The human body self-regulates its internal environment to maintain homeostasis during stress, injury, or disease. Various organs in the body regulate blood flow to maintain perfusion during otherwise ischemic conditions.
Blood flow through a vessel can be approximated by Poiseuille’s law:
Q = flow rate; P = pressure; r = radius; η = fluid viscosity; l = length of tubing.
Blood flow through a vessel is directly proportional to the pressure difference but proportional to the fourth power of the radius. This law underscores how regulation of regional blood flow occurs throughout the body.
The heart continually exercises, with “resting” cardiac blood flow approximately 250 mL/min (5% of CO). The heart’s O2 extraction ratio is 75%-80%, and is therefore dependent on increased blood flow to sustain oxygenation during stress. Myocardial O2 consumption (MVo2) is determined by the rate of force development (dP/dt) and the left ventricular wall tension (T). T is related to the left ventricle (LV) diameter and LV pressure by LaPlace’s law:
During systole, the LV and right ventricle (RV) pump blood to systemic, high-pressure circulation and pulmonary, low-pressure circulation, respectively. The LV needs to achieve such a high pressure so that LV myocardial blood flow occurs only during diastole. Right and left coronary arteries originate from the aorta just distal to the aortic valve, so aortic diastolic pressure is the driving force for cardiac perfusion. Blood flow to the myocardium can be approximated by the equation:
Coronary perfusion pressure (CPP) = DBPAorta − LVEDP
Blood flow through the right coronary artery (RCA) to the RV is not interrupted during systole due to lower RV pressures and less extravascular compression. Blood flow through the coronary sinus is maximal during late systole, secondary to extravascular compression and minimal right atrial pressure.
The LV subendocardium is exposed to higher pressures during systole than the subepicardium, and is therefore more susceptible to ischemia. Disease states such as coronary artery disease (CAD) (decreased vessel radius), pressure overload hypertrophy (increased LVEDP), severe tachycardia (decreased diastolic period), or aortic insufficiency (decreased aortic diastolic pressure) expose LV subendocardium to ischemia.
Regulation of cardiac blood flow is controlled by neural and metabolic factors. Both the sympathetic and parasympathetic nervous system influence cardiac blood flow. Sympathetic vascular tone results from a feed-forward, beta adrenergic induced vasodilation of small arterioles. The parasympathetic system controls heart rate via the vagus nerve to the SA node. Decreased parasympathetic activity allows HR elevation with minimal direct effect on blood flow.
Metabolic factors considered to influence coronary blood flow include nitric oxide (NO), Katp, Ca2+, pH, O2 and CO2...