Vein is a blood vessel that carries blood toward the heart. The blood circulates in the body through a system of tubes called blood vessels. The three kinds of vessels are arteries, capillaries, and veins. Most veins return blood to the heart after the blood has delivered nourishment to the tissues and taken up waste products. Blood in veins is called venous blood.
The blood returning from the body cells has lost much of its oxygen and is dull, brownish-red. It circulates through the right side of the heart and then goes to the lungs. Here it gives off its waste carbon dioxide and takes on a new supply of oxygen. Bright red blood from the lungs returns to the left side of the heart through the pulmonary veins. Then it begins its trip through the body.
The veins begin at the capillaries. At first they are very tiny, and are called venules. Small veins join to form larger ones. Finally, all the venous blood of the body flows into two very large veins that enter into the heart. One of these, the superior vena cava, carries venous blood from the head and arms. The other, the inferior vena cava, carries it from the trunk and legs.
Veins, like arteries, have walls made of three layers. But the vein walls are thinner, less elastic, and less muscular than those of the arteries. The inner layer of the veins is the intima. In many of the larger veins, the intima has folds that serve as valves. These folds lie against the wall when the blood is flowing freely. Several things can cause the blood to slow down or stop—the effects of gravity, pressure on a vein, the tendency of blood to clot, faulty valves, or low blood pressure. When the valves are closed, they stop the blood from flowing backward. The valves are usually just above the place where two veins join. There are no valves in the veins of the abdomen, brain, and lungs, or in the smaller veins.
Veins that are swollen, stretched, or coiled on themselves are varicose veins. Phlebitis is inflammation of a vein. Phlebitis may produce redness, tenderness, swelling, and pain in the area of the vein.
