Capillaries

In the veins, blood is forced to move against gravity much of the time. To help keep the flow toward the heart, many veins are equipped with one-way valves. In addition, the action of the diaphragm and muscles in the arms and legs exert a massaging effect that helps to move blood back to the heart. The purpose of all this pumping and transporting is to exchange oxygen and other nutrients for waste products. The transfer takes place across microscopic, one-cell-thick vessels called "capillaries" that connect the tiny muscular branches of arteries, called arterioles, with tiny veins called venules. The capillaries allow nutrients, oxygen, and fluids to pass into tissue and also collect carbon dioxide waste materials and fluids for return to veins and lymph system. The capillaries are where the important functions of the circulation take place: the exchange of material between circulation and cells.