module 3 (exchange surfaces)

• Surface area to volume ratio

  The surface area of an organism divided by its volume
• The larger the organism, the smaller the ratio
• Factors affecting gas exchange

  • Diffusion distance
  • Surface area
  • Concentration gradient
  • Temperature
• Adaptations of large organisms to maximise diffusion


  • Projections on root hair cells (increase surface area)
  • Ventilation, good blood supply and countercurrent flow mechanism (maintain concentration gradient)
  • Alveoli wall is one thin layer of squamous epithelial cells (short diffusion distance)
• Route that air takes during ventilation

  1. Trachea

  2. Bronchi

  3. Bronchioles

  4. Alveoli

  
• Ratio


  Surface area / volume
• Ventilation


  1. Inhaling and exhaling in humans

  2. Controlled by diaphragm and antagonistic interaction of internal and external intercostal muscles
• Inspiration


  1. External intercostal muscles contract and internal relax

  2. Pushing ribs up and out

  3. Diaphragm contracts and flattens

  4. Air pressure in the lungs drops below atmospheric pressure as lung volume increases

  5. Air moves in down pressure gradient

  
• Expiration


  1. External intercostal muscles relax and internal contract

  2. Pulling ribs down and in

  3. Diaphragm relaxes and domes

  4. Air pressure in lungs increases above atmospheric pressure as lung volume decreases

  5. Air forced out down pressure gradient
• Alveoli


  • Tiny air sacs located at the end of the bronchioles
  • The site of gas exchange
  • Highly abundant in each lung- 300 million in each human lung
  • Surrounded by the capillary network
  • Epithelium one cell thick

  
• Why do large organisms need a specialised exchange surface?
• Vital capacity


  The maximum volume of air an individual can inhale and exhale during a deep breath
• Tidal volume


  The air inhaled and exhaled when at rest
• Spirometer


  An apparatus that measures the volume of air inspired and expired by the lungs

  
• Ventilation rate


  • The volume of air inhaled per minute
  • Calculated using the formula: ventilation rate = tidal volume x breathing rate
• Fish gill anatomy


  • Gill filaments are stacks of gill lamellae at right angles
  • Many gill filaments covered in many gill lamellae are positioned at right angles
  • Creates a large surface area for efficient diffusion
• Countercurrent flow


  • When water flows over gills in opposite directions to the flow of blood in the capillaries
  • Equilibrium is not reached
  • Diffusion gradient is maintained across entire length of gill lamellae
• How the tracheal system provides a large surface area

  • Highly branched tracheoles
  • Large number of tracheoles
• Fluid-filled tracheole ends


  • Adaptation to increase movement of gases
  • When insect flies and muscles respire anaerobically- lactate produced
  • Water potential of cells lowered, so water moves from tracheoles to cells by osmosis
  • Gases diffuse faster in air
• Why don't small organisms need breathing systems?

  *They have a large surface area to volume ratio *No cells are far from the surface