Further atrial development

As septation of the primitive atrium is occurring, there are a number of developments within the forming right and left atria:

• right atrium:
  • incorporates the remains of the primitive sinus venosus posteriorly:
    • this region forms the smooth-walled part of the mature right atrium, termed the sinus venarum
    • the junction between the sinus venosum and the original atrium is visible as the crista terminalis
  • at the entrance to the right atrium from the sinus venosum, there are two folds, the right and left venous valves:
  • superiorly, both folds fuse to form an elevation termed the septum spurium; eventually, the septum spurium contributes to the septum secundum
    • the right valve has several fates:
      • superiorly it degenerates
      • inferiorly it forms the valve of the inferior vena cava and the valve of the coronary sinus
    • the left valve contributes to the development of the septum secundum

the left atrium:

þþ a single primitive vein originates from the dorsosuperior wall of the chamber

• it grows posteriorly to merge with the veins developing from the lung buds
• expansion of the chamber consumes the single vein as part of the posterior wall; eventually, the connection that this vein made to the veins from the lung buds also becomes incorporated into the wall of the left atrium; finally, expansion results in individual veins opening by separate orifices into the chamber
• the region of fusion with the veins forms the majority of the surface of mature left atrium; it is smooth walled
• the only remnant of the original left atrium in the mature heart is the auricle