The systemic arterial circulation is an extensive high-pressure system. The structure of its vessels reflects the high pressures to which they are subjected. The output of the left ventricle of the heart is carried in large diameter vessels with a high component of elastic tissue in their walls which smoothes the systolic pressure wave. These are called the large elastic arteries (i.e. the aorta and its large branches such as the carotid, subclavian and renal arteries).

Moving away from the heart these large elastic arteries branch off into smaller vessels in which the artery walls become proportionately more muscular.

Elastic arteries are the largest arteries and receive the main output of the left ventricle: thus they are subjected to the high systolic pressures of 120-160 mmHg.

These large vessels are adapted to smooth out the surges in blood flow, since blood is impelled through them only during the systolic phase of the cardiac cycle. The elastic tissue in their walls provides the resilience to smooth out the pressure wave.

The intima of large elastic arteries is composed of endothelium with a thin layer of underlying fibrocollagenous tissue.

Elastic arteries have a thick, highly developed media of which elastic fibres are the main component. These are gathered together in sheets arranged in concentric layers throughout the thickness of the media. In the

largest artery, the aorta, there are often 50 or more layers.

The elastic fibres are arranged so that they run in bands around the circumference (circumferentially) rather than along the length of the artery (longitudinally) in order to counteract the tendency for the vessel to over distend during systole.

When the heart contracts and forces blood into the aorta, the elastic fibres are stretched and, as a result the aorta distends.

At the end of the left ventricular contraction, the force generated by the heart diminishes and so the force stretching the fibres is removed. As a result of this they tend to return to their starting size.

The effect of this recoil is to maintain the pressure head and thus force the blood pumped out of the left ventricle away from the heart and into the systemic vascular system.

This occurs as a result of the conversion of potential energy created by the distension of the elastic wall of the aorta into kinetic energy which continues to move blood away from the heart even in the diastolic phase.

Return of the elastic fibres from their stretched to unstretched state during diastole maintains a diastolic pressure within the aorta and large arteries of about 60-80 mmHg. It is this mechanism which helps to smooth out the explosive expulsion of blood from the left ventricle into a steady flow throughout the arterial system. Interposed between the elastic layers are smooth muscle cells and some collagen.

The adventitia of the large vessels carries vasa vasorum and nerves.

Muscular arteries have a media composed almost entirely of smooth muscle.

The large elastic arteries gradually merge into muscular arteries by losing most of their medial elastic sheets, usually leaving only two layers, an internal elastic lamina and an external elastic lamina at the junction of the media with the intima and adventitia, respectively.

Although the walls of muscular arteries are distensible to a certain extent, as they become smaller and smaller with each successive branching the amount of elastic tissue decreases whilst the muscular component proportionately increases.

In a muscular artery the media is composed almost entirely of smooth muscle. These arteries are therefore highly contractile, their degree of contraction or relaxation being controlled by the autonomic nervous system as well as by endothelium-derived vasoactive substances. A few fine elastic fibres are scattered among the smooth muscle cells, but are not organised into sheets. These are most numerous in the large muscular arteries, which are a direct continuation of the distal end of the elastic arteries.

Muscular arteries vary in size from about 1 cm in diameter close to their origin at the elastic arteries, to about 0.5 mm in diameter. In the larger arteries there may be 30 or more layers of smooth muscle cells, whereas in the smallest peripheral arteries, there are only 2 or 3 layers. The smooth muscle cells are usually arranged circumferentially at right angles to the long axis of the vessel.

The internal elastic lamina is commonly a distinct prominent layer, but the external elastic lamina is less well defined, and is often incomplete.