Cardiac Output (Heart Rate × Stroke Volume)

This is the definition rather than a model, which makes it one of the few relations on this page with no fitted constants in it. Whatever the left ventricle ejects per beat, multiplied by the beats in a minute, is what leaves the heart in a minute. A resting adult sits near 70 mL a beat at 70 beats a minute, so $70 \times 70 = 4900$ mL/min, near enough 5 L/min, which is the number every physiology course opens with.

What makes it interesting is how the two terms trade against each other. A trained endurance athlete may rest at 45 beats a minute with a stroke volume above 100 mL and arrive at the same 5 L/min. Under stress the body raises output to 20 L/min or more, and it does that mostly by raising rate, because stroke volume runs out of room fast: past roughly 160 beats a minute, filling time gets short enough that stroke volume starts falling, and pushing the rate higher can lower output rather than raise it. That non-monotonic behaviour is invisible in the formula and it is the reason a very fast heart is not a good heart.

Two practical cautions. Cardiac output scales with body size, so it is often reported as cardiac index, the same number divided by body surface area, with a normal range near 2.5 to 4.0 L/min/m². And stroke volume is not measured directly at the bedside; it comes from echocardiography, thermodilution or a pulse contour algorithm, each with its own error of 10 to 20 percent, which is far larger than any rounding in this arithmetic.