The valves between the atria and ventricles are called the atrioventricular valves. Cardiac dysrhythmia is an irregular heartbeat that can occur in the ventricles or atria. It electrically connects the atria and ventricles. Heart valves separate the atria from the ventricles, or the ventricles from a blood vessel. Cardiac muscle forms both the atria and the ventricles of the heart. It is a conduction block between the atria and ventricles. Pacemakers that control both the atria and ventricles are called dual-chamber pacemakers. This is located between the atria and the ventricles. The cardiomyocytes make up the bulk (99%) of cells in the atria and ventricles. The muscle cells make up the bulk (99%) of cells in the atria and ventricles. Pacemakers that control not only the ventricles but the atria as well have become common. The hearts of amphibians have three chambers, two atria and one ventricle. It functions to carry the electrical impulse from the atria to the ventricle. There is a heart with three chambers&colon; two atria and one ventricle. They start from either the atria or atrioventricular node.