Excretion

• Excretion in Humans


  • Unlike plants, humans have organs which are specialised for the removal of certain excretory products
  • They include the lungs and kidneys
  • The liver also has a vital role in excretion
• Organs involved in excretion

  • Lungs
  • Liver
  • Kidneys
• Lungs


  Excrete carbon dioxide
• Kidneys


  Excrete excess water, salts and urea
• Excretion is the removal of the waste substances of metabolic reactions, toxic materials and substances in excess of requirements
• Carbon dioxide must be excreted as it dissolves in water easily to form an acidic solution which can lower the pH of cells
• Too much carbon dioxide in the body is toxic
• Urea is also toxic to the body in higher concentrations and so must be excreted
• The urinary system in humans

  • Kidney regulates water content and filters blood
  • Ureter carries urine from the kidneys to the bladder
  • Bladder stores urine as it is produced by the kidney
  • Urethra connects the bladder to the exterior where urine is released
• Kidney


  Two bean-shaped organs that filter the blood
• Ureter


  Tube connecting the kidney to the bladder
• Bladder


  Organ that stores urine (excess water, salts and urea) as it is produced by the kidney
• Urethra


  Tube that connects the bladder to the exterior; where urine is released
• Note the difference between the 'ureter' and the 'urethra'. These two names are commonly confused by students so take care to learn them and know which tube is which - they are NOT interchangeable!
• The Kidney


  • They regulate the water content of the blood (vital for maintaining blood pressure)
  • They excrete the toxic waste products of metabolism (such as urea) and substances in excess of requirements (such as salts)
• Nephrons are located between the cortex and medulla of the kidney
• Waste substances that need to be removed from the blood


  • UREA
  • CREATININE
  • AMMONIA
  • URIC ACID
  • EXCESS SALT/IONS (e.g. Na+, Cl-, Mg2+, Ca2+, HCO3-)
• Nephron


  Tiny structures in the kidney, also known as kidney tubules or renal tubules
• Structure of a nephron

  1. Bowman's capsule

  2. Glomerulus

  3. Proximal convoluted tubule

  4. Loop of Henle

  5. Distal convoluted tubule

  6. Collecting duct
• Ultrafiltration


  1. Blood in the glomerulus is under high pressure, forcing substances out of the blood into the Bowman's capsule

  2. Substances filtered out: water, salts, glucose, urea
• Selective reabsorption


  • Reabsorption of glucose by active transport in the proximal convoluted tubule
  • Reabsorption of water by osmosis and salts by diffusion in the Loop of Henle and collecting duct
• In a person with normal blood glucose, all glucose is reabsorbed in the proximal convoluted tubule
• In diabetes, not all glucose can be reabsorbed, so it ends up in the urine
• Deamination of amino acids in the liver

  1. Amino group (NH2) is removed

  2. Nitrogen-containing part is converted to urea

  3. Carbon-containing part is converted to glycogen
• Urea is less toxic than ammonia and is excreted by the kidneys
• Excretion


  Removal from the body of waste products of metabolic reactions, toxic substances and substances in excess of requirements
• Egestion


  Expulsion of undigested food waste from the anus