Question

How do parasympathetic neural signals affect the working of the heart?
A. Reduce both heart rate and cardiac output
B. Heart rate is increased without affecting the cardiac output
C. Both heart rate and cardiac output increases
D. Heart rate decreases but cardiac output increases

Answer 1

Cardiac output is defined as the amount of blood pumped into the ventricles per minute. The right ventricle pumps blood into the pulmonary circulation, and at the same time, an equal amount of blood is pumped from the left ventricle into the systemic circulation. Cardiac output is the result of stroke volume and heart rate.

Complete answer:
Cardiac output is defined as the amount of blood pumped into the ventricles per minute.
Normal cardiac output is about $5$ liters per ventricle per minute. Stroke volume (SV) is the volume of blood pumped into the stroke ventricle, usually about $70ml$. Normal heart rate (HR) can be around $70$ days per minute. In this case, heart rate $= HR \times SV = 70 \times 70 = 4900ml$ .
Cardiac output is directly related to the body surface. Consequently, the presentation of the absolute value of cardiac output is very imprecise. It is good to express it as an indicator cardiac index..
Regulation of cardiac output and heart rate by neural system:
Effect of parasympathetic nervous system on heart-Stimulation of the parasympathetic (Vagus) nerve has a negative associative effect. Therefore, the heart rate and cardiac decrease .
A moderate decrease in heart rate does not change the cardiac output. This is because heart rate increases ventricular charging time, which increases end-diastolic volume. The stroke volume is based on Starling's Law, so the stroke volume increases.
Effect of sympathetic nervous system on heart-Stimulation of the sympathetic nerve has a positive Chrono tropic effect, increasing heart rate which increases cardiac output.
As the heart rate increases, cardiac output increases somewhat, but when heart rate increases further, heart rate starts to decrease. When the heart rate increases above a certain range, the ventricular charging time decreases. This decreases end diastolic volume, stroke volume, and cardiac output.

Hence, the correct answer is option (A).

Note: Thyroxine increases the sensitivity of the beta receptors of the heart to the effects of catecholamines. In addition, the number of beta receptors in the heart muscle increases. Because of this, hyperthyroidism increases the heart rate and cardiac output.
Catecholamines (epinephrine and norepinephrine) increase heart rate due to their effect on the heart muscle via beta receptors. This behaviour is similar to that caused by sympathetic stimulation. Sympathetic stimulation not only increases heart rate but also increases contractile ability. This leads to a significant increase in cardiac output.