Although the words breathing and respiration are sometimes used interchangeably, they have distinct meanings. Breathing is the process of moving oxygen-rich air into and out of the lungs. Respiration refers to all of the processes involved in getting oxygen to tissues, including breathing, diffusion of oxygen from the lungs to the blood, transport by the blood, and diffusion from the blood to tissues. Respiration is essential for aerobic respiration, the process within cells in which nutrients and oxygen are used to build the energy molecule adenosine triphosphate (ATP). In aerobic respiration, body cells use oxygen to metabolize glucose, forming carbon dioxide as a waste product that is exhaled.
As the diaphragm contracts and moves downward, the pectoralis minor and intercostal muscles pull the rib cage outward. The chest cavity expands, and air rushes into the lungs through the trachea to fill the resulting vacuum. When the diaphragm relaxes to its normal, upwardly curving position, the lungs contract, and air is forced out.
Because body cells are constantly using up oxygen and producing carbon dioxide, the lungs work continuously. An adult normally breathes from 14 to 20 times per minute, but vigorous exercise can raise the rate to 80 breaths per minute. A child’s rate of breathing at rest is faster than an adult’s at rest, and a newborn baby has a rate of about 40 breaths per minute. In general, smaller animals have faster breathing rates than larger animals. A rat, for example, breathes about 60 times per minute, while a horse breathes only about 12 times per minute.
The process of breathing is generally divided into two phases, inspiration and expiration. In inspiration, air is moved into the lungs. In expiration, air is forced out of the lungs. The lungs themselves have no muscle tissue. Their movements are controlled by the rib cage and the diaphragm. During inspiration the muscles around the rib cage contract, lifting the ribs upward and outward, and lowering the dome of the diaphragm until it forms a nearly flat sheet. As a result of these changes, the chest cavity expands. Because the lungs are attached to the chest cavity, they also expand. With the enlargement of the lungs, air pressure inside the lungs falls below the pressure of the air outside the body, creating a partial vacuum, and air from outside the body rushes into the lungs.
The amount of air normally taken into the lungs in a single breath during quiet breathing is called the tidal volume. In adults the tidal volume is equal to about 0.5 liters (about 1 pt). The lungs can hold about ten times this volume if they are filled to capacity. This maximum amount, called the vital capacity, is generally about 4.8 liters (about 1.3 gal) in an adult male, but varies from one individual to the next. Athletes, for example, can have a vital capacity of as much as 5.7 liters ( 1.5 gal). The vital capacity is reached only during strenuous exercise.
In expiration the muscles that lift the rib cage and lower the diaphragm relax. As a result, the rib cage and the diaphragm return to their original positions, and the lungs contract with them. With each contraction of the lungs the air inside them is forced out.
A person can alter the rate of breathing and can even stop breathing for a short time. But it is impossible to voluntarily stop breathing permanently because breathing, like the heartbeat, is an involuntary activity controlled by nerve centers in the brain stem, the lower part of the brain. These centers are connected with the muscles of the rib cage and diaphragm, and they increase or decrease the rate of breathing according to the needs of the body.