Illustration copyright 2000 by Nucleus Communications, Inc. All rights reserved. http://www.nucleusinc.com

The ductus arteriosus is a blood vessel in an unborn baby (fetus) that connects the pulmonary artery, which carries blood to the lungs, and the aorta, which carries blood to the body. This allows blood flow to bypass the lungs. (The baby does not need blood to flow through the lungs before it is born, because the baby is not breathing air.)

Normally, this blood vessel closes shortly after birth; when it does not close, it is known as a patent (open) ductus arteriosus. Extra blood from the aorta then flows into the lungs and back to the heart, rather than out into the body. In severe cases, this can stress the heart, increase pressure in the pulmonary artery that carries blood to the lungs, and cause fluid to build up in the lungs.