Peritoneal Dialysis Mechanism and Physiology

Core Physiological Principles

The peritoneum serves as the natural semipermeable membrane, with blood flow provided by the peritoneal microcirculation rather than an artificial extracorporeal circuit, allowing for solute and water exchange 1, 2
Peritoneal dialysis uses the peritoneal membrane as a natural semipermeable barrier, where dialysate solution instilled into the peritoneal cavity removes uremic toxins through diffusion and convection, while ultrafiltration is achieved by creating an osmotic gradient using hypertonic glucose or icodextrin solutions 1, 2

Diffusion is the primary mechanism for small molecule clearance, where solutes move from areas of high concentration to low concentration across the peritoneal membrane, effectively removing uremic toxins like creatinine, urea, and electrolytes 1, 2
Convection provides additional solute clearance, where water carries dissolved solutes across the membrane through solvent drag, and solutes are removed together with fluid during ultrafiltration 1, 2

Peritoneal dialysis achieves only 10-20% of normal kidney clearance for index substances like urea and creatinine, with even lower clearance for higher molecular weight solutes 1, 2
Critical functions not replaced by peritoneal dialysis include tubular secretive and reabsorptive function, and endocrine function of the kidney 1, 2
Nutrient losses occur during treatment, including low molecular weight macronutrients and micronutrients, and continuous protein loss can contribute to malnutrition 1, 2, 3

Middle molecule clearance is maximized by continuous 24-hour peritoneal dialysis without dry periods, as it depends more on total dialysis time than dialysate flow rate 4

The American Journal of Kidney Diseases recommends that peritoneal dialysis patients have higher dietary protein requirements (1.2-1.3 g/kg/day) compared to hemodialysis patients (1.2 g/kg/day) due to protein losses into peritoneal dialysate (5-15 g/24 hours) 5
Peritoneal amino acid losses average about 3 g/day, contributing to malnutrition 5
Continuous protein loss occurs (averaging 5-15 g/24 hours) and can contribute to malnutrition 5

Haematologica reports that uric acid clearance is approximately 70-100 mL/min with plasma uric acid falling by about 50% with each 6-hour treatment in hemodialysis 6
Peritoneal dialysis has lower efficiency in removing solute and fluid compared to intermittent hemodialysis and continuous renal replacement therapies, limiting its usefulness when significant rapid solute removal (uric acid, urea, potassium, phosphate) is required 6
The American Heart Association, as reported in Circulation, notes that peritoneal dialysis offers smaller hemodynamic shifts compared to hemodialysis, making it advantageous for hemodynamically unstable patients 7