Now Playing: Cardiovascular Systems

Arteries are blood vessels that carry blood from the heart out to all tissues of the body.

Veins are vessels that return blood back to the heart.

The heart is a muscular organ that pumps blood throughout thousands of miles of blood vessels in the circulatory system. There are four chambers in the heart. Oxygen-poor blood returning from the body enters the upper right chamber, called the right atrium, from there it enters the lower right chamber, called the ventricle. It is then pumped to the lungs where the waste product carbon dioxide is exchanged for the nutrient, oxygen. The oxygen-rich blood returns to the upper left chamber, called the left atrium, and from there it enters the lower left chamber, called the left ventricle. From the lower left chamber, the oxygen-rich blood is pumped out to all parts of the body.