Breathing, or pulmonary ventilation, is the process that exchanges air between the atmosphere and the alveoli of the lungs. Air moves into and out of the lungs along an air pressure gradient-from regions of higher pressure to regions of lower pressure. There are three pressures that are important in breathing:

  1. Atmospheric pressure is the pressure of the air that surrounds the earth. Atmospheric pressure at

    sea level is 760 mm Hg, but at higher elevations it decreases because there is less air at higher elevations.

  2. Intra-alveolar (intrapulmonary) pressure is the air pressure within the lungs. As we breathe in and out, this pressure fluctuates between being lower than atmospheric pressure and higher than atmospheric pressure.
  3. Intrapleural pressure is the pressure within the pleural cavity. It is about 2 to 6 mm Hg below the atmospheric pressure during various phases of breathing. This lower intrapleural pressure

    is often described as “negative pressure,” and it keeps the lungs stuck to the internal walls of the thoracic cage and helps expand the lungs, even as the thoracic cage expands and contracts during breathing. If the intrapleural pressure were to equal atmospheric pressure, the lungs would collapse and be nonfunctional.

    Inspiration

    The process of moving air into the lungs is called inspiration, or inhalation. When the lungs are at rest, the air pressure in the lungs is the same as the atmospheric pressure. In order for air to flow into the lungs, the intraalveolar pressure must be decreased to below atmospheric pressure. This change allows for air to flow from the higher air pressure in the atmosphere towards the lower air pressure within the lungs. The contraction of the diaphragm and the external intercostals during i nspiration causes an increase in lung volume, which results in a decrease in intra-alveolar pressure.

    The dome-shaped diaphragm is a thin sheet of skeletal muscle separating the thoracic and abdominal cavities. When it contracts, the diaphragm pulls inferiorly and becomes flattened, which increases the volume of the thoracic cavity. At the same time, contraction of the external intercostals elevates and protracts the ribs and pushes the sternum anteriorly, which further increases the volume of the thoracic cavity.

    Because the negative intrapleural pressure and the surface tension of the pleural fluid keep the visceral pleura stuck to the parietal pleura, the lungs are pulled along when the thoracic cage expands. Therefore, the expansion of the thoracic cavity increases the volume of the lungs, which decreases the intra-alveolar pressure. Then, the higher atmospheric pressure forces air through the air passageways into the lungs until intra-alveolar and atmospheric pressures are equal. Quiet inspiration requires the contraction of the diaphragm and the external intercos- tals only. Forceful inspiration requires the involvement of additional muscles in the neck and chest, such as the sternocleidomastoid, scalenes, serratus anterior, and pectora- lis minor. The contraction of these muscles elevates and protracts the ribs to a greater extent, leading to a greater increase in the volume of the thoracic cavity. Through this further increase in thoracic volume, intraalveolar pressure decreases to a greater extent, which results in greater airflow into the lungs.

    Expiration

    Expiration, or exhalation, occurs when the diaphragm and external intercostals relax, allowing the thoracic cage and lungs to return to their original size. This results in a decrease in the volume of the thoracic cavity and lungs. The decrease in lung volume increases intra-alveolar pressure to a level higher than atmospheric pressure. The higher intra-alveolar pressure forces air out of the lungs until intra-alveolar and atmospheric pressures are equal.

    Expiration during quiet breathing is a rather passive process because the abundant elastic connective tissue in the lungs and thoracic wall causes them to return to their original size as soon as the muscles of inspiration relax. However, a forceful expiration is possible by contraction of the internal intercostals, which depresses and retracts the ribs, and by the muscles of the abdominal wall, which move the abdominal viscera and diaphragm superiorly. These contractions further decrease the volume of the thoracic cavity and lungs, which increases the intraalveolar pressure, causing more air to flow out of the lungs.