This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Heart: Cardiac Muscle, Subject name: Physiology
According to Starling law, force of contraction is directly proportional to: 
Select and explain. 
A. Contractility of heart, B. End systolic length of muscle fibres, C. End diastolic length of muscle fibres, D. Muscle tension
C: End diastolic length of muscle fibres
Answer: C: End diastolic length of muscle fibres 
Explanation: Ans: C (End diastolic length of muscle fibres) Ref: Ganong's Review of Medical Physiology, 21st edExplanation:"The Frank-Starling law of heart states that within physblogicai limits the force of cardiac contraction is proportional to its end diastolic volume." Ref: Khurana"For the heart, the length of the muscle fibers (i.e., the extent of the preload) is proportionate to the end-diastoiic volume. The relation between ventricular stroke volume and end-diastolic volume is called the Frank-Starling curve." Ref: GanongThe Frank-Starling Law of the HeartAlso known as Starling's law or the Frank-Stariing mechanism or Maestrini heart's law.It states that the stroke volume of the heart increases in response to an increase in the volume of blood filling the heart (the end diastolic volume) when all other factors remain constant.The increased volume of blood stretches the ventricular wall, causing cardiac muscle to contract more forcefully.