The estimation of peripheral vascular resistance to varying rates of flow in the isolated rat hindquarter.

IN THE INTACT ANIMAL and m man, variations in the peripheral resistance to flow, and hence in the arterial pressure, are produced by variation in the size of the arterioles. The relationship between flow and pressure is complicated in the intact animal by the tact that blood is not a homogeneous fluid. Its apparent viscosity varies, being greater in large than in small vessels. Hemodynamic studies on the relationship between flow and pressure in isolated organs have been reported by several authors. Whittaker and Winton, and Pappenheimer and Maes thought that the resistance to flow was unaltered by variations in pressure. More recently, Nichol and co-workers found the resistance to be almost independent of the pressure, as long as this was well above the critical closing pressure. Green and co-workers, however, suggested that a decrease in resistance occurred in response to increasing intravascular pressure. Most of the studies on the relationship between flow and pressure were made by measurement of the venous outflow at various fixed pressures. This method has the disadvantage that the measured venous outflow may be less than the total flow through that part of the vascular bed in which the main resistance to flow occurs—namely, the arterioles—since some fluid may be lost as edema or through alternative venous channels. Furthermore,