From the earliest stage of lung development, there is an accompanying blood circulation. In
the adult lung, the pulmonary arteries are closely associated with the airways. During early
fetal development, the airways act as a template for pulmonary blood vessel development in
that the vessels form by vasculogenesis around the branching airways. In later lung
development, as the alveoli multiply, new capillaries form by angiogenesis. As blood vessels
increase in size, they develop a muscle wall that is relatively thick during fetal life and shows …