Patent Ductus Arteriosus (PDA)
There are a number of anatomical structures that arise as a puppy fetus develops, but then disappear or change before or shortly after birth. This makes sense, as a fetal puppy (just like a human fetus) is living in a liquid environment, with many of the bodily functions provided by the mother, but then transitions immediately to a self-contained, breathing newborn puppy. Patent Ductus Arteriosus is in the category of conditions where one of those necessary fetal structures remains after birth, when it should have naturally gone away or ceased its function.
In the normal dog, the pulmonary artery delivers blood to the lungs, where it is oxygenated and returned to the heart. The oxygen-rich blood is then pumped into the aorta and delivered to the body. However, a fetal puppy is not breathing... in fact, the lungs are basically collapsed... and its blood is getting the necessary oxygen from the mother. Therefore the body has a mechanism to divert that blood flow so that it bypasses the lungs and goes straight to the body. That mechanism is the Ductus Arteriosus, a "shunt" (bypass) from the pulmonary artery to the descending aorta.
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