The heart is a single organ, but it functions as a double pump sending oxygen-depleted blood to the lungs to pick-up oxygen and unload carbon dioxide, while simultaneously sending oxygen-loaded blood coming from the lungs to cells throughout the body. The heart's right ventricle pumps blood to the lungs, while the left ventricle pumps the oxygenated blood to the body cells. The left ventricle is the stronger of the two and performs the more demanding part of the double-pump work.
In addition to moving oxygen throughout the body, the heart also helps deliver nutrients and pick-up waste products. The adult human heart is around the size of a fist, and its double-pumping action moves blood throughout the body about 1,000 times a day. All animals with a circulatory system possess a heart, including invertebrates, although the organ's actual structure can vary among different species.