Question

What is cardiac output? What is its value in humans?

Answer 1

Cardiac output is also known as heart output that is used to describe the volume of blood being pumped by the heart. It is dependent on the heart as well as the circulatory system veins and arteries. It is calculated by multiplying stroke volume with heart rate.

Complete answer:
The heart is composed of 4 chambers that are in a relaxed state i.e they are in joint diastole. As the tricuspid and bicuspid valves are open, blood from the pulmonary veins and vena cava flows into the left and right ventricle respectively through the left and right atria. The semilunar valves are closed at this stage. The SAN now generates an action potential that stimulates both the atria to undergo a simultaneous contraction n the atrial systole. The action potential is conducted to the ventricular side by the AVN and AV bundle from where the bundle of his transmits it through the entire ventricular musculature.

This cause the ventricular muscles to contract, the atria undergoes relaxation, coinciding with the ventricular systole. Ventricular systole increases the ventricular pressure causing the closure to tricuspid and bicuspid valves due to attempted backflow of blood into the atria. As the ventricular pressure increases further, the semilunar valves guarding the pulmonary artery and the aorta are forced open allowing the blood in the ventricles to flow through these vessels. The ventricles are now relaxed and its pressure declines further, tricuspid and bicuspid valves are pushed open by the pressure in the atria being emptied by the veins.

This sequential event in the heart which is cyclically repeated is called the cardiac cycle and it consists of systole and diastole of the atria and ventricle. The heart beats 72 times per minute, i.e many cardiac cycles are performed per min. From this, it could be deduced that the duration of a cardiac cycle is 0.8 sec. During a cardiac cycle, each ventricle pumps out an approx 70 ml of blood which is called the stroke volume. The stroke volume multiplied by the heart rate gives the cardiac output.

The cardiac output can be defined as the volume of blood pumped out by each ventricle per min and the average is 5000 ml or 5 L per minute in an individual.

Note: During each cardiac cycle two prominent sounds are produced which can be easily heard through a stethoscope. The first heart sound is associated with the closure of the tricuspid and bicuspid valves whereas the second heart sound is associated with the closure of the semilunar valves. These sounds are of clinical diagnostic significance.