Adaptive Features for Gas Exchange in Insects

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					Adaptive Features for Gas Exchange in Insects.
     Terrestrial environment – 20% oxygen but big problems
      with drying out.                                                 The amount of material
     The tracheal system opens the way to rapid and sustained         an organism needs to
      movements but imposes strict limitations on the size of          exchange is proportional
      the organism.                                                    to its volume; the
     The tracheal system permits a very high metabolic rate.          amount of material it
     Flying insects use a huge amount of oxygen /g of body            can exchange is
      weight – even more than birds.                                   proportional to its
     Chitin exoskeleton – cuts down water loss but is                 surface area.
      impermeable to gases. Require spiracles to allow gas
      exchange to take place.
     However, 25% pf CO2 produced is lost through the cuticle (joints)
     Spiracles have valves that open and close in response to humidity and
      the rate of air movement.
     Spiracles can open independently according to the CO2 level within.
     Abdominal pumping in some insects helps ventilation.
     Some insects have air sacs that store a little air and can expand and contract.
     Respiratory gases move into and out of tissues by diffusion.
     Diffusion is fast but can only operate over short distances – the largest insect still
      only has diffusion operating over 1 cm to the centre of its body. LIMITS SIZE.
     Tracheae are lined with water-proofing chitin,(arranged in spirals for
      support) tracheoles are not, so allowing the diffusion of gases in and
      out of the cells.
     The spirals can squeeze together like a concertina to expel air.
     The opening of spiracles is lined with hairs which filter dust.
     Valves in the spiracles are important in preventing water loss.
     Tracheae have fine tough rings that prevent them from collapsing.
     Some insects (weta) have air sacs (air reservoirs) that can act like bellows, forcing
      air into the system.
     The tiniest tracheoles (the air capillaries) pass within a few micrometres
      of every respiring cell.
     The end of the tracheole contains a watery fluid which limits the
      penetration of the gases for diffusion. However, when oxygen demand builds up eg
      when an insect is flying, lactic acid in the tissues causes water to be withdrawn
      from the tracheoles by osmosis and exposes additional surface area
      for gaseous exchange.
     Open blood system, no respiratory pigment in blood.
Adaptive Features for Gas Exchange in Fish.
     Environment – oxygen-poor but with no worries about drying out.
                                Fish exterior is impermeable to gases.
                                Needs gills – large surface area, but delicate so need to
                                  be internal (fast swimming would damage them)
                                Gills need constant flow of water over them to extract
                                  enough oxygen.
                                Inhalation – operculum closed and mouth open, water
                                  sucked in.
     Exhalation – mouth closed and operculum open for water to exit.
     Countercurrent system keeps the diffusion gradient effective.
     Closed single circuit circulatory system, with respiratory
      pigment in blood.
     Salt water has less dissolved oxygen than fresh.
     Gills can extract up to 80% of the dissolved oxygen in water
      (compare with lungs)
     There is 30 times more oxygen in air than in water(0.7%)
     Water is more dense than air – it takes more energy to move it over the gas
      exchange surface.
     In fish 1/5 total energy is used in gas exchange compared with 1/50 in terrestrial
     Air flows faster than water.(50x more)
     Oxygen can be absorbed more quickly from air than from water (diffusion constant
      for water 300,000 times faster)
     CO2 taken away more quickly by air than water (11,000 times faster)
     Water is denser than air so provides more support.
     Gill filaments are so thin walled that without the support of water they collapse –
      reduce surface area drastically so despite the increased availability of oxygen, fish
      can’t breathe.
     Gill arches each have two rows of gill filaments. Ends of gill filaments interlock
      forming a continuous mesh through which water must pass.
     Ventilation of gills – some fish have to swim forwards all the time to ensure the
      water flow is continuous. Others ventilate with double pumping system – mouth and
      gill. No back flow – effectively there are non-return valves (soft flaps of tissue
      around the mouth and operculum)