As the heart tube elongates, four regions began to dilate to give four continuous chambers, from the cephalic end to the caudal end:
- bulbus cordis
- ventricle
- atrium
- sinus venosus
The sinus venosus receives blood from the primitive venous system - see submenu. The bulbus cordis is in continuity with and passes blood to the aortic arches. It can be divided into three regions:
- section next to the ventricle:
- relatively dilated
- eventually becomes incorporated into the mature right ventricle as its rough - trabeculated - part
- conus cordis:
- mid-section
- forms the smooth-walled outflow tracts of the mature ventricles
- truncus arteriosus:
- distal part of bulbus cordis
- forms the roots of the aorta and pulmonary trunk in the mature heart
- its distal section, the aortic sac, divides up from the fourth week to produce the branchial arch arteries The region between ventricle and bulbus cordis is narrow and forms the primary interventricular foramen. Both the region proximal to it, the ventricle, and the immediately distal part of the bulbus cordis begin to form trabeculae on their inner surfaces. This demarcates them as the primitive left and right ventricles respectively. This division is visible on the outside of the heart tube as the interventricular sulcus. Due to the subdivision of the atrium, the conus cordis and truncus arteriosus are displaced from their position on the right of the primitive heart to a more central position, intermediate to the primitive right and left atria.