Practice question

Artificial kidney or dialysis device replaces glomerular filtration by exploiting physical principles of diffusion across synthetic semipermeable membranes. Blood pumped through thousands of hollow fibers with wall pore size cut off around 10 kilodaltons separates from dialysate fluid flowing countercurrently. Small metabolic wastes urea 60 daltons, creatinine 113 daltons and electrolytes potassium accumulate at higher concentration in plasma than dialysate creating concentration gradient. According to Fick's first law diffusive flux proportional to concentration difference times membrane permeability and surface area divided by thickness. Consequently solutes diffuse passively from blood into dialysate without need for ATP driven pumps. Large proteins like albumin retained due to size exclusion. Countercurrent flow maintains gradient along fiber length enhancing clearance. Osmotic or hydrostatic gradients may adjust water balance but waste removal itself driven by diffusion. Active transport, endocytosis or vesicular trafficking not involved in this extracorporeal circuit. Therefore driving force allowing removal of nitrogenous waste during dialysis is passive diffusion down concentration gradient.