3.1.1 - exchange surfaces

• specialised exchange surfaces are needed because:

  • metabolic activity is greater in multicellular organisms
  • oxygen and carbon dioxide need to be transported at higher rates
  • surface area to volume ratios are smaller
  • diffusion will not achieve adequate rate of gas exchange
• increased surface area - overcomes reduced SA:V ratio in larger organisms
• thin layers - reduces diffusion distance
• good blood supply + ventilation - maintains steep concentration gradients through the quick supply and removal of gases
• nasal cavity:

  key features

  • good blood supply
  • lined with hairs and mucus-secreting cells
  • moist surface
• nasal cavity:

  key features + functions

  • good blood supply - warms air entering body
  • lined with hairs and mucus-secreting cells - traps dust and bacteria
  • moist surface - increases humidity, reduces evaporation from lungs
• trachea:

  key features + functions

  • supported by flexible cartilage - prevents collapse
  • lined with mucus secreting goblet cells - traps dust and bacteria
  • ciliated epithelium cells - moves mucus away from the lungs
• bronchus:

  • cartilage - prevents collapse
• bronchioles:

  key features + functions

  • smooth muscle + no cartilage - can constrict and dilate to vary amount of air reaching lungs
  • flattened epithelium cells - some gas exchange is possible
• alveoli:

  key features + functions

  • single layer of flattened epithelium cells - short diffusion pathway = increase in diffusion rate
  • elastic fibres and collagen - enables stretching and elastic recoil during ventilation
  • large surface area - increased rate of diffusion
  • good blood supply + ventilation - oxygen is supplied more quickly, carbon dioxide is removed quicker, maintains steep concentration gradient
  • layer of surfactant - remains inflated
• inspiration (inhalation):

  1. external intercostal muscles contract
  2. ribs move up and out
  3. diaphragm contracts and flattens
  4. thorax volume increases
  5. air pressure in lungs drops below atmospheric pressure
  6. air moves into lungs
• expiration (exhalation):

  1. external intercostal muscles relax
  2. ribs move down and inwards
  3. diaphragm relaxes and reverts to domed shape
  4. thorax volume decreases
  5. air pressure in lungs rises above atmospheric pressure
  6. air moves out of lungs
• peak flow meter - measure the rate at which patients expel air into a handheld tube, can be used to monitor conditions such as asthma
• spirometer - patients breathe in and out of a mouthpiece attached to a sealed chamber; oxygen form the chamber is used up, can measure several aspects of lung volume
• total lung capacity = vital capacity + residual volume
• residual volume - volume remaining in the lungs even after a person has exhaled with maximum force
• vital capacity - maximum volume that can be breathed out following the strongest possible inhalation
• tidal volume - the volume inhaled with each resting breath
• inspiratory reserve + expiratory reserve volumes - the additional volumes of air that can be breathed in and out during inhalation and exhalation