The blood capillaries are where the important functions of the circulation take place: the exchange of material between circulation and cells. Capillaries are the smallest of the body’s blood vessels. They are only one cell thick, and they are the sites of the transfer of oxygen and other nutrients from the bloodstream to other tissues in the body; they also collect carbon dioxide waste materials andmycontentbreak fluids for return to the veins. They connect the tiny muscular branches of arteries, called arterioles, with tiny veins (called venules). Ultimately, the capillary is the site of internal or cellular respiration and is responsible for the utilization of oxygen by the tissue and the transporting of carbon dioxide as waste to the veins for elimination by the lungs. The arterial blood system branches extensively to deliver blood to over a billion capillaries in the body. The extensiveness of these branches is much more readily appreciated by knowing that the capillaries provide a total surface area of 1,000 square miles for exchanges of gases, waste, and nutrients between blood and tissue fluid. Oxygen rich blood flows from arterioles or small branches of the artery into the capillary bed and the pressure inside of the arteries is roughly fifty times that on the inside of the veins. It is this pressure difference that forces the blood into the capillary bed. Although the amount of blood flowing through a particular capillary bed is determined in part by a small circular muscle around the arteriole branches, the absence of smooth muscle and connective tissue layers permits a more rapid rate of transport between the blood and the tissue.