2.6 bio- Role of Kidney in homeostasis

• Kidney


  • Removes toxic waste substances from the body
  • Osmoregulation
  • Controls the volume and concentration of urine
• Osmoregulation


  Maintenance of constant water levels in the body fluids of an organism
• Importance of osmoregulation


  • Prevents cells bursting or shrinking when water enters or leaves by osmosis
  • Cellular reactions occur in aqueous solution, therefore water levels affect concentrations and the rate of reactions in cells
• Structures of the human excretory system

  • Renal artery
  • Renal vein
  • Ureter
  • Urethra
• Nephrons


  Functional units of the kidney where filtration and selective reabsorption take place to produce urine (a waste product of the kidney containing urea, excess water and excess ions)
• Stages of the process of excretion

  • Filtration
  • Selective reabsorption
  • Osmoregulation
• Filtration


  1. Blood flows through the capillary knot under high pressure

  2. The arteriole leading into the capillary knot is wider than the arteriole taking blood from the capillary knot, creating a build up of pressure

  3. Small molecules (e.g. urea, glucose), water and salts are filtered out of the blood and into the Bowman's capsule

  4. Large molecules (e.g. RBCs, proteins) remain in the blood as they are too large to fit through the pores in the capillary walls
• Selective reabsorption


  1. Glucose, some water and some salts are reabsorbed into the bloodstream

  2. The molecules not selectively reabsorbed travel down the kidney tubule as urine and are transported to the bladder via the ureter, where they are stored and eventually excreted
• Osmoregulation


  1. The water content of the blood is adjusted

  2. If blood water levels are high, more dilute urine is produced

  3. If blood water levels are low, more concentrated urine is produced

  4. The volume of water reabsorbed is controlled by anti-diuretic hormone (ADH), which is secreted by the pituitary gland and causes the kidneys to reabsorb more water into the blood producing more concentrated urine
• Composition of blood, filtrate and urine

  • Water
  • Salts
  • Urea
  • Glucose
  • Proteins
  • Cells
• The presence of glucose in the urine may indicate diabetes
• The presence of blood or cells in the urine may indicate kidney disease
• Methods of treating kidney disease

  • Kidney dialysis
  • Kidney transplant
• Kidney dialysis

  • A machine artificially filters a patient's blood
  • Selectively permeable barrier separates patient's blood from dialysis fluid
  • Materials exchanged across the barrier (e.g. urea, excess ions and water) move out of the blood and into the dialysis fluid
  • Large cells and proteins remain in the blood
• Advantages of kidney dialysis

  • Does not involve surgery
  • Patient can undergo kidney dialysis while waiting for a donor kidney
• Disadvantages of kidney dialysis

  • Connected to a dialysis machine for many hours a week
  • May have to travel to hospital
  • Must control diet (e.g. fluid and salt intake)
  • Not a permanent solution
• Kidney transplant


  • Taking a kidney from a living donor or someone recently deceased and implanting it into the patient
  • Tissue typing ensures that the transplanted organ is 'compatible' with the recipient
  • Immunosuppressant drugs help prevent the immune system from rejecting the organ
• Advantages of kidney transplant

  • More permanent solution
  • Improves patient's quality of life
• Disadvantages of kidney transplant

  • Difficult to find a suitable donor
  • Involves major surgery
  • Transplanted kidney has a limited life-span
  • Risk of organ rejection
  • May have to take immunosuppressant drugs for life
  • Immunosuppressants increase the risk of other infections