119. The Right and Left Atrial Walls and The Venous Valves

  1. The right atrial walls
    1. THE SINUS VENARUM (smooth part of the wall of the right atrium into which the great veins open) is derived from the sinus venosus
      1. The rest of the atrium and its muscular extension, the auricle, have a rough trabeculated surface and are derived from the primitive right atrium
    2. THE SINUS VENARUM AND THE PRIMITIVE ATRIUM are demarcated internally by a vertical ridge, the crista terminalis, and externally, by an inconspicuous groove, the sulcus terminalis
      1. Thus, the crista represents the cranial part of the right sinoatrial valve
        1. The lower portion of the right sinoatrial valve forms the valves of the inferior vena cava and coronary sinus
      2. The left sinoatrial valve fuses with the septum secundum and is incorporated into the interatrial septum
  2. The left atrial walls
    1. MOST OF THE LEFT ATRIUM is smooth and is derived from the primitive pulmonary vein, which develops as an evagination from the dorsal wall of the atrium in the sinoatrial region
      1. Initially, the single common pulmonary vein opens into the primitive left atrium, but as the latter expands, parts of the vein are gradually absorbed into the wall of the left atrium
      2. Progressively, the proximal parts of the branches of the pulmonary vein are also absorbed, thus, the 4 pulmonary veins all open independently into the left atrium
      3. Only the left auricle (derived from the primitive atrium) has a rough, trabeculated appearance
  3. The venous valves
    1. THE ENTRANCE OFTHE SINOATRIAL OPENING is flanked on each side by a valvular fold, the right and left venous valves
      1. On the right, this fold is formed by a sinoatrial fold
      2. On the left, there is a smaller fold, called the left venous valve
    2. DORSOCRANIALLY, THE VALVES FUSE to form a ridge called the septum spurium
    3. INITIALLY, THE VALVES ARE LARGE, but when the right horn is incorporated into the atrial wall, the left sinus venosus valve and septum spurium fuse with the developing atrial septum
      1. The superior portion of the right venous valve disappears completely, while its inferior part fuses with the septum that develops between the orifice of the right vitelline vein (inferior vena cava) and the orifice of the coronary sinus. The remainder of the valve is divided into 2 parts: valve of the inferior vena cava and valve of the coronary sinus

the right and left atrial walls and  the venous valves: image #1