121. The Cardiac Valves and Conducting System

  1. The cardiac valves
    1. THE SEMILUNAR VALVES form from 3 valve swellings or ridges of subendocardial connective tissue at the openings of the aorta and pulmonary trunk
      1. The swellings hollow out and reshape themselves to form 3 thin cusps, which are covered by endocardium
    2. THE ATRIOVENTRICULAR VALVES [the tricuspid (right AV valve) and mitral (left AV valve)] develop simultaneously from localized proliferation of subendocardial tissue around the atrioventricular canals
      1. They are hollowed out on their ventricular sides
  2. The ventricular walls
    1. CAVITATION OF THE VENTRICULAR WALLS forms a spongelike mass of muscle bundles
      1. Some muscle remains as the trabeculae carneae of the ventricular walls
      2. Other muscles form the papillary muscles and modify to form the chordae tendineae which connect the ventricular walls with the atrioventricular valves
  3. The conducting system
    1. THE MUSCULAR LAYERS (myocardium) of the atrium and ventricle are initially continuous, and the primitive atrium serves as a pacemaker for the primitive heart until the sinus venosus takes over that function
      1. The sinoatrial (SA) node is originally in the right wall of the sinus venosus, but as it is incorporated with the sinus venosus into the wall of the right atrium, it comes to lie near where the superior vena cava enters the right atrium
      2. After the sinus venosus is incorporated into the right atrium, cells from its left wall, which are found in the base of the interatrial septum just anterior to the opening of the coronary sinus, and cells along the atrioventricular canal make up the atrioventricular (AV) node and the bundle of His
      3. As the 4 heart chambers form, a band of connective tissue grows in from the epicardium and separates the atrial muscle from the ventricular muscle to form the cardiac skeleton. Thus, the conducting system remains as the only pathway from the atria to the ventricles
      4. The sinoatrial (SA) node, the atrioventricular (AV) node, and the atrioventricular bundle of His are soon supplied by external nerves (the parasympathetic vagus and sympathetic fibers). Histologic differentiation of these specialized tissues is continued up to, and after, birth

the cardiac valves and  conducting system: image #1