At any given time, the distribution of blood in a person's body will be approximately: 64% veins, 13% arteries, 9% pulmonary vessels, 7% heart and 7% arterioles&capillaries. Although the heart weighs less than 1% of the total body weight, it relentlessly receives nearly 5% of the total blood flow (which may explain why the arteries of the heart can so easily develop problems). Congenital defects and infectious disease can strike anywhere, but by far the most common disease occurs in the arteries: atherosclerosis.

Blockages can occur in veins as well as in arteries, but these tend to be caused by blood clots (thrombi) rather than by atherosclerosis. Thrombophlebitis (often simply called phlebitis) most commonly involves clotting of blood and inflammation of a vein in the leg. This can be serious if a portion of the clot becomes detached, travels through the heart, and gets pumped to the lung where it blocks a pulmonary artery as a pulmonary embolism. About 10% of people with pulmonary embolism die within an hour.

Clotting of blood in the veins can occur when blood flow is slow or stagnant. This can occur during long periods of immobilization such as when confined to a hospital bed, cramped in a crowded airplane on a long flight or driving for an extended period. Periodic walking, tightening/relaxing of lower body muscles and stretching within the confines of space can reduce the stagnation of the blood. Low-doses of aspirin under may reduce possible clotting.