Gas Exchange

• Gas Exchange


  Interchange of respiratory gases (O2 and CO2) between cells of an organism and its environment
• Gas Exchange


  1. Needed to obtain O2 for aerobic respiration to produce energy in form of ATP + get rid of toxic waste product CO2

  2. 3 stages: 1) Breathing, 2) Transport in circulatory system, 3) Exchange with interstitial fluid
• Gas Exchange


  Occurs by diffusion across a gas exchange surface
• Adaptations of Gas Exchange Surfaces for Efficient Diffusion

  • Permeable - Allows gas to pass through
  • Thin - Short distance for diffusion (Fick's Law)
  • Increased surface area - Increased surface area for faster diffusion (Fick's Law)
  • Good blood supply/well-vascularised
  • Moist
• Gas Exchange occurs only in organisms which use blood as transport medium
• Fick's Law


  Rate of diffusion = Surface area / Thickness of surface x Concentration gradient across / on either side of surface
• Adaptations of gas exchange surface maximise rate of diffusion of gases
• Cells


  • Need constant supply of O2 + continuous removal of CO2 by diffusion with body fluids
• Gas Exchange in Aquatic Habitats

  • Amount of O2 in H2O is 0.8% of O2 in air
  • O2 diffuses more slowly in H2O than in air
  • More energy required to pass certain volume of H2O over gas exchange surface than same volume of air
• At higher temperatures


  Amount of O2 dissolved in H2O decreases, metabolic rate of aquatic organism increases
• Partial Pressure


  Fraction of total atmospheric pressure that an individual gas exerts
• At high altitudes


  Atmospheric partial pressure of O2 decreases, rate of diffusion decreases
• Outward diffusion of CO2

  Inward diffusion of O2
• Atmospheric PCO2 is low


  CO2 is very soluble in H2O, terrestrial organisms have steep PCO2 gradient between gas exchange surface and air, aquatic organisms have low difference in PCO2 between surface and air
• Stagnant H2O


  CO2 level of H2O increases compared to well-aerated water bodies, loss of CO2 to external environment very difficult, can lead to death
• Organisms with large surface area to volume ratio

  • Can exchange O2 and CO2 with external environment by diffusion across body surface, no specialised gas exchange system
• As size of organism increases

  • Surface area to volume ratio decreases, simple diffusion across body surface no longer sufficient, develop specialised gas exchange surfaces with special ventilation mechanisms
• Respiratory pigments


  Coloured molecules which reversibly bind to O2, act as O2 carriers
• Haemoglobin (Hb)


  Respiratory pigment that forms oxyhaemoglobin, helps with equilibrium and concentration gradient
• Plants have lower metabolic rates, gas exchange requirements less than in animals
• Leaves


  • Main site of gas exchange between plant and environment
  • Stomata allow unhindered diffusion of gases
  • Thin, flat, high stomatal density
  • Internal air spaces allow quick diffusion of O2 and CO2 for absorption and release from cells
  • Spongy mesophyll adapted for gas exchange with many air spaces and fewer chloroplasts
• Guard cells


  Control gas exchange, respond to internal CO2 concentration, light and water availability
• Roots and root hair cells

  • Exchange gases across high surface area with air spaces in soil
• Cork cells


  Impermeable to O2 and CO2, pores called lenticels allow gas exchange between internal tissues and external air
• Submerged plants


  Obtain CO2 in form of HCO3- ions from surrounding water by diffusion across surface
• Insects


  • Terrestrial, breathe in air for gas exchange
  • Air has higher PO2 than H2O, less dense and viscous than H2O, reducing water loss problem
  • Develop internal gas exchange surface - tracheal system
• Insect Ventilation


  Chemoreceptors detect high CO2, abdominal spiracles open, dorso-ventral muscles contract to force air out, abdominal spiracles close and thoracic spiracles open, dorso-ventral muscles relax to draw air in
• Insect Gas Exchange


  • O2 diffuses directly to cells through highly branched tracheal system without circulatory system
  • At rest, O2 enters through tracheae, dissolves in fluid-filled tracheoles and diffuses into cells
  • When active, water leaves tracheole fluid, air fills tracheal system and O2 diffuses faster to respiring cells
• Fish


  • Aquatic, have specialized gas exchange structure - gills
  • Gills are extensions of body in pharyngeal region between buccal cavity and oesophagus, connected to external environment
  • Gill filaments bear numerous plate-like folds called lamellae which are highly vascularised with thin epithelium for fast diffusion
  • Gill rakers prevent debris from damaging gills
• Fish Ventilation


  2 pump system - fish open mouth to increase volume of buccal cavity, causing water to rush in, water then moves from buccal cavity to opercular cavity over gills
• Gills

  • Inefficient on land
  • Prone to H2O loss
  • Stick together when not supported by supportive medium H2O
• Gill Structure


  • Gill rakers
  • Gill arch
  • Gill slit
  • Lamellae
  • Gill filament
  • Gill
• Gill filaments


  • 2 rows in 'V' shape
  • Bear numerous plate-like folds = LAMELLAE
  • Highly vascularised
  • Thin epithelium
  • Increase surface area for fast diffusion
• Opercular Cavity


  • Space between inside of operculum and gills
  • Afferent blood vessels carry blood to gills
  • Efferent blood vessels carry blood away
  • Ends of adjacent gills overlap, increasing resistance to flow of H2O to slow it down for exchange
• Gill rakers


  • Tooth-like structures attached to gill arches
  • Prevent particles of debris and food from damaging gills
• Ventilation


  1. Fish open mouth, volume of buccal cavity increases, pressure decreases, H2O rushes into mouth

  2. H2O moves from buccal cavity to opercular cavity where gills are present

  3. Operculum flap closes due to external pressure

  4. Fish closes mouth, volume of buccal cavity decreases, internal pressure increases, forces H2O towards opercular cavity passing over gills

  5. Opercular cavity contracts and flap opens due to internal pressure, H2O flows out of cavity
• Ventilation


  • Ensures unidirectional continuous flow of H2O over gills (unlike tidal ventilation)
  • Important for counter-current flow
• Gaseous Exchange


  1. Deoxygenated blood carried to gills by afferent artery

  2. Passes over lamellae and is oxygenated by diffusion of dissolved O2 in H2O through lamellae wall

  3. O2-rich blood flows to rest of body from gills in efferent vessel

  4. Blood and H2O move in opposite directions = COUNTERCURRENT FLOW
• Countercurrent flow


  • More efficient as it ensures diffusion gradient which favours movement of O2 from H2O to blood along whole length of lamella, maximising O2 uptake
• In concurrent flow, equilibrium is eventually reached when Hb is 50% saturated with O2, maximum O2 saturation possible