Therefore, any factor that shortens this phase (total diastolic phase/min) during the cardiac cycle such as increasing heart rate or prolonged ventricular systole due to depressed contractility will impair the supply of blood to the subendocardium, rendering this region most susceptible to ischemia. Changes in the diastolic phase probably have minimal impact on the blood flow to the epicardial layers which are perfused throughout the cardiac cycle. Buy Asthma Inhalers Online
Humeral factors: These substances are a group of circulating agents such as angiotensin II, serotonin, thromboxane, prostacycline, and bradykinin that modulate coronary resistance. Their effect in most cases is mediated through interaction with endothelial cells. Coronary endothelial cells seem to have receptors for several humeral agents. Binding to a specific receptor stimulates release of substances with smooth muscle relaxation properties (endothelial-derived relaxing factor, EDRF) or constrictor properties. EDRF causes vascular smooth muscle relaxation through a rise in the intracellular concentration of cyclic GMP (guanosine monophosphate). It is a labile compound which has not been isolated yet. However, it remains the subject of intense research.