Role of Kidney in Homeostasis

• Osmoregulation


  The maintenance of constant water levels in the body fluids of an organism
• Osmoregulation is important because:

  • Prevents cells from bursting or shrinking when water enters or leaves by osmosis
  • Cellular reactions occur in aqueous solutions ∴ water levels affect concentrations and the rates of reaction in cells
• Functions of the kidneys


  • Removes toxic waste substances from the body
  • Maintain the water levels of body fluids
  • Control the volume and concentration of urine
• Renal artery


  Supplies blood to the kidneys
• Renal vein


  Drains blood from the kidneys
• Ureter


  Takes urine to the bladder from the kidneys
• Urethra


  Releases urine from the bladder, out of the body
• Structure of the kidneys

  • Outer cortex
  • Pelvis (leads to ureter)
  • Inner medulla
• Nephron


  Functional unit of the kidney where filtration and selective reabsorption takes place
• Structures of the nephron

  • Capillary knot
  • Bowman's capsule
  • Tubule
  • Collecting duct
  • Arteriole to capillary knot
  • Arteriole from capillary knot
• Stages in the formation of urine

  • Ultrafiltration
  • Selective reabsorption
  • Osmoregulation
• Filtration in the kidneys

  1. Blood flows through the capillary knot under high pressure

  2. Small molecules (e.g. urea, glucose), water and salts are filtered out of the blood and into the Bowman's capsule
• Reason for high pressure in capillary knot

  Arteriole leading into the capillary knot is wider than the arteriole taking blood from the capillary knot
• Large molecules (e.g. RBCs, proteins) remain in the blood because they are too large to fit through the pores in the capillary walls
• Substances selectively reabsorbed from the nephron tubule

  • All sugars
  • Some water
  • Some ions
• What happens to molecules not selectively reabsorbed


  They travel down the kidney tubule as urine and are transported to the bladder via the ureter. Here they are stored and eventually excreted.
• Urine


  Waste product of the kidney, contains urea, excess water, excess ions
• If blood water levels become too high, the kidney produces more dilute urine
• If blood water levels become too low, the kidney produces more concentrated urine
• How the concentration and volume of urine is controlled


  Controlled by the secretion of anti-diuretic hormone (ADH)
• What produces ADH


  Pituitary gland
• How ADH affects the kidneys

  1. ADH causes the kidneys to reabsorb more water into the blood

  2. More concentrated urine produced
• Composition of blood


  • Cells
  • Proteins
  • Water
  • Glucose
  • Salts
  • Urea
• Composition of filtrate


  • Water
  • Glucose
  • Salts
  • Urea
• Composition of urine


  • Some water
  • Some salts
  • Urea
• Glucose in the urine may indicate diabetes
• Blood or cells in the urine may indicate kidney disease
• Kidney disease


  Detected by the presence of cells or blood in a person's urine
• How can kidney failure be treated?

  1. Kidney dialysis

  2. Kidney transplant
• Kidney dialysis


  A machine artificially filters a patient's blood
• Dialysate


  The dialysis fluid
• How does kidney dialysis work? (higher)

  1. Selectively permeable barrier separates patient's blood from dialysate

  2. Materials exchanged across the barrier e.g. urea, excess ions and water move out of the blood and into the dialysis fluid

  3. Large cells and proteins remain in the blood
• Composition of dialysate (higher)

  • Fewer salts and water than the patient's own blood so excess salts and water diffuse out of the blood into the dialysate
  • No urea
• Kidney transplant


  Taking a kidney from a living donor or someone recently deceased and implanting it into the patient
• Kidney 'rejection'


  The immune system detects the foreign tissue and attacks it
• Precautions taken to minimise the risk of rejection

  1. Tissue typing ensures that the transplanted organ is 'compatible' with the recipient

  2. Immunosuppressant drugs help prevent the immune system from rejecting the organ