1.12.1 Cardiac Output Definition

Cardiac Output Definition is the total volume of blood ejected by a single ventricle into its corresponding circulation over one minute, calculated as the product of stroke volume, the amount of blood ejected per beat, and heart rate, the number of beats occurring per minute. Cardiac output represents the overall pumping capacity of the heart and is the primary determinant of the rate at which oxygenated blood and nutrients are delivered to the tissues of the body.

Mathematical Definition

Cardiac output is derived directly from the two quantities that describe ventricular activity per beat and per minute.

The Cardiac Output Equation

Cardiac output equals stroke volume multiplied by heart rate.

Typical Units

Cardiac output is conventionally expressed in liters of blood per minute, reflecting the combined contribution of the volume ejected with each beat and the frequency of beating.

Left and Right Ventricular Output

Because the pulmonary and systemic circulations are arranged in series, the outputs of the two ventricles must be equal over time.

Balanced Circulatory Flow

The left ventricle ejects blood into the systemic circulation while the right ventricle ejects an equal volume into the pulmonary circulation; sustained imbalance between the two outputs would result in pooling of blood in one circulation at the expense of the other.

Determinants of Cardiac Output

Cardiac output reflects the combined regulation of its two component variables.

Stroke Volume Influences

Preload, afterload, and myocardial contractility together determine how much blood is ejected with each individual beat.

Heart Rate Influences

Autonomic nervous input to the sinoatrial node, along with circulating hormones, determines the rate at which the heart beats, directly scaling total output.

Cardiac Index

To allow meaningful comparison across individuals of different body sizes, cardiac output is often expressed relative to body surface area as the cardiac index.

Diagrammatic Summary

Physiological Significance

Cardiac output must continuously rise and fall to match the metabolic demands of the body, increasing markedly during exercise and decreasing during rest, and its adequacy is fundamental to normal organ perfusion; sustained inability of the heart to generate sufficient cardiac output defines the clinical syndrome of heart failure.