If there is only one lower chamber (ventricle), it is called univentricular heart or simply single ventricle. Both arteries (aorta and pulmonary artery) arise from this single chamber.
But sometimes both arteries can arise from right ventricle and blood from left ventricle can reach right ventricle through a septal defect. Then it is called double outlet right ventricle. In this situation there are two ventricles (lower chambers), while in the former situtation there is only one ventricle. Similarly then can be a double outlet left ventricle also. All these are rare congenital abnormalities of the heart.
