Blood vessels

The circulation can be divided into the systemic and the pulmonary circulation. The systemic circulation receives oxygenated blood from the left side of the heart via the aorta and returns desaturated blood to the right side of the heart in the venae cavae. The desaturated blood is delivered to the pulmonary circulation from the right ventricle via the pulmonary artery to be oxygenated and to exchange carbon dioxide. Oxygenated blood is then returned to the left atrium via the pulmonary veins (Figure CR1).

Structure and function

Blood vessel walls are basically structured in three layers. The adventitia is the outer layer and is made up of connective tissue with nerve fibres. The middle layer or media is of varying thickness and contains mainly smooth muscle. The innermost layer is the intima and consists of the endothelium, basement membrane and supporting connective tissue (Figure CR2). The composition of blood vessel walls is mainly a mixture of elastic tissue, fibrous tissue and smooth muscle. This mixture again varies according to the type of vessel. The aorta walls are predominantly elastic and fibrous tissue with little smooth muscle, whereas the vena cava walls consist largely of smooth muscle and fibrous components. The composition of vessel walls reflects their function.

Figure CR3 lists functional aspects relating vessel characteristics to function.

Blood vessel diameter and wall thickness

A major factor determining thickness is mean arterial pressure.