Overview of vasculature

The vascular system of the developing embryo is quite complex, imposed by (1) the need to bypass developing lungs; and (2) increased demand from proliferating tissues. The following is a schematic representation of the left side of the paired embryonic circulatory system.

There are several important points to consider:

• Oxygenated blood is supplied to the embryo via the left and right umbilical veins. Although "vein" may seem to be a misnomer, these vessels are afferent to the developing heart.
  
• The sinus venosus serves as the main collecting chamber for blood in the developing heart. Of note, both oxygenated blood from the placenta and deoxygenated blood from the embryonic tissues are mixed within the sinus. The blood pumped from the heart is of an intermediate oxygen tension, which is sufficient to meet the developing tissues' demands.
  
• The surface of the yolk sac is the site of hematopoesis (especially erythropoesis during early stages). Afferent blood from the vitelline arteries serve to flush newly-formed blood cells into circulation, returning to the heart via the vitelline veins.
  
• Blood pumped from the heart exits through the aortic arches before flowing through the paired dorsal aortae. Tissues are primarily supplied via efferent segmental arteries arising to supply each developing somite. The dorsal aortae end in the umbilical arteries, which return mixed blood to the placenta for reoxygenation.
  
• The cardinal veins (anterior, posterior and common) drain blood from developing tissues. The deoxygenated blood is mixed in the sinus venosus and recirculated.
  

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