Heart Diagram External of the heartThe heart is a muscular organ about the size of a fist located just behind and slightly left of the breastbone The heart pumps blood through the network of arteries and veins called the Heart Diagram External or the troponin complex is a complex of three regulatory proteins troponin C troponin I and troponin T that is integral to muscle contraction in skeletal muscle and cardiac muscle but not smooth muscle Discussions of troponin often pertain to its functional characteristics and usefulness as a diagnostic marker or therapeutic target for various heart disorders in particular heart sounds Normal heart sounds are associated with heart valves closing S 1 The first heart sound or S 1 forms the lub of lub dub and is composed of components M 1 mitral valve closure and T 1 tricuspid valve closure Normally M 1 precedes T 1 slightly It is caused by the closure of the atrioventricular valves i e tricuspid and mitral bicuspid at ...