Blood vessels are hollow tubes that are responsible for circulating blood throughout the body and maintaining our pulse and blood pressure. There are three basic types of blood vessels--veins, arteries and capillaries. Each has unique characteristics that help it to carry out its vital role in the human body.
Arteries
Arteries are large blood vessels that carry blood away from the heart to the rest of the human body. According to the Franklin Institute, arteries have three layers--a thick, rough outer layer of tissue, a muscular middle layer of tissue that helps the heart pump blood, and an inner layer that is made up of epithelial cells (closely packed cells that help to enclose or protect blood vessels and organs). Arteries are elastic, able to expand to accommodate blood volume and then contract after the heart relaxes, pushing the blood through and relieving blood pressure. The inner layer of arteries is smooth to facilitate blood passing through. Pulsating arteries can be felt through the skin and, because arteries expand and contract with each heart beat, they allow us to feel for our pulse.
The main artery is the dorsal aorta, which is responsible for carrying blood from the heart. The aorta divides into additional arteries, branching off into smaller and smaller arteries as it moves further away from the heart.
Capillaries
Capillaries are thin and fragile blood vessels. According to the Franklin Institute, they are only one epithelial cell thick, forcing blood cells to pass through them single file. Capillaries are embedded throughout the body, connecting veins and arteries. Capillaries are responsible for two critical bodily functions. First, they facilitate the exchange of carbon dioxide and oxygen. After receiving oxygen-rich blood from arteries, capillaries release oxygen into the surrounding tissues and pass waste-rich blood containing carbon dioxide to veins, which carry it back toward the heart. Second, capillaries help the body release excess heat. When we exercise and our body temperature rises, or when our skin is exposed to hot substances (water) or temperatures, the capillaries release the heat into the tissue beneath the skin. This can sometimes cause a flushed or reddish appearance on the face or hands.
Veins
Veins are large blood vessels that carry blood back to the heart. According to the Franklin Institute, veins are structured similarly to arteries in that they have three layers--an outer tissue layer, a middle muscular layer and a smooth inner layer of epithelial cells. However, veins carry blood at a lower blood pressure, so they are not as thick or strong as arteries. The primary responsibility of veins is to carry waste-rich blood back to the heart and lungs. To ensure that this blood does not flow backward, veins are equipped with valves that act like gates that keep blood from moving in the wrong direction. Valves are particularly important for keeping blood flowing back to the heart and lungs against the flow of gravity (for example, moving up from the foot).
