The blood vessels of the cardiovascular system are for convenience of description classified into arteries (elastic and muscular), resistance vessels (small arteries and arterioles), capillaries, venules and veins. Typical dimensions for the different types of vessel are illustrated.

The Systemic Circulation

Arteries

The systemic (or greater) circulation begins with the pumping of blood by the left ventricle into the largest artery, the aorta. This ascends from the top of the heart, bends downward at the aortic arch and descends just anterior to the spinal column. The aorta bifurcates into the left and right iliac arteries, which supply the pelvis and legs. The major arteries supplying the head, the arms and the heart arise from the aortic arch, and the main arteries supplying the visceral organs branch from the descending aorta. All of the major organs except the liver (see below) are therefore supplied with blood by arteries that arise from the aorta. The fundamentally parallel organization of the systemic vasculature has a number of advantages over the alternative series arrangement, in which blood would flow sequentially through one organ after another. The parallel arrangement of the vascular system ensures that the supply of blood to each organ is relatively independent, is driven by a large pressure head, and also that each organ receives highly oxygenated blood.

The aorta and its major branches (brachiocephalic, common carotid, subclavian and common iliac arteries) are termed elastic arteries. In addition to conducting blood away from the heart, these arteries distend during systole and recoil during diastole, damping the pulse wave and evening out the discontinuous flow of blood created by the heart’s intermittent pumping action.

Elastic arteries branch to give rise to muscular arteries with relatively thicker walls; this prevents their collapse when joints bend. The muscular arteries give rise to resistance vessels

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