The Pathway

  1. Air enters the nostrils
  2. Passes through the nasopharynx,
  3. Enters the oral pharynx
  4. Passes through the glottis
  5. Enters the trachea
  6. Divides into the right and left bronchi, which branches and re-branches into
  7. bronchioles, each of which terminates in a cluster of
  8. alveoli
Human respiratory system

Only in the alveoli does actual gas exchange takes place. There are some 300 million alveoli in two adult lungs. These provide a surface area of some 160 m2 (almost equal to the singles area of a tennis court and 80 times the area of our skin).


The diaphragm divides the body cavity into the abdominal cavity, which contains the viscera (e.g., stomach and intestines) and the thoracic cavity, which contains the heart and lungs. The inner surface of the thoracic cavity and the outer surface of the lungs are lined with pleural membranes which adhere to each other. If air is introduced between them, the adhesion is broken and the natural elasticity of the lung causes it to collapse. This can occur from trauma. And it is sometimes induced deliberately to allow the lung to rest. In either case, reinflation occurs as the air is gradually absorbed by the tissues.

Because of this adhesion, any action that increases the volume of the thoracic cavity causes the lungs to expand, drawing air into them. During inspiration (inhaling),

The external intercostal muscles contract, lifting the ribs up and out.

The diaphragm contracts, drawing it down. During expiration (exhaling), these processes are reversed and the natural elasticity of the lungs returns them to their normal volume. At rest, we breath 15–18 times a minute exchanging about 500 ml of air.

In more vigorous expiration, The internal intercostal muscles draw the ribs down and inward. The wall of the abdomen contracts pushing the stomach and liver upward.

Under these conditions, an average adult male can flush his lungs with about 4 liters of air at each breath. This is called the vital capacity. Even with maximum expiration, about 1200 ml of residual air remain. The table shows what happens to the composition of air when it reaches the alveoli. Some of the oxygen dissolves in the film of moisture covering the epithelium of the alveoli. From here it diffuses into the blood in a nearby capillary. It enters a red blood cell and combines with the hemoglobin therein.

Gas exchange in humans

At the same time, some of the carbon dioxide in the blood diffuses into the alveoli from which it can be exhaled.