Exchange Surfaces

• Surface Area is the amount of tissue in contact with the environment
• The smaller the organism, the bigger the surface area to volume ratio
• Unicellular organisms rely on diffusion alone as they have a low metabolic rate and a large SA: Vol ratio
• Larger organisms have to adapt as they can't just rely on diffusion alone
• Some examples of specialised exchange surfaces include:

  • Increased surface area
  • Thin layers
  • Good blood supply
  • Ventilation to maintain diffusion gradient
• Mass flow is movement of large volumes of substances within a transport system
• Mammals have....

  • Small SA: Vol ratio
  • Large volume of cells
  • High metabolic rate
  • To maintain body temperature

  Therefore they require lots of oxygen and a quick removal of waste (CO2)
• Nasal Cavity

  • Large SA
  • Rich blood supply (warms the air that enters the body)
  • Hairy lining with mucus (traps bacteria/viruses)
  • Moist Surface (increases humidity to reduce evaporation)
• Trachea has incomplete rings of cartilage which prevents collapsing and eases food passing
• Trachea is line with goblet cell that secrete mucus and ciliated epithelium which wafts the mucus backwards into the oesophagus where they are swallowed and killed by stomach acid
• Bronchioles

  • No cartilage
  • Covered with smooth muscle
  • Thin flattened epithelium
• Alveoli

  • Large SA
  • One cell-thick (short diffusion distance)
  • Moist lining
  • Rich capillary network
  • Good Blood Supply (maintains steep concentration gradient)
  • Lined with lung surfactant (keeps them from sticking)
  • Contains collagen and elastic fibres (elastic recoil)
• Lung surfactant lowers the surface tension and reduces the forces of cohesion in the alveoli
• What is ventilation?

  Movement of air in and out of the lungs due to changes of pressure in the thorax
• Inspiration:

  1. Uses energy
  2. Diaphragm contracts, flattens and lowers
  3. External intercostals contract - ribs up + out, internal intercostals relax
  4. Thoracic volume (space between neck and abdomen) increases
  5. Pressure inside lungs is lower than atmospheric so air enters
• Expiration:

  1. Passive process
  2. Diaphragm relaxes external intercostals relax - ribs down and in, internal intercostals contract
  3. External intercostals relax - ribs down and in, internal intercostals contract
  4. Thoracic volume decrease
  5. Pressure inside lungs is higher than atmospheric so air leaves
• Where does gas exchange in humans occur?

  Alveoli
• Gas exchange pathway in humans:

  Nose, Trachea, Bronchus, Bronchioles, Alveolus
• An asthma attack is when the smooth muscle in the bronchioles get triggered and go into spasm, constricting the airway and therefore a wheezing noise is made
• Tidal Volume

  The 'normal' volume of air that moves in and out of the lungs with each resting breath
• Vital Capacity

  The largest volume of air that can be breathed in straight after the strongest possible exhalation of breath
• Inspiratory Reserve Volume

  Maximum volume of air you can breathe in over and above a normal inhalation
• Expiratory Reserve Volume

  The extra amount of air you can force out of your lungs over and above the normal tidal volume of air you breathe out
• Residual Volume

  The volume of air that is left in your lungs after you have exhaled as hard as possible (maintains concentration gradient)
• Total Lung Capacity

  The residual volume + vital capacity
• Equipment used to measure....

  • Peak flow meter (measures the rate at which air is expelled from the lungs)
  • Vitalographs
  • Spirometer (measures different lung volumes/breathing patterns)
• Breathing rate is the number of breaths taken per minute
• Ventilation rate is the total volume of air inhaled in one minute
• Ventilation Rate = Tidal Volume* Breathing Rate
• What skeleton does an insect have?

  Exoskeleton
• Insects have an open circulatory system where body fluid acts as blood & tissue fluid
• Insects have no blood pigment (haemoglobin) to carry O2
• Insects breathe through spiracles which are small holes in the abdomen and thorax. Air enters the spiracle allowing oxygen to travel down the tracheae to the tracheoles where gas exchange takes place
• Spiracles are protected by sphincters which are like guard cells that open or close to prevent water loss
• What is the tracheae made of?

  Chitin
• Gas exchange in insects pathway

  Spiracle, Tracheae, Tracheoles
• Insects Gas Exchange Steps 1-3:

  1. Spiracles along the thorax and abdomen allow water and air to enter
  2. May have sphincters. Inactive insect means low oxygen demand therefore spiracles are closed however opposite occurs when CO2 builds up
  3. Spiracles lead to tracheae which is lined with chitin (impermeable to gases) and it carries air to the tracheoles
• Insects Gas Exchange Steps 4-5

  4. Gas exchange takes place in the tracheoles

  5. There is tracheal fluid at the end of the tracheoles which limits air penetration
• Process of insect gas exchange:

  • Tracheoles have a large SA
  • Oxygen dissolves in the moisture on walls of tracheoles
  • High oxygen demand causes lactic acid build up in tissues so water moves out of the tracheoles via osmosis so more surface area is exposed for gas exchange
• Ventilation in larger insects

  • Mechanical ventilation - air pumped in by abdomenal/thoracic muscular movement
  • Collapsible enlarged air sacs act as reservoirs - inflated/deflated by ventilation of thorax/abdomen