2007A12 Explain the physiological principles underlying the use of peritoneal dialysis
in a patient with chronic renal failure with this dialysate solution:
Na⁺ 132mmol/L, Mg²⁺ 0.5mmol/L, K⁺ 0mmol/L, Lactate 40mmol/L, Cl^- 96mmol/L, glucose 2.5%, Ca²⁺ 3.5mmol/L.

Definition

·  Exchange between dialysate and blood across a membrane

Purpose

·  Maintain fluid, electrolyte, acid-base homeostasis

·  Excrete metabolic waste

Diffusion

·  Movement of solute from high concentration to low concentration

o Slow process

·  Modulated by Gibbs-Donnan effect (?)

o Proteins are anionic, membrane-permeable and attract small cations e.g. Na⁺

o (Not sure how this is relevant)

Osmosis

·  Movement of solvent from low osmolality to high osmolality

o Dialysate 400mOsm and hypertonic (mainly due to glucose)

o Plasma 290mOsm, by definition isotonic

o Osmolarity: number of osmotically active particles per L

§ ≈= 2 x Na⁺ + urea + glucose

o Tonicity: effect of a solution on cell size (hypertonic -> swelling)

Solvent drag

·  Movement of solute due to bulk flow of solvent

·  May allow movement of a substance against a concentration gradient

Lactate

·  Metabolized by liver to produce HCO₃^-

·  i.e. buffer for H⁺

Glucose

·  Large amount

·  Can cause hyperglycaemia, obesity, insulin resistance