When you exercise, your cardiovascular system works harder. It needs to deliver oxygen to muscles, transport heat to the skin, transport hormones to meet metabolic demands and deliver nutrients and fuel to tissues. The volume delivered by each beat of your heart -- your stroke volume -- increases when you exercise to increase the circulating blood in your system so your body responds appropriately to exercise.
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Stroke volume is the amount of blood that is pumped out of the left ventricle to the body with each heartbeat. Stroke volume increases according to how you exercise because your body needs more oxygen and nourishment, which are both received from the blood. Stroke volume increases depending on the type of physical activity your are doing and your training level. For example, during an upright physical activity like jogging, stroke volume increases from about 50 mL at rest to 120 mL at maximal exercise intensity. In a trained Olympic runner, stroke volume can increase from 80 mL at rest to 200 mL during maximal exercise intensity as the heart pumps more efficiently.
Your stroke volume increases during exercise but reaches a plateau, as there is a limit to how much blood your body can pump during physical activity. At this point, stroke volume may remain steady up to the point of exhaustion, which causes you to stop exercising. It is also important to note that in supine -- lying on your back with your face upward -- physical activities like certain swimming positions, there may be a smaller increase in stroke volume. This is because a supine activity prevents blood from pooling in the lower extremities, which enhances venous return and decreases the need for increased stroke volume to meet the body's needs.
One anatomical explanation for the increase in stroke volume during exercise is the Frank-Starling mechanism. Blood is pumped to the body from the left ventricle and when this ventricle fills more completely, it stretches further and produces a more forceful contraction. In other words, more blood entering the heart results in more blood being ejected. This mechanism results in a greater amount of blood being circulated through your body during exercise.
An increase in stroke volume is only seen during aerobic exercises like running, swimming or cycling. Many anaerobic exercises like weightlifting are of short duration and affect your heart differently.