The heart consists of four chambers, two atria and two ventricles. The main valves of the heart are the mitral and aortic valves on the left and the tricuspid and pulmonary valves on the right. They are complex avascular structures, which will open and close 2700 million times during a normal lifetime.
This lies to the right and slightly behind the right ventricle, it is a thin, walled chamber which receives the venous return to the heart from the superior and inferior vena cava. On the floor of the right atrium is the tricuspid valve, which is the gateway to the right ventricle.
The tricuspid valve has three triangular cusps, septal, anterior and posterior. It is held in position and prevents reflux back into the right atrium by the chordae tendinae which are attached to the free border of the cusps.
Located directly beneath the sternum it is approximately one third of the thickness of the left ventricle. The right ventricular receives venous blood from the right atrium during diastole; this is then ejected superiorly and to the right through the pulmonary valve to the pulmonary artery.
Located to the midline behind the right ventricle. The left atrium receives blood from the pulmonary veins. It has an irregular shape and although smaller than the right atrium has a slightly thicker muscle mass.
In the floor of the left atrium is situated the mitral valve which is the gateway to the left ventricle. Two stronger papillary muscles and chordae tendineae embedded within the left ventricle are attached to both cusps of the mitral valve.
Located posteriorly to the left of the right ventricle. It is conical in shape and its apex lies approximately in the 5th intercostal space. The left ventricle receives blood from the left atrium during diastole and ejects blood against high resistance into the systemic circulation during ventricle systole. Therefore the left ventricle is more muscular than the right ventricle to combat this resistance.
Above and anteriorly the left ventricle opens into the aorta though the aortic valve. The aortic valve has three semilunar cusps, which are stronger than those of the pulmonary valve. At the origin of each cusp, the walls of the aorta show a slight deviation, this is the origin of the sinuses of valsalva. The right coronary artery arises from the right aortic sinus, and the left coronary artery from the left aortic sinus.
THE ATRIOVENTRICULAR JUNCTION.
There is no muscular continuity between the atria and the ventricles except through the conducting tissue of the Atrioventricular node and AV bundle.