In utero, the placenta is the main site for gas exchange for the developing foetus and the blood flow to the foetal lung is minimal. Blood from the right ventricle bypasses the lungs and passes directly from the pulmonary artery to the aorta via a foetal vessel called the arterial duct. After birth, a number of changes occur in transition from the foetal to the newborn circulation including expansion of the lungs (which reduces pulmonary vascular resistance) and closure of the arterial duct, so that blood now perfuses the lungs. There are certain severe congenital cardiac lesions that are only.
