Cardiac Anatomy
The detailed anatomy of the heart can be found in anatomy textbooks. The following presents only a brief description of cardiac anatomy so that the physiology of the cardiac cycle can be understood.
The heart consists of four chambers: the right atrium, right ventricle, left atrium, and left ventricle. The right atrium receives blood from the superior and inferior vena cavae, which returns venous blood to the heart from the body. The right atrium is a highly distensible chamber so that it can accommodate the venous return and maintain a low pressure (0-3 mmHg). The actual pressure within the right atrium depends upon the volume of blood within the atrium and the compliance of the atrium. Blood flows from the right atrium, across the tricuspid valve, and into the right ventricle. The free wall of the right ventricle is not as thick as the left ventricle, and anatomically, it wraps itself around part of the larger, and thicker, left ventricle. The outflow tract of the right ventricle is the pulmonary artery which is separated from the ventricle by the semilunar pulmonic valve. Blood returns to the heart from the lungs via four pulmonary veins that enter the left atrium. The left atrium, like the right, is highly compliant, although quantitatively less compliant than the right. Therefore, the left atrial pressure is higher than the right atrial pressure (6-10 mmHg compared to 0-3 mmHg). Blood flows from the left atrium, across the mitral valve, and into the left ventricle. The left ventricle has a very thick muscular wall so that it can generate high pressures during contraction. Blood from the left ventricle is ejected across the aortic valve and into the aorta.
The tricuspid and mitral valves (also called atrioventricular, or AV valves) have fibrous strands (chordae tendineae) on their leaflets that attach to papillary muscles located on the respective ventricular walls. The papillary muscles contract during ventricular contraction and generate tension on the valve leaflets via the chordae tendineae to prevent the AV valves from bulging back into the atria and becoming incompetent. The semilunar valves (pulmonic and aortic) do not have analogous attachments.
