Three different types of vessels transport blood pumped by the heart, delivering it throughout the body and returning it to the heart. The first is an artery, a blood vessel that transports blood away from the heart. Arteries receive blood as it surges at high pressure from the ventricles. This explains why they are highly muscular and have a large diameter. Arteries are classified by size into three categories:
- Conducting, or elastic, arteries: These arteries are large in size. Many are greater than 10 mm in diameter. They expand and relax as they receive blood from the pumping heart.
- Distributing, or muscular, arteries: These arteries carry blood farther from the heart to the various organs. They average about 6 mm in diameter. Most arteries identified by name are conducting or distributing arteries.
- Resistance arteries: Resistance arteries function to help the body distribute blood across organs. An arteriole, a resistance artery that has a diameter range of 40–200 μm, is the smallest of all arteries.
The second type of blood vessel is a vein, which collects blood and returns it back to the heart after it has been distributed by capillaries. Venous return is the flow of blood back from the body to the heart. It is accomplished by a combination of pressure, gravity, and contraction of skeletal muscles. In the venous system, small veins converge to form larger vessels, with vessel size increasing as they approach the heart. Their average diameter is approximately 5 mm. A venule is the smallest type of vein, with a diameter of 15–100 μm. It collects blood that has just passed through capillary beds. Veins above the heart use gravity to return blood. Veins elsewhere push blood upward against gravity, a feat accomplished in part by pressure within veins and contraction by skeletal muscles. Pressure is supplied in this manner to keep blood constantly moving upward. Veins larger than 2 mm in diameter also possess one-way valves that prevent blood from flowing downward when skeletal muscles are relaxed.
The third type of blood vessel is a capillary. A capillary is any of a vast network of tiny vessels that enable the exchange of nutrients and gases between blood and interstitial fluid, which bathes cells. Capillaries are grouped into capillary beds that are filled with blood by arteries and drained by veins. A capillary is narrower at its proximal end, where it receives blood from arteries, averaging 5 μm in diameter. It widens to about 9 μm in diameter at its distal end, where it meets veins.