Why coarctation of aorta in infantile, cause passes of the blood from pulmonary artery to the aorta through pda?

Plumbing! Blood must get to the body, else the fetus would not survive. The same is true once the baby is born. In utero, blood flows from pulm artery (pa) to aorta (ao) normally. Once born, the blood continues to flow this way so that blood can get to the body. When the PDA closes, the child becomes very sick.
Ductal dependence. Coarctation is usually associated with a small ao arch and/or aortic stenosis. If this is the case, the only way blood can get to the body (or systemic circulation) is thru the pda. The PDA is often associated with the coarctation and the narrowing may worsen as the PDA closes. This is why the coarctation may be "ductal dependent".
Coarctation . The narrowed aorta causes a lower pressure beyond the narrowing which allows flow from the PDA into the aorta.
Other. Blood flows from higher pressure to lower pressure. Aorta higher, pa lower usually. Coarctation distal to PDA cause higher aortic pressure spikes with obstruction and higher pa flow from aorta to pa through the pda-patent ductus arteriosus.

Related Questions

Why coarctation of aorta in infants, causing blood flow from pulmonary artery to aorta?

To Survive! Blood must get to the head & body; coarctation is an obstruction to this flow. So blood flows through a vessel called the PDA (normal in a fetus), to go from the pulmonary artery to aorta ( getting blood and nutrients to the head and body). The infant will become very ill if nothing is done and the PDA closes. For more info, visit: http://www.Ncbi.Nlm.Nih.Gov/pubmedhealth/pmh0001242/. Read more...
Types of Coarctation. There are 3 types of aortic coarctation. If there is blood flow from the pulmonary artery to the aorta, the infant has preductal coarctation. This can be associated with turner syndrome, and genetic problem that affects female infants. If there is ongoing blood flow from the pulmonary artery to the aorta, repair is likely needed, which could be surgical or, less likely, through catheters alone. Read more...
PDA . It would only do that if the coarctation was severe and there was a patent ductus arteriosus (pda) which is a connecting artery between the aorta and pulmonary artery, leading to blood flow from pa to the descending aorta. Read more...