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Heart Rate, Stroke Volume and Cardiac Output
The primary function of the heart is to act as a pump. How can we measure its
performance and ability to pump and circulate blood around the body? Simple
– measure its output….how much blood the heart is pumping out per min.
Heart rate (HR) – the number of times the heart ventricles beat in one minute.
Stroke Volume (SV) – the volume of blood ejected by the heart each time a
ventricle contracts. SV is the difference in the volume of blood in the ventricle
before and after ventricle contraction. The following terms are used to
measure SV:
End-diastolic volume (EDV) – before, refers to the volume of blood in the
ventricle at the end of the relaxation filling phase.
End-systolic volume (ESV) – after, refers to the volume of blood remaining
in the ventricles at the end of the contraction phase.
The average resting values for EDV and ESV shows that only 40/50% of the
blood in the ventricles is pumped out – this equates to about half a glass of
wine at rest.
EDV – before =130ml
ESV – after = 60ml
Therefore SV = 70ml
Cardiac Output (Q) – the amount of blood circulated by the heart per min
(the volume of blood ejected by the heart ventricles in one minute).
This can be expressed in word form
Q = SV x HR
(L/min) = (ml per beat) x (beats per min)
Homework
Answer the following:
a) What is the cardiac output at rest of a person with a RHR of 75bpm
and stroke volume of 70ml? (1 mark)
b) If an athlete has a resting Q of 5L/min, but a RHR of 60, what is their
resting SV? Suggest reasons why the resting SV has increased.
(3 marks)
c) At rest, cardiac output is roughly the same for each individual. How can
this be so when the RHR of fit and unfit people can vary greatly?
Discuss this in relation to a trained athlete with RHR of 50bpm and SV
of 100ml and an untrained person with RHR of 70bpm and SV of
70/72ml. (4 marks)