The peritoneal membrane is a complex heterogenous, semi-permeable membrane with multiple pores. The early models of peritoneal membrane transport included multiple sites of resistance to the flow of solutes across the membrane. These included the capillary fluid film overlying the capillary endothelium, the capillary endothelium per se, the endothelial basement membrane, the interstitium, the mesothelial cells and the fluid overlying the peritoneal membrane. Newer concepts such as the Three Pore Model suggest that the major resistance to peritoneal transport is in the capillary endothelium and its basement membrane.
The Three Pore Model is a theoretical model validated by clinical observations1,2. It suggests that the peritoneal capillary is the critical barrier to trans-peritoneal transport. Solute and water transport across the peritoneal capillary is mediated by pores of three different sizes.
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Large pores (100-200 A) exist in small numbers and constitute < 0.1% of all pores. The transport macromolecules and anatomically are identified as clefts between endothelial cells. Small pores (40-60 A) are more numerous and believed to transport small solutes and water. Ultra-small or transcellular pores (4-6 A) are water channels or aquaporin-1. They are numerous and resemble the water channels present in red blood cells and renal proximal tubules. They transport water only (sieving) and are present in the endothelial cells of the peritoneal capillaries.
During ultrafiltration in PD, and unlike HD, solutes do not move across the membrane in direct proportion to their concentration in blood. Sodium is held back or sieved at the aquaporin barrier while water moves through. Sieving makes ultrafiltration a less effective form of convective solute transport.
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The semi-permeable peritoneal membrane allows solutes and water to be transported from the vascular system to the peritoneal cavity and vice versa through diffusion. Diffusion is the process of solutes moving from an area of higher concentration to an area of lower concentration, which is the case when dialysate is instilled into the peritoneal cavity. Actually, solutes move randomly in both directions, but there is simply more solute moving from the high to low concentration side than in the opposite direction. Eventually, the concentrations become equal on both sides of the membrane. This is termed equilibrium.
The movement of solute molecules is random and driven by thermal energy. This energy is proportional to absolute temperature (degrees Centigrade above -273). This thermal energy is transferred to kinetic energy which is the multiple of mass and velocity. Since this energy is the same for different sized molecules at the same temperature, the larger molecules must move more slowly in order to have the same energy as the smaller molecules. Thus, the diffusive rate depends on molecular weight.
Solute transport is influenced by the membrane permeability and size, characteristics of the solute, the volume of dialysate instilled, and blood flow to the membrane. Solute transport can be increased by maximizing the contact of dialysis solution with the membrane by placing the patient in a supine position or increasing the exchange volume.
Osmosis is the movement of water across a semi-permeable membrane from an area of low solute concentration to an area of high solute concentration. The hydrostatic pressure gradient and the osmotic gradient between the blood and the dialysis solution influence osmosis.
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Increasing the dextrose concentration of dialysis solution enhances fluid removal by increasing the osmotic gradient between the plasma and the peritoneal fluid. The higher the dextrose concentration, the higher the fluid removal. The osmotic gradient is always greatest at the beginning of the dialysis exchange. As osmotic equilibration is achieved, the gradient decreases. Some reabsorption of fluid occurs when dialysate dwells beyond the point of equilibration.
Alternatively, a glucose polymer can be used instead of glucose. The polymer is not absorbed, so the fluid removal is sustained. This process is called colloid osmosis.
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There is a significant amount of protein lost in dialysate. The amount lost varies from patient to patient but averages between 5-8 gm/day. Protein loss stabilizes and remains relatively constant unless the patient experiences peritonitis, then the protein loss increases during the infection. It is very important that protein intake be adequate in the PD patient. Daily protein requirements average 1.2-1.5 gm/kg of body weight. Other substances lost in dialysate are amino acids, water-soluble vitamins, hormones and some medications.
Substances that are absorbed from dialysis solution into the systemic circulation include dextrose and calcium. The increased concentration of dextrose in dialysis solution causes dextrose to move into the systemic circulation.
The major factor influencing the systemic calcium absorption is the amount of ionized calcium in the plasma and in the dialysis solution. A calcium concentration of 0.8 mmol/L in dialysis solution provides an ionized calcium of 1.6 mmol/L. Half of a standard serum calcium level of 2.20 - 2.58 mmol/L is ionized when the serum albumin is normal. Thus, with an ionized calcium in dialysis solution of 1.6 mmol/L and an ionized serum calcium of 1.0 mmol/L, calcium can be absorbed into the systemic circulation.
References:
- Rippe B. A three-pore model of peritoneal transport. Perit Dial Int 13 Suppl 2:S35-S38, 1993
- Rippe B, Krediet RT. Peritoneal physiology-tranport of solutes. In: Gokal R, Nolph KD, eds. The textbook of peritoneal dialysis. Dordrecht: Kluwer Academic Publishers, 1994:69-113