-caliber arteries or arterioles : rich in muscle tissue, they have a small light and a thick, contractile wall, thanks to which they regulate and control the resistance of the flow in the capillary bed. They are governed by a rich innervation of sympathetic fibers and by several local factors. Arterioles represent the extreme branches of the arterial tree and continue into the capillaries.

According to the above, the caliber and elasticity of the arteries progressively decrease from the aorta to the periphery, while the smooth muscular component increases by reflex. Blood pressure and speed also decrease as one moves away from the heart; the total transverse section, on the other hand, increases, because the caliber of the sum of the collateral and terminal branches of each artery is always greater than that of the original vessel. There are therefore gradual passages from one type of artery to another; it is also possible to identify arteries of mixed type, which have intermediate characteristics between the different vessel types.

All arteries carry oxygen-rich blood. The pulmonary artery is an exception, which carries deoxygenated blood to the lungs - where red blood cells release carbon dioxide to enrich themselves with oxygen - and umbilicals in the fetus. One speaks therefore of systemic arteries, to indicate the blood vessels assigned to the transport of oxygenated blood from the heart to the rest of the body, and of pulmonary arteries, used for the transport of deoxygenated blood from the heart to the lungs; consequently the pulmonary veins, unlike the systemic ones, carry oxygen-rich blood.

The walls of the arteries

The wall of all the arteries is made up of three concentric tunas: the inner, the inner, the medium and the adventitia (or external frock).

The intimate or simply intimate habit represents the innermost layer of the vessel wall; it delimits the lumen and is formed by a thin layer of endothelial cells resting on an equally small layer of connective tissue; acts as a protective coating and ensures the regulation of the transport of material between blood and tissues. The cells that compose it play very important roles, in some ways still to be clarified, such as the release of paracrine substances capable of regulating blood flow.

The average habit is made up of smooth muscle fibrocells and elastic fibers; it is generally the thickest and variable according to the size and type of artery. The average cassock has the purpose of giving elasticity to the vessel (in large caliber arteries the elastic fibers are abundant, while the contractile ones are relatively few) and contractility (in the muscular arteries there is a predominance of the smooth muscle content compared to the elastic).

The more external adventitious frock is made up of loose connective tissue, with bundles of smooth muscular fibrocellulas, and is mainly used for storage; in large and medium-caliber vessels it contains the vasa vasorum (small vessels that nourish and nourish the vascular walls) and the nerva vasorum (sympathetic vegetative fibers essentially responsible for controlling the smooth muscle fibers of the intermediate garment).

Between one frock and the other there are elastic laminae; the internal elastic lamina is a dense elastic membrane that separates the intimate from the average, while the external elastic lamina, less developed, represents the external limit of the average tunic.