The rhythmic and incessant beating of the heart is critical to human life. Your heart beats on average about 100,000 times a day and will pump about 42 million gallons of blood during your lifetime.

The heart's main purpose is to pump blood throughout the body to supply cells and tissues with the food and oxygen they need to survive. The body contains approximately six quarts of blood. By completely circulating all six quarts through the body every one to three minutes, the heart keeps the living system -- our body -- running.

As the video describes, oxygen-poor blood is taken into the heart and pumped to the lungs where it picks up oxygen. It then travels back to the heart before being pumped to cells throughout the body. Along the way, blood also passes through the abdomen, past cells of the intestines and the stomach, where it picks up nutrients that it can distribute to nourish other parts of the body.

How exactly does blood in the capillaries exchange oxygen, other gases, and nutrients with nearby cells? In most cases it happens automatically. Differences in pressure and/or concentration are key to these critical exchanges.

For example, the oxygen-rich blood travelling through a capillary is under a slightly greater pressure than the cells adjacent to the capillary. Gases like oxygen move from areas of higher pressure to areas of lower pressure. And even though the pressure difference between the capillaries and the cells is slight, it is enough to cause oxygen to move freely from the blood to the surrounding cells.

Further along, when the blood begins its journey back to the heart and enters the slightly larger-diameter veins, its pressure drops to a point that is lower than the pressure in the adjacent cells. This allows gases like carbon dioxide -- a waste product -- to move out of the cells and into the blood stream, where it can be carried to the lungs and released.