excretion

• Excretion


  The removal of waste products of metabolism from an organism

  
• Metabolism


  The sum of all the chemical reactions in an organism
• Homeostasis


  • The maintenance of a constant internal environment
  • Necessary for an organism to be independent of its surroundings, and for its metabolism to function efficiently
• Functions of the skin

  • Sense organ
  • Protection
  • Vitamin production
  • Fat store
  • Excretion
  • Temperature regulation
• Temperature regulation in warm conditions

  1. Blood vessels dilate (vasodilation) to allow more heat loss through radiation

  2. Sweating - glands release sweat onto the surface of the skin, cooling the skin by evaporation
• Temperature regulation in cold conditions

  1. Blood vessels narrow (vasoconstriction) to reduce heat loss

  2. Hairs stand up (piloerection) to trap a layer of warm air and insulate the body

  3. Shivering - muscles contract to generate heat
• Excretory system


  • Lungs (excrete CO2 and H2O)
  • Skin (excrete sweat - salt and water)
  • Kidneys (excrete urine - urea, salt and water)

  
• Urea


  Produced when amino acids are broken down in the liver - deamination
• Functions of the kidneys


  • Excretion - elimination of metabolic waste
  • Osmoregulation - keeping the blood at the correct concentration
  • Homeostasis - maintaining a suitable internal environment, controlling blood plasma pH
• Filtration in the nephron

  1. Blood in the glomerulus is under high pressure, forcing plasma into Bowman's capsule

  2. Small molecules like water, glucose, amino acids, vitamins, minerals, salts and urea are filtered out (glomerular filtrate)

  3. Large substances like proteins and blood cells do not enter Bowman's capsule
• Reabsorption in the nephron

  1. Glomerular filtrate passes into the proximal convoluted tubule where reabsorption of substances needed by the body takes place

  2. All glucose and amino acids are reabsorbed by active transport

  3. Some salts are reabsorbed by active transport and diffusion

  4. Water is reabsorbed by osmosis

  5. More water and salt are reabsorbed in the loop of Henle and distal convoluted tubule

  6. Fluid then enters the collecting duct where water is reabsorbed by osmosis

  
• Secretion in the nephron

  Salts and hydrogen ions from the blood are secreted into the tubule to help control the pH of the blood
• If you drink a great deal of water

  ADH is made in the hypothalamus and stored in the pituitary gland for release
• If the blood is too concentrated (too little water)


  Receptors in the hypothalamus signal the pituitary gland to secrete ADH, causing the DCT and CD to become more permeable to water, so more water is reabsorbed and a smaller volume of urine is produced
• If the blood is too dilute (too much water)


  The hypothalamus signals the pituitary to turn off ADH production, meaning the DCT and CD are impermeable to water, no more water is reabsorbed, and a relatively large volume of urine is produced