Concerning the physiology of the coronary circulation, the following is/are true:

c. Adenosine is the most important metabolic regulator of coronary blood flow

d. Sympathetic nerve stimulation constricts coronary arteries despite the need for increased cardiac output

Since myocardium maximally extracts O2 from blood at rest, increased demand requires increased delivery. Systolic pressures compress intramyocardial vessels, so maximal coronary flow is during diastole. Adenosine, a breakdown product of ATP, is a vasodilator and the most important metabolic regulator of coronary blood flow. Although sympathetic nerves produce coronary vasoconstriction, the autoregulatory vasodilatory responses to increased myocardial demand overwhelm that effect.