The embryo's vein system develops out of a very irregular network of capillaries, from which finally individual ones transform themselves definitively into veins while others disappear again. The result of this is that the venous system is not very uniform, and in the adult far more variants of venous outflows than on the arterial side exist. One can nevertheless distinguish among three basic systems: cardinal, umbilical and omphalomesenteric. The pulmonary veins cannot be counted as belonging to these three systems and are treated separately.
The cardinal system
The cardinal veins form as the basis for the intraembryonic venous part of the circulatory system. Various venous systems appear in various stages of the embryogenesis and partially disappear again
Very early in the development two paired systems appear:
- The superior cardinal veins bring the blood from the head region via the left and right common cardinal vein
- The inferior cardinal veins drain the blood from the lower half of the body into the two common cardinal veins
From here, the blood is emptied into the sinus venosus and into the atrium via the sinus horns.
The adult venous system is much more variable as the arterial system. Through transformative processes during the development the individual sections arise from various portions of the embryonic venous system.