The atria are the chambers of the heart in which the blood dwells prior to entering the ventricles, which are the main pumping chambers. The right and left atrial lie next to each other in the heart and there is a membranous wall or septum between the two. In some people, there can be a hole in the muscular wall or septum, known as an atrial septal defect. Tiny defects called patent foramen ovale are present in up to 30 percent of people and are of no consequence except in unusual circumstances. Moderate size to larger size defects should be corrected and may require heart surgery.