The human respiratory system allows air to enter through the nose or mouth, pass through the pharynx and larynx, and travel into the lungs through the trachea and bronchial tubes. In the lungs, the bronchial tubes branch into smaller passages called bronchioles and alveolar ducts, ending in tiny air sacs called alveoli which have a total surface area of around 70 square meters. Oxygen diffuses from the alveoli into the blood in the pulmonary capillaries, while carbon dioxide diffuses in the opposite direction.

1. The human respiratory system

5. Respiratory animation

6. Gas exchange in fish

10. Lining of the trachea Back