Gas Exchange

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Gas exchange in single-celled organism

  • Relies on diffusion 
  • Such a small distance between envionment & organism that gas simply diffuses into the body 
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Gas exchange in an insect

  • Tracheal system (system of air tubes inside insect)
  • They have holes in the cuticle going from their body surface to the air tubes
  • The holes are called spiracles
  • Spiracles lead into the tracheae which are lined with chitin
  • Chitin supports the tracheae and makes them impermeable to gases
  • The tracheae branches off into tracheoles where respiratory gases are exchanged
  • Large surface area for gas exchange due to large number of tracheoles
  • When insects are active, water in the ends of the tracheole is withdrawn
  • This minimizes diffusion distance for gases
  • Also tracheoles are tiny and extend into individual cells e.g muscle fibres
  • Tracheal system lets gas exchange happen independently of circulatory system so insects blood doesn't transport any respiratory gases
  • Small insects: air moves along tracheal system by diffusion
  • Larger/ active insects: air actively pumed by by thorax/abdomen movements
  • These also have air sacs (collapsible tracheae) which are inflated & deflated by respiratory movements which moves more air through tracheal system when needed
  • Ventilatory movements are more forceful & frequent when activity increases
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Gas exchange in fish

  • gills= respiratory structures helping gas exchange
  • fish have 4 pairs of gill arches that support gill filaments
  • gill filament subdivided into 2 stacks of gill lamellae (main site for gas exchange)  whcih lie on top of each other
  • water keeps lamellae apart exposing surfaces for gas exchange
  • surface area of gill lamellae maximised by gill plates on upper & lower surfaces
  • high concentration gradient between water (carrying oxygen to gills) & blood (carrying oxygen from gills to body)
  • Counter-current principle: water moves across gill plates in opposite direction to blood flowing through them
  • Causes water to have higher oxygen concentration than the blood it flows past so diffusion can occur over whole surface of gill plate
  • gill plates are very thin and delicate so blood flowing through them is only a short distance from seawater
  • Water is taken in through the mouth and then passes over gills 
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