Excretion

• Excretion


  The removal of metabolic waste from the body.
• Carbon dioxide, produced in respiration, and oxygen, produced in photosynthesis, are waste products that are excreted from plants. Describe how these waste products are excreted from plants.


  Gases diffuse from the air spaces in the spongy mesophyll layer of the leaf, moving from an area of high concentration to an area of low concentration, through the stomata. Regulated by guard cells.
• How is urea excreted from the kidneys?

  Excess amino acids are

  broken down into urea

  within the liver. This urea is

  filtered from the blood

  within the kidneys and is

  passed from the body in

  urine. Urea needs to be removed because it is toxic.
• How is carbon dioxide excreted from the lungs?

  Carbon dioxide diffuses

  from respiring cells into

  blood. Carbon dioxide then

  diffuses from blood within

  the capillaries into alveoli

  within the lungs.
• How does the skin excrete water and salt?

  Water and salts are

  excreted from the body in

  sweat. Excess salt and water can

  affect water concentration.

  This can result in cell damage

  due to the effect on osmosis.
• Kidney

  The kidney filters the blood of substances that not required by the body - toxic urea, excess water and excess mineral ions. The kidney also reabsorbs, back into the blood, substances that are needed - glucose, water and mineral ions.
• Renal vein


  Newly filtered blood

  passes from kidney to

  Vena Cava
• Renal artery


  Supplies the kidneys

  with blood to be

  filtered. Blood under

  high pressure.
• Bladder


  Muscular bag that

  stores urine.
• Sphincter muscles


  Two ring like

  muscles that

  contract to close

  the urethra & hold

  back the urine.
• Urethra


  Tubes that carry urine

  from bladder to the

  outside environment.
• Ureter


  Tubes that carry urine from

  kidney to bladder.
• Ultrafiltration


  The filtration of small molecules from the blood into the nephron
• Blood in the glomerulus

  • Under high pressure due to the blood vessel entering the glomerulus being wider than the blood vessel leaving the glomerulus
• Glomerulus & Bowman's capsule walls


  • Only one cell thick with gaps
  • Basement membrane in the middle acts as a filter
• Ultrafiltration process


  Small molecules are squeezed from the blood in the glomerulus into the Bowman's capsule of the nephron
• Glomerular filtrate


  • Glucose
  • Minerals
  • Water
  • Hormones
  • Urea
• The glomerular filtrate formed contains small molecules such as glucose, minerals, water, hormones & urea, but no large molecules such as blood cells and proteins
• Selective re-absorption


  The reabsorption of useful substances from

  the nephron filtrate, in the proximal

  convoluted tubule, back into the blood

  The substances reabsorbed are glucose,

  mineral ions, vitamins and water.

  Glucose is reabsorbed by active transport

  and water is reabsorbed by osmosis.
• Water re-absorption

  The reabsorption of water from the

  collecting duct back into the blood.

  The state of hydration of the body and the

  levels of ADH determine the volume of water

  reabsorbed.
• Urine release


  Urine contains water, salts, urea and uobilin.
• Osmoregulation


  The control of the concentration of water in the blood
• Water is essential for many cell reactions and acts as a solvent
• A low concentration of water in the blood


  Causes dehydration of cells, as a result of osmosis, resulting in cellular damage
• A high concentration of water in the blood


  Causes over-hydration of cells, potentially resulting in burst / lysed cells
• The body controls the concentration of water in the blood


  By adjusting the amount of water that is reabsorbed from the collecting duct in the nephron
• ADH (antidiuretic hormone)


  Regulates the reabsorption of water
• Role of ADH when the concentration of water in the blood is low

  Receptors in the hypothalamus are highly stimulated ->

  Increased ADH hormone released from pituitary gland ->

  Collecting duct becomes more permeable to water ->

  More water reabsorbed into the blood from the kidney tubule (+ thirst)

  Urine is produced in a smaller quantity but more highly concentrated.
• Role of ADH when the concentration of water in the blood is high

  Receptors in the hypothalamus are less stimulated -> Decreased ADH hormone released

  from the pituitary gland -> Collecting duct becomes less permeable to water -> Less water

  reabsorbed into the blood from the kidney tubule -> Urine produced in a larger quantity

  and very dilute.