congenital abnormality in which there may be a single atrium with two ventricles or a single ventricle with two atria. Rudimentary parts of the atrial and ventricular septa may be present but are incompetent to prevent a virtual single chamber in either case.
amphibians and most reptiles have a heart with three chambers—two atria and a single ventricle. These animals also have separate circuits of blood vessels for oxygenating blood and delivering it to the body. Deoxygenated blood returning from the body empties into the right atrium. From there, blood is conducted to the ventricle and is then pumped to the lungs. After picking up oxygen and getting rid of carbon dioxide in the lungs, blood returns to the heart and empties into the left atrium. The blood then enters the ventricle a second time and is pumped out to the body. The second trip through the heart keeps blood pressure strong and blood flow rapid as blood is pumped to the tissues, helping the blood deliver oxygen more efficiently.