Mammalian gaseous exchange system

• Gas exchange occurs in the lungs.
• Key structures in mammalian gas exchange

  Nasal cavity, Trachea, Bronchus, Bronchioles and Alveoli.
• Nasal Cavity

  • Large surface area, which warms the air to body temperature.
  • A hairy lining, which secretes mucus to trap dust and bacteria, protecting delicate lung tissue from irritation and infection.
  • Moist surfaces, which increase humidity of the incoming air, reducing evaporation from the exchange surfaces.
• Trachea

  • Wide tube supported by c-shaped cartilage rings, which stop the trachea collapsing.
  • Rings are incomplete to allow food to move down the oesophagus behind the trachea.
  • Trachea and its branches are lined with ciliated epithelium with goblet cells between and below the epithelial cells.
  • Goblet cells secrete mucus onto the lining of the trachea to trap dust and microorganisms.
  • Cilia beat and move the mucus away from the lungs, towards the throat to be swallowed.
• Cigarette smoke stops cilia beating.
• Bronchus

  • Trachea divides to form the left and right bronchus.
  • Similar structure to the trachea with the same supporting cartilage rings but they are smaller.
• Bronchioles

  • In the lungs, the bronchi divide to form many small bronchioles.
  • Smaller bronchioles have no cartilage.
  • Walls of the bronchioles contain smooth muscle. When the smooth muscle contracts, the bronchioles constrict. When it relaxes, the bronchioles dilate. This changes the volume of air reaching the lungs.
  • Bronchioles are lined with a thin layer of flattened epithelium, making some gaseous exchange possible.
• Alveoli

  • Tiny air sacs which are the main gas exchange surfaces in the body.
  • Unique to mammalian lungs.
  • Each alveolus has a diameter of around 200-300μm and consists of a layer of flattened epithelial cells, along with some collagen and elastin fibres.
  • These elastic tissues allow the alveoli to stretch and when they return to resting size, they help to squeeze air out. This is the elastic recoil of the lungs.
• Adaptation of alveoli: Large surface area

  300-500 million alveoli per adult lung. The alveolar surface area for gas exchange in the two lungs is 50-75m².

  If the lungs were balloon-like structures, the surface area would not be large enough for the amount of oxygen needed to diffuse into the body.
• Adaptation of alveoli: Thin layers

  Both the alveoli and the capillaries that surround them have single-cell thick walls so the diffusion distances between air in the alveolus and blood in the capillaries are shorter.
• Adaptation of alveoli: Good blood supply

  Network of 280 million capillaries surrounding the alveoli. The constant flow of blood brings carbon dioxide and carries away oxygen, maintaining a steep concentration gradient for gaseous exchange.
• Adaptation of alveoli: Good ventilation


  Breathing moves air in and out of the alveoli, helping to maintain the steep concentration gradient for gaseous exchange.
• Adaptation of alveoli: Inner surface

  The inner surface of the alveoli is covered in a thin layer of a solution of water, salts and lung surfactant. This surfactant makes it possible for alveoli to stay inflated. Oxygen dissolves in the water before diffusing into the blood, but the water can also evaporate into the air in the alveoli.
• Inspiration

  External intercostal muscles contract to raise the ribs upwards and outwards. The diaphragm contracts, becoming flatter and moving down, which increases the volume of the chest cavity. Pressure falls below atmospheric pressure and air moves into the lungs.
• Expiration

  The external intercostal muscles relax (internal intercostal muscles may also contract) and the ribs move downwards and inwards. The diaphragm relaxes and is pushed upwards so the volume of the chest cavity decreases and this increases pressure in the lungs above atmospheric pressure so air is forced out of the lungs.