In hypoplastic left heart syndrome, the left side of the heart - including the aorta, aortic valve, left ventricle and mitral valve - is underdeveloped. Blood returning from the lungs must flow through an opening in the wall between the atria (atrial septal defect). The right ventricle pumps the blood into the pulmonary artery and blood reaches the aorta through a patent ductus arteriosus.