Editing Coronary circulation (section)

===Changes in diastole===
During contraction of the [[ventricle (heart)|ventricular]] myocardium ([[Systole (medicine)|systole]]), the subendocardial coronary vessels (the vessels that enter the myocardium) are compressed due to the high ventricular pressures.
This compression results in momentary retrograde blood flow (i.e., blood flows backward toward the aorta) which further inhibits perfusion of myocardium during systole.
However, the epicardial coronary vessels (the vessels that run along the outer surface of the heart) remain open.
Because of this, blood flow in the subendocardium stops during ventricular contraction.
As a result, most myocardial perfusion occurs during heart relaxation ([[diastole]]) when the subendocardial coronary vessels are open and under lower pressure.
Flow never comes to zero in the right coronary artery, since the right ventricular pressure is less than the diastolic blood pressure.<ref>{{cite journal |last1=Algranati |first1=Dotan |last2=Kassab |first2=Ghassan S |last3=Lanir |first3=Yoram |title=Mechanisms of myocardium-coronary vessel interaction |journal=Am J Physiol Heart Circ Physiol |date=March 2010 |volume=298 |issue=3 |page=H861–873 |doi=10.1152/ajpheart.00925.2009 |pmid=19966048 |url=https://journals.physiology.org/doi/pdf/10.1152/ajpheart.00925.2009 |access-date=26 May 2021|pmc=2838558 }}</ref>