Trachea and bronchi These are the airways that bring gases to and from the lungs. C-shaped rings of cartilage give strength and support to the trachea and bronchi. Cartilage is a strong and flexible tissue. Cartilage is also present in the outer ear and the end of the nose and bones. The trachea and bronchi contain mucus and tiny hair-like structures called cilia which trap dust particles and push them back up towards the mouth.
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Fig. 3 Cilia lining the bronchi.
Lungs There are two lungs which contain the bronchioles and alveoli. The ribs surround the lungs to protect them (and the heart).
Bronchioles These are tiny tubes branching off from the bronchi. They are surrounded by bands of muscle but do not have cartilage. They carry air to and from the alveoli.
Alveoli Alveoli are the tiny air sacs in the lungs. The alveoli are adapted to allow rapid exchange of gases, oxygen, carbon dioxide and water vapour between the respiratory system and the circulatory system.
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Gas exchange The rapid transfer of the gases oxygen, carbon dioxide and water vapour between the respiratory system and the cirrculatory system.
There are approximately 300 million alveoli in each lung. This provides a huge surface area. Each alveolus is only one cell thick and is surrounded by a dense capillary network which allows for rapid transfer of gases to and from the bloodstream. Alveoli are moist. Gases diffuse more quickly across a moist surface than a dry one.
11.2 Gas exchange in the alveoli
Each alveolus is surrounded by a network of capillaries. This is the point of interaction between the respiratory and circulatory systems. The thin walls of the alveoli and the capillaries allow the rapid exchange of gases between the lungs and the blood. Oxygen diffuses through the walls of the alveoli and the capillaries into the bloodstream. Once oxygen enters the bloodstream it is picked up by the
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Fig. 5 Thin-walled alveoli are surrounded by capillaries.
