The Chambers and Valves of the Heart

Each of the pumps in the heart has two chambers. The upper chamber
is called the atrium, and the lower chambers the ventricle (“belly”).
The atria accumulate the blood arriving at the heart and then inject it
into the ventricles. The ventricles eject the blood from the heart into
the arteries; they perform most of the work required for the pumping.
Each pump has two flap valves. The right pump has the tricuspid valve
(between the atrium and the ventricle) and the pulmonary valve (at the
beginning of the pulmonary artery). The left pump has the mistral
valve (between the atrium and the ventricle) and the aortic valve (at the
beginning of the aorta). These flap valves operate passively; their
leaflets bend open to permit the flow of blood in the forward direction;
but they flip shut when there is an incipient flow in the backward
direction.

We can best understand the operation of the chambers and the valves
of the heart by tracing the flow of a parcel of blood through the heart
and around the entire circulatory system. The blood arrives at the heart
via the inferior and the superior vena cava and enters the right atrium,
where it accumulates until the atrium begins to contract and the
tricuspid valve opens. The blood then flows into the relaxed right
ventricle, filling and expanding it. When the ventricle begins to
contract a moment later, the leaflets of the tricuspid valve flip into their
shut position, while the leaflets of the pulmonary valve opens. The
contraction of the ventricle then propels the blood into the pulmonary
artery. When the ventricle completes its contraction and begins to
relax, the pulmonary valve shuts behind the parcel of blood, preventing
any backflow. Within the lungs, the pulmonary artery branches into
many small arterioles and even smaller capillaries. While passing
through these pulmonary capillaries, the bloodsheds its load of carbon
dioxide and absorbs oxygen. The capillaries connect to veins, which
merge into the main pulmonary veins. These veins carry the blood to
the left atrium of the heart, where it accumulates until the mitral valve
opens. The blood then flows into the relaxed left ventricle, filling and
expanding it. The ventricle begins to contract and the mistral valve
shuts, while the aortic valve opens. The ventricle completes its
contraction, pushing the blood into the aorta. When the ventricle
begins to relax, the aortic valve shuts, preventing backflow of the
parcel of blood. The aorta branches out into arteries, arterioles, and
ultimately into capillaries, which distribute the blood throughout the
body. While passing through these capillaries, the blood delivers
oxygen to the cells and absorbs carbon dioxide dumped by the cells
during their metabolic activity. The systemic capillaries merge into
veins, and finally, the blood returns to the heart via the inferior and the
superior vena cava, completing the circuit.

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