adaptations-for-gas-exchange-in-animals

• How much oxygen an organism needs


  • Depends on its volume
• The rate that oxygen is absorbed


  • Depends on the surface area available for gas exchange
• Surface area to volume ratio of an organism

  • Affects the surface adapted for use for gas exchange
  • Affects the level of activity of the organism
• As organisms increase in size, their surface area to volume ratio decreases and so specialised respiratory surfaces are needed
• Why fish require a specialised gas exchange surface

  • They have a smaller surface area to volume ratio
  • They are relatively active and so have high metabolic rates making oxygen requirements high
  • They require a ventilation mechanism to maintain concentration gradients for gas exchange
• Ventilation in fish


  1. Mouth opens, floor of buccal cavity lowers so volume increases, pressure decreases and water rushes in

  2. Mouth closes, floor of buccal cavity raises, increasing pressure pushing water over the gills

  3. Pressure in gill cavity increases and water forces operculum open and leaves through it
• Gill filaments


  • Made of gill plates/lamellae (the gas exchange surface across which the water flows)
  • Gill rakers prevent large particulates entering and blocking the gills
• Gas exchange surfaces must

  • Be moist in terrestrial animals
  • Be thin (short diffusion pathway)
  • Have a large surface area
  • Be permeable to gases
  • Have a good blood supply to maintain concentration gradients (larger organisms only)
• If water and blood flow in the same direction across the gill plate

  Equilibrium is reached and oxygen diffusion reaches no net movement halfway across the gill plate
• If water and blood flow in opposite directions across the gill plate


  The concentration gradient is maintained and oxygen diffuses into the blood across the entire gill plate
• How some organisms adapt to the challenge of gas exchange

  • Amoeba: Single cell, large surface area to volume ratio, rate of oxygen diffusion through external surface meets demand, low metabolic rate means oxygen demand is low, short diffusion distance to the middle of the cell
  • Flatworm: Multicellular, smaller surface area to volume ratio, flattened body to reduce diffusion distance so rate of oxygen diffusion through body surface meets demand
  • Earthworm: Multicellular, even smaller surface area to volume ratio, body surface still used for gas exchange but circulatory system needed to distribute oxygen, blood vessels are close to skin surface and blood has haemoglobin with a high affinity for oxygen, mucus secreted to moisten surface and slow moving to reduce oxygen demand
• Insects cannot use their external surface for gas exchange as they are covered in an impermeable cuticle to reduce water loss by evaporation
• Gas exchange in insects

  1. Pairs of spiracles on segments of the thorax and abdomen

  2. These holes lead to tubes called tracheae leading to tracheoles

  3. Tracheoles enter muscle cells directly, they have fluid at the end for dissolving and diffusion of oxygen

  4. During flight, when oxygen requirements increase, fluid in tracheoles decreases to shorten diffusion path and whole-body contractions ventilate the tracheal system by speeding up air flow through spiracles
• Spiracles may close to reduce water loss
• Tracheoles are no more than 20µm in size to ensure short diffusion path
• Gas exchange in humans

  • Rib, intercostal muscles, pleural membranes with pleural cavity between, diaphragm, chest cavity, alveoli, bronchiole, larynx leading to trachea
  • The gas exchange surface, lined with surfactant that reduces surface tension and prevents collapse on exhalation
• Ventilation in humans - inspiration

  1. External intercostal muscles contract and pull the rib cage up and out

  2. Outer pleural membrane is pulled out, this reduces pressure in the pleural cavity and the inner pleural membrane is pulled outward

  3. This pulls on the surface of the lungs and causes an increase in the volume of the alveoli

  4. Alveolar pressure decreases to below atmospheric pressure and air is drawn into the lungs
• Gas exchange in amphibia

  • Aquatic tadpoles have feathery gills, they don't ventilate like fish but movement of the gills through water maintains a concentration gradient
  • Adult amphibia have soft, moist skin and exchange gases over their surface at rest, oxygen and carbon dioxide circulate through a closed circulation system containing haemoglobin
  • When active, movements of the buccal cavity ventilate lungs, which are simple with few alveoli