The law of the heart is a concept that states that the heart works according to a certain rule. This law was discovered by William Starling in 1902 and has since become one of the most famous laws in physiology.
According to Starling's Law, after the heart contracts, it must relax to prepare for the next contraction. This occurs due to the fact that the myocardium (muscle tissue of the heart) tends to a state of rest, which is achieved by reducing the volume of the ventricles. Thus, the heart cannot continue beating indefinitely and must rest to prepare for the next contraction.
Starling's Law is an important understanding of cardiac physiology and helps explain why the heart beats at a certain rate and force. It also has practical uses in medicine, such as in the treatment of heart failure or arrhythmia.