Haemodialysis

When a patient's own kidneys can no longer clear urea and other toxic wastes from the blood, an external machine can temporarily take over that job through a process called haemodialysis. The patient's blood, drawn from a convenient artery and mixed with an anticoagulant such as heparin to stop it clotting in the equipment, is pumped into a dialysing unit built around a long, coiled cellulose tube immersed in a water bath of dialysing fluid. The tiny pores in this tube let small molecules - glucose, salts and urea - pass through by diffusion into the surrounding fluid, in a process functionally similar to glomerular filtration, while blood cells and large protein molecules are too big to cross and stay in the blood. The dialysing fluid itself is carefully formulated with a matching concentration of salts and sugar so that only excess wastes diffuse out, without stripping away the glucose and essential salts the body needs to keep. Once cleared of urea and other toxins, the purified blood is pumped back into the patient through a vein. Because haemodialysis only m …