Stroke volume is affected by preload and afterload, according to Cheetah Medical. The preload is the load placed on the ventricle by the entering blood volume. This is like a balloon being filled with water. The afterload is the resistance the ventricle pumps against to push the blood out of the heart and into the rest of the body.
Stroke volume is also affected by the body's demand for oxygen, delivery of nutrients to the muscles and tissues and the removal of toxins, explains Cheetah Medical. For example, exercise increases the demand for oxygen, which results in a faster heart rate and increased stroke volume. People with a high level of cardiovascular fitness have larger stroke volumes and lower heart rates at rest than people who have a lower level of cardiovascular fitness. This is because the trained heart has naturally higher stroke volumes, thus the heart pumps less often to meet the body's demand for oxygen.
Cheetah Medical notes that changes in stroke volume can be an indicator of cardiac problems. This is measured through the stroke volume/stroke volume index. Stroke volume is measured as the amount of blood ejected by the left ventricle in one contraction. It is measured in ml/beat.