Adaptations of gas exchange animals

• Gas exchange


  How much oxygen an organism needs depends on its volume

  The rate that oxygen is absorbed at depends on the surface area available for gas exchange
• Relationship between surface area to volume ratio and gas exchange

  • As organisms increase in size, their surface area to volume ratio decreases and so specialised respiratory surfaces are needed
• Gas exchange in insects

  Insects cannot use their external surface for gas exchange as they are covered in an impermeable cuticle to reduce water loss by evaporation

  Pairs of spiracles on segments of the thorax and abdomen

  These holes lead to tubes called tracheae leading to tracheoles

  Tracheoles enter muscle cells directly

  They have fluid at the end for dissolving and diffusion of oxygen

  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
• Gas exchange in fish

  • Fish 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


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

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

  Pressure in gill cavity increases and water forces operculum open and leaves through it
• Gills of fish


  • 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
• Requirements for gas exchange surfaces

  • 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)
• Continuous flow


  If water and blood flow in the same direction, equilibrium is reached and oxygen diffusion reaches no net movement halfway across the gill plate
• Counter current flow


  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
• Adaptations for gas exchange in 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
• Adaptations for gas exchange in Flatworm

  • Multicellular

  Smaller surface area to volume ratio

  Flattened body to reduce diffusion distance so rate of oxygen diffusion through body surface meets demand
• Adaptations for gas exchange in 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
• Gas exchange in humans

  Intercostal muscles contract and pull the rib cage up and out

  Outer pleural membrane is pulled out, reducing pressure in the pleural cavity and pulling on the surface of the lungs, causing an increase in the volume of the alveoli

  Alveolar pressure decreases to below atmospheric pressure and air is drawn into the lungs

  The gas exchange surface is the alveoli, lined with surfactant that reduces surface tension and prevents collapse on exhalation
• Gas exchange in amphibia

  • Aquatic tadpoles have feathery gills, but don't ventilate like fish - 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