The primary role of the heart is to pump oxygenated blood around the body to the working muscles and organs. In order for this to occur, deoxygenated blood from the heart must pass through the lungs before it goes out tot eh rest of the body. Deoxygenated blood that has entered the heart is then immediately pumped into the lungs to become oxygenated, through diffusion of CO2 and O2 in the capillaries on the lung surface (sacs called alveoli). Once the O2 is picked up by the blood cells, it is then free to pass on throughout the body through the blood stream, to the working muscles. Here, oxygen will be exchanged for CO2 once again (this blood is now DEoxygenated), and will then return to the heart, and the process will start again.
