3.3.2: Gas Exchange

• Inspiration:

  • Diaphragm contracts and moves up
  • External intercostal muscles contract -> ribs move up and out
  • Internal intercostal muscles relax
  • Volume of thorax/thoracic cavity INCREASES
  • Pressure of thorax/thoracic cavity DECREASES (to equalise [atmospheric] pressure difference)
• Expiration:

  • Diaphragm relaxes and moves down
  • External intercostal muscles relax -> ribs move down and inwards
  • Internal intercostal muscles contract
  • Volume in thorax/thoracic cavity DECREASES
  • Pressure in thorax/thoracic cavity INCREASES (to equalise [atmospheric] pressure difference)
• Gas exchange in INSECTS

  • Spiracles enable diffusion of O2 in & out down conc. grad
  • … into trachae which branch into tracheoles (which are numerous & highly branched, extending throughout the entirety of insect)
  • Tracheoles have permeable, thin walls & are connected directly to respiring tissues -> shortens diffusion pathway
  • End of tracheoles filled with H2O -> muscle cells respiring anaerobically -> producing lactate, lowering Ψ in muscle cells -> H2O moves from tracheoles to muscle cells via osmosis -> enables air to be drawn in to fill in space -> ∴ oxygen diffuses in
• Gas exchange in FISH:

  • COUNTERCURRENT FLOW: opposite flow of blood in capillaries & flow of water
  • Water will (always) have a higher conc. of O2 relative to conc. of O2 in blood in lamellae
  • ∴ O2 continously diffuses into blood capillaries across entire length of gill plate
  • This maintains a (steep) O2 concentration gradient in the gas exchange system of fish that enables its continous diffusion into (deoxygenated) blood in capillaries of lamellae
• Structure of gas exchange system in fish

  • Gill plates -> Gill filaments -> large SA
  • Lamellae -> increase SA further
  • Lamellae embedded with blood capillaries -> thin layer/endothelium -> increases diffusion
• Gas exchange in single-celled organisms

  • Single-celled organisms have a large SA:V ratio & thin surface -> short diffusion pathway
  • O2 absorbed directly through diffusion across cell-surface membrane of their outer surface
• Gas exchange in INSECTS pt.2

  • Respiring cells use up O2 -> conc. of O2 lower towards tracheole ends
  • Creates (diffusion) gradient that enables gaseous O2 to diffuse from atmosphere into spiracles -> trachae -> tracheoles
• Label the cross-section of the leaf of a dicotyledenous plant

  • A - palisade mesophyll
  • B - xylem & phloem
  • C - spongy mesophyll
  • D - waxy cuticle

  
• Gas exchange in the leaves of dicotyledenous plants

  • Mesophyll cells -> large surface area
  • Diffusion of gases occurs through the stomata
  • Stomata open & close via guard cells
• Adaptations of xerophytic plants

  • Stomata sunken in pits -> traps moist air -> reduces concentration gradient of H2O/H2O potential between leaf & air -> reduces transpiration rate / diffusion of H2O out
  • Reduced number of stomata
  • Curled/corrigated leaves with stomata inside -> protects from wind
• Adaptations of the alveolar epithelium

  • Many alveoli -> large SA for O2 & CO2 diffusion
  • Alveolar epithelium 1 cell thick -> thin exchange surface -> short diffusion pathway
  • Steep concentration gradient of O2 & CO2 between alveoli & capillary endothelium -> maintained by blood flow & ventilation