Excretion in humans

• Unlike plants, humans have organs which are specialised for the removal of certain excretory products
• Organs involved in excretion

  • Lungs
  • Kidneys
  • Liver
• 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, making it toxic
• Urea


  Toxic to the body in higher concentrations and so must be excreted
• Main structures of the urinary system

  • Kidneys
  • Ureters
  • Bladder
  • Urethra
• The kidneys regulate the water content of the blood and excrete the toxic waste products of metabolism and substances in excess of requirements
• Nephrons


  • Also known as kidney tubules or renal tubules
  • Each kidney contains around a million of them
  • They start in the cortex, loop down into the medulla and back up to the cortex
  • The contents of the nephrons drain into the innermost part of the kidney and the urine collects there before it flows into the ureter
• Ultrafiltration


  1. Arterioles branch off the renal artery and lead to each nephron, where they form a knot of capillaries (the glomerulus) sitting inside the cup-shaped Bowman's capsule

  2. The capillaries get narrower as they get further into the glomerulus which increases the pressure on the blood moving through them

  3. This eventually causes the smaller molecules being carried in the blood to be forced out of the capillaries and into the Bowman's capsule, where they form the filtrate

  4. The substances forced out are: glucose, water, urea, salts
• Reabsorption of glucose


  • Glucose is the first substance to be reabsorbed at the proximal (first) convoluted tubule by active transport
  • The nephron is adapted for this by having many mitochondria to provide energy for the active transport of glucose molecules
  • Reabsorption of glucose cannot take place anywhere else in the nephron as the gates that facilitate the active transport of glucose are only found in the proximal convoluted tubule
• In a person with a normal blood glucose level, there are enough gates present to remove all of the glucose from the filtrate back into the blood
• People with diabetes cannot control their blood glucose levels and they are often very high, meaning that not all of the glucose filtered out can be reabsorbed into the blood in the proximal convoluted tubule
• As there is nowhere else for the glucose to be reabsorbed, it continues in the filtrate and ends up in the urine
• Reabsorption of water and salts

  1. As the filtrate drips through the Loop of Henle, necessary salts are reabsorbed back into the blood by diffusion and active transport

  2. As salts are reabsorbed back into the blood, water follows by osmosis

  3. Water is also reabsorbed from the collecting duct in different amounts depending on how much water the body needs at that time
• Many digested food molecules absorbed into the blood in the small intestine are carried to the liver for assimilation
• Deamination


  • The amino group of all amino acids - NH2 (which contains the nitrogen atoms) is removed
  • The part of the molecule which contains carbon is turned into glycogen and stored
  • The other part, which contains nitrogen, is turned into ammonia, which is highly toxic, and so is immediately converted into urea, which is less toxic
  • The urea dissolves in the blood and is taken to the kidney to be excreted
• The toxic consequences of high urea levels, if it is not excreted effectively, are very serious: cell death, reduced response to insulin leading to diabetes, deposits inside blood vessels
• Excretion


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


  The expulsion of undigested food waste from the anus