Excretion in Humans

  • Excretion: the removal from organisms of toxic materials, the waste products of metabolism (chemical reactions in cells including respiration) and substances in excess of requirements.
  • Substances should include carbon dioxide, urea and salts.


Function of Kidney

  • Removal of urea and excess water and the re-absorption of glucose and some salts
  • Urea is formed in the liver from excess amino acids
  • Alcohol, drugs & hormones are broken down in the liver

  • Cortex: contains Bowman’s capsules and coiled tubules
  • Ureter: carries urine from kidney to bladder
  • Medulla: contains loops of Henlé and collecting ducts
  • Loop of Henlé: selectively absorbs water/solutes
  • Collecting ducts: reabsorbs water into blood and store wastes until they are passed into ureter
  • Urethra: carried urine from bladder to the outside.
  • Bladder: stores urine
  • Renal capsule: filters from blood: water, glucose, urea and salts.
  • Tubule: (yellow) reabsorbs 100% of glucose, most of the water and some salts back into the blood (red), leading to concentration of urea in the urine as well as loss of excess water and salts into the tubule.
  • Renal artery: brings wastes and water from blood
  • Renal vein: reabsorbs water and useful molecules and leaves wastes behind


Structure of the Kidney

  1. Ultrafiltration: blood from renal artery enters the glomerulus. Water, urea, salts and glucose are forced into the Bowman’s capsule. Blood cells and large proteins cannot pass through.
  2. Selective reabsorption: in the proximal tubule two thirds of the salt and water and all the glucose moves out of the nephron, by active transport. These substances are reabsorbed back into the blood capillary.
  3. Loop of Henlé: this part of the loop of Henlé is permeable to water but not salt. Water is drawn out of the filtrate in the nephron by osmosis because of the low water potential of the medulla tissue fluid.
  4. Loop of Henlé: this part is permeable to salt but not water. The loss of water from the filtrate in the previous stage increases the salt concentration. Some salt passively diffuses out of the loop here.
  5. Collecting duct: the remaining substances move through the second coiled tubule (distal tubule), into the collecting duct. The permeability of this part of the nephron to water is controlled



  • When a kidney machine takes a patient’s blood and cleans it, then returns the blood to circulation.
  • This is how it works:
    • Blood enters machine from patient
    • The pump passes the blood through the dialysis tubing which is semi-permeable therefore acting as a filter
    • The surrounding liquid contains some salts, glucose but no urea so waste materials pass from blood by diffusion;
    • The ‘cleaned’ blood returns to patient’s circulation/body


Dialysis Transplant
More expensive in the long run Less expensive in the long run
Very disruptive (three 6-8 hrs sessions per week) Not very disruptive (only have to take medication)
Do not need to find kidney Need a kidney
Need a machine & must live near one Can go anywhere, anytime
Risk of rejection




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