Stroke volume, ejection fraction, and cardiac output

The main job of the heart is to pump oxygenated blood with nutrients through the arteries to the body’s tissues and receive back deoxygenated blood full of waste products through the veins.

Now, let’s zoom into the left ventricle. There’s a moment when the left ventricle is fully relaxed. It occurs at the end of filling or diastole, also called the end-diastolic point, and the volume of blood within the left ventricle is called the end-diastolic volume, and it’s about 120 milliliters. Then the left ventricle contracts, forcing blood through the aorta and into the whole arterial system. After that is another moment when the left ventricle is fully contracted. It occurs at the end of contraction or systole, also called the end-systolic point, and the volume of blood within the left ventricle is called end-systolic volume, and it’s about 50 milliliters. So, end-diastolic volume minus end-systolic volume, gives us the stroke volume, which is the volume of blood that the left ventricle ejects with every heartbeat, or stroke. In this case, the stroke volume is 120 minus 50, which equals 70 milliliters.

Stroke volume is a useful measurement, but it can vary based on the size of a person. For example, a stroke volume of 50 milliliters might be absolutely fine for a small person with a small heart volume, but may be low for a large person with a bigger heart volume. So another helpful measurement is the ejection fraction, which is the stroke volume divided by the end-diastolic volume, Ejection fraction = Stroke Volume / End- Diastolic Volume. In a normal individual that’s 70/120, or about 58%, but it can fluctuate between 50 and 65% and still be considered normal. In other words, at least half of the blood volume in the left ventricle should get pumped out during each heartbeat. In hearts that have a low contractility - a low force of contraction - the ejection fraction can fall below 50%.