Characteristics of special circulations

Blood flow through vascular beds is usually determined by the pressure gradient and the diameter of the precapillary resistance vessels. Special circulations have additional features of intrinsic blood flow control, allowing autoregulation. Coronary blood flow is linked to myocardial oxygen consumption by a metabolic mechanism, where increased metabolism releases vasodilator metabolites. Cerebral blood flow is primarily regulated by a myogenic mechanism whereby increases in transmural pressure stretch the vascular smooth muscle, which responds by contracting. Renal blood flow is regulated by myogenic mechanisms and tubuloglomerular feedback. Hepatic blood flow is delivered via the portal vein and hepatic artery, and flow varies in these vessels reciprocally to maintain constant total flow. The pulmonary circulation receives the entire cardiac output, and blood flow is regulated both passively and actively to allow increases in flow without significant increases in pressure.