There are two stages to breathing—inspiration and expiration. These are better known as breathing in and out. This movement of air into and out of the lungs is controlled by muscles that change the size of the pleural cavity (the thin, fluid-filled space between the two membranes that surround the lungs). This change in pleural cavity size affects pressure in the cavity relative to the external atmospheric pressure. Air naturally moves from a higher pressure to a lower pressure.

During inhalation, the pressure is greater in the atmosphere than it is in the pleural cavity because the size of the pleural cavity is increased, thereby decreasing its pressure. This causes the air to rush into the lungs. 

 Watch

Observe respiration in this animation. The diaphragm and the rib muscles (internal and external intercostals) control the air pressure inside the lungs that causes air to move in and out of the lungs. The video below provides a good summary of breathing.
 

 Read

Read "The Mechanics of Breathing" on pages 249 and 250 of your textbook.

Spirometers

A spirometer is an instrument that is used to measure air exchange. It produces a graph called a spirograph, which represents the amount of air that moves into and out of the lungs with each breath.     

Spirometers are given to people who have undergone heart or lung surgery. Doctors use incentive spirometers to encourage patients to regain lung capacity after surgery.

The spirometer allows practitioners to compile the information in a spirograph that provides information about the patient's tidal volume, vital capacity, and reserve volumes.

Watch

Watch this video to learn how to interpret a spirograph.