The transformation processes of the ventricles
While the common atrium is arranged through septation into right and left atria the ventricle is separated into two chambers by the interventricular septum (IVS). In contrast with septation of the atria, which is an active process, the subdivision of the ventricles is largely passive. In fact, whereas the two ventricles increase in size, the part between them (septum portion) does not grow with the heart and forms a constriction, the sulcus interventricularis.
The change from a serial to a parallel flow of blood is accomplished during stage 13 (ca. the 32nd day). This development is mainly facilitated by hemodynamic factors in that the blood flow comes together from the two sinus horns and travels in a spiral stream to the outflow tract. The following changes occur in the heart:
- The sinus venosus is shifted from the middle to the right side. The blood flows thereby only via the right atrium into the heart.
- The atrioventricular canal is shifted from the left side where it was initially to the middle so that the right atrium can communicate directly with the right ventricle. Thus a part of the blood gets directly from the right atrium into the right ventricle. The definitive separation of atria from ventricles follows by the growing together of the dorsal and ventral av-septa.
- The common atrium and ventricle become divided by septa. However, the left ventricle keeps its access to the outflow tract.