Introduction

In blood vessel disorders, blood vessels may become blocked, bulge abnormally, or tear. Or they may become stretched out or inflamed. As a result, blood does not circulate through the body as it normally does.

Essentially, blood vessels are the body's plumbing system. Blood vessels (arteries, capillaries, and veins) carry blood throughout the body, delivering oxygen and nutrients to tissues and carrying waste products away from tissues. The body's tissues need a constant supply of oxygen and nutrients to survive.

See the figure Arteries and Veins: Circulating the Blood.

Arteries carry blood from the heart to the body's tissues. The heart pumps blood into the body's main artery, the aorta. The aorta divides into smaller and smaller arteries and finally into capillaries—tiny blood vessels that connect arteries and veins. In capillaries, oxygen and nutrients leave the blood and enter the tissues, and waste products leave the tissues and enter the blood. Capillaries lead to veins, which become larger and larger as they approach the heart. Veins carry blood back to the heart.

The heart pumps the blood that has returned from the veins to the lungs (through the pulmonary arteries). In the lungs, oxygen is added to blood (the blood becomes oxygenated). Oxygenated blood flows from the lungs to the heart (through the pulmonary veins). The heart then pumps this blood into the arteries and to the tissues.

Both arteries and veins are elastic. Arteries can change size: They expand (dilate) and contract (constrict). Thus, they help control blood pressure and can deliver enough blood to tissues. For example, when arteries dilate, they provide more space for blood to flow through, so blood pressure decreases. Veins dilate when blood flow through them increases and constrict when blood flow decreases. Thus, they help adjust how much blood the bloodstream can hold.

The heart pumps blood through the arteries, so blood travels through arteries under high pressure. In contrast, the heart does not pump blood through the veins, so blood travels through veins under low pressure. Blood in some veins (such as the veins in the legs) has to travel against gravity to reach the heart. These veins have one-way valves with two flaps (cusps) that prevent blood from flowing backward. The flaps open to allow blood to flow toward the heart and close if blood starts to flow backward. Blood in the leg veins is propelled toward the heart by contractions of the leg muscles, which squeeze the veins in the legs.

Blood vessels hold up quite well to a lifetime of use, considering that blood makes more than 1,000 trips around the body each day. To some degree, the body can repair damaged blood vessels and create new vessels when needed. Nevertheless, aging itself and disorders that may develop as people age take a toll on blood vessels.

If a disorder affects arteries, severe symptoms can occur within seconds because lack of oxygen affects tissues immediately. If a disorder affects veins, symptoms usually are less severe or develop more slowly. Many blood vessel disorders can be effectively prevented or treated.