Biology F211- Lungs

The Lungs

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Why organisms need special exchange surfaces

All living cells need certain substances to keep them alive these include:

  • oxygen for aerobic respiration
  • glucose as a source of energy
  • proteins for growth and repair
  • fats to make membranes and to be stored as energy
  • minerals to maintain their water potential and to help enzyme action

Organisms absorb these substances  from their environment or make them inside their cytoplasm as Metablosim

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What exchange surfaces look like

All good exhange surfaces have certain features in common:

  • Large surface area to volume ration
  • Thin barrier to reduce diffusion distance
  • Supply of molecules on one side keep the conc. gradient high
  • removal of required  molecules on other side keep the conc. graident low

Examples of exchange surfaces:

  • Small intestine, Liver, Root hair cell and Hyphae of fungi
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The lungs as an organ of exchange

The lungs

  • The lungs are a pair of large inflateable structures lying in the chest cavity.
  • Air can pass into the lungs through the nose and along the trachea, bronchi and bhronchioles.
  • The air finally reaches the alvioli and this is where the exchage of gases takes place

Gases pass both ways O2 from the air to the alveoli ->to blood in capilaries. CO2 passes from blood to the air in the alveoli

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How the lungs are adapted for exchange

Large surface area:

  • Provides more space for molecules to pass through
  • Individual alveoli are small but there are lots so many that the surface area is large

Barrier permeable to O2 and CO2:

  • The plasma membrane that surrond the cytoplasm of the cell form the barrier to exchange.


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How the lungs are adapted for exchange

Thin barrier to reduce diffusion distance 

  • Alveolus wall is one cell thick
  • Cappilary wall is one cell thick
  • Both walls consist of squamous cells (flattend cells)
  • cappilaries are in close contact with alveolus wall


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Inhaling and Exhaling


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Tissue in the Lung


  • The cartilage plays a structural role. It supports the trachea and bronchi, holding them open. This stops them colapsing when air pressure is low during inhalation.

Smooth muscle:

  • This muscle can contract, when the muscle contracts it will constrict the airway. Constriction mean the fow of air to the alveoli is restricted


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Tissue in the Lung

Elastic fibers

  • When the airway constricts, it deforms the elastic fibers in the loose tissue. as the smooth muscle relaxes the elastic fibers will recoil. this helps dialte (widen) the airway

Goblet cells

  • Secret muscus to trap tiny particles from the air these particles may include bacteria and pollen. Trapping these will reduce the risk of infection
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