The ductus arteriosus is a large blood vessel that connects the main pulmonary trunk (blood vessel to lungs) with the descending aorta (blood vessel to body organs). In utero, pulmonary resistance (blood pressure in lungs) remains high, as the work of lungs i.e getting oxygen from air, is done by the placenta. This resists the heart from pumping the blood into the lungs. The bulk of the right ventricular (right lower chamber of heart) output travels through the ductus arteriosus, maintaining flow in the descending aorta and to the placenta.
At delivery, a series of complex changes occur. With the first breath, the tiny blood vessels in the lungs (pulmonary capillaries) open and resistance to pulmonary blood flow drops. This allows the blood from the heart to be pumped into the lungs. At the same time, the blood pressure in the rest of the body (systemic BP) increases. As systemic pressures start to exceed pulmonary pressures, blood flow through the ductus arteriosus reverses and oxygenated blood from the systemic circulation flows into the pulmonary circulation. Final functional closure of the duct occurs by 24 hours of age in around half of healthy full-term infants but may be considerably delayed in the premature.