Figure 3. Vascularization of organs.

Blood vessels emerging in the mesoderm form an initial net-like plexus around the organ primordium. As development proceeds and organs grow in size, the vasculature is remodeled coordinately. Branching organs of endodermal origin, like pancreas and lung, co-develop epithelial branches with arteriovenous networks. In these organs, arteries and veins align with nearly every branch, both large and small.