Gas exchange

• Gas exchange in single-celled organisms and insects

  • Thin, flat shape and large surface area to volume ratio
  • Short diffusion distance to all parts of cell for rapid diffusion eg. of O2 / CO2
• Tracheal system of an insect

  1. Air moves through spiracles (pores) on insect surface

  2. Air moves through tracheae

  3. Which divide into tracheoles where gas exchange occurs directly to/from cells

  
• O2 used by cells during respiration


  Establishes a conc. gradient for O2 to diffuse down
• CO2 produced by respiration


  Diffuses down conc. gradient from respiring cells
• Adaptations for gas exchange in insect tracheal system

  • Tracheoles have thin walls
  • High numbers of highly branched tracheoles
  • Tracheae provide tubes full of air
  • Fluid in end of tracheoles drawn into tissues by osmosis during exercise (lactate produced in anaerobic respiration lowers water potential of cells)
  • Contraction of abdominal muscles (abdominal pumping) changes pressure in body causing air to move in / out

  
• Structural and functional compromises between opposing the needs for efficient gas exchange and the limitation of water loss as shown by terrestrial insects
• Adaptations for gas exchange in insect tracheal system to limit water loss

  • Thick waxy exoskeleton
  • Spiracles can open & close (not open all the time)
  • Tiny hairs around spiracles
• Adaptations of gas exchange surfaces: across the gills of fish

  • Each gill made of many filaments (thin plates) covered with many (secondary) lamellae (projections)
  • Thin lamellae wall / epithelium
  • Lamellae have a large number of capillaries
• Counter current flow adaptation in fish gills


  1. Blood and water flow in opposite directions through/over lamellae

  2. So oxygen concentration always higher in water (than blood near)

  3. So maintains a concentration gradient of O2 between water and blood

  4. For diffusion along whole length of lamellae

  
• If water and blood flowed in same direction (parallel flow) equilibrium would be reached, so oxygen wouldn't diffuse into blood along the whole gill plate.
• Adaptations of gas exchange surfaces: by the leaves of dicotyledonous plants

  • Many stomata (high density)
  • Spongy mesophyll cells contain air spaces
  • Thin
• Structural and functional compromises between opposing the needs for efficient gas exchange and the limitation of water loss as shown by xerophytic plants
• Adaptations of xerophytic plants for gas exchange and limiting water loss

  • Thicker waxy cuticle
  • Sunken stomata in pits
  • Rolled leaves
  • Hairs
  • Spines / needles

  
• The gross structure of the human gas exchange system

  • Many alveoli / capillaries
  • Large surface area
  • Alveoli / capillary walls are thin
  • Short diffusion distance
  • Ventilation / circulation
  • Maintains concentration gradient
• The essential features of the alveolar epithelium as a surface over which gas exchange takes place

  • Thin / flattened cells / one cell thick → short diffusion distance
  • Folded → large surface area
  • Permeable → allows diffusion of oxygen and carbon dioxide
  • Moist → gases can dissolve
  • Good blood supply from network of capillaries → maintains concentration gradient

  
• Gas exchange in the lungs

  1. Oxygen diffuses from alveolar air space into blood down its concentration gradient

  2. Across the alveolar epithelium then across the capillary endothelium

  3. Opposite for carbon dioxide
• Inspiration (breathing in)

  1. External intercostal muscles contract, internal intercostal muscles relax (antagonistic) → ribcage moves up / out

  2. Diaphragm muscles contract → flattens

  3. Increasing volume in thoracic cavity (chest)

  4. Decreasing pressure in thoracic cavity

  5. Atmospheric pressure higher than pressure in lungs → air moves down pressure gradient into lungs

  
• Expiration (breathing out)

  1. Internal intercostal muscles can contract, external intercostal muscles relax → ribcage moves down / in

  2. Diaphragm relaxes → moves upwards

  3. Decreasing volume in thoracic cavity

  4. Increasing pressure in thoracic cavity

  5. Atmospheric pressure lower than pressure in lungs → air moves down pressure gradient out of lungs

  
• Normal expiration is passive (no muscle contraction required), aided by elastic recoil in alveoli
• Forced expiration is active because internal intercostal muscles contract
• Why is ventilation needed?

  • Maintains an oxygen concentration gradient
  • Brings in air containing higher concentration of oxygen
  • Removes air with lower concentration of oxygen
• Tidal volume

  Volume of air in each breath
• Ventilation rate

  Number of breaths per minute
• Forced expiratory volume (FEV)

  Maximum volume of air a person can breathe out in 1 second
• Forced vital capacity (FVC)

  Maximum volume of air a person can breathe out in a single breath
• Reduced elasticity

  Lungs may expand / recoil less → reduced tidal volume / FVC

  
• Narrower airways / reduced airflow in / out of lungs

  Reduced FEV
• Thicker tissue in alveoli

  Increased diffusion distance → reduced rate of gas exchange
• Walls of alveoli break down

  Reduced surface area → reduced rate of gas exchange
• If gas exchange reduces

  Ventilation rate often increases to compensate for reduced oxygen in blood
• Cells receive less oxygen


  Rate of aerobic respiration reduced → less ATP made → fatigue
• Standard deviation

  • Gives an indication of spread of values around the mean
  • ± 2 standard deviations from the mean includes over 95% of the data
  • Presence of overlaps of standard deviations of different means - overlap → differences in means are likely to be due to chance, no overlap → differences in means are likely to be significant
• Statistical tests

  • Correlation coefficient → examining an association between 2 sets of data
  • Student's t test → comparing means of 2 sets of data
  • Chi-squared test → when data are categoric
  • Used to calculate probability of the link between a lung disease and a risk factor being due to chance
  • If the probability (P) of the difference in results being due to chance is equal to, or less than 5% (P ≤ 0.05), the difference/association is significant
• Evaluating a conclusion


  • Use all information provided - read, underline and annotate information in the question carefully
  • Evaluate data - overall trend, use of stats tests / standard deviation
  • Evaluate method of collecting data - sample size, bias, control variables eg. age, duration of experiment
  • Evaluate context - has a broad generalisation been made from a very specific set of data?
• Correlation vs causation


  • Correlation = when a change in one variable is reflected by a change in another
  • Causation = when a change in one variable causes a change in another variable - a correlation between two variables may be identified on a scatter diagram
  • Correlation does not mean causation → there may be other factors involved
• Haemoglobin

  The role of haemoglobin and red blood cells in the transport of oxygen

  
• Red blood cells

  • Contain (lots of) haemoglobin (Hb)
  • No nucleus → more space for Hb
  • Biconcave shape → ↑ SA:V / short diffusion distance

  
• Role of haemoglobin

  1. Associates with / binds / loads O2 near gas exchange surfaces where partial pressure of O2 (pO2) is high

  2. Forming oxyhaemoglobin which transports O2 (each can carry 4 O2 - one at each Haem group)

  3. Dissociates from / unloads O2 near cells / tissues where pO2 is low

  
• Structure of haemoglobin


  • Protein with a quaternary structure
  • Made of 4 polypeptide chains
  • Each chain contains a Haem group containing an iron ion (Fe2+)

  
• Affinity for oxygen

  • Ability of Hb to attract, or bind, oxygen
  • In regions with low pO2 - respiring tissues, Hb has a low affinity for O2 so O2 readily unloads / dissociates with Hb, so % saturation is low
  • In regions with high pO2 - gas exchange surfaces, Hb has a high affinity for O2 so O2 readily loads / associates with Hb, so % saturation is high