LYMPHATIC VERSUS NONLYMPHATIC FLUID ABSORPTION FROM THE PERITONEAL-CAVITY AS RELATED TO THE PERITONEAL ULTRAFILTRATION CAPACITY AND SIEVINGPROPERTIES

In this article we discuss the role of capillary fluid absorption via Starling mechanisms (the transcapillary hydrostatic pressure gradient opposed by the colloid osmotic pressure gradient as multiplied by the capillary UF coefficient) vs. lymphatic fluid absorption as determinan ts of the total fluid loss from the peritoneal cavity during continuou s ambulatory peritoneal dialysis (CAPD). We also mention that, under n onsteady state conditions, there is in addition some net absorption of fluid into the interstitium of tissues surrounding the peritoneal cav ity. Support for the contention that nonlymphatic fluid absorption dir ectly into the capillaries is the major mode of fluid transport from t he peritoneal cavity to the blood is given by measurements of the peri toneal-to-blood clearance of tracer albumin (or other proteins). Such measurements yield clearance values of the order of 0.2-0.3 ml/min in CAPD. This represents only about 20% of the total peritoneal fluid los s rate (1.2-1.3 ml/min) in ordinary CAPD dwells. Indirect support for a relatively low lymph flow is also derived from capillary physiology. Like continuous capillary walls, the peritoneal membrane shows a bimo dal selectivity towards molecules of graded molecular size. Thus, smal l solute transport can be described as occurring by diffusion through numerous 'small' (approximately 50 angstrom radius) pores, whereas lar ge solute transport is consistent with blood-peritoneal convection thr ough smaller numbers of 'large' (radius approximately 250 angstrom) po res. Furthermore, peritoneal sieving data are compatible with the noti on that large crystalloid osmotic pressure gradients cause fluid flow through a water-exclusive ('ultra-small' pore) pathway. A three-pore m odel of peritoneal selectivity can explain why small solute sieving co efficients are only 0. 5-0.6, even though small solute reflection coef ficients are close to zero. Another important implication of the three -pore concept is that the peritoneal UF- coefficient is much higher th an previously thought, emphasizing the role of capillary absorption in the fluid loss from the peritoneal cavity in CAPD. It is concluded th at fluid loss from the peritoneal cavity is dominated by capillary flu id absorption. Hence, lymphatic absorption accounts for just a small f raction of the peritoneal-to-blood absorption of fluid in peritoneal d ialysis.