Peritoneal dialysis is a technique whereby infusion of dialysis solution into the peritoneal cavity is followed by a variable dwell time and subsequent drainage. Continuous ambulatory peritoneal dialysis (CAPD) is a continuous treatment consisting of four to five 2-L dialysis exchanges per day (Fig. 4-1A). Diurnal exchanges last 4 to 6 hours, and the nocturnal exchange remains in the peritoneal cavity for 6 to 8 hours. Continuous cyclic peritoneal dialysis, in reality, is a continuous treatment carried out with an automated cycler machine (Fig. 4-1B). Multiple short-dwell exchanges are performed at night with the aid of an automated cycler machine. Other peritoneal dialysis treatments consist of intermittent regimens (Fig. 4-2A-C).
During peritoneal dialysis, solutes and fluids are exchanged between the capillary blood and the intraperitoneal fluid through a biologic membrane, the peritoneum. The three-layered peritoneal membrane consists of 1) the mesothelium, a continuous monolayer of flat cells, and their basement membranes; 2) a very compliant interstitium; and 3) the capillary wall, consisting of a continuous layer of mainly non-fenestrated endothelial cells, supported by a basement membrane. The mesothelial layer is considered to be less of a transport barrier to fluid and solutes, including macromolecules, than is the endothelial layer . The capillary endothelial cell membrane is permeable to water through aquaporins (radius of approximately 0.2 to 0.4 nm) . In addition, small solutes and water are transported through ubiquitous small pores (radius of approximately 0.4 to 0.55 nm). Sparsely populated large pores (radius of approximately 0.25 nm, perhaps mainly venular) transport macromolecules passively. Diffusion and convection move small molecules through the interstitium with its gel and sol phases, which are restrictive owing to the phenomenon of exclusion [3,4]. The splanchnic blood flow in the normal adult ranges from 1.0 to 2.4 L/min, arising from celiac and mesenteric arteries . The lymphatic vessels located primarily in the subdiaphragmatic region drain fluid and solutes from the peritoneal cavity through bulk transport.
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