11.1 Structure and function of the respiratory system
The respiratory system mainly consists of the airways and a pair of lungs. The airways include the nose, mouth, trachea and two bronchi (singular: bronchus) which branch into smaller and smaller bronchioles. At the end of these are tiny air sacs called alveoli. It is in the alveoli that gas exchange takes place between the respiratory system and the circulatory system.
Nose
l l l
Gases enter and leave the body through the nose and mouth. Hairs and mucus in the nose filter and remove dust particles from the air. The nose has a dense blood supply to warm the air.
Bronchiole Heart
Alveoli Where gas exchange takes place
Diaphragm moves during inhaling and exhaling
Fig. 2 The human respiratory system
Epiglottis The epiglottis is a flap of tissue attached to the entrance of the trachea. It is usually upright to allow air to flow into the trachea. When you swallow food the epiglottis closes over the trachea to prevent food from entering the airways. If you talk while you are eating, the epiglottis will be upright, so food can ‘go down the wrong way’ into the trachea instead of the oesophagus. This can lead to choking.
Voice box (larynx) The voice box, or larynx, is located at the top of the trachea. Sound can be made as air passes through the voice box, causing the two vocal cords to vibrate.
Diaphragm The diaphragm is a large sheet of muscle, located under the lungs. It contracts to draw air into the lungs, and relaxes to push air out.
Intercostal muscles The intercostal muscles are attached to the ribs. They contract to move the ribs up and out during inhalation and relax to allow the ribs to move back in during exhalation.