What Factors Affect Heart Rate, Stroke Volume, and Cardiac Output?

Heart rate, stroke volume, and cardiac output are affected mainly by exercise intensity, autonomic and hormonal control, venous return, myocardial contractility, and resistance to ventricular ejection. Cardiac output depends directly on both heart rate and stroke volume.

$$\text{Cardiac output} = \text{heart rate} \times \text{stroke volume}$$

or

$$Q = HR \times SV$$

Factors affecting each variable

During exercise, muscle and respiratory pumps increase venous return, producing a greater end-diastolic volume. The myocardium stretches more and contracts with greater force, increasing stroke volume. Sympathetic stimulation also increases both heart rate and contractility.

For example, if heart rate is $150\ \text{beats min}^{-1}$ and stroke volume is $100\ \text{mL beat}^{-1}$:

$$Q = 150 \times 100 = 15{,}000\ \text{mL min}^{-1} = 15\ \text{L min}^{-1}$$

Endurance training typically increases resting and exercising stroke volume through greater blood volume, ventricular filling, and cardiac adaptation. A trained athlete may therefore have a lower resting heart rate while maintaining a similar resting cardiac output.

A common misconception is that cardiac output rises only because heart rate rises. In reality, both heart rate and stroke volume contribute, although stroke volume may plateau at higher exercise intensities in some individuals.

Exam technique

For an IB SEHS response on A.1.3 Transport, state $Q = HR \times SV$, identify the relevant factor, and explain its physiological mechanism. When calculating cardiac output, show substitutions and convert $\text{mL min}^{-1}$ to $\text{L min}^{-1}$ where required.