This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
[{'content': 'Single most impoant factor in control of automatic contractility of hea is: \nSelect and explain. \nA. Myocordial wall thickness, B. Right atrial volume, C. SA node pacemaker potential, D. Sympathetic stimulation', 'role': 'user'}, {'content': 'Answer: D \nExplanation: D i.e. Sympathetic stimulation Intrinsic factor Length of activations - Most impoant factor Frank- starling law: "Energy of contraction is propoional to initial length of cardiac muscle fiber." Length of muscle fiber is propoionate to end diastolic blood volume (preload)Q. Extrinsic factor Adrenergc neural stimulation/ Sympathetic stimulationQ - Inotropic effect is smaller, Maximum in atria - It becomes impoant when the hea is increased as it permits adequate diastolic filling.', 'role': 'assistant'}]