(e) the effects of kidney failure and its potential treatments

The idea

Kidney failure means the nephrons can no longer filter blood and reabsorb useful solutes effectively, so the composition of body fluids drifts from the norm. Learners should link reduced filtration to accumulation of urea and other nitrogenous waste, and reduced reabsorption to loss of glucose, amino acids and useful ions in urine. Loss of water regulation disturbs water potential and blood volume, and loss of osmoregulation plus acid–base balance changes blood pH. Treatments must be matched to these effects: dialysis removes waste by diffusion down a concentration gradient across a partially permeable membrane, while a kidney transplant replaces filtration and reabsorption and needs immunosuppression. Diet and fluid restriction reduce the waste and water load.

What earns the marks

• Kidney failure reduces filtration at the glomerulus and reabsorption in the proximal convoluted tubule, so urea accumulates in the blood rather than being excreted.
• Reduced reabsorption means glucose, amino acids and useful ions may be lost in urine, and water regulation fails so water potential and blood volume are poorly controlled.
• Loss of osmoregulation and acid–base control changes blood pH and the solute concentration of body fluids, producing the symptoms of kidney failure.
• Haemodialysis removes urea and excess ions by diffusion down a concentration gradient across a partially permeable membrane into dialysis fluid, which is kept at the correct concentration.
• A kidney transplant restores filtration and reabsorption but requires immunosuppressant drugs to reduce rejection of the donated organ.
• Dietary and fluid management reduces the intake of protein breakdown products and water, lowering the load the failing kidneys or dialysis must handle.

Common mistakes and how to put them right

• Thinking dialysis actively pumps urea out of the blood.

  Put it right by explaining that urea moves by diffusion down its concentration gradient into dialysis fluid.

• Assuming a transplant removes the need for any medication.

  Put it right by stating that immunosuppressants are needed to reduce rejection.

• Confusing kidney failure with simply producing less urine.

  Put it right by linking failure to loss of filtration, reabsorption and osmoregulation, not just urine volume.

Exam tips

• Name the process (filtration, reabsorption, osmoregulation) each time you describe an effect, so the biology is explicit.
• When comparing treatments, contrast the mechanism of dialysis with the replacement of organ function in a transplant.
• Use the phrase 'diffusion down a concentration gradient' precisely when explaining how dialysis fluid removes waste.