Development of Heart

The heart begins its development as a simple tube called the heart tube. This heart tube is established as a ventral middle blood vessel in the chest at the 4th week of development. Direction of blood flow in this lood vessel is caudal to cranial. So if that is the direction of blood flow then
as the middle part of the tube becomes the heart thus it immediately tells us that the lower part of the tube will be the veins as according to the direction of flow this part carries blood to the region of the tube that will become the heart and similarly the upper part of the tube becomes the arteries as it carries blood away from the heart.

Now the parts of the heart tube and there Adult derivatives are as follows:

Embryonic Dilatation	Adult Structure
Truncus arteriosus	Aorta, Pulmonary trunk and semilunar valves.
Bulbus cordis	Smooth part of right ventricle (conus arteriosus) Smooth part of left ventricle (aortic vestibule)
Primitive ventricle	Trabeculated part of right ventricle Trabeculated part of left ventricle
Primitive atrium	Trabeculated part of right atrium (pectinate muscle) Trabeculated part of left atrium (pectinate muscle)
Sinus venosus	Right- Smooth part of right atrium (sinus venarum) Left- Coronary sinus and oblique vein of left atrium.

The region of the tube that forms the heart are the primitive atrium and ventricle, the sinus venosus and the bulbus cordis.

The region of the tube that forms the heart becomes larger. Now the question is how does it get larger?

The primitive atrium incorporates part of sinus venosus and the primitive ventricle incorporates part of the bulbus coridis and thus become large.

The primitive Atrium and Ventricle have a lumenal surface that is trabeculated and the lumenal surface of the bulbus cordis and sinus venosus is smooth, thus when the ventricle fuses with the bulbus cordis and atrium fuses with the sinus venosus this gives rise to the inner walls of the adult atrium and ventricle which have some region that is smmoth and some that is trabeculated.

If you see the heart tube from the lateral aspect, it folds into an S-shaped curve thus the arteries now come out of the front wall of the heart and the vein enter through the back wall of the heart. Also the atrium now lies majorly at the posterior aspect of the heart and the ventricles lie at the anterior aspect. One more thing to remember is that after the folding the atria come to lie a little above the ventricles.