A resting adult has a heart rate of 72 beats per minute and a stroke volume of 70 mL. (i) Calculate his restin

Concept understanding — Cardiac Output Formula

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.

Note

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 …

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Cardiac output, the volume of blood pumped by the heart per minute, increases significantly during exercise to meet the body's elevated demands for oxygen and nutrient delivery.

Cardiac output is a fundamental measure in cardiovascular physiology, representing the total volume of blood pumped by the heart's left ventricle into the aorta per minute. Understanding cardiac output is crucial in Physical Education because it directly reflects the heart's efficiency and its ability to supply oxygen and nutrients to the working muscles, especially during physical activity. When we exercise, our muscles need more oxygen and fuel to generate energy, and they also produce more waste products. The heart must respond by increasing its output to meet these heightened demands.

Cardiac output is determined by two primary factors: the heart rate and the stroke volume.

Cardiac Output (CO) = Heart Rate (HR) \times Stroke Volume (SV)

Where:

Heart Rate (HR) is the number of times the heart beats per minute, measured in beats per minute (bpm).
Stroke Volume (SV) is the volume of blood pumped out by the left ventricle in a single beat, typically measured in millilitres (mL).
Cardiac Output (CO) is the total volume of blood pumped per minute, usually expressed in millilitres per minute (mL/min) or litres per minute (L/min).

(i) Calculate his resting cardiac output in litres per minute.

To calculate the resting cardiac output, we use the given resting heart rate and stroke volume.

Given resting heart rate (HR) = 72 beats per minute
Given resting stroke volume (SV) = 70 mL per beat
Using the formula, Cardiac Output (CO) = HR × SV CO(resting) = 72 bpm × 70 mL/beat CO(resting) = 5040 mL/minute
To convert millilitres per minute to litres per minute, divide by 1000: CO(resting) = 5040/1000 L/minute CO(resting) = 5.04 L/minute

(ii) During maximal exercise his heart rate rises to 190 bpm and his stroke volume to 120 mL — calculate his maximal cardiac output.

To calculate the maximal cardiac output, we use the given maximal exercise heart rate and stroke volume.

Given maximal exercise heart rate (HR) = 190 beats per minute
Given maximal exercise stroke volume (SV) = 120 mL per beat
Using the formula, Cardiac Output (CO) = HR × SV CO(maximal) = 190 bpm × 120 mL/beat CO(maximal) = 22800 mL/minute
To convert millilitres per minute to litres per minute, divide by 1000: CO(maximal) = 22800/1000 L/minute CO(maximal) = 22.8 L/minute

(iii) State by what factor the cardiac output has increased and explain why this increase is necessary.

First, let's calculate the factor by which the cardiac output has increased.

Factor of increase = (Maximal Cardiac Output)/(Resting Cardiac Output) Factor = (22.8 L/minute)/(5.04 L/minute) Factor ≈ 4.52 The cardiac output has increased by a factor of approximately 4.52.

This significant increase in cardiac output during maximal exercise is absolutely necessary to meet the dramatically elevated metabolic demands of the body, particularly the working muscles. Here's why:

Increased Oxygen Delivery: During intense exercise, muscles require a much greater supply of oxygen to fuel aerobic respiration, which produces ATP (energy). A higher cardiac output means more oxygenated blood is pumped to the muscles per minute, ensuring they receive enough oxygen to sustain activity. …

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Q.A resting adult has a heart rate of 72 beats per minute and a stroke volume of 70 mL.

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