Characteristics of Special Circulations

Blood flow through a vascular bed is usually determined by the pressure gradient across it and the diameter of the precapillary resistance vessels. Special circulations have additional specific features of blood flow control. Several organs control their blood supply by autoregulation. Coronary blood flow is linked to myocardial oxygen consumption, primarily by a metabolic mechanism, thus increases in demand, or decreases in supply, of oxygen cause the release of vasodilator metabolites, which act on vascular smooth muscle to cause relaxation and hence increase blood flow. Cerebral blood flow is primarily regulated by a myogenic mechanism whereby increases in transmural pressure stretch the smooth muscle, which responds by contracting, however, this can be overridden by cerebral tissue PCO2. Further, the cerebral circulation is contained in a rigid structure (the skull) that has additional consequences for blood flow after brain injury, when the pressure within this compartment may rise dramatically. Renal blood flow is regulated by both extrinsic and intrinsic mechanisms; sympathetic vasoconstriction of the afferent arterioles reduces renal blood flow in response to a decrease in effective circulating volume, myogenic mechanisms and tubuloglomerular feedback, as well as the release of vasoactive metabolites from the vascular endothelium regulate renal blood flow intrinsically. Hepatic blood flow is delivered via the hepatic artery and the portal vein, and the amount varies reciprocally to maintain total blood flow constant. The pulmonary circulation receives the entire cardiac output and blood flow is regulated both passively and actively. Pulmonary vessels are highly distensible and can accommodate increases in blood flow without significant increases in pressure. However, this leads to regional differences in blood flow throughout the lung. Regional reductions in alveolar oxygen tension constrict local arterioles to divert blood to better ventilated areas. In ‘special circulations’ additional factors govern the control of blood flow, beyond the ‘standard’ mechanisms that prevail in most organ systems (see article Control of the Circulation). In this article the coronary, cerebral, renal, hepatic and pulmonary ‘special’ circulations are considered. Blood flow through the cardiovascular system is dependant on the force driving blood along the vessel (pressure gradient) and is restricted by the resistance of the vessels. The resistance to blood flow, in turn is dependent on the radius and length of the blood vessel and the viscosity of the blood flowing through it. In practice it is the resistance of the arterioles that normally has the greatest effect on blood flow. An approximate relationship between vessel dimensions and blood viscosity is shown in Equation 1 (below). Poiseuille’s law is an approximation for the cardiovascular system since Poiseuille’s law strictly should only be applied to Newtonian fluids flowing through a straight unbranched, non-distensible, tube; conditions that do not prevail in the cardiovascular system. However, the equation does give a useful approximation for the cardiovascular system. Q = (P1 – P2) r 8l Poiseuille’s law; Q = blood flow, P1 – P2 = the pressure gradient (normally arterial minus venous), r = radius of the vessel,   viscosity, l = length of the vessel, /8 = constant of proportionality. In Poiseuille’s Law radius of the vessel is raised to the power 4, thus if the radius were to double, flow would increase 16-fold. The most important vessels regulating blood flow in this manner are the small arteries and arterioles as they contain an abundance of vascular smooth muscle arranged in a circular manner along the length of the vessel, the tone of which is regulated by both extrinsic (neural and humoral) and intrinsic (myogenic and metabolic) factors.