The lungs do not have muscle tissue, so they do not perform the movements themselves. During inhalation and exhalation, the lungs passively follow the walls of the chest cavity. Outside, each lung is covered with pulmonary pleura. The walls of the chest cavity are lined with parietal pleura. Between the two pleura is the pleural cavity filled with a small amount of fluid. The pressure in the pleural cavity is below atmospheric.
The diaphragm plays an important role in respiratory movements.
The diaphragm is a domed muscular septum that separates the chest cavity from the abdominal cavity.
Inhalation occurs due to the contraction of the external intercostal muscles and the diaphragm. The volume of the chest increases, the pressure in the cavity between the pulmonary and parietal pleura drops, and the pulmonary vesicles expand. Air enters the lungs.
Exhalation occurs when the intercostal muscles and diaphragm relax and the volume of the chest decreases.
During deep breathing, the internal intercostal muscles, the muscles of the abdominal wall, all the muscles of the chest and some others are also involved in the respiratory movements.
The vital capacity of the lungs
The respiratory rate in an adult is 15–17 respiratory movements per minute. With physical exertion, it can increase several times. The depth of breathing also changes depending on the needs of the body.
Respiratory volume is the amount of air inhaled and exhaled at rest. It is equal to approximately 500 cm³.
Inspiratory reserve volume – the amount of air that a person can inhale additionally after a calm inhalation (about 1500 cm³).
Expiratory reserve volume – the amount of air that a person can exhale with volitional exertion after a calm exhalation (approximately 1500 cm³).
After the deepest exhalation, about 1000 cm³ of air remains in the lungs. This is the residual volume.
Tidal volume, inspiratory reserve volume and expiratory reserve volume constitute the vital capacity of the lungs. The average vital capacity of the lungs is approximately 3500 cm³. For trained people, it can be much larger and reach 6000 cm³.
The vital capacity of the lungs is determined using a special device – a spirograph (spirometer).