The heart is a large muscle that pumps blood throughout the body. Blood carries oxygen to all the organs, muscles, and skin of your body. After the body takes the oxygen out of the blood, the blood returns to the heart. The right side of the heart collects that blood and pumps it to the lungs to get fresh oxygen. This oxygen-rich blood from the lungs then returns to the left side of the heart, where it is pumped back out to the rest of the body, starting the process all over.