Blood is circulated throughout the body by means of three different types of blood vessels: arteries, veins, and capillaries. ARTERIES are large vessels that carry blood away from the heart, VEINS are large vessels that carry blood back to the heart, and CAPILLARIES form a connecting network of smaller vessels between the arteries and the veins.

Hemorrhage (bleeding) occurs whenever there is a break in the wall of one or more blood vessels. In most small cuts, only the capillaries are injured. Deeper wounds result in injury to veins or arteries. Injury to the capillaries is not serious and can generally be controlled by a small bandage strip or pad. Injury to veins or arteries is serious and may endanger life.

One twelfth to one fifteenth of the body weight is blood. A person weighing 150 pounds has approximately 10 to 12 pints of blood. One pint of blood can usually be lost without harmful effect; in fact, this is the amount usually given by blood donors. However, the loss of 2 pints usually causes shock, and shock becomes greater and greater as the amount of blood loss increases. If one half of the blood in the body is lost, death usually results.

Capillary blood is usually brick red in color. When capillaries are cut, the blood oozes out slowly. Blood from the veins is dark red. When a vein is cut, the blood escapes in a steady flow. When an artery near the surface is cut, the blood gushes out in spurts that are synchronized with heart beats; but if the cut artery is deeply buried, the bleeding appears in a steady stream. Arterial blood is usually bright red in color.

In actual practice, you may find it difficult to decide whether the bleeding is venous or arterial, but the distinction is usually not important. A person can bleed to death quickly from a cut artery; prolonged bleeding from any large cut can, of course, have the same effect. The important thing to know is that all bleeding must be controlled as quickly as possible.