In the embryo, formation of the heart begins in the pharyngeal, or throat, region. The first visible indication of the embryonic heart occurs in the undifferentiated mesoderm, the middle of the three primary layers in the embryo, as a thickening of invading cells. An endocardial (lining) tube of flattened cells subsequently forms and continues to differentiate until a young tube with forked anterior and posterior ends arises. As differentiation and growth progress, this primitive tube begins to fold upon itself, and constrictions along its length produce four primary chambers. These are called, from posterior to anterior, the sinus venosus, atrium, ventricle, and truncus arteriosus. The characteristic bending of the tube causes the ventricle to swing first to the right and then behind the atrium, the truncus coming to lie between the sideways dilations of the atrium. It is during this stage of development and growth that the first pulsations of heart activity begin.

Endocardial cushions (local thickenings of the endocardium, or heart lining) “pinch” the single opening between the atrium and the ventricle into two portions, thereby forming two openings. These cushions are also responsible for the formation of the two atrioventricular valves (the valves between atria and ventricles), which regulate the direction of blood flow through the heart.

The atrium becomes separated into right and left halves first by a primary partition with a perforation and later by a secondary partition, which, too, has a large opening, called the foramen ovale, in its lower part. Even though the two openings do not quite coincide in position, blood still passes through, from the right atrium to the left. At birth, increased blood pressure in the left atrium forces the primary partition against the secondary one, so that the two openings are blocked and the atria are completely separated. The two partitions eventually fuse.

The ventricle becomes partially divided into two chambers by an indentation of myocardium (heart muscle) at its tip. This developing partition is largely muscular and is supplemented by membranous connective tissue that develops in conjunction with the subdivision of the truncus arteriosus by a spiral partition into two channels, one for systemic and one for pulmonary circulation (the aorta and the pulmonary artery, respectively). At this time, the heart rotates clockwise and to the left so that it resides in the left thorax, with the left chambers posterior and the right chambers anterior. The greater portion of blood passing through the right side of the heart in the fetus is returned to the systemic circulation by the ductus arteriosus, a vessel connecting the pulmonary artery and the aorta. At birth this duct becomes closed by a violent contraction of its muscular wall. Thereafter the blood in the right side of the heart is driven through the pulmonary arteries to the lungs for oxygenation and returned to the left side of the heart for ejection into the systemic circulation. A distinct median furrow at the apex of the ventricles marks the external subdivision of the ventricle into right and left chambers.

Valves of the heart

To prevent backflow of blood, the heart is equipped with valves that permit the blood to flow in only one direction. There are two types of valves located in the heart: the atrioventricular valves (tricuspid and mitral) and the semilunar valves (pulmonary and aortic).

The atrioventricular valves are thin, leaflike structures located between the atria and the ventricles. The right atrioventricular opening is guarded by the tricuspid valve, so called because it consists of three irregularly shaped cusps, or flaps. The leaflets consist essentially of folds of endocardium (the membrane lining the heart) reinforced with a flat sheet of dense connective tissue. At the base of the leaflets, the middle supporting flat plate becomes continuous with that of the dense connective tissue of the ridge surrounding the openings.

Tendinous cords of dense tissue (chordae tendineae) covered by thin endocardium extend from the nipplelike papillary muscles to connect with the ventricular surface of the middle supporting layer of each leaflet. The chordae tendineae and the papillary muscles from which they arise limit the extent to which the portions of the valves near their free margin can billow toward the atria. The left atrioventricular opening is guarded by the mitral, or bicuspid, valve, so named because it consists of two flaps. The mitral valve is attached in the same manner as the tricuspid, but it is stronger and thicker because the left ventricle is by nature a more powerful pump working under high pressure.

Blood is propelled through the tricuspid and mitral valves as the atria contract. When the ventricles contract, blood is forced backward, passing between the flaps and walls of the ventricles. The flaps are thus pushed upward until they meet and unite, forming a complete partition between the atria and the ventricles. The expanded flaps of the valves are restrained by the chordae tendineae and papillary muscles from opening into the atria.

The semilunar valves are pocketlike structures attached at the point at which the pulmonary artery and the aorta leave the ventricles. The pulmonary valve guards the orifice between the right ventricle and the pulmonary artery. The aortic valve protects the orifice between the left ventricle and the aorta. The three leaflets of the aortic semilunar and two leaflets of the pulmonary valves are thinner than those of the atrioventricular valves, but they are of the same general construction with the exception that they possess no chordae tendineae.

Closure of the heart valves is associated with an audible sound, called the heartbeat. The first sound occurs when the mitral and tricuspid valves close, the second when the pulmonary and aortic semilunar valves close. These characteristic heart sounds have been found to be caused by the vibration of the walls of the heart and major vessels around the heart. The low-frequency first heart sound is heard when the ventricles contract, causing a sudden backflow of blood that closes the valves and causes them to bulge back. The elasticity of the valves then causes the blood to bounce backward into each respective ventricle. This effect sets the walls of the ventricles into vibration, and the vibrations travel away from the valves. When the vibrations reach the chest wall where the wall is in contact with the heart, sound waves are created that can be heard with the aid of a stethoscope.

The second heart sound results from vibrations set up in the walls of the pulmonary artery, the aorta, and, to a lesser extent, the ventricles, as the blood reverberates back and forth between the walls of the arteries and the valves after the pulmonary and aortic semilunar valves suddenly close. These vibrations are then heard as a high-frequency sound as the chest wall transforms the vibrations into sound waves. The first heart sound is followed after a short pause by the second. A pause about twice as long comes between the second sound and the beginning of the next cycle. The opening of the valves is silent.