Blood vessels are hollow tubes that allow blood to flow throughout the body. If all the blood vessels of an adult, including the veins, arteries and capillaries, were laid end to end, they would stretch for 100,000 miles, according to information published by the Franklin Institute. Each type of blood vessel performs distinct and important functions.
Arteries
The arteries are strong elastic blood vessels that carry oxygen-rich blood from the heart through the body. The walls of the arteries must be strong because as the heart pumps the blood, the pressure in the arteries is high. The elastic ability of the arteries is important, because as the heart contracts the arteries expand to fill with blood, and as the heart relaxes the arteries contract to help blood continue to flow. For this reason the pulse, or heartbeat, can be felt and measured in the arteries.
The main artery that leaves the heart is called the aorta. This is also the largest artery. As blood flows farther from the heart, it travels through vessels that get progressively smaller and smaller. Blood leaving the heart is full of oxygen, so this blood is bright red in color. The arteries flow into the capillaries.
Capillaries
The capillaries are the tiny vessels, as thin as a strand of hair, that supply blood to all the cells in the body. The capillaries are very thin and fragile. Their thin walls allow the oxygen and nutrients to escape from the blood into the cells. The carbon dioxide and other waste products from the cells flow through the capillary walls into the blood. The capillaries then flow into the network of veins.
In addition to connecting the arteries and veins, the capillaries are responsible for releasing excess heat in the body. During exercise, for example, the heat produced is delivered to the capillaries, where it is released to the tissues. This is why the skin appears flushed when you’re hot.
Veins
Veins are not as strong and elastic as arteries, because the pressure of the blood is not as high. Veins are responsible for carrying the blood full of waste products, such as carbon dioxide, from the capillaries back to the heart. Veins have valves in them that help keep the blood flowing in one direction: toward the heart.
Because the blood in the veins has very little oxygen, the color of the blood is a deep red, almost maroon color. Many of the veins run close to the skin and because the walls of the veins are thin the blood is often visible. The way the light refracts, the blood can appear blue.
Pulmonary Arteries and Veins
The network of blood vessels that connect the heart and the lungs work slightly differently from the other vessels in the body. The pulmonary artery is the main artery that leads from the right side of the heart to the lungs. Just as the other arteries in the body, the blood flows away from the heart. The difference is that this artery carries oxygen-poor blood to the lungs where the carbon dioxide is exchanged for the oxygen. The oxygenated blood then flows from the lungs back to the heart in the pulmonary vein.
About this Author
Stephanie Chandler is a freelance writer whose master’s degree in biomedical science and over 15 years’ experience in the scientific and pharmaceutical professions provide her with the knowledge to contribute to health topics. Chandler has been writing for corporations and small businesses since 1991. In addition to writing scientific papers and procedures, her articles are published on LIVESTRONG.com, eHow, Overstock.com and Helium.com.
