Mechanisms of solute clearance and ultrafiltration in peritoneal dialysis
- Madhukar Misra, MD, FRCP
Madhukar Misra, MD, FRCP
- Professor of Medicine
- University of Missouri School of Medicine, Columbia
- Ramesh Khanna, MD, FACP
Ramesh Khanna, MD, FACP
- Karl D Nolph, MD Chair in Nephrology, Professor of Medicine
- Vice Chair, Department of Medicine
- Director, Division of Nephrology
- University of Missouri-Columbia
Chronic peritoneal dialysis (as with continuous ambulatory peritoneal dialysis [CAPD]) can be complicated by problems with either solute clearance (which occurs by solute diffusion from the plasma into dialysate) or ultrafiltration (which is driven by the osmotic gradient between the hyperosmotic dialysate and the plasma). The mechanisms of solute clearance and ultrafiltration across the peritoneal membrane will be reviewed in this topic review. The clinical issues that can arise are discussed separately. (See "Problems with solute clearance and ultrafiltration in continuous peritoneal dialysis".)
GENERAL PRINCIPLES OF TRANSPORT ACROSS THE PERITONEAL MEMBRANE
The average surface area of the peritoneal membrane is between 1 and 1.3 m2 in adults [1,2]. During peritoneal dialysis, it is principally the parietal peritoneum that participates in peritoneal transport since only approximately one-third of the visceral peritoneum is in contact with the dialysis solution at a given time . In addition to the capillary surface area, the diffusion length between the dialysate and the mesothelium also plays an important role in the overall transport characteristics of the peritoneum.
There are three barriers between the dialysate in the peritoneum and capillary blood: the capillary wall, which is most important; the interstitium; and the mesothelial cell layer. The mesothelial cell layer does not constitute a major barrier to solute or water transport across the peritoneum, while the interstitium offers some resistance to solute transport that is mainly restricted to large solutes .
Pores for solute transport — According to the three-pore model of solute transport, the capillary wall consists of a system of pores of three sizes, which are size selective in restricting solute transport [5,6]:
●There is an abundance of small pores (average radius 40 to 50 Å) that mediate the transport of lower-molecular-weight solutes. The transport of these solutes is limited by the number of small pores.
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- GENERAL PRINCIPLES OF TRANSPORT ACROSS THE PERITONEAL MEMBRANE
- Pores for solute transport
- - Aquaporin-1 and water transport
- Solute transport
- - Low-molecular-weight solutes
- - High-molecular-weight solutes
- - Electrolytes
- Fluid transport
- CLINICAL CORRELATES
- Large or small peritoneal vascular surface area
- Impaired aquaporin-1-mediated water transport
- - Assessment of aquaporin-1 function
- Increased lymphatic absorption
- SUMMARY AND RECOMMENDATIONS