Cardiac Output (OCR A Level Biology): Revision Note
Calculating cardiac output
Cardiac output (CO) is the term used to describe the volume of blood that is pumped by the heart 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 = number of times a heart beats per minute
This can also be described as the number of cardiac cycles per minute
Stroke volume = the volume of blood pumped out of the heart during 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, so:
heart rate = cardiac output ÷ stroke volume
stroke volume = cardiac output ÷ heart rate
Worked Example
It took a woman 1 second to complete a single cardiac cycle. Her stroke volume was measured at 73 cm3.
Calculate the cardiac output. Give your answer in dm3 per minute.
Step 1: find the heart rate
1 cardiac cycle takes 1 second
1 minute = 60 seconds
60 x 1 = 60 bpm
Step 2: insert relevant figures into the equation
Cardiac output = heart rate x stroke volume
Cardiac output = 60 x 73 = 4 380 cm3 per minute
Step 3: convert to dm3
1 dm3 = 1000 cm3
4 380 ÷ 1000 = 4.38
CO = 4.38 dm3 per minute
Examiner Tips and Tricks
While this has not been demonstrated above, it can be useful to convert all the figures found in the question into the units required before starting your working; this way you are less likely to forget to do it later on!
Most cardiac cycle graphs show the changes in pressure in the left ventricle, left atrium and aorta. Remember that in order to work out the stroke volume you need to know the change in volume, not the change in pressure!
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