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The heart is comprised of four chambers; two atria and two ventricles. The atria collect blood coming into the heart, that will then be transmitted into the ventricles, and by doing so, the atria facilitate circulation.
The atria rhythmically contract and relax their walls, which allows them to push blood back into the ventricles that is returning into them during ventricular systole via the major venous vessels. The left and right atria are situated above the left and right ventricles, respectively, and are separated from each other by the inter-atrial septum. The atria communicate with the ventricles through an opening known as the atrioventricular orifice. That is to say that the left atrium communicates with the left ventricle through the left atrioventricular orifice and the right atrium communicates with the right ventricle through the right atrioventricular orifice. These two oval apertures between the atria and ventricles are guarded by semilunar valves known as the cuspid valves.
The right atrioventricular orifice faces anteroinferiorly and it is situated anterior to the opening for the inferior vena cava into the right atrium. The semilunar valve that guards the right atrioventricular orifice is known as the tricuspid valve due to the fact that this valve possesses three cusps or septae.
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