The renal compartment: a hydraulic view

Abstract

Background: The hydraulic behavior of the renal compartment is poorly understood. In particular, the role of the renal capsule on the intrarenal pressure has not been thoroughly addressed to date. We hypothesized that pressure and volume in the renal compartment are not linearly related, similar to other body compartments.

Methods: The pressure-volume curve of the renal compartment was obtained by injecting fluid into the renal pelvis and recording the rise in intrarenal pressure in six anesthetized and mechanically ventilated piglets, using a catheter Camino 4B® inserted into the renal parenchyma.

Results: In healthy kidneys, pressure has a highly nonlinear dependence on the injected volume, as revealed by an exponential fit to the data (R (2) = 0.92). On the contrary, a linear relation between pressure and volume is observed in decapsulated kidneys. We propose a biomechanical model for the renal capsule that is able to explain the nonlinear pressure-volume dependence for moderate volume increases.

Conclusions: We have presented experimental evidence and a theoretical model that supports the existence of a renal compartment. The mechanical role of the renal capsule investigated in this work may have important implications in elucidating the role of decompressive capsulotomy in reducing the intrarenal pressure in acutely injured kidneys.

Figure 1

Schematic of the experimental setup design for measuring the intrarenal pressure. Fluid is injected through a Foley catheter into the renal pelvis, and intrarenal pressure is measured using a Camino catheter located in the lower renal pole.

Figure 2

Pressure-volume curve in the renal compartment. Data is expressed as mean value ± SEM. Solid line represents the exponential fit to the data. p, intrarenal pressure; V, volume injected to the renal pelvis.

Figure 3

Schematic of the biomechanical model for the kidney capsule. p, intrarenal pressure; σ, capsule stress; h, capsule thickness; r, capsule radius.

Figure 4

Comparison between the biomechanical model of the renal capsule, the capsulated and decapsulated kidneys. Data is expressed as mean value ± SEM.

Figure 5

Comparative plot of pressure-volume curves for the intact kidney, the decapsultaed kidney, and the biomechanical model. The capsule confinement can explain the nonlinear relation between pressure and volume for small amounts of fluid. Tissue recruitment provides additional stiffness to the renal continent in order to bear the intrarenal overpressure.