<prompt> Which of the following statements about myocardial oxygen demand is true?Correlates with hea rateIs directly propoional to external cardiac workIs negligible when hea is at restDepends upon duration of systole  <response>: Myocardial oxygen demand is the amount of oxygen that the hea requires to maintain optimal function, and myocardial oxygen supply is the amount of oxygen provided to the hea by the blood which is controlled by the coronary aeries Ref: guyton and hall textbook of medical physiology 12 edition page number:246,247,248 