9.1 The Gas Exchange System

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Describe the structure of the human gas exchange system (lungs, trachea, bronchi, bronchioles, alveoli, capillary network)
  2. Describe where cartilage, ciliated epithelium, goblet cells, squamous epithelium, smooth muscle and capillaries are found in the gas exchange system
  3. Recognise these tissues in microscope slides, photomicrographs and electron micrographs
  4. Recognise the trachea, bronchi, bronchioles and alveoli in images and make plan diagrams of transverse sections of the trachea and bronchus walls
  5. Describe the functions of ciliated epithelial cells, goblet cells and mucous glands in keeping the gas exchange system healthy
  6. Describe the functions of cartilage, smooth muscle, elastic fibres and squamous epithelium in the gas exchange system
  7. Describe how gas exchange takes place between the alveoli and the blood capillaries

1. Structure of the Human Gas Exchange System

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The gas exchange system is the set of organs and tubes in your chest (the thorax) that bring air into your body and allow oxygen and carbon dioxide to be exchanged between the air and your blood.

Air travels through a series of structures, getting deeper and deeper into the lungs, before it reaches the tiny air sacs where the actual exchange of gases happens. Here is the pathway air follows:

Trachea → Bronchi → Bronchioles → Alveoli

The Lungs

  • You have two lungs, one on each side of your chest.
  • The lungs are the main organs of the gas exchange system.
  • All of the tubes and air sacs described below are found inside or leading into the lungs.

The Trachea (Windpipe)

  • The trachea is the large tube that connects your mouth and nose to the lungs.
  • It is roughly 1.8 cm in diameter — wide enough to let plenty of air through.
  • Its walls contain C-shaped rings of cartilage (a tough, flexible material) that hold the tube open so it never collapses.
  • The inside of the trachea is lined with specialised cells that clean the air as it passes through.

The Bronchi (singular: Bronchus)

  • At the bottom of the trachea, the tube splits into two bronchi — one leading to the left lung and one to the right lung.
  • Each bronchus is narrower than the trachea (about 1.2 cm in diameter).
  • Like the trachea, the bronchi have cartilage for support, but here the cartilage forms irregular plates rather than neat C-shaped rings.
  • The bronchi also have a lining of specialised cells similar to the trachea.

The Bronchioles

  • Inside the lungs, the bronchi branch again and again into smaller and smaller tubes called bronchioles.
  • Bronchioles are very narrow — less than 1 mm in diameter.
  • Unlike the trachea and bronchi, bronchioles do not have cartilage in their walls.
  • They do have smooth muscle (a type of muscle you cannot control consciously), which can tighten or relax to change how much air flows through.

The Alveoli (singular: Alveolus)

  • At the end of the bronchioles are millions of tiny air sacs called alveoli.
  • These are the actual site of gas exchange — where oxygen moves into the blood and carbon dioxide moves out.
  • Their walls are extremely thin (just one cell thick) and they have a very large surface area, both of which make gas exchange fast and efficient.
  • An adult human has around 480–500 million alveoli — giving a total surface area of 40–75 m², roughly the size of a tennis court!

The Capillary Network

  • Each alveolus is wrapped in a dense network of tiny blood vessels called capillaries.
  • Capillaries are incredibly narrow — only about 3–4 µm in diameter (a micrometre, µm, is one millionth of a metre).
  • This is so narrow that red blood cells have to pass through in single file, giving them maximum time to pick up oxygen.
  • Oxygen diffuses from the air in the alveoli into the blood in the capillaries; carbon dioxide diffuses the opposite way.

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