The physiology of peritoneal solute transport and ultrafiltration

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Few studies have been published on the magnitude of the surface area of the peritoneum. Wegener mentioned a surface area of 1.72 m2 in one adult woman [1] and Putiloff a value of 2.07 m2 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 m2 [3] to 1.3 m2 [5]. Using CT scanning in CAPD patients a value of 0.55 m2 has been found [6], but this method needs further validation. 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].