The Peritoneal Microcirculation in Peritoneal Dialysis

Abstract

This paper deals with the peritoneal microcirculation and with peritoneal exchange occurring in peritoneal dialysis (PD). The capillary wall is a major barrier to solute and water exchange across the peritoneal membrane. There is a bimodal size-selectivity of solute transport between blood and the peritoneal cavity, through pores of radius approximately 40-50 A as well as through a very low number of large pores of radius approximately 250 A. Furthermore, during glucose-induced osmosis during PD, nearly 40% of the total osmotic water flow occurs through molecular water channels, termed "aquaporin-1." This causes an inequality between 1 - sigma and the sieving coefficient for small solutes, which is a key feature of the "three-pore model" of peritoneal transport. The peritoneal interstitium, coupled in series with the capillary walls, markedly modifies small-solute transport and makes large-solute transport asymmetric. Thus, although severely restricted in the blood-to-peritoneal direction, the absorption of large solutes from the peritoneal cavity occurs at a high clearance rate ( approximately 1 mL/min), largely independent of molecular radius. True absorption of macromolecules to the blood via lymphatics, however, seems to be occurring at a rate of approximately 0.2 mL/min. Several controversial issues regarding transcapillary and transperitoneal exchange mechanisms are discussed in this paper.

Water transport across the peritoneal membrane

“…The pathways available for solute and water exchange between the plasma in the peritoneal capillaries and the fluid in the peritoneal cavity include i) the continuous capillary endothelium; ii) the peritoneal interstitial space; and iii) the mesothelium. Of these potential barriers, it is the capillary endothelium that seems to offer the rate-limiting hindrance, restricting the solute exchange to less than 0.1% of its total surface area (4,(8)(9)(10). Solutes larger than glucose are hindered in their transport across the permeable pathways ("pores") of the capillary wall.…”

“…The peritoneal membrane (Fig. 1A) has a relatively large surface area (~1 m2) (3), a high degree of capillarization, and a relatively high blood flow (100-150 mL/min) in adults (4)(5)(6). The compact zone of the visceral peritoneum (that forms most of the peritoneal surface area) is about 20 m-thick in PD patients, whereas the parietal peritoneum can be thickened up to 500 m in long-term PD patients, compared to 50 m in controls (7).…”

Acute Peritoneal Dialysis in Rats Results in a Marked Reduction of Interstitial Colloid Osmotic Pressure

“…Previously, Leypoldt and Mistry (20) as well as Rippe et al (12) estimated how much the Starling forces have to change during PD to account for the observed volume changes. The authors calculated that the COP i in the peritoneal tissues needs to fall from~10 mmHg to 1 to 2 mmHg to account for such changes.…”

“…It is conceivable that the COP i will drop even further during the subsequent dwells, as a result of washout of colloids, to provide an even larger shift in the Starling equilibrium in chronic PD. Theoretically, a 7-to 8-mmHg drop in COP i would be sufficient to promote an effective absorptive force during the late phase of the PD dwell (12,20).…”

“…In PD, a hyperosmotic solution is instilled into the PC to remove excess fluid and metabolites from the blood of patients with impaired renal function (12). In this study, PD was performed by injecting 20 ml of 3.86% Dianeal (Baxter Health Care, Castlebar, Ireland), which essentially is a lactated Ringer solution that contains glucose as osmotic agent, into the PC.…”