The effects of flow rate, length and external pressure upon the pressure required for fluid to flow through a ureter.

Abstract

To determine in vitro the effects of increments of external pressure on the pressure required to conduct fluid through ureters of various lengths at different flow rates, as the flow of a fluid through a collapsible tube is influenced by various factors (e.g. external pressure, the pressure gradient between the ends, the length and diameter of the tube, and the viscosity of fluid). Two in vitro systems were designed, composed of three parts, i.e. a perfusion line, an exit line and a container of two different widths in which short or long ureteric segments, obtained from cattle, could be placed; the ureter was connected to the perfusion and exit lines. Physiological saline was added to the container until the desired external pressure was applied to the ureter. The flow pressure (height of the perfusion line) was recorded when producing flows through ureters of varying length at 1.5 and 6 mL/min, and determined under various external pressures. The intra-ureteric pressure during flow was also monitored by a pressure transducer. The four combinations of long and short ureters with high and low flow rates were compared using analysis of variance, with the Pearson correlation coefficient used to evaluate the relationships between the various pressures. There were close relationships between flow pressure and external pressure (r = 0.727), intra-ureteric and external pressure (r = 0.766), and the flow pressure and intra-ureteric pressure (r = 0.940, all P < 0.001). Increments in external pressure resulted in greater flow and intra-ureteric pressure (P < 0.05). Increases in flow pressure were more pronounced than increases in intra-ureteric pressure at the same external pressure (P < 0.05) at high flow rates. A longer ureter and higher flow rates caused greater intra-ureteric pressure (P < 0.05). External pressure increases the pressure required to conduct fluid through a ureter and the effect is more pronounced at high flow rates. The length of the ureter also affects the flow pressure at high flow rates. Therefore, flow through the ureter follows the Poiseuille equation only at high flow rates. Thus, increases in intra-abdominal pressure may cause greater intrapelvic pressure and induce ureteric obstruction, contributing to the pathogenesis of hydronephrosis.