Cardiac Output: Definition, Formula, Factors and Regulation

Introduction

The heart continuously pumps blood to supply oxygen and nutrients to different parts of the body. The amount of blood pumped by the heart in one minute is known as cardiac output.

Cardiac output is an important measure of how effectively the heart supplies blood to the body's tissues.

For example, during exercise, muscles require more oxygen. The heart responds by increasing its pumping activity, which generally increases cardiac output.

1. What Is Cardiac Output?

Cardiac output (CO) is the volume of blood pumped by one ventricle of the heart in one minute.

It is usually expressed in litres per minute (L/min) or millilitres per minute (mL/min).

Cardiac output depends mainly on two factors:

• Heart rate (HR): Number of heartbeats per minute.

• Stroke volume (SV): Amount of blood pumped by one ventricle during each heartbeat.

The right and left ventricles normally pump approximately equal amounts of blood over time.

2. Formula of Cardiac Output

Cardiac output is calculated by multiplying heart rate by stroke volume.

<LearningViz type_id="CARDIAC_OUTPUT_PRODUCT" initial_values={{"heartRateBeatsPerMinute":75,"strokeVolumeMilliliters":70}} locale_override="en-US"/>

Where:

• CO = Cardiac output

• HR = Heart rate (beats per minute)

• SV = Stroke volume (mL per beat)

Example Calculation

Suppose a person's:

• Heart rate = 75 beats/minute

• Stroke volume = 70 mL/beat

The cardiac output is:

75 × 70 = 5,250 mL/minute

Therefore, the cardiac output is 5.25 L/minute.

This means the heart pumps approximately 5.25 litres of blood per minute under these conditions.

3. Normal Value of Cardiac Output

In a healthy resting adult, cardiac output is approximately 5 L/minute, although it varies with body size, age, physical activity and physiological condition.

During exercise, cardiac output can increase considerably to meet the higher oxygen requirements of muscles.

Important: The normal value is an approximate resting value, not a fixed measurement for every person.

4. Factors Affecting Cardiac Output

Cardiac output changes according to the body's requirements. Its two main components are heart rate and stroke volume.

A. Heart Rate

An increase in heart rate generally increases cardiac output if stroke volume remains unchanged.

For example, during running, the heart beats faster to supply more blood to active muscles.

However, an extremely high heart rate may reduce the time available for ventricular filling and can decrease stroke volume.

B. Stroke Volume

Stroke volume is the amount of blood ejected by one ventricle during a single heartbeat.

It is influenced mainly by three factors:

1. Preload

Preload refers to the degree of stretching of the ventricular muscle before contraction. It is related to the amount of blood filling the ventricles.

Generally, increased ventricular filling can increase the force of contraction and stroke volume within physiological limits.

2. Contractility

Contractility is the ability of the heart muscle to contract forcefully.

Increased contractility generally increases the amount of blood pumped with each heartbeat.

3. Afterload

Afterload refers to the resistance or pressure that the ventricles must overcome to eject blood.

Increased afterload can make it more difficult for the ventricles to pump blood and may reduce stroke volume.

[IMG 1 — Horizontal educational infographic showing the three major factors affecting stroke volume: preload (ventricular filling), contractility (strength of contraction) and afterload (resistance to blood ejection).]

5. Regulation of Cardiac Output

The body regulates cardiac output according to its physiological requirements.

A. Autonomic Nervous System

The autonomic nervous system influences both heart rate and the force of contraction.

• Sympathetic stimulation: Increases heart rate and contraction strength, generally increasing cardiac output.

• Parasympathetic stimulation: Mainly decreases heart rate, which can reduce cardiac output when other factors remain unchanged.

B. Venous Return

Venous return is the amount of blood returning to the heart through the veins.

An increase in venous return generally increases ventricular filling. This can increase stroke volume and therefore cardiac output.

C. Physical Activity

During exercise, sympathetic activity increases and more blood returns to the heart from active muscles.

These changes help increase cardiac output to meet the body's increased oxygen requirements.

6. Clinical Importance of Cardiac Output

Cardiac output is important for maintaining adequate blood supply to organs and tissues.

Changes in cardiac output may occur in several physiological and medical conditions.

A. During Exercise

Cardiac output increases to deliver more oxygen and nutrients to working muscles.

B. In Heart Failure

The heart may become unable to pump sufficient blood to meet the body's requirements. Cardiac output may be reduced, particularly in certain forms of heart failure.

C. During Severe Blood Loss

Blood volume and venous return may decrease, reducing ventricular filling, stroke volume and cardiac output.

D. During Shock

In some types of shock, inadequate cardiac output or abnormal blood distribution can result in insufficient oxygen delivery to tissues.

Clinical Note: Cardiac output is one important measure of cardiovascular function. It is interpreted along with blood pressure, heart rate, symptoms and other clinical findings.

Key Concepts

• Cardiac output is the volume of blood pumped by one ventricle in one minute.

• It depends mainly on heart rate and stroke volume.

• The approximate resting cardiac output in a healthy adult is 5 L/minute.

• Stroke volume is influenced by preload, contractility and afterload.

• Sympathetic stimulation generally increases cardiac output.

• Exercise increases cardiac output to meet the body's higher oxygen requirements.

• Reduced cardiac output can affect the delivery of oxygen and nutrients to body tissues.