BPK 142 Lecture Notes - Lecture 10: Cardiac Output, Stroke Volume, Fick'S Laws Of Diffusion

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Cardiac Output During Exercise:
Cardiac Output and Oxygen Transport
Cardiac output ("Q"): The amount of blood pumped by either the left or right
ventricle of the heart per minute.
Both the left and right ventricles must have the same cardiac output so that
blood flow through the pulmonary and systemic circuits is maintained equally.
Stroke volume: The amount of blood pumped by either the left or right
ventricle per beat.
Cardiac output = heart rate X stroke volume
Since the blood transports oxygen, when cardiac output increases in exercise
more oxygen will be transported to the working muscles.
This relationship can be expressed by the Fick equation: VO2 = HR X SV
X (a-vO2) diff
Where: VO2 = oxygen uptake or utilization by the tissues in the body
(a-vO2)
diff = arterial-mixed venous oxygen difference
Therefore, in order to increase oxygen uptake, you must increase cardiac
output and/or extract more oxygen from the arterial blood.
In general, the higher the maximal stroke volume --> higher maximal cardiac
output --> higher maximum oxygen uptake (VO2max.)
Exercise Heart Rate
For any given subject, heart usually increases linearly with increasing
workload until the subject's maximum heart rate is reached.
The heart rate at a given oxygen uptake is higher when the exercise is
performed with the arms than with the legs.
Since:
1) the cardiac output required for a given workload is reasonably
similar for trained and untrained subjects
2) trained subjects have a higher stroke volume than untrained
subjects
--> then, for any given workload, trained subjects will have a
lower exercise heart rate.
Stroke Volume During Exercise
Stroke volume: End-diastolic volume minus end-systolic volume
○ Systole: The contraction phase of the cardiac cycle, when the ventricles
pump out their stroke volumes.
○ Diastole; The resting phase of the cardiac cycle, between heart beats
End-diastolic volume (EDV); The volume of blood in each ventricle
at the end of diastole – 120 ml in an untrained person at rest
End-systolic volume (ESV): The volume of blood that remains in
each ventricle after the ventricles have finished contracting – 50 ml in
an untrained person at rest
Ejection fraction: The percentage of EDV ejected with each contraction
Stroke volume increases to its highest values during submaximal exercise
(40% VO2max., HR = 110 - 120) and then remains constant during the
progression from moderate to maximal work.
Mechanism of increase in stroke volume during exercise: Greater systolic
emptying = greater ejection fraction. The heart has a functional residual
volume - at rest in the upright position, only 50 - 60% of the blood in the
ventricle is pumped out of the ventricle during the contraction - 50 to 80 ml of
blood remains in the ventricle.
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