The is the muscular wall that makes up the bulk of the heart. The ventricle is the thicker and larger ventricle. The atrium and ventricle of each side are connected by a/an valve. A single cardiac cycle consists of the simultaneous contractions of both followed by the simultaneous contractions of both . The heart supplies blood to itself via arteries. Contraction of the ventricles (and relaxation of the atria) is called , whereas relaxation of the ventricles is called . The heart pumps blood to the lungs for oxygenation via the pulmonary .