The main function of the heart is to pump blood to all parts of the body.
The blood carries oxygen and food substances to the cells of the body, and
collects the waste products from those cells. The circulating blood also ensures
that the whole body remains at an even temperature.
The heart has really two pumps working side by side. One side pumps
blood to the lungs and one to the body. The side pumping to the lungs normally
works at a lower pressure than the side to the body, and sends the blue blood
coming back from the body to the lungs to be made red, whilst the higher
pressure side pumps the red oxygenated blood to the body.