Calculating Cardiac Output
- 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
Graph showing the changes in the volume of the left ventricle within one cardiac cycle; this is the stroke volume
Cardiac output formula
- 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:
- Heart rate = cardiac output ÷ stroke volume
- Stroke volume = cardiac output ÷ heart rate
- Cardiac output is measured in cm3 min-1
- Heart rate is measured in beats per min (bpm)
- Stroke volume is measured in cm3
Worked example
A woman took 1.2 seconds to complete a single cardiac cycle. The stroke volume of her heart was measured at 75 cm3. Calculate the cardiac output, give your answer in dm3.
Step 1: Find the heart rate
One cardiac cycle (atrial systole, ventricular systole and diastole) takes 1.2 seconds
To find the number of cardiac cycles completed in a minute, multiply by 60
60 x 1.2 = 72 bpm
Step 2: Insert relevant figures into the equation
Cardiac output = heart rate x stroke volume
Cardiac output = 72 x 75 = 5,400 cm3
Cardiac output = 5.40 dm3
Worked example
An athlete runs a 10 km marathon, after which his heart rate was measured at 110 bpm and his cardiac output was determined to be 9,800 cm3.
Calculate the stroke volume after the marathon.
Stroke volume = cardiac output ÷ heart rate
Stroke volume = 9,800 ÷ 110
Stroke volume = 89.09 cm3
Exam Tip
1 dm3 is equal to 1000 cm3. It can be useful to convert all the figures found in the first question into the same units before starting your workout, that way you are less likely to make any mistakes!
