Cardiac Output, Heart Rate, and Stroke Volume

Cardiac output is the volume of blood the heart pumps per minute and is the product of heart rate and stroke volume. During exercise it can rise several-fold above rest as heart rate climbs and stroke volume increases, providing the central engine that delivers oxygen to working muscle. How heart rate and stroke volume each contribute, and how they reach their ceilings, defines the central limit to whole-body exercise capacity.

Definition

Cardiac output is the quantity of blood ejected by the heart per minute, equal to heart rate multiplied by stroke volume; stroke volume is the blood ejected per beat, and heart rate is the number of beats per minute.

Scope

The topic covers the definition of cardiac output, the separate behaviour of heart rate and stroke volume from rest to maximal exercise, the mechanisms that raise each, and the way their product sets oxygen delivery. It treats these as reference physiology and does not provide testing thresholds or training prescriptions.

Core questions

• How much does cardiac output rise from rest to maximal exercise, and what sets its ceiling?
• How do heart rate and stroke volume each change as exercise intensity increases?
• Which mechanisms raise stroke volume during dynamic exercise?
• Why is central cardiac output considered a principal limit to maximal oxygen uptake?

Key concepts

• Cardiac output equals heart rate times stroke volume
• Frank-Starling mechanism and venous return
• Ejection fraction and end-diastolic volume
• Maximal heart rate
• Stroke volume plateau
• Fick principle linking output to oxygen uptake

Mechanisms

At exercise onset, parasympathetic withdrawal followed by rising sympathetic drive increases heart rate, while the muscle pump and venoconstriction enhance venous return; greater filling (Frank-Starling mechanism) together with increased contractility raises stroke volume (Rowell, 1974). Stroke volume rises steeply at low-to-moderate intensities and then tends to plateau, so that further increases in cardiac output at high intensity depend largely on heart rate (Åstrand et al., 1964). Maximal cardiac output is achieved when maximal heart rate coincides with a near-maximal stroke volume, and this central oxygen-delivery capacity is a principal determinant of maximal oxygen uptake (Joyner & Casey, 2015).

Clinical relevance

The behaviour of cardiac output, heart rate, and stroke volume underlies the interpretation of exercise testing and helps explain how reduced pump function can limit exercise tolerance. This entry is descriptive reference physiology and is not intended to guide individual diagnosis, prognosis, or treatment.

Evidence & guidelines

Quantitative descriptions come from classic invasive physiology studies and from synthesising reviews rather than from clinical guidelines. Åstrand and colleagues measured cardiac output across submaximal and maximal work, and Rowell's review integrates these data into the broader picture of circulatory control.

History

Direct measurement of cardiac output during work, using dye-dilution and the Fick principle, established in the mid-twentieth century how the heart's output scales with exercise intensity. Åstrand and colleagues' 1964 study is a benchmark description of cardiac output during submaximal and maximal work, and later reviews placed these findings within the integrated control of the circulation.

Debates

Does stroke volume truly plateau during upright exercise?

Whether stroke volume reaches a plateau at moderate intensity or continues to rise toward maximal effort has been debated, with the answer depending on posture, fitness, and measurement method; in upright dynamic exercise a plateau is commonly observed.

Key figures

• Per-Olof Åstrand
• Bengt Saltin
• Loring Rowell
• Michael Joyner

Seminal works

• astrand-1964
• rowell-1974
• joyner-casey-2015

Frequently asked questions

How is cardiac output calculated?

Cardiac output equals heart rate multiplied by stroke volume, so it reflects both how fast the heart beats and how much blood it ejects with each beat.

Does heart rate or stroke volume drive the rise in cardiac output during exercise?

Both contribute, but stroke volume rises mainly at lower intensities and tends to plateau, so increases in cardiac output at high intensity depend largely on the continuing rise in heart rate.