Stroke Volume and Cardiac Output: How Much Blood?

Stroke volume: the blood per beat

The ventricle never empties completely. It fills to a maximum — the end-diastolic volume, around 120 mL — then squeezes down to a leftover amount, the end-systolic volume, around 50 mL. The difference is the stroke volume: the blood actually ejected on that beat. Here, 120 − 50 = 70 mL per beat.

Stroke volume (SV) = End-diastolic volume − End-systolic volume

  End-diastolic volume (EDV) = 120 mL   (full ventricle, end of filling)
  End-systolic volume  (ESV) =  50 mL   (leftover, end of squeeze)

  SV = 120 mL − 50 mL = 70 mL per beat

Stroke volume is just what came out: the full chamber minus what stayed behind.

Cardiac output: the heart's total delivery

One beat is a drop in the bucket. What the body actually cares about is flow over time: the cardiac output, the litres of blood the heart delivers each minute. The formula could not be simpler — multiply the blood per beat by the beats per minute.

Cardiac output (CO) = Stroke volume × Heart rate

  AT REST
  SV = 70 mL = 0.07 L      HR = 70 beats/min
  CO = 0.07 L × 70 = 4.9 L/min   (~5 litres per minute)

  DURING HARD EXERCISE
  SV rises to ~110 mL      HR rises to ~170 beats/min
  CO = 0.110 L × 170 = 18.7 L/min  (almost 4× resting!)

Two knobs — volume per beat and rate — let output climb nearly four-fold from rest to exercise.

Notice the two ways to raise output: pump more per beat or pump more often. A trained athlete's heart leans on a large stroke volume and can run a low resting rate; a frightened heart leans on speed. When cardiac output can't keep up with what the tissues need, that mismatch is the root of the symptoms we group under heart failure — though, importantly, output can be low for many different reasons.