Arteries transport blood away from the heart under high pressure to body tissues. Their structure adapts them for both high pressure and changes in pressure. The main artery of the heart is the aorta, which originates from the left ventricle of the heart and then gives rise to branches that course upward and downward to supply all body tissues with oxygen and nutrients.

The structure of arteries permits them to expand and contract. This is due to the presence of elastic fibers that enable the arteries to stretch outward with each pulse of blood pumped by the heart and then recoil back to their original shape when tension is released. Like all blood vessels, arteries have an inner layer, the tunica intima, which is composed of a single layer of flattened endothelial cells fitted together to form a smooth, continuous tube. In large arteries, this inner layer is interspersed with elastic fibers and surrounded by a thick band of elastic fibers. The middle layer or tunica media of large arteries is very thick and consists largely of smooth muscle and elastic fibers. In very large arteries, the outer layer or tunica adventitia also contains some elastic fibers in addition to connective tissue.

Veins

Veins transport blood under low pressure toward the heart and also act as a reservoir of variable capacity to maintain a steady return of blood to the heart. The veins of the systemic circulation lead into the body's largest veins, the superior and inferior vena cavae, which empty into the right atrium of the heart.

Veins differ from arteries in three ways: their walls are thinner and contain little elastic fiber, and their internal diameter is greater. These structural properties make them able to stretch outward with ease and thereby adapt them to their function as a reservoir for blood. Since veins contain blood under low pressure, some structural modification is needed to prevent the downward pull of gravity from leading to the accumulation of blood in the legs and feet. The veins in the lower body therefore contain special one-way valves that prevent this from occurring. When the muscles in the extremities are active (eg, during exercise), their alternating relaxations and contractions squeeze the veins in such a way that blood is forced upward toward the heart. (It has been observed that soldiers kept standing at rigid attention for more than about 15 minutes may faint. This is because without the "muscle pump" working on the veins, insufficient blood returns to the heart and blood flow to the brain is reduced.)

The tunica media of arteries is thick and heavily reinforced with elastic fibers and smooth muscle; the tunica media of veins is thinner and contains less elastic fiber and smooth muscle. Arteries are thus well adapted to their function of carrying blood under pressure and veins to their function of serving as a reservoir to maintain a steady return of blood to the heart.

Arterioles

In addition to their function of distributing blood, arterioles also act as pressure-reducing valves between the arteries and capillaries; they also play an important role in determining blood pressure.

Of all the blood vessels, arterioles have the greatest proportion of smooth muscle in their walls. This makes the muscular tension in their walls such that they do not stretch under pressure. Instead, they act as pressure-reducing valves between the arteries and capillaries, buffering the delicate capillaries from the high pressure of blood in the arterial system. The degree of muscular tension in the walls of the arterioles dictates their internal diameter, and this in turn dictates the resistance to blood flow through the arterioles. The arterioles exert a profound effect on blood pressure because they account for a large component of the peripheral resistance to blood flow; blood pressure is a product of total peripheral resistance and cardiac output.

Venules

The function of venules is to drain blood from the capillary beds into the venous section.

Capillaries

Capillaries are functionally unique. They are the only blood vessels in the cardiovascular system that have the all-important function of permitting the exchange of substances (eg, water, oxygen, carbon dioxide, glucose) between the bloodstream and the surrounding tissues.

Capillaries are composed of a single layer of flattened endothelial cells fitted together to form a continuous tube. This structure adapts them to their function of permitting the exchange of substances between the bloodstream and the surrounding tissues. It is at the level of the capillaries that cells take in oxygen and give out carbon dioxide and that they take in nutrients and give out waste products of cellular metabolism. Branching of the capillaries is extensive; although they are very small, there are so many of them (approximately 10 billion) that their total cross-sectional area is more than six times that of all the other blood vessels combined. The net effect is that no cell in the body is ever very far away from a capillary and its life-sustaining contents.

Relation to other systems and organs

The vascular system stands in a unique relation to the heart; these two components have the same function: To provide oxygen, nutrients, and hormones to the cells of all body tissues. It is, in a way, more accurate to think of the heart and the vascular system as one unit rather than two, because each is equipped to carry out half of that function.

The vascular system is also closely related to the adrenergic receptors and the autonomic nervous system, which together control important aspects of its function. The smooth muscle cells in the arteries, veins, arterioles, and venules bear on their surfaces alpha-adrenergic receptors; the receptors bind molecules released by cells of the autonomic nervous system and respond by contracting. These molecular messengers are called norepinephrine and epinephrine.

The contraction of the smooth muscle cells results in constriction of arterioles, which in turn causes (1) an increase in arteriolar resistance (and therefore an increase in blood pressure) and (2) an increase in return of venous blood to the heart. Another class of adrenergic receptors, called beta-2, causes dilation of blood vessels when stimulated by epinephrine.