haemodialysis

• • During dialysis the patient is connected to a machine which continually pumps their blood through a system of coiled tubes (increases SA) with semipermeable membranes separating the blood from the dialysate (fluid the coiled tubes are submerged in). 
• • The dialysate fluid has the same concentration as blood plasma (without the metabolic wastes) and flows in the opposite direction to blood being constantly replaced in order to maintain a high concentration gradient to ensure the removal of urea via diffusion. Dialysis only utilises passive transport.  
• • The dialysate fluid has:
  • no urea creating a large concentration gradient for diffusion of urea from blood to dialysate (ensures urea is removed)
• • glucose concentration equal to normal blood glucose levels so no concentration gradient exists, therefore ensuring glucose is retained in the blood
• • ion concentration similar to ideal blood plasma concentration so ions only move if an imbalance exists