3.4.6 Cardiac Output

Key Formula:

$$\text{Cardiac Output} = \text{Stroke Volume} \times \text{Heart Rate}$$

1. Stroke Volume (SV):

• The volume of blood ejected by the left ventricle during one contraction.
• Measured in millilitres per beat (ml/beat).
• Typically, around 70 ml in a resting adult.

2. Heart Rate (HR):

• The number of heartbeats per minute.
• Measured in beats per minute (bpm).
• Resting heart rate in adults is usually ~70 bpm.

Units of Cardiac Output:

• Measured in litres per minute (L/min).
• E.g., with a stroke volume of 70 ml and a heart rate of 70 bpm: $\text{Cardiac Output} = 70 \, \text{ml} \times 70 \, \text{bpm} = 4900 \, \text{ml/min} = :highlight[4.9 \, \text{L/min}].$

Factors Affecting Cardiac Output:

1. Exercise:

• Increases heart rate and stroke volume, leading to a higher cardiac output.
• Provides more oxygen to muscles and removes carbon dioxide efficiently.

2. Fitness Levels:

• Athletes may have a larger stroke volume and a lower resting heart rate, resulting in a more efficient cardiac output.

3. Adrenaline:

• Stimulates the heart to beat faster and stronger, increasing cardiac output during stress or danger.

4. Health Conditions:

• Heart failure can reduce stroke volume, lowering cardiac output.
• Anaemia or low oxygen levels can trigger an increase in heart rate to compensate.

Why Cardiac Output is Important:

1. Delivers Oxygen:

• Ensures tissues receive adequate oxygen for cellular respiration.

2. Removes Waste Products:

• Transports carbon dioxide and other metabolic waste for removal.

3. Maintains Blood Pressure:

• Sustains sufficient pressure to transport blood through arteries and capillaries.

Key Definitions:

• Cardiac Output: Volume of blood pumped by the heart per minute.
• Stroke Volume: Volume of blood pumped by the left ventricle in one contraction.
• Heart Rate: Number of heartbeats per minute.