Renal dialysis
Dialysis is the term used to describe the separation of small and large molecules using a partially permeable membrane Renal dialysis is used as a treatment for kidney failure and comes in two forms: Haemodialysis Peritoneal dialysis For haemodialysis: The patient requires regular treatment in a hospital or at home using a machine known as a haemodialyser, which acts as an artificial kidney In this dialysis machine, partially permeable dialysis membranes separate the patient's blood from the dialysis fluid (known as the dialysate) The blood is passed through tubes of dialysis membrane, which are surrounded by dialysate The dialysate contains substances needed in the blood (e.g. glucose and sodium ions) in the right concentrations (i.e. concentrations similar to a normal level in blood) As the dialysate contains a glucose concentration equal to a normal blood sugar level, this prevents the net movement of glucose across the membrane as no concentration gradient exists As the dialysis fluid contains a salt concentration similar to the ideal blood concentration, movement of salts across the membrane only occurs where there is an imbalance (if the blood is too low in salts, they will diffuse into the blood; if the blood is too high in salts, they will diffuse out of the blood) The fluid in the machine is continually refreshed so that concentration gradients are maintained between the dialysis fluids and the blood Importantly, the dialysate contains no urea This causes urea to diffuse down its concentration gradient from the blood into the dialysate and is eventually disposed of The haemodialyser is designed so that the patient's blood and the dialysate flow in opposite directions, creating a concentration gradient along the length of the dialyser component of the machine This means that each time blood circulates through the machine, some more of the urea it contains passes into the dialysate, until almost all of it is removed (after approximately 3 hours) The drug heparin is added to the blood as it is an anticoagulant (blood thinner) that prevents the formation of blood clots
