Mechanics of breathing

The mechanics of breathing involve volume and pressure changes occurring during ventilation that allow air to move in and out of the lungs. Air will move from an area of high pressure to an area of low pressure. Therefore, a pressure gradient between the atmosphere and the alveoli must be developed. This section will explain how changes in thoracic volume, lung volume, and pulmonary pressures occur in order to cause the pressure gradients responsible for inspiration and expiration.

Thoracic volume. The volume of the thoracic cavity increases during inspiration and decreases during expiration.

Inspiration. The most important muscle of inspiration is the diaphragm, a thin, dome-shaped muscle inserted into the lower ribs. A skeletal muscle, it is supplied by the phrenic nerves. When the diaphragm contracts, it flattens and pushes downward against the contents of the abdomen. Therefore, contraction of the diaphragm causes an increase in the vertical dimension of the thoracic cavity and an increase in thoracic volume. In fact, the diaphragm is responsible for 75% of the enlargement of the thoracic cavity during normal, quiet breathing.

Assisting the diaphragm with inspiration are the external intercostal muscles, which connect adjacent ribs. When the external intercostal muscles contract, the ribs are lifted upward and outward (much like a handle on a bucket). Therefore, contraction of these muscles causes an increase in the horizontal dimension of the thoracic cavity and a further increase in thoracic volume. The external intercostal muscles are supplied by the intercostal nerves.

Deeper inspirations are achieved by more forceful contraction of the diaphragm and external intercostal muscles. Furthermore, accessory inspira-tory muscles, including the scalenus and sternocleidomastoid muscles, contribute to this process. Located mainly in the neck, these muscles raise the sternum and elevate the first two ribs. As a result, the upper portion of the thoracic cavity is enlarged.

Expiration. Expiration during normal, quiet breathing is passive. In other words, no active muscle contraction is required. When the diaphragm is no longer stimulated by the phrenic nerves to contract, it passively returns to its original preinspiration position under the ribs. Relaxation of the external intercostal muscles allows the rib cage to fall inward and downward, largely due to gravity. As a result, these movements cause a decrease in thoracic volume.

During exercise or voluntary hyperventilation, expiration becomes an active process. Under these conditions, a larger volume of air must be exhaled more rapidly. Therefore, two muscle groups are recruited to facilitate this process. The most important muscles of expiration are the muscles of the abdominal wall. Contraction of these muscles pushes inward on the abdominal contents and increases abdominal pressure. As a result, the diaphragm is pushed upward more rapidly and more forcefully toward its preinspiration position. Assisting the muscles of the abdominal wall are the internal intercostal muscles. These muscles are also found between the ribs; however, they are oriented in a direction opposite to that of the external intercostal muscles. Contraction of these muscles pulls the ribs inward and downward.

Lung volume. No real physical attachments exist between the lungs and the thoracic wall. Instead, the lungs literally float in the thoracic cavity, surrounded by pleural fluid. Therefore, the question arises of how the volume of the lungs increases when the volume of the thoracic cavity increases. The mechanism involves the pleural fluid and the surface tension between the molecules of this fluid. As mentioned previously, the surface tension of the pleural fluid keeps the parietal pleura lining the thoracic cavity and the visceral pleura on the external surface of the lungs "adhered" to each other. In other words, the pleural fluid keeps the lungs in contact with the chest wall. Therefore, as the muscles of inspiration cause the chest wall to expand (thus increasing the thoracic volume), the lungs are pulled open as well. As a result, lung volume also increases.

Pulmonary pressures. Changes in thoracic volume and lung volume cause pressures within the airways and the pleural cavity to change. These pressure changes create the pressure gradients responsible for airflow in and out of the lungs. Four pressures must be considered (see Figure 17.1):

• Atmospheric

• Intrapleural

• Transpulmonary

In-between breaths During inspiration During expiration

Figure 17.1 Pulmonary pressures. (a) In between breaths, alveolar pressure (Palv) is equal to atmospheric pressure (Patm), which is 0 cmH2O. No air flows in or out of the lungs. (b) During inspiration, as lung volume increases, alveolar pressure decreases and becomes subatmospheric (-1 cmH2O). The pressure gradient between the atmosphere and the alveoli allows air to flow into the lungs. (c) During expiration, the lungs recoil and lung volume decreases. Alveolar pressure increases and becomes greater (+1 cmH2O) than atmospheric. The pressure gradient between atmosphere and alveoli forces air to flow out of the lungs.

In-between breaths During inspiration During expiration

Figure 17.1 Pulmonary pressures. (a) In between breaths, alveolar pressure (Palv) is equal to atmospheric pressure (Patm), which is 0 cmH2O. No air flows in or out of the lungs. (b) During inspiration, as lung volume increases, alveolar pressure decreases and becomes subatmospheric (-1 cmH2O). The pressure gradient between the atmosphere and the alveoli allows air to flow into the lungs. (c) During expiration, the lungs recoil and lung volume decreases. Alveolar pressure increases and becomes greater (+1 cmH2O) than atmospheric. The pressure gradient between atmosphere and alveoli forces air to flow out of the lungs.

As it does with all objects on the surface of the Earth, gravity exerts its effects on the molecules of the atmosphere. The weight generated by these molecules is referred to as atmospheric, or barometric, pressure (Patm). At sea level, atmospheric pressure is 760 mmHg. In order to simplify this discussion, atmospheric pressure will be normalized to 0 mmHg (or 0 cmH2O) and all other pressures are referenced to this.

Intrapleural pressure (Ppl) is the pressure within the pleural cavity. Under equilibrium conditions, the chest wall tends to pull outward and the elastic recoil of the lungs tends to pull them inward (like a collapsing balloon). These opposing forces create a subatmospheric or negative pressure within the pleural space. In between breaths, intrapleural pressure is -5 cmH2O. During inspiration, the lungs follow the chest wall as it expands. However, the lung tissue resists being stretched, so the intrapleural pressure becomes even more negative and is -8 cmH20.

Alveolar pressure (Palv) is the pressure within the alveoli. In between breaths, it is equal to 0 cmH2O. Because no pressure gradient exists between the atmosphere and the alveoli, there is no airflow. However, in order for air to flow into the lungs, alveolar pressure must fall below atmospheric pressure. In other words, alveolar pressure becomes slightly negative. According to Boyle's law, at a constant temperature, the volume of a gas and its pressure are inversely related:

Therefore, as lung volume increases during inspiration, the pressure within the alveoli decreases. Atmospheric pressure is now greater than alveolar pressure and air enters the lungs. Because the lungs are normally very compliant, or distensible, only a small pressure gradient is necessary for air to flow into the lungs. During inspiration, alveolar pressure is -1 cmH2O. During expiration the opposite occurs. Lung volume decreases and pressure within the alveoli increases. Alveolar pressure is now greater than atmospheric pressure and air flows out of the lungs. Alveolar pressure during expiration is +1 cmH2O.

Transpulmonary pressure (Ptp) is the pressure difference between the inside and outside of the lungs. In other words, it is the pressure difference between the alveoli and the pleural space:

tp alv pl

In between breaths, the transpulmonary pressure is +5 cmH2O. The transpulmonary pressure is also referred to as the expanding pressure of the lungs. A force of +5 cmH2O expands, or pushes outward on, the lungs so that they fill the thoracic cavity. As might be expected, during inspiration, the transpulmonary pressure increases, causing greater expansion of the lungs:

The entry of air into the pleural cavity is referred to as a pneumothorax. This may occur spontaneously when a "leak" develops on the surface of the lung, allowing air to escape from the airways into pleural space. It may also result from a physical trauma that causes penetration of the chest wall so that air enters pleural space from the atmosphere. In either case, the pleural cavity is no longer a closed space and the pressure within it equilibrates with the atmospheric pressure (0 cmH2O). As a result, the transpulmonary pressure is also equal to 0 cmH2O and the lung collapses.

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