THE CARDIOVASCULAR SYSTEM

The circulatory system of vertebrates is composed of three elements : the heart, a muscular pump that we shall consider later in this chapter; the blood vessels, a network of tubes that pass through the body; and the blood, which circulates within these vessels. The plumbing of the closed circuit, the heart and vessels, is known collectively as the cardiovascular system. Blood moves within this cardiovascular system, leaving the heart through vessels known as arteries. From the arteries, the blood passes into a larger network of arterioles, or smaller arteries. From these, it eventually is forced through the capillaries, a fine latticework of very narrow tubes, which get their name from the Latin word capillus, "a hair"". It is while passing through these capillaries that the blood exchanges gases and metabolites with the cells of the body. After traversing the capillaries, the blood passes into a third kind of vessel, the venules or small veins. A network of venules and larger veins collects the circulated blood and carries it back to the heart.

Arteries
The walls of the arteries are made up of three layers of tissue. The innermost one is composed of a thin layer of endothelial cells. Surrounding these cells is a tick layer of smooth muscle and elastic, the artery is able to expand its volume considerably in response to a pulse of fluid pressure, much as a tubular balloon might respond to air blown into it. The steady contraction of the muscle layer strengthens the wall of the vessel against overexpansion.

Capillaries
Capillaries have the simplest structure of any element in the cardiovascular system. They are little more than tubes one cell tick and on the average about 1 millimeter long; they connect the arterioles with the venules. The internal diameter of the capillaries is, on the average, about 8 micrometers. Surprisingly, this is little more than the diameter of a red blood cell (5 to 7 micrometer).

Veins
Veins do not have to accommodate the pulsing pressures that arteries do, because much of the force of the heartbeat is weakened by the high resistance and great cross-sectional area of the capillary network. The walls of veins, although similar in structure to those of the arteries, have much thinner layer of muscle and elastic fiber. An empty artery is still a hollow tube, like a pipe, but when a vein is empty, its walls collapse like an empty balloon.