Air is inhaled into the lungs where oxygen is exchanged through tiny gas permeable sacs within the lungs for carbon dioxide from the blood. The heart pumps the oxygen rich blood from the left atrium through the arteries then through tiny vessels called capillaries to the tissues of the body. At the cell level, oxygen is given up for metabolism and the carbon dioxide produced by this action is picked up by the blood. The oxygen depleted and carbon dioxide rich blood is then pumped back to the heart, through the veins to the right atrium to the lungs where the process is repeated.
Aerobic activity increases the strength of the heart muscle. The result is a greater volume of blood per stroke. This is referred to as Stroke Volume or the amount of blood ejected from each ventricle of the heart during one stroke. Cardiac Output is a measure of the amount of blood pumped through each ventricle in one minute. Vital Capacity is the volume of air that can be forcibly ejected from the lungs in a single expiration. Aerobic activity provides a Training Effect on Vital Capacity, Stroke Volume and Cardiac Output. By definition an artery carries blood away from the heart while veins carry blood toward the heart.