In the early stages of development the embryo obtains its nourishment from the surrounding tissues, which reaches the embryo by diffusion across the extraembryonic coelom and yolk sac. However, as the embryo increases in size, simple diffusion can not meet the increasing nutritional requirements of the rapidly growing embryo and the early development of a transport system becomes necessary. The primitive cardiovascular system, therefore, begins to develop during the 3rd week of development.The primitive CVS becomes functional in the 4th week as the developing heart begins to beat.
The two major components of the cardiovascular system are: (1) heart, and (2) blood vessels. The heart itself is a specialized blood vessel with
a large lumen and very thick muscular walls.