VENTRICLE

muscular chamber that pumps blood out of the heart and into the circulatory system. Ventricles occur among some invertebrates. Among vertebrates, fishes and amphibians generally have a single ventricle, while reptiles, birds, and mammals have two. In humans, the ventricles are the two lower chambers of the heart. The walls of the chambers, and particularly the walls of the left ventricle, are far more heavily muscled than the walls of the atria, or upper chambers (see atrium), because it is in the ventricles that the major force is exerted in the process of pumping the blood to the bodily tissues and to the lungs. Each opening leading into or away from the ventricles is guarded by a valve. These openings are the following: those from the two upper chambers; the opening from the right ventricle into the pulmonary artery, which transports blood to the lungs; and the opening from the left ventricle into the aorta, the main trunk by which oxygen-rich blood starts its course to the tissues. The interior surfaces of the ventricles are ridged with bundles and bands of muscle, called trabeculae carneae. The papillary muscles project like nipples into the cavities of the ventricles. They are attached by fine strands of tendon to the valves between the atria and ventricles and prevent the valves from opening when the ventricles contract. See also heart.