Identify whether each item would increase or decrease stroke volume.
- Sympathetic stimulation
- Increasing contractility
- Stress
- Dehydration
- Increasing EDV
- Increasing afferioad
- Increased heart rate i
- Acute exercise
- Decreased venous retum
- Increasing preload
The Asnwer
Increase stroke volume
- Sympathetic stimulation
- Increasing contractility
- Stress
- Increasing EDV
- Acute exercise
- Increasing preload
Decrease stroke volume
- Dehydration
- Increasing after load
- Decreased venous return
- Increased heart rate
Explanation: - Stroke volume is the amount of blood pumped by the left ventricle into the systemic circulation during each heart beat. Its value is around 70 ml in a normal healthy individual.
- Cardiac output is the amount of blood pumped by the left ventricle into the systemic circulation in one minute.
Cardiac output = Stroke volume × heart rate. As stroke volume is around 70 ml and normal heart rate is roughly around 72 beats per minute therefore cardiac output is around 5000 ml per minute or 5 litres/minute.
- Stroke volume can also be expressed as :-
Stroke volume = End diastolic volume (EDV) - End systolic volume (ESV).
End diastolic volume is the amount of blood remaining in the left ventricle at the end of diastolic phase of cardiac cycle just before the beginning of systole. It is usually around 120 ml.
End systolic volume is the amount of blood remaining in the left ventricle at the end of systole or just before the beginning of diastole. It is normally around 50 ml.
Factors increasing stroke volume :-
- Sympathetic stimulation causes the release of neurotransmitters like nor epinephrine and epinephrine which increases the force of contraction of cardiac muscle cells and also increases the heart rate. The over-all effect of sympathetic stimulation is increase in the stroke volume thereby increasing the cardiac output.
- Increasing contractility increases stroke volume as more blood is pumped into the circulation by the ventricular musculature of the heart.
- Stress causes increase in the sympathetic stimulation as a part of fight and flight response of the body and sympathetic stimulation increases the stroke volume.
- Increased end diastolic volume (EDV) means increased amount of blood remains in the ventricles of the heart at the end of diastolic phase which causes increased streching of the cardiac myocytes which in turn causes cardiac myocytes to contract with greater strength during the upcoming systole thus increasing the stroke volume.
- Acute exercise acts a stress on the body and therefore causes sympathetic stimulation increasing the stroke volume and cardiac output both.
- Preload is the amount of stretching force on the cardiac muscle fibers at the end of the diastole. Increased streching of the cardiac muscle cells before systole or at the end of diastole causes the cardiac muscle cells to contract with greater strength during the upcoming systole thus increasing the stroke volume.
Factors decreasing stroke volume :-
- Dehydration causes decrease in the volume of circulating blood which increases the heart rate as a compensatory mechanism to supply blood to the body organs. Increase in the heart rate decreases the stroke volume. Additionally, decrease in the blood volume caused by dehydration causes decrease in venous return which reduces the end diastolic volume which further reduces the stroke volume.
- Afterload is the vascular resistance that the ventricles must overcome to produce outflow of the blood during systole. Thus, an increase in the afterload causes decrease in the stroke volume as the ventricles now need to overcome more resistance to pump the blood.
- Decreased venous return causes less blood to return back into the heart through the venous system. Decrease in the venous return causes decrease in the end diastolic volume which decreases the stroke volume.
- Increased heart rate alone decreases the stroke volume as less time is left for the heart to get filled up during the diastolic phase of cardiac cycle which decreases the end diastolic volume.