Breathing, or pulmonary ventilation, is the
movement of air from outside the body into and out of the bronchial
tree and alveoli. The actions responsible for these air movements
are termed inspiration (inhalation) and expiration (exhalation).
Atmospheric pressure, due to the weight of air, is the force that causes air to move into the lungs. At sea level, this pressure is sufficient to support a column of mercury (the original means of measuring this pressure) about 760 millimeters (mm) high in a tube. thus, normal air pressure is equal to 760 mm of mercury (Hg).
Air pressure is exerted on all surfaces in contact with the air, and since persons breathe air, the inside surfaces of their lungs also are subjected to pressure. In other words, the pressures on the inside of the lungs and alveoli and on the outside of the thoracic wall are about the same.
If the pressure inside the lungs and alveoli decreases, outside air will be pushed into the airways by atmospheric pressure. That is what happens during inspiration. Muscle fibers in the dome-shaped diaphragm below the lungs are stimulated to contract by impulses carried on the phrenic nerves, which are associated with the cervical plexuses. As this happens, the diaphragm moves downward, the size of the thoracic cavity is enlarged , and the pressure within the alveoli is reduced about 2 mm Hg below that of atmospheric pressure. In response to this decreased pressure, air is forced into the airways by atmospheric pressure, and the lungs expand.
While the diaphragm is contracting and moving downward, the external intercostal muscles between the ribs may be stimulated to contract. This action raises the ribs and elevates the sternum, so that the size of the thoracic cavity increase even more. As a result, the pressure inside is further reduced and more air is forced into the airways by the relatively greater atmospheric pressure.
The expansion of the lungs is aided by the fact that the parietal pleura, on the inner wall of the thoracic cavity, and the visceral pleura, attached to the surface of the lungs, are separated only by a thin film of serous fluid. The water molecules in this fluid have a great attraction to one another, creating a force called surface tension. This force holds the moist surfaces of the pleural membranes tightly together. Consequently, when the thoracic wall is moved upward and outward by the action of the external intercostal muscles, the parietal pleura is moved too, and the visceral pleura follows it. This action helps to expand the lungs in all directions.
The surface tension between the adjacent moist membranes is sufficient to cause the collapse of the alveoli, which have moist inner surfaces. Certain alveoli cells, however, synthesize a mixture of lipoproteins, called surfactant. Surfactant, which is secreted into the alveoli air spaces continuously, acts to reduce the surface tension and decreases the tendency of the alveoli to collapse, when the lung volume is low.
If a person needs to take a deeper breath than normal breath, the diaphragm and external intercostal muscles may be contracted to an even greater extent. Additional muscles, such as the pectoralis minors and sternocleidomastoids, can also be used to pull the thoracic cage farther upward and outward, enlarging the thoracic cavity and decreasing the internal pressure still more.
The forces responsible for normal expiration come from the elastic recoil of tissues and from surface tension. The lungs and thoracic wall, for example, contain a considerable amount of elastic tissues, and as the lungs expand during inspiration, these tissues are stretched. As the diaphragm lowers, the abdominal organs beneath it are compressed. As the diaphragm and external intercostal muscles relax following inspiration, these elastic tissues cause the lungs and thoracic cage to recoil, and they return to their original shapes. Similarly, the abdominal organs spring back into their previous shapes, pushing the diaphragm upwards. At the same time, the surface tension that develops between the moist surfaces of the alveolar linings tend to cause a decrease in the diameter of the alveoli. Each of these factors tends to increase the alveolar pressure about 1 mm Hg above atmospheric pressure, so that the air inside the lungs is forced out through the respiratory passages. Thus, normal expiration is a passive process.
If a person needs to exhale more air than normal, the posterior internal intercostal muscles can be contracted. These muscles pull the ribs sternum downward and inward increasing the pressure in the lungs. Also, the abdominal wall muscles, including the external and internal obliques, transversus abdominis, and rectus abdominis, can be used to squeeze the abdominal organs inward. Thus, the abdominal wall muscles can cause pressure in the abdominal cavity to increase and force the diaphragm still higher against the lungs. As a result of these actions, additional air ca be squeezed out of the lungs.