9.1.2 Distribution of Tissues

• Cartilage is a strong and flexible tissue found in various places around the body
  • Cartilage forms 'c' shaped rings that are arranged along the trachea, called tracheal rings
    • These rings help to support the trachea and ensure it stays open, while allowing it to move and flex while we breathe
  • Cartilage also supports the bronchi, though it does not form c-shaped rings here
  • Cartilage is usually not found in the bronchioles
• Ciliated epithelium is a specialised tissue which lines all of the tubes of the airways; each cell has small projections of cilia which sweep mucus, dust and bacteria upwards and away from the lungs and the epithelium itself

Ciliated epithelium showing cilia, basement membrane, goblet cell and nucleus

• Goblet cells can be found scattered throughout the ciliated epithelium in the trachea, and also in the bronchi, though usually not in the bronchioles
  • They are mucus-producing cells that secrete viscous mucus which traps dust, bacteria and other microorganisms and prevents them from reaching the lungs
  • The mucus is then swept along by the cilia of the ciliated epithelium upwards and is swallowed
  • The mucus and any microorganisms will then be destroyed by the acid in the stomach
• The alveoli have a lining of thin squamous epithelium, that allows for gas exchange
  • The squamous epithelium forms the structure of the alveolar wall and is very thin and permeable for the easy diffusion of gases
• Smooth muscle can be found in the walls of the trachea, bronchi and bronchioles
  • It helps to regulate the flow of air into the lungs by relaxing when more air is needed and constricting when less air is needed
• Elastic fibres, sometimes known as elastin, are found in the lining of the airways and in the walls of the alveoli
  • The role of elastic fibres is to recoil and enable the structures of the airways to regain their original shape, e.g. the alveoli stretch to accomodate air during inhalation, and then recoil to their original shape during exhalation
• Each alveolus is surrounded by an extensive network of capillaries
  • Carbon dioxide diffuses out of the capillaries and into the alveoli to be exhaled, while oxygen diffuses the other way from alveoli and into the capillaries to be carried around the body
  • These capillaries have a diameter of around 3-4 µm, which is only wide enough for one red blood cell to travel through at any one time
    • This ensures that there is sufficient time and opportunity for gas exchange to occur

Worked example: Protection against pathogens

Explain how the lining of the trachea, bronchus and bronchioles provide protection against pathogens.

Cilia beat to move mucus away from the lungs and up towards the mouth. Mucus is produced by goblet cells and acts as a barrier to pathogens from entering the ciliated epithelium. The mucus also traps bacteria and microorganisms, while blood vessels bring macrophages to engulf any pathogens in phagocytosis.