Cardiac Output (AQA A Level Biology)

Maths Skill: Using the Cardiac Output Equation

• Cardiac output (CO) is the term used to describe the volume of blood that is pumped by the heart (the left and right ventricle) per unit of time

• An average adult has a cardiac output of roughly 4.7 litres of blood per minute when at rest

• Individuals who are fitter often have higher cardiac outputs due to having thicker and stronger ventricular muscles in their hearts

• Cardiac output increases when an individual is exercising

  • This is so that the blood supply can match the increased metabolic demands of the cells

• The CO of an individual can be calculated using their heart rate and stroke volume

• Heart rate is the number of times a heart beats per minute

  • This can also be described as the number of cardiac cycles per minute

• Stroke volume is the volume of blood pumped out of the left ventricle during one cardiac cycle

Image showing the changes in volume of the left ventricle within one cardiac cycle.

Calculating cardiac output

• Cardiac output is found by multiplying the heart rate by the stroke volume:

Cardiac output = heart rate x stroke volume

• The equation can be rearranged to find the heart rate and stroke volume if required

Step 1: Find the heart rate

1 cardiac cycle (atrial systole, ventricular systole and diastole) takes 1 second

To find the number of cardiac cycles completed in a minute, multiply by 60

60 x 1 = 60 bpm

Step 2: Insert relevant figures into the equation

Cardiac output = heart rate x stroke volume

Cardiac output = 60 x 77 = 4,620 cm³

CO = 4.62 dm³

Step 1: Rearrange the equation

Cardiac output = heart rate x stroke volume

Stroke volume = cardiac output ÷ heart rate

Step 2: Insert relevant figures into the equation

4.3 ÷ 72 = 0.0597 dm³

Stroke volume = 59.7 cm³