contractility
Contractility is the heart muscle's own punching power — how hard it squeezes regardless of how full it is. Two ventricles filled to the same degree can still eject different amounts if one muscle simply contracts more vigorously.
Formally, contractility is the force of contraction at a given preload and afterload. It rises with adrenaline and stimulation of the sympathetic nervous system, and with positive inotropic drugs; it falls with damaged or oxygen-starved muscle, certain drugs, and acid-base disturbances.
Because contractility is defined as independent of loading conditions, it is genuinely hard to capture with a single bedside number. Clinicians infer it from how well the ventricle empties and from measures such as ejection fraction, while remembering those measures are also affected by preload and afterload.
Contractility is the lever many heart-failure and shock treatments pull on. Boosting it can rescue a failing pump in the short term, but at the cost of higher oxygen demand, which is why such drugs are used carefully.
During exercise, sympathetic nerves and circulating adrenaline raise contractility, so each beat ejects a larger fraction of its blood even before preload changes.
Adrenaline boosts contractility, helping the heart meet the demands of exertion.
Contractility and inotropy describe the same property; a drug that raises it is a positive inotrope, one that lowers it is a negative inotrope.