Factors Affecting Cardiac Output

The three quantities and the equation that links them

Heart rate (HR) is the number of times the heart beats each minute, measured in beats per minute (bpm). Stroke volume (SV) is the volume of blood ejected from the left ventricle in one beat, measured in millilitres (ml). Cardiac output (CO) is the volume of blood ejected from the left ventricle in one minute, measured in litres per minute (l/min).

The three are locked together by one equation, and you must be able to state it and use it in both directions:

Cardiac output = heart rate × stroke volume → CO = HR × SV

Because stroke volume is in millilitres and cardiac output is in litres, the calculation has two stages: multiply, then divide by 1000 to convert ml to l. Rearranging the equation gives stroke volume = cardiac output ÷ heart rate and heart rate = cardiac output ÷ stroke volume.

Typical values: untrained individual versus trained athlete

You are expected to know the values in these tables, with their units, and to be able to compare an untrained individual with a trained athlete at the same intensity. Learn the shape of each row, not just single numbers: heart rate and cardiac output climb steadily with intensity, while stroke volume climbs and then levels off.

Maximum heart rate is the same for both, because it is worked out from age: 220 minus age. A 17-year-old performer has a maximum heart rate of 220 − 17 = 203 bpm.

• Trained athlete, rest: heart rate 30–70 bpm, stroke volume 80–120 ml, cardiac output 4.8–6.4 l/min
• Trained athlete, sub-maximal intensity: heart rate 95–120 bpm, stroke volume 160–200 ml, cardiac output 15–20 l/min
• Trained athlete, maximal intensity: heart rate 220 minus age, stroke volume 160–200 ml, cardiac output 30–40 l/min
• The two resting cardiac outputs are the same. The trained athlete reaches theirs with a much lower heart rate and a much larger stroke volume.

Worked calculations

Calculations are easy marks if you show your working in stages. Write the equation, substitute, multiply, then convert.

• Untrained at rest: HR 70 bpm, SV 75 ml → CO = 70 × 75 = 5250 ml/min = 5.25 l/min (inside the resting range 4.8–6.4 l/min)
• Trained at maximal intensity: HR 195 bpm, SV 180 ml → CO = 195 × 180 = 35 100 ml/min = 35.1 l/min (inside the trained maximal range 30–40 l/min)
• Rearranged, to find stroke volume: CO 20 l/min at a heart rate of 200 bpm → SV = 20 000 ÷ 200 = 100 ml
• Rearranged, to find heart rate: CO 18 l/min with a stroke volume of 160 ml → HR = 18 000 ÷ 160 = 113 bpm (a sub-maximal intensity for a trained athlete)

The relationship between heart rate, stroke volume and cardiac output

Cardiac output has to rise during exercise because the working muscles need more oxygen delivered and more carbon dioxide and heat removed. Both heart rate and stroke volume increase, but they increase differently.

Heart rate rises in a straight line with exercise intensity, from around 60–80 bpm at rest to a maximum of 220 minus age. It is the fastest way for the body to raise cardiac output, because the cardiac control centre can change it within a single beat.

Stroke volume rises with intensity too, but not in a straight line. It rises steeply at first, up to a moderate intensity, and then levels off and holds steady, because a very fast heart rate shortens diastole, the filling phase, so the ventricle has less time to fill and cannot eject more blood per beat.

• At rest: CO is 4.8–6.4 l/min for both a trained and an untrained performer
• During sub-maximal exercise: CO rises to 10–15 l/min (untrained) or 15–20 l/min (trained)
• During maximal exercise: CO reaches 20–30 l/min (untrained) or 30–40 l/min (trained)
• Immediately before exercise: an anticipatory rise in heart rate is caused by adrenaline release, so heart rate is already above resting value at the start

Starling's law of the heart

Starling's law says that the more the myocardium is stretched by the blood returning to the heart, the greater the force of contraction, and therefore the greater the stroke volume.

The chain to learn is: more venous return → greater stretch of the myocardium → stronger contraction → larger stroke volume → larger cardiac output. This is why stroke volume rises as exercise intensity rises, and why a trained athlete, who has a larger blood volume and stronger venous return, has a much bigger stroke volume than an untrained performer.

Regulation of heart rate

Heart rate is controlled by the cardiac control centre, which sits in the medulla oblongata of the brain. It receives information from receptors and then alters the balance of nerve impulses sent along the autonomic nervous system to the sinoatrial node.

The sympathetic nervous system increases heart rate and the force of contraction. The parasympathetic nervous system, working through the vagus nerve, decreases heart rate. At rest the parasympathetic system dominates; during exercise the sympathetic system takes over.

• Baroreceptors detect changes in blood pressure; a fall in blood pressure causes the cardiac control centre to increase sympathetic stimulation and raise heart rate
• Chemoreceptors detect changes in carbon dioxide, oxygen and pH in the blood; a rise in carbon dioxide during exercise raises heart rate
• Mechanoreceptors detect the movement and stretch of muscles and joints; their impulses contribute to the anticipatory rise before exercise begins
• Hormonal control: adrenaline released from the adrenal glands travels in the blood and increases heart rate and the force of contraction
• Intrinsic control: stretch of the myocardium (Starling's law) increases the force of contraction, and an increase in temperature raises heart rate

Responses to exercise and changes caused by long-term training

During exercise of increasing intensity, heart rate and cardiac output rise all the way to maximum while stroke volume rises and then plateaus. During recovery the reverse happens: heart rate drops quickly at first, then more slowly, and stroke volume and cardiac output return to resting levels as venous return falls and the need for oxygen at the muscles falls.

Long-term training changes the resting picture, not just the maximum. The heart undergoes cardiac hypertrophy: the myocardium, especially of the left ventricle, becomes thicker and the chamber becomes larger.

• Resting heart rate falls (30–70 bpm in a trained athlete) because a larger stroke volume means the same cardiac output can be achieved with fewer beats
• Resting stroke volume rises (80–120 ml) because of cardiac hypertrophy, a larger blood volume and stronger venous return
• Resting cardiac output is unchanged at 4.8–6.4 l/min — a lower heart rate multiplied by a larger stroke volume gives the same answer
• Maximum cardiac output rises from 20–30 to 30–40 l/min because maximum stroke volume rises while maximum heart rate is unchanged
• Recovery is faster in a trained athlete because the heart returns to resting values sooner after exercise stops