Question

Imagine the moment when the heart ventricles begin to contract. At this moment, what is happening with ECG, the heart sounds, the ventricular blood volume, and the tricuspid/bicuspid as well as the semilunar valves? Also, describe how ventricular contraction and blood ejection are affected by mitral regurgitation and by aortic stenosis

Answer 1

In the initial stages of ventricular contraction, the muscles begin to contract rising the pressure in ventricles to a bit higher than earlier. But this pressure, will not be strong enough to open up the pulmonary and aortic semilunar valve, present in right and left heart chamber, which was closed earlier in atrial systole (producing the second heart sound "DUB"). So when the ventricle contraction just begins. It does not change the semilunar valves. They remain closed.

When the pressure inside the ventricles, rises more than the atrial pressure, it rushes the blood towards the atrium closing the tricuspid valve in right chamber and bicuspid valve in left chamber of heart producing the first heart sound "LUB". Since, the blood doesn't rushes out of ventricles in the early phase, the volume of blood remains constant. This phase is known as isovolumetric contraction.

We know that blood always moves from the higher pressure to lower pressure. In the later phase, ejection phase of ventricles, the pressure due to muscle contraction increases more than the pulmonary artery or aorta which results in flow of blood from ventricles to pulmonary trunk (on right side) and to aorta (on left side) . This increased pressure force opens the semilunar valves.

Both the ventricles pump the same amount of blood of of ventricles though left ventricle generate higher pressure. This blood volume escapes out is known as stroke volume which is about 70-80ml. In the resting phase of ventricles, an adult heart contains about 130ml of blood. This voulme is known as end diastolic volume. since, blood flowed out of ventricle would be 70-80ml and in intial diastolic phase it have 130 ml of blood, approximately 50 - 60ml of blood remain in ventricles. This is known as end systolic volume.

ventricular systole occurs at depolarisation which is represented by the QRS complex. It shows the contraction of ventricles followed by increase in pressure of ventricle. It occurs in two phases and lasts for about 270ms. T wave shows ventricular reploarization and relaxation.

Mitral regurgitation is characterised by backward flow of blood into the atrium. When the ventricular pressure arises more than that of atrium, the mitral valve on the left side of heart gets open before the opening of the aortic semilunar valve and thus blood rushes back into the left atrium. Because of mitral regurgitation, the afterload on the ventricle is reduced (total outflow resistance is reduced) so that end-systolic volume can be smaller than normal. Even when the aortic valve are closed the mitral valve remains open resulting in continuous back flow of blood into left atrium. In this condition there is no isovolumetric contraction. During ventricular diastolic filling, the elevated pressure within the left atrium is transmitted to the left ventricle during filling so that left ventricular end-diastolic volume (and pressure) increases.

In aortic stenosis, the left ventricular emptying is affected because of increased outflow resistance due to minor opening aperture of aortic valve for ejection. This outflow resistance causes the pressure gradient within the aortic valve such that the peak systolic pressure within the ventricle is greatly increased. This leads to an increase in ventricular wall stress (afterload), a decrease in stroke volume, and an increase in end-systolic volume. Since, the end systolic volume Is increases, the excess residual volume added to the incoming venous return causes the end-diastolic volume to increase.