When air is breathed in it passes from the nose to the alveoli of the lungs down a series of tubes (see diagram 9.3). After entering the nose the air passes through the nasal cavity, which is lined with a moist membrane that adds warmth and moisture to the air as it passes. The air then flows through the pharynx or throat, a passage that carries both food and air, to the larynx where the voice-box is located. Here the passages for food and air separate again. Food must pass into the oesophagus and the air into the windpipe or trachea. To prevent food entering this, a small flap of tissue called the epiglottis closes the opening during swallowing (see chapter 11). A reflex that inhibits breathing during swallowing also (usually) prevents choking on food.
The trachea is the tube that ducts the air down the throat. Incomplete rings of cartilage in its walls help keep it open even when the neck is bent and head turned. The fact that acrobats and people that tie themselves in knots doing yoga still keep breathing during the most contorted manoeuvres shows how effective this arrangement is. The air passage now divides into the two bronchi that take the air to the right and left lungs before dividing into smaller and smaller bronchioles that spread throughout the lungs to carry air to the alveoli. Smooth muscles in the walls of the bronchi and bronchioles adjust the diameter of the air passages.
The tissue lining the respiratory passages produces mucus and is covered with miniature hairs or cilia. Any dust that is breathed into the respiratory system immediately gets entangled in the mucous and the cilia move it towards the mouth or nose where it can be coughed up or blown out.