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Embryology of Patent ductus arteriosus

Ductus arteriosus arise from the left 6th aortic arch.
Ductus closes physiologicallt within 2 hours of birth.
Ductus anatomically closes by 2 weeks.
Closure of ductus can take upto 8 weeks in term infants and upto 6 months in preterm infants.

Why does the ductus closes after birth?
1.Ductus artenosus closes immediately after birth due to sudden increase in partial pressure of oxygen.
2.Decreased level of vasoactive prostaglandins.

What are the ductus dependent circulations
In certain situations ductus artenosus is the source of pulmonary blood flow. They include 
  1. Pulmonary atresia with intact inter ventricular septum.
  2. Ductus is the only source of systemic blood flow. 
  3. Hypoplastic left heart syndrome.
  4. Aortic atresia.
  5. Complete interruption of aortic arch.
  6. Preductal coarctation of aorta.

Ductus as the site of bidirectional mixing of blood seen in TGA (Transposition of great arterires)