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The ductus arteriosum (DA) (or arteriosus) is the thick short conduit for blood to bypass the non-ventilated lungs in the fetus. It is located between and connects the proximal left pulmonary artery and the undersurface of the aortic arch distal to the origin of the last branch of the arch, at the aortic isthmus. It allows blood to flow from the right circulation to the left.
In the immediate postnatal period, the ductus arteriosus closes under the influence of falling prostaglandin concentrations and changing pressures in the cardiac chambers (placental flow ceases raising right atrial pressure and increased venous return from the lungs increases left atrial pressures). The muscle in the walls of the ductus contract, thereby forcing blood from the pulmonary trunk into the newly ventilated lungs.
After closure it persists as the fibrous ligamentum arteriosum.