The Physiology of Peritoneal Solute, Water, and Lymphatic Transport

Abstract

Few studies have been published on the magnitude of the surface area of the peritoneum. Wegener mentioned a surface area of 1.72 m 2 in one adult woman [1] and Putiloff a value of 2.07 m 2 in one adult male [2]. More recent autopsy studies reported lower values [3–5]; the average peritoneal surface area in adults ranged from 1.0 m 2 [3] to 1.3 m 2 [5]. Using CT scanning in continuous ambulatory peritoneal dialysis (CAPD) patients a value of 0.55 m 2 has been found [6]. Some studies reported relationships between peritoneal surface area and body weight and/or body surface area, while others did not. The ratio between peritoneal surface area and body weight in adults is about half of that found in newborn infants [3]. A difference between adults and infants is barely present when peritoneal surface area is related to body surface area [3]. The peritoneal/body surface area averaged 0.6–0.8 in adults and 0.5–0.6 in infants. About 60% of the peritoneum consists of visceral peritoneum, 10% of which covers the liver, 30% of mesenterium and omentum, and 10% is parietal peritoneum [3–5]. The latter includes the diaphragmatic peritoneum, which comprises 3–8% of the total peritoneal surface area. Species differences are present, especially with regard to the contribution of diaphragmatic peritoneum, which is larger in humans than in rodents [5]. The contribution of the various parts of the peritoneum to solute transport during peritoneal dialysis may vary. Evisceration was found to cause a marked reduction in the transport of creatinine in rabbits [7] but not in rats [8, 9]. Effective peritoneal dialysis has been described in a neonate with extensive resection of the small intestine [10]. It has been hypothesized that the peritoneum covering the liver might be especially important in solute transport during peritoneal dialysis because of the close proximity with the liver sinusoids, but this could not be confirmed in experimental studies in rats [11, 12]. The diaphragmatic part of the peritoneum is especially involved in the absorption of solutes and fluid from the peritoneal cavity into the lymphatic system [13]. Observations in rats have shown that the peritoneal surface area increases with the age of the animals, with a proportional increase in dialysate/plasma (D/P) ratios of urea and creatinine [14]. The proportion of the peritoneum that is involved in transport during peritoneal dialysis is not known. The abovementioned evisceration experiments suggest that the relative contribution of the parietal peritoneum may be more important than that of the visceral peritoneum. Although similar diffusion rates were found during experiments in rats undergoing peritoneal dialysis using a diffusion chamber, placed at various parts of the peritoneum, it appeared that only 25–30% of the visceral peritoneum was in contact with the dialysis solution [11]. Furthermore, it has been shown in cats that commercial dialysis solutions increase blood flow to the mesentery, omentum, intestinal serosa, and parietal peritoneum without altering total splanchnic blood flow [15]. This study, using microspheres, points to hyperemia of these tissues, thereby increasing the peritoneal capillary surface area. It appears from the above data that the surface area of the peritoneal membrane involved in peritoneal dialysis is not a static property but should be defined in a functional way. The functional surface area of the peritoneal membrane cannot be measured directly, but the functional cross-sectional exchange pore area divided by the effective diffusion path length can be estimated. This parameter takes not only the capillary surface area into account, but also the distance to the dialysate/mesothelial contact. Using kinetic modeling, values of 117–250 m have been reported [16–19]. When the surface area of the peritoneum that is involved in transport is 0.6 m 2 and the unrestricted area over diffusion distance is set at 190 m, it can be calculated that the length from the capillary wall to the peritoneal cavity would be 3 mm in case of unrestricted diffusion in water. As the peritoneum is much thinner, this means that the resistance to diffusion of interstitial tissue must greatly exceed that of water. This will be discussed further in the following section.