1. We discussed many factors associated with determination of stroke volume. A.

Question

1.      We discussed many factors associated with determination of stroke volume.

A. State the equations used to determine SV and CO. State the average values for all components.

B. What happens to stroke volume when:

                           i.          ESV increases or decreases

                         ii.          EDV increases or decreases

                        iii.          increased sympathetic innervation

                        iv.          increased parasympathetic innervation

                          v.          afterload increases or decreases

Explanation / Answer

Answer:

1.A. Cardiac output (CO), is a term used in cardiac physiology that describes the volume of blood being pumped by the heart, in particular by the left or right ventricle, per unit time.

Cardiac output is the product of the heart rate (HR), which is the number of heart beats per minute, and the stroke volume (SV), which is the volume of blood pumped from the ventricle per beat i.e.

CO = HR X SV

Normal range: In a subject weighing 70 kg, the cardiac output at rest would be around 5 liters/minute assuming a heart rate of 70 beats/min, the stroke volume would be approximately 70 ml.

Stroke volume:
Its value is obtained by subtracting end-systolic volume (ESV) from end-diastolic volume (EDV) for a given ventricle.

SV = EDV ESV

In a healthy 70-kg man, ESV is approximately 50 mL and EDV is approximately 120mL, giving a difference of 70 mL for the stroke volume.

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1.B.
i) If ESV increases, SV decreases and if ESV decreases, SV increases.
ii) EDV increases, SV increases and if EDV decreases, SV decreases.
iii)Autonomic nerves not only innervate the SA node, but also are found elsewhere in the heart. Norepinephrine released by sympathetic nerves increases the force with which ventricular muscle fibers contract (by increasing the Ca++ effect). Norepinephrine, by increasing the force of contraction, would tend to increase the ejection fraction and thus the stroke volume.
increased sympathetic innervation will increase stroke volume.
iv) increased parasympathetic innervation will decrease stroke volume.
v) Elevated afterload (commonly measured as the aortic pressure during systole) reduces stroke volume and decreased afterload increases stroke volume.

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