Vasculogenesis
Vasculogenesis marks the beginning of the circulatory system development in the embryonic stage of growth. Different parts of the embryo facilitate the development of the human venous and arterial systems. Aortic arches (six pairs) are the main parts of the upper embryo where the development of the arterial system commences. Moreover, during the first eight weeks of development, three bilateral veins stem the formation of the venous system. Notably, fetal circulation commences in the eighth week of development. Nevertheless, this circulation is independent of the lungs. The lungs are bypasses through the truncus arteriosus. Additionally, the fetus is nourished with nutrients and oxygen via the umbilical cord from the maternal placenta.
Furthermore, a closed circulatory system is found in organisms such as vertebrates, annelids, and cephalopods. The human circulatory system is a typical example of a closed system. However, some vertebrates, such as fish, have a single circuit, single cycle circulation facilitated by the two chambers of the heart. Most reptiles and amphibians have a double circulatory system. Nevertheless, their hearts lack a complete separation, for example, a three-chambered heart in amphibians. Although vital for the propagation of organisms, the circulatory system might be absent in some organisms, such as the flatworm (Simões-Costa, Marcos et al. 8).