Cardiac Output

Introduction

Cardiac output is a crucial physiological parameter that quantifies the volume of blood pumped by each ventricle of the heart in one minute. It serves as an essential indicator of cardiovascular function and reflects the heart's ability to meet the body's circulatory demands. Cardiac output is determined by two primary factors: heart rate and stroke volume. Understanding how these variables interact and their significance in maintaining overall health is fundamental in the study of human biology.

Cardiac Output Definition

Cardiac output refers to the volume of blood ejected by each ventricle of the heart (the left ventricle and the right ventricle) in one minute.

Factors Determining Cardiac Output

Heart Rate (HR)

Heart rate represents the number of heartbeats (contractions) per minute. It is typically measured in beats per minute (bpm).

Stroke Volume (SV)

Stroke volume refers to the volume of blood ejected by each ventricle with each heartbeat. It is usually expressed in millilitres per beat (mL/beat).

Calculation of Cardiac Output

Formula

Cardiac output (CO) can be calculated using the following formula

• CO = HR x SV
• Where:
  • CO: Cardiac Output (in mL/minute or L/minute)
  • HR: Heart Rate (in bpm)
  • SV: Stroke Volume (in mL/beat or L/beat)

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Cardiac Output

Significance of Cardiac Output

Circulatory Efficiency

Cardiac output is a critical parameter in assessing the efficiency of the circulatory system. It determines the amount of blood that is circulated through the body's tissues, delivering oxygen and nutrients while removing waste products.

Response to Demand

Cardiac output can adapt to changing physiological demands. For example, during exercise, the body requires increased oxygen delivery to active muscles. To meet this demand, both heart rate and stroke volume can increase, resulting in a higher cardiac output.

Ventricular Symmetry

Equal Output

It is essential to note that the left and right ventricles of the heart pump the same volume of blood per minute. While the left ventricle ejects oxygenated blood into the aorta for distribution to the body's tissues, the right ventricle pumps deoxygenated blood into the pulmonary artery for oxygenation in the lungs.

Examples of Cardiac Output

1. Resting State

In a resting adult, the typical cardiac output may be around 4.5 to 5.5 litres per minute. This is achieved with a moderate heart rate and stroke volume.

2. Exercise

During intense physical activity or exercise, cardiac output can increase significantly. For example, in elite athletes, cardiac output can reach up to 30 to 35 litres per minute, demonstrating the heart's ability to adapt to increased oxygen demand.

3. Pathological Conditions

Certain medical conditions, such as heart failure, can reduce cardiac output, leading to inadequate circulation and potential symptoms of fatigue, shortness of breath, and oedema.

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Cardiac Output

Summary

Cardiac output is a vital physiological parameter that quantifies the volume of blood ejected by each ventricle of the heart in one minute. It is determined by two primary factors: heart rate (the number of heartbeats per minute) and stroke volume (the volume of blood ejected by each ventricle with each heartbeat). Cardiac output is a key indicator of circulatory efficiency and the heart's ability to meet the body's oxygen and nutrient demands. The left and right ventricles of the heart pump the same volume of blood per minute, with the left ventricle supplying oxygenated blood to the body and the right ventricle pumping deoxygenated blood to the lungs for oxygenation. Cardiac output can vary under different conditions, such as during exercise, and is crucial for overall health and well-being.