Kidney Failure (OCR A Level Biology)

• Kidney failure can occur in one or both kidneys due to a variety of reasons, including (but not limited to):
  • Blood loss in an accident
  • High blood pressure
  • Diabetes
  • Overuse of certain drugs (e.g. aspirin)
  • Certain infections

• Kidney failure is dangerous and can be fatal within a relatively short time period
• In some cases, kidney failure happens suddenly and only lasts for a short time but in other cases, it can become a long-term condition
• If the kidneys fail:
  • Urea, water, salts and various toxins are retained and not excreted
  • Less blood is filtered by the glomerulus, causing the glomerular filtration rate (GFR) to decrease
  • This leads to a build-up of toxins in the blood
  • In addition, the electrolyte balance in the blood is disrupted (the concentrations of ions and charged compounds are not maintained)

The importance of the balance of electrolytes

• The disruption of the electrolyte balance in the blood as a result of kidney failure can have very harmful consequences
• Excess potassium ions in the blood can lead to abdominal cramps, tiredness, muscle weakness and even paralysis
• If potassium ion concentrations continue to increase, the frequency of impulses from the sinoatrial node in the heart may decrease, potentially leading to arrhythmia and cardiac arrest
• Depending on the body's needs, the kidneys either conserve or secrete sodium, which plays an important role in neuromuscular function, fluid balance and acid/base balance
• A build-up of sodium can cause disorientation, muscle spasms, higher blood pressure and general weakness

Potential treatments for kidney failure

• Humans can survive with one functioning kidney, but if both are damaged then there will quickly be a build-up of toxic wastes in the body which will be fatal if not removed
• There are two forms of treatment for kidney failure
  • Renal dialysis - toxins, metabolic waste products and excess substances are removed from the blood by diffusion via a dialysis membrane)
  • Kidney transplant - the non-functioning kidney (or kidneys) is replaced with a functioning kidney

Treatment options for total kidney failure

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 is uses 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
• Renal dialysis imposes heavy restrictions on the lives of patients who rely on it, as they have to make regular trips to the hospital to receive the treatment, which takes at least three hours each time
• In addition, patients have to carefully control their diet to minimise their urea production and their salt intake

How haemodialysis works

Kidney transplant

• An alternative to these potentially restricting dialysis treatments is to have a kidney transplant
• Although only one kidney is required for a transplant (one functioning kidney can replace two non-functioning kidneys) a donor with a compatible blood group must be found
• Even with this, a kidney transplant patient must still take medication to stop their immune system from rejecting the donated kidney as this kidney will often be of a different tissue type
• Kidney transplants are a better long term solution to kidney failure than dialysis; however, there are several disadvantages to kidney transplants, including:
  • Donors won’t have the same antigens on cell surfaces so there will be some immune response to the new kidney (risk of rejection is reduced – but not removed –  by ‘tissue typing’ the donor and the recipient first)
  • This has to be suppressed by taking immunosuppressant drugs for the rest of their lives – these can have long term side effects and leave the patient vulnerable to infections
  • There are not enough donors to cope with the demand

• However, if a healthy, close matched kidney is available, then the benefits of a transplant over dialysis include:
  • The patient has much more freedom as they are not tied to having dialysis several times a week in one place
  • Their diets can be much less restrictive than they are when on dialysis
  • The use of dialysis machines is very expensive and so this cost is removed
  • A kidney transplant is a long-term solution whereas dialysis will only work for a limited time