Unit 1.2.1: Exchange Surfaces and Breathing

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The Lungs

The Lungs

  • The Lungs are protected by the ribs.
  • The plasma membranes surrounding the thin cytoplasm of cells form a barrier permeable to oxygen and carbon dioxide.
  • The lungs produce a surfactant which reduces the cohesive forces between the water molecules. (Without it the alveolus would collapse as it would be unable to expand). 

The Alveoli

  • Provides a large surface area for gaseous exchange.
  • Both the alveolus and the capillary wall is one cell thick and consists of squamous cells. 
  • The capillaries are in close contact with the alveolus walls and are so narrow that the RBC's are squeezed against the capillary wall, reducing the rate at which they flow and making them closer to the air in the alveoli.
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Maintaining the Diffusion Gradient

Maintaining the Diffusion Gradient

  • A rich blood capillary network ensures that there's always a high concentration of carbon dioxide and a low concentration of oxygen as the oxygenated blood is moved out of the lungs via the pulmonary veins. 
  • Ventilation ensures that there is always a high concentration of oxygen and a low concentration of carbon dioxide in the alveoli sacs to maintain the diffusion gradient. 

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The Trachea And Bronchi

The trachea and bronchi have a similar structure and only differ only in size: 

  • Much of the wall consists of cartilage; wich is in the form of C-rings.
  • The inside surface contains elastic fibres, smooth muscle and blood vessels.
  • The inner lining is a ciliated epithelium layer and among them, there are goblet cells.

The Bronchioles

  • The bronchioles are much narrower than the bronchi.
  • The larger bronchioles may have some cartilage, but smaller ones have no cartilage. 
  • The wall is mostly made up of smooth muscle and elastic fibres.
  • The smallest bronchioles have clusters of alveoli at their ends.               
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Tissue Functions

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The Spirometer

  • The spirometer consists of a chamber filled with oxygen that floats in a tank of water.
  • The person breathes from the mouthpiece attached to a tube that connects to the chamber.
  • Breathing intakes air from the chamber, making it sink.
  • Breathing out does the opposite.
  • The movements of the chamber are recorded using a data logger so a spirometer trace can be produced.
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Spirometer. PT 2


  • Exhaling in the spirometer can increase CO2 levels dangerously.
  • Soda Lime is used to absorb the CO2 exhaled. 
  • The volume of CO2 exhaled is the same as the volume of O2 inhaled, the CO2 removed would equal to the volume of O2 used b the person breathing in and out.

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