Kidney Loss, Dialysis and Transplantation

Each kidney contains roughly one million nephrons, and each nephron works in five stages. Knowing which stage does what is the difference between describing dialysis and explaining it: dialysis assists the broad outcome of removing wastes and excess fluid, but it does not reproduce nephron filtration, selective tubular regulation or endocrine function.

At the glomerulus and Bowman's capsule, blood pressure forces water, glucose, urea and ions out of the capillary knot into the capsule. Cells and plasma proteins are too large to cross, so they stay in the blood. This is pressure filtration, and it is not selective about which small molecules it removes.

The proximal convoluted tubule then reclaims the useful material: about 65% of the water, all of the glucose and most ions return to the blood. The loop of Henle runs a counter-current multiplier that builds a salt gradient in the medulla, with the descending limb losing water and the ascending limb losing salt.

Finally the distal convoluted tubule fine-tunes the result under ADH and aldosterone control, and the collecting duct carries out the last ADH-regulated water reabsorption to produce concentrated urine. Those are the same two hormones you met in Lesson 4, acting on the same tubule.

A patient is connected to a haemodialysis machine by two blood lines. In an enlarged dialyser, blood travels inside many hollow fibres while dialysate travels outside them in the opposite direction. A magnified fibre wall shows that blood cells and large plasma proteins remain in the blood, small wastes diffuse into dialysate and excess water crosses by pressure-controlled ultrafiltration.

Trace it: Follow a urea molecule from the patient's blood into the dialysate. Then explain how opposite flow helps maintain its concentration gradient.

Pause and copy the highlighted principle into your notes.

Dialysis can remove urea and excess water, but it is intermittent, time-consuming and less precise than healthy kidneys. It also does not fully replace every kidney role, so diet, fluid limits and monitoring remain important.

Limits of dialysis compared with healthy kidneys

Healthy kidneys are endocrine organs as well as filters. They secrete erythropoietin, which drives red blood cell production in bone marrow, so kidney failure causes anaemia and persistent fatigue that dialysis alone does not fix. They also activate vitamin D, and without it calcium absorption falls and bone disease follows.

The third limit is not a lost kidney function at all, it is timing. Kidneys work continuously, while haemodialysis compresses the same job into about twelve hours a week. Between sessions, urea, potassium and fluid build up again, which is why strict fluid and dietary potassium limits are part of the treatment rather than an optional extra.

A transplant replaces the organ rather than one of its functions. The failed kidneys are usually left in place and the donor kidney is implanted low in the pelvis, connected to the iliac artery and vein with its ureter joined to the bladder, because those vessels are easier to reach surgically.

The obstacle is immunological. Donor and recipient are tissue-typed for HLA compatibility, and a cross-match before surgery checks for pre-formed antibodies. Rejection then comes in three forms: hyperacute within minutes from pre-formed antibodies, acute over days to weeks from a T-cell attack on donor antigens, and chronic over months to years as slow immune-mediated fibrosis.

Lifelong immunosuppressants such as tacrolimus, mycophenolate and prednisolone hold that response down, and they carry the central trade-off of transplant medicine. Suppressing the immune system to protect the graft also raises the risk of infection and of certain cancers, particularly skin cancer and lymphoma.