Cardiac Output: The Heart's Delivery System
Think of your heart as the delivery engine of your body. Every cell needs oxygen and nutrients to survive, and every cell produces waste that must be carried away. The heart's job is to pump blood — the delivery vehicle — through a closed network of pipes (arteries, veins, capillaries) to reach every tissue. But how much blood does the heart actually move in a given time? That quantity is called cardiac output.
The Core Idea
Cardiac output is simply the volume of blood the heart pumps out in one minute. It tells you how hard the heart is working to meet the body's demands. If the body needs more oxygen — say, during exercise or stress — cardiac output rises. If the heart is weak or damaged, cardiac output falls, and the body struggles to get enough supply.
Two factors determine cardiac output:
- Stroke volume — the amount of blood pumped out by one ventricle in a single beat (about 70 mL at rest in a healthy adult)
- Heart rate — the number of beats per minute (about 72 beats per minute at rest)
Cardiac output is the product of these two: stroke volume multiplied by heart rate. At rest, this works out to roughly 5 litres per minute — which is nearly the entire blood volume of an average adult. That means your heart pumps all your blood through your body once every minute.
The heart has two sides — right and left — but cardiac output refers to the output of one ventricle (usually the left ventricle, which pumps blood to the whole body). Both ventricles pump the same volume per minute; otherwise blood would pool on one side.
Why It Matters
Cardiac output is not a fixed number. It changes constantly to match the body's needs. When you run, your muscles demand more oxygen, so your heart rate rises and stroke volume increases — cardiac output can climb to 20–25 litres per minute in a trained athlete. When you sleep, demand drops, and cardiac output falls.
Doctors and physiologists care about cardiac output because it reveals how well the heart is functioning. A low cardiac output can indicate heart failure, where the pump is too weak to meet demand. A high cardiac output can occur in conditions like anaemia or hyperthyroidism, where the body demands more blood flow even at rest.
Several factors influence cardiac output:
- Physical activity — increases both heart rate and stroke volume
- Emotional state — fear or excitement raises heart rate
- Body temperature — fever increases metabolic demand and cardiac output
- Blood volume — if you lose blood, stroke volume drops and cardiac output falls
- Nervous system signals — the autonomic nervous system speeds up or slows down the heart …