THE PUMPING ACTION

Deoxygenated blood (blood low in oxygen content) from various parts of the body first returns to the right atrium through the two venae cavae. Blood from the head, neck and arm goes to anterior (upper) vena cava and blood from the lower parts of the body is brought back by the posterior (lower) vena cava. From the venae cavae open into the right atrium

The right atrium contracts, blood flows into the right ventricle. You may think that the contraction of the right atrium causes the blood to flow down into the ventricle, but that’s not the case. The ventricle first relaxes, creating an empty cavity for blood to flow into the ventricle. The right atrium then contracts, forcing the remaining blood in it to go down into the right ventricle.

As the right ventricle contracts, blood in it is forced out into the pulmonary artery. There is a valve separating the atrium and ventricle, that prevents blood from flowing back into the atrium as ventricle contracts. This valve is known as the Tricuspid valve, because it has three flaps. These flaps are attached to cord-like tendons called chordae tendineae. As the ventricle contracts, the blood pressure forces the flaps to close. The tendons prevent the flaps from being turned back into the atrium.

Similarly, there is a valve in the pulmonary artery just as it leaves the heart. When the right ventricle relaxes, the valves are forced close to prevent back-flow of blood.

The pulmonary artery then carries the deoxygenated blood to the two lungs, where the blood becomes oxygenated again and returns the heart via the pulmonary vein.

Now the blood comes to the right side of the heart. The left side of the heart works much like the right side.

Blood from the pulmonary vein flows into the left atrium, and enters the left ventricle. Between the left atrium and the left ventricle is the bicuspid valve. This is similar in structure and function to the tricuspid valve but as the name suggests, it has only two flaps instead of three. When the left ventricle contracts, blood leaves by way of a large artery, the aortic arch. From the aortic arch blood is distributed to all parts of the body. The aortic arch curves upwards from the left ventricle as a U-shaped tube and it also possesses valves to prevent the back-flow of blood.

You may have noticed that the heart muscle of the left ventricle is thicker than muscle of the right ventricle. This is because, the right ventricle only needs to pump blood to the lungs, which are not far away from the heart. Less force is required to send the blood to the lungs. However, the left ventricle has a much harder job of pumping blood to the rest of the body.

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©Copyright TQ Team 25896, 1999. The Circulatory System- Online Learning.