The Process of Respiration
Respiration involves ventilation of the lungs, exchange of gases, and their transport in the blood. Respiratory needs are met by central and peripheral controls of breathing.
Ventilation is the movement of air into and out of the lungs, normally accomplished by breathing. There are two phases of ventilation (Fig. 14-7):
* Inhalation, or inspiration, is the drawing of air into the lungs.
* Exhalation, or expiration, is the expulsion of air from the lungs.
In inhalation, the active phase of breathing, respiratory muscles contract to enlarge the thoracic cavity. During quiet breathing, the movement of the diaphragm accounts for most of the increase in thoracic volume. The diaphragm is a strong, dome-shaped muscle attached to the body wall around the base of the rib cage. The contraction and flattening of the diaphragm cause a piston-like downward motion that increases the vertical dimension of the chest. Other muscles that participate in breathing are the external and internal intercostal muscles. These muscles run at different angles in two layers between the ribs.
Figure 14-7 Pulmonary ventilation. (A) Inhalation. (B) Exhalation.
As the external intercostals contract for inhalation, they lift the rib cage upward and outward. Put the palms of your hands on either side of the rib cage to feel this action as you inhale. During forceful inhalation, the rib cage is moved further up and out by contraction of muscles in the neck and chest wall.
As the thoracic cavity increases in size, gas pressure within the cavity decreases. This phenomenon follows a law in physics stating that when the volume of a given amount of gas increases, the pressure of the gas decreases. Conversely, when the volume decreases, the pressure increases. If you blow air into a tight balloon that does not expand very much, the gas particles are in close contact and will hit the wall of the balloon frequently, creating greater pressure (Fig. 14-8). If you tap this balloon, it will spring back to its original shape. When you blow into a soft balloon that expands easily under pressure, the gas particles spread out into a larger area and will not hit the balloon’s wall as often.
Figure 14-8 The relationship of gas pressure to volume. (A) Inflation of a stiff balloon creates strong air pressure against the wall of the balloon. (B) The same amount of air in a soft balloon spreads out into the available space, resulting in lower gas pressure.
If you tap the balloon, your finger will make an indentation. Thus, pressure in the chest cavity drops as the thorax expands. When the pressure drops to slightly below the air pressure outside the lungs, air is drawn into the lungs, as by suction. The ease with which one can expand the lungs and thorax is called compliance. Normal elasticity of the lung tissue, aided by surfactant, allows the lungs to expand under pressure and fill adequately with air during inhalation. Compliance is decreased when the lungs resist expansion. Conditions that can decrease compliance include diseases that damage or scar lung tissue, fluid accumulation in the lungs, deficiency of surfactant, and interference with the action of breathing muscles.
Air enters the respiratory passages and flows through the ever-dividing tubes of the bronchial tree. As the air traverses this route, it moves more and more slowly through the great number of bronchial tubes until there is virtually no forward flow as it reaches the alveoli. The incoming air mixes with the residual air remaining in the respiratory passageways, so that the gases soon are evenly distributed. Each breath causes relatively little change in the gas composition of the alveoli, but normal continuous breathing ensures the presence of adequate oxygen and the removal of carbon dioxide.
In exhalation, the passive phase of breathing, the respiratory muscles relax, allowing the ribs and diaphragm to return to their original positions. The lung tissues are elastic and recoil to their original size during exhalation. Surface tension within the alveoli aids in this return to resting size. During forced exhalation, the internal intercostal muscles contract, pulling the bottom of the rib cage in and down. The muscles of the abdominal wall contract, pushing the abdominal viscera upward against the relaxed diaphragm. A lung capacity is a sum of volumes. These same values are shown on a graph as they might appear on a tracing made by a spirometer, an instrument for recording lung volumes (Fig. 14-9). The tracing is a spirogram.
Figure 14-9 A spirogram. The tracing of lung volumes is made with a spirometer.