Chp - 13 Excretion

• Excretion


  The removal from the body of the waste products of metabolism, toxic materials and substances in excess of requirements
• Excretory products


  • Carbon dioxide (from respiration)
  • Urea
• Urea formation


  1. Breaks down excess amino acids

  2. Removes the nitrogen containing part of the amino acid molecule (amino group)

  3. Ammonia (NH3) combines with carbon dioxide to form urea

  4. Urea is carried to the kidneys in the blood where it is filtered out and excreted in urine
• Liver


  • All the blood from the digestive system flows to liver first before going into the rest of the circulation
  • Converts glucose to glycogen to regulate blood glucose concentration
  • Converts fatty acids and glycerol into fat
  • Produces cholesterol from fats
  • Uses amino acids to make proteins
  • Breaks down toxins
• The liver breaks down toxins, for example drugs such as alcohol and paracetamol
• The body cannot store the amino acids which are broken down
• One molecule from the breakdown of amino acids is converted to carbohydrate or fat and used as a source of energy
• The other molecule from the breakdown of amino acids is ammonia (NH3) which combines with carbon dioxide to form the excretory product urea
• Kidney


  • Part of the urinary system
  • Responsible for the excretion of urea and excess salts from the body
  • Control the water and ion content of the blood
• Blood filtration in the kidney

  1. Blood enters the kidneys through the renal arteries

  2. Blood flows through a complex network of filtering units called kidney tubules

  3. Small molecules like glucose, salts, water and urea are filtered out of the blood plasma to form a fluid called filtrate

  4. Filtrate flows to the end of the tubule and becomes urine

  5. Urine flows into the ureter and to the bladder where it is stored

  6. Urine leaves the body through the urethra
• Glomerulus


  • The main filtering unit of the kidney
  • Formed by a network of capillaries enclosed within the Bowman's capsule
  • Capillaries have numerous fenestrations or pores that allow rapid filtration of the blood
• Ultrafiltration in the glomerulus


  1. Blood filters into the Bowman's capsule

  2. Blood cells and large molecules stay in the blood

  3. Small molecules like urea, glucose, salts and water pass out of the glomerulus and into the Bowman's capsule
• Reabsorption in the kidney tubule

  1. All of the glucose and some salts are reabsorbed back into the blood from the kidney tubule

  2. Reabsorption involves active transport

  3. Microvilli and mitochondria facilitate the reabsorption process

  4. Urea and excess salts are left after reabsorption

  5. Water may be reabsorbed if the body is low in water
• The fluid that enters the collecting ducts is urine
• The blood leaving the kidney in the renal vein has a much lower concentration of waste chemicals