The cardiovascular system is the first system to begin functioning in the embryo. Blood circulation begins in the third week.
Development of blood vessels and primitive blood cells
Blood vessels take shape in very small empty spaces that appear between cells, called the lacunae.
These merge to form blood vessels. The cells caught in the vessels become primitive blood cells.
The development of blood vessels begins simultaneously in the different structures of the embryo and the placenta.
The formation of fetal blood does not begin before the 2nd month.
A drawing to understand
To understand the development of the heart and where it happens, imagine a sphere. Inside, the cells of the future embryo form a flat disk that separates two cavities. Above the disk will be the amniotic sac, and below, the vitelline duct. On the outside, a blood network begins to form to feed and oxygenate the embryo.
The cells programmed for the heart are very far from the thorax ...
At this stage, the cells programmed to form the heart are found in the cardiogenic region located at one end of the disc, in front of the head, the brain and other structures.
In this area, two parallel tubes form, one at each end. Curiously, these tubes, one on each side of the cardiogenic region, will merge to form the heart.
The heart must fall back on itself during its growth
Some parts of the embryo develop more quickly than others. This is the case for the head and brain. This asymmetric growth causes the disk to begin to bend.
Simultaneously, the disk also grows widthwise, causing another, deeper, fold from side-to-side, like a bird folding its wings over its belly.
The folding of these "wings" places the two parallel tubes one next to the other. In the third week, the tubes will partially fuse along their central part.
During the 4th week, the newly formed single tube lengthens and develops unevenly, widening in some places and narrowing in others. Its growth being rapid and the available space being limited, the tube is forced to fold back on itself, toward the right, forming a U. Then, it grows further and forms an S.
And so, what will become the atria and the sinus venosus is now positioned behind the future ventricles and the bulbus cordis.
Rapid growth of the head pushes the heart inside the thorax
While the embryonic disk bends sideways to allow the cardiac tubes to fuse, the growth of the head causes a flexing which moves the cardiogenic region, pushing it under the embryo. The heart is now placed in what will eventually become the chest cavity.
And place in a small bag
This movement of the heart into the future chest cavity leads it to invaginate into a small gap that envelopes it and becomes the pericardium.