Role of the Liver in Small-Solute Transport During Peritoneal Dialysis1

Peritoneal dialysis (PD) is dependent on the transport of water and solutes from the blood capillaries within the tissues that surround the peritoneal cavity. Because of their large blood supply and surface area, the viscera have been considered the most important tissues for PD transport. In animals, however, removal of the gastrointestinal tract decreases PD small-solute mass transfer by only 10 to 27% (9,10). To investigate the theoretical basis for these observations, a distributed model of peritoneal transport was extended to take into account the transport characteristics of four tissue groups that surround the cavity: the liver, the hollow viscera, the abdominal wall, and the diaphragm. The mass transfer-area coefficient (MTAC) of sucrose for each tissue was calculated from the following: MTAC = 1(D(pa))#{176}5 A, where D is the effective solute interstitial diffusivity, pa is the solute transcapillary permeability-area per unit tissue volume, and A is the apparent peritoneal surface area of the tissue. Our results for the adult human predict that the MTAC for the liver is comparable to that of all of the other viscera and makes up 43% of the total MTAC for the peritoneal cavity. The predicted MTAC is 4 cm3/min (plasma) or 6 cm3/min , Received December 20, 1993. Accepted February 21, 1994. 2 Correspondence to Dr. M.F. Flessner, Box 675, Nephrology Unit, University of Rochester Medical Center, 601 Elmwood Avenue, Rochester, NY 14642. 1046-6673/0501-01 16$03.OO/O Journal of the American Society of Nephrology Copyright C 1994 by the American society of Nephrology D espite a lange number of theoretical articles on the subject of small-solute transfer during penitoneal dialysis, all of the published conceptual models portray the penitoneal transport system as a single tissue with one set of characteristic transport parameters. Simple membrane models ( 1 -5) use a single mass transfer-area coefficient (MTAC) to characterize the entire “peritoneal membrane. “ More complex physiologic models, which calculate diffusion, convection, and transcapillary transport within the tissue space surrounding the penitoneal cavity (6-8), have also failed to account for the variability of the microcirculation in the different peritoneal tissues and have used a single set of parameters to describe these processes. Recent experimental studies (91 1) have demonstrated only small decreases in the MTAC of small solutes after the removal of the hollow viscera, which compose more than half of the available transport surface area (1 2). These indicate that the penitoneal transport system may be more correctly treated as a collection of different surfaces in Its transport function. We have examined the theoretical mass transfer characteristics of specific penitoneal tissues. We use our previously published “distributed model” (6,7), which simulates solute transport from blood contamed within capillaries that are uniformly distributed In each tissue space surrounding the peritoneal cavity. The penitoneal tissue space is divided into four tissue groups, and parameters that characterize each of these tissue groups are estimated from the literature. These parameters are used to calculate the equivalent MTAC for each tissue group. Our analysis shows that the liver, which possesses only 1 3% of the peritoneal area, may account for nearly half of the small-solute transfer.