3.1.3 The Mammalian Gas Exchange System

• Gas exchange takes place in the human thorax. This is a collection of organs and tissues in the chest cavity

Tissues of the gas exchange system

• There are a number of different tissue types present in the mammalian gas exchange system
• Each tissue is structurally adapted to perform a very specific purpose
• Ciliated epithelial cells, goblet cells and mucous glands play vital roles in maintaining the health of the gas exchange system
• Cartilage, smooth muscle, elastic fibres and squamous epithelial tissue all play important structural roles in maintaining the gas exchange system

Cartilage

• Cartilage is a strong and flexible tissue found in various places around the body
  • One place is in rings 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 micrograph

Ciliated epithelium

• Ciliated epithelium is a specialised tissue found along the trachea down to the bronchi
• 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 microvilli, basement membrane, goblet cell and nucleus

Ciliated epithelium – electron micrograph

Goblet cells

• Goblet cells can be found scattered throughout the ciliated epithelium in the trachea
• 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

Goblet cell – light micrograph

Squamous epithelium

• The alveoli have a lining of thin and squamous epithelium, that allows for gas exchange
  • The squamous epithelium forms the structure of the alveolar wall and so is very thin and permeable for the easy diffusion of gases

Squamous epithelium – light micrograph

Smooth muscle

• Smooth muscle can be found throughout the walls of the bronchi and bronchioles
  • It helps to regulate the flow of air into the lungs by dilating when more air is needed and constricting when less air is needed

Smooth muscle – light micrograph

Elastic fibres

• Elastic fibres are present in all lung tissues. They are very important as they enable the lung to stretch and recoil. This ability to recoil is what makes expiration a passive process

Capillaries

• 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 3-4µm, 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

Cross-section of capillaries – light micrograph

Structures & Functions in the Gas Exchange System Table

Components of the gas exchange system

• The different tissues make up a number of important components within the gas exchange system
• Trachea
  • The trachea is the channel that allows air to travel to the lungs
  • C-shaped rings of cartilage ensure that this air channel remains open at all times
    • They are C-shaped to prevent any friction from rubbing with the oesophagus located close behind

  • The trachea is lined with ciliated epithelium
  • There is a substantial covering of mucus inside the trachea (produced by goblet cells and mucous glands) that helps to trap dust and bacteria to prevent them from entering the lungs
  • The wall of the trachea contains smooth muscle and elastic fibres

Structure of the trachea. The ends of the cartilage rings are connected by smooth muscle and elastic fibres.

• Bronchi
  • Bronchi have a similar structure to the trachea but they have thinner walls and a smaller diameter
  • The cartilage in the bronchi does not form a c-shape, but can form full rings, and can also form irregular blocks
• Bronchioles
  • Bronchioles are narrow self-supporting tubes with thin walls
    • They are not usually supported by cartilage, though a few bronchioles may contain some cartilage
  • A large number of bronchioles are present in the gas exchange system
  • Bronchioles are lined with ciliated epithelium in the same way as the trachea and bronchi, though the usually do not contain any goblet cells
  • Bronchioles vary in size and structure, getting smaller as they get closer to the alveoli
    • The larger bronchioles possess elastic fibres and smooth muscle that adjust the size of the airway to increase or decrease airflow
    • The smallest bronchioles do not have any smooth muscle but they do have elastic fibres
• Alveoli
  • Groups of alveoli are located at the ends of the bronchioles
  • The alveolar wall consists of a single layer of epithelium
    • Elastic fibres are located in the extracellular matrix
  • There is an extensive capillary network
  • A watery fluid lines the alveoli, facilitating the diffusion of gases

The main structures of the human gas exchange system

Thorax Structures Table