Form and Function: The Physiological Implications of the Anatomy of the Respiratory System

The Ventilatory Pump

The ventilatory pump is composed of the bones, muscles, and soft tissue of the thorax, the pleura lining the chest wall and the lungs, the peripheral nerves connecting the CNS to the ventilatory muscles, and the airways of the lung. All of these structures are necessary for the "bellows" function of the respiratory system, which is the movement of gas by bulk flow measured in liters per minute (in contrast to movement by diffusion measured in milliliters per minute) from the atmosphere to the alveoli and back out (note that we use the term "gas" as a generic term for the components of the atmosphere (primarily nitrogen and oxygen) and the contents of the alveoli (primarily nitrogen, oxygen, carbon dioxide, and water vapor); there is a tendency to think of oxygen being inhaled and carbon dioxide being exhaled but, as you will see in Chapter 5, there are times when you must pay attention to the presence of these other gases as well).

Requirements of the Ventilatory Pump ↑

To move 5 L/min of gas into and out of the lungs, the ventilation level seen in a healthy, resting person, the ventilatory pump must create a negative pressure within the thorax (note: all pressures in respiratory physiology are taken relative to atmospheric pressure, which, by convention, is considered to be the zero reference point). In addition, the pump must provide a system for distribution of the inhaled gas to the alveoli. Finally, the pump must minimize energy expenditure to achieve its purpose while adapting to a range of required metabolic needs (e.g., exercise) and to diseases such as asthma that adversely affect airflow.

As we examine the form and function of the ventilatory pump, we need to consider each of the key elements that contribute to the pump. These elements are the bones, muscles, pleura, peripheral nerves, and airways (Fig. 2-2).

Bones ↑

The skeleton forms the superstructure on which muscles and connective tissue are placed. The internal organs of the thorax are encased and protected by the vertebral column, ribs, and sternum. In addition, these structures provide a relatively rigid cage that enables the muscles to generate a negative intrathoracic pressure during inspiration.

The ventilatory muscles, acting on the bones, expand the dimensions of the thorax. Viewed from the side of a person, the orientation of the ribs at the end of exhalation is "superior to inferior," that is, from the head downward toward the feet, as one goes from the vertebral column in the posterior region of the chest to the sternum anteriorly. During inspiration, the action of the external intercostal muscles lifts the ribs and makes them more horizontal. This movement has the effect of increasing the cross-sectional area of the thorax and contributes to the generation of negative intrathoracic pressure.

The sternocleidomastoid muscle, one of the accessory muscles of inhalation (see the section "Muscles of Inspiration"), inserts on the clavicle and sternum and helps lift the superior portions of the chest when greater force is needed to assist ventilation. These two motions of the rib cage—the upward lift of the ribs laterally and the sternum anteriorly—have been likened to the motion of a bucket handle, in the case of the ribs, and the handle of an old-fashioned pump, in the case of the sternum (Animated Fig. 2-3). Both movements work to increase the volume of the thoracic cavity.

Use Animated Figure 2-3 to view the pump handle and bucket handle motions of the ribs during breathing. You can press the play button to watch the motion during inspiration and expiration or use the slider to control the animation yourself.

Muscles ↑

Inspiration, the movement of gas into the lungs, requires the generation of a negative pressure within the alveolus. Expiration, the movement of gas out of the lungs, requires positive pressure in the alveolus. The inspiratory muscles are necessary to initiate each breath. In normal individuals at rest, expiration is a passive phenomenon as the recoil of the lungs generates the pressure necessary to expel gas from the lungs. If airway resistance is high, as in asthma, or ventilatory requirements are increased, as during exercise, however, expiratory muscles must be activated.

Muscles of Inspiration

During quiet breathing at rest, inspiration is achieved largely as a consequence of the action of the diaphragm, with some assistance from the external intercostal muscles, located between the ribs, which lift and stabilize the rib cage. The diaphragm is innervated by cervical spinal roots that exit the spinal cord between C3 and C5. The intercostal muscles, in contrast, are activated by nerve roots that emanate from the spinal cord along the range of the thoracic vertebrae.

Under conditions that require higher levels of ventilation or greater muscular force because of stiffness of the chest wall, obstruction of the airways, or weakness of the diaphragm, additional muscles, the accessory muscles of ventilation, must be used to assist in the generation of negative intrathoracic pressure. The major accessory muscles of inhalation include the scalenus and sternocleidomastoid muscles in the neck and the pectoralis muscles in the chest. For the pectoralis muscle, which normally acts to move the arm, to function as a ventilatory muscle, the arm must be fixed in position. Typically patients in respiratory distress learn to do this on their own and assume a "tripod position" by resting their hands on their knees or on a nearby table or chair (Fig. 2-4). With the position of the arms secure, contraction of the pectoralis results in elevation of the anterior wall of the chest. Chances are you have also done this at some point after heavy exercise as you bent over with your hands on your knees and tried to catch your breath. A patient in the tripod position is likely to have a problem with the ventilatory pump or a need for high levels of ventilation.

Muscles of Expiration

During quiet breathing at rest, expiration is a passive process. When the inspiratory muscles relax at the end of inspiration, the normal tendency of the lungs to recoil inward, which is the consequence of elastic and surface forces (Chapter 3 discusses the exact nature of these forces), produces positive pressure within the lung. A pressure gradient is thus established between the alveolus and the mouth. Consequently, air moves out of the thorax. In a sense, the energy stored in the lungs by the action of the inspiratory muscles is now used for expiration, a system design that minimizes energy expenditure.

To increase ventilation, which is the volume of air going into and out of the lungs each minute, one can increase tidal volume (the size of the breath) and/or the rate or frequency of breathing. For small to moderate increases in ventilation, changes in tidal volume may suffice. Above a ventilation of about 30 to 40 L/min, however, the rate of breathing must also be increased. To increase the breathing rate, you must reduce the amount of time that the system spends in exhalation (i.e., you must increase expiratory flow). Because expiration is passive during normal, quiet breathing, expiratory flow is determined under resting conditions by the pressure in the alveolus, which is produced by the recoil of the lungs, and the resistance of the airways (see Chapter 4). Therefore, to increase expiratory flow, you must increase the pressure gradient between the alveolus and the pharynx by recruiting accessory muscles of exhalation. The major accessory muscles of exhalation are the internal intercostal and the abdominal muscles. The nerves that innervate the intercostal and abdominal muscles originate from the thoracic and lumbar spine.

Pleura ↑

The lungs are surrounded by a thin layer of tissue called the visceral pleura. A similar layer, called the parietal pleura, lines the inside of the chest wall. The space between these two layers, the pleural space, is a virtual space containing only a few milliliters of fluid. Physiologically, the pleurae and the space they create play a critical role in linking the motion of the chest wall and the lungs. The chest wall and the lungs possess elastic properties that vary over the course of the breath. At the end of exhalation, the elastic properties of the lung exert a force inward (i.e., a collapsing force); at the same volume, because the resting volume of the chest wall is greater than the volume at the end of exhalation, the elastic properties of the chest wall exert a force in an outward direction (i.e., an expanding force). At this point in the respiratory cycle, these forces are equal and opposite to each other and are linked via the pleural space. The outward recoil of the chest wall and the collapsing forces of the lung create a negative pressure within the pleural space. When the chest wall moves outward because of the action of the inspiratory muscles, the pressure within the pleural space decreases (i.e., becomes more negative), thereby leading to a decrease in alveolar pressure and flow of air into the lung. When the inspiratory muscles relax at the end of inspiration, the pleural pressure increases, or becomes less negative, and the elastic forces of the lung produce a positive alveolar pressure and flow of gas out of the lung. This interaction and sequence of these events are explained in more detail in Chapter 3.

Peripheral Nerves ↑

Motor nerves emanate from the spinal cord and link the controller to the muscles of ventilation. As already discussed, the diaphragm receives its innervation from cervical nerve roots (C3-C5), and the intercostal muscles are activated via nerves from the thoracic spinal cord. The abdominal muscles, which are the strongest expiratory muscles, are innervated by nerves from the thoracic and lumbar spine (Fig. 2-5).

The lungs are rich in sensory receptors that detect changes in flow and pressure in the airways as well as volume changes in the lung parenchyma. In addition, C fibers in airways and near pulmonary capillaries may play a role in sensations associated with the buildup of fluid in the lung, as is seen in heart failure. One could argue that the lungs are, in many ways, sensory organs (see Chapter 8). All of the information arising from these receptors is conveyed to the brain via the vagus nerve.

Finally, the autonomic nervous system also plays a role in regulating the ventilatory pump. Fibers from the sympathetic nervous system terminate near the airways. When stimulated, these fibers lead to bronchodilation. Conversely, stimulation of the parasympathetic nervous system, which innervates the airways via cholinergic receptors, causes bronchoconstriction. The balance between these two components of the autonomic nervous system determines the tone of the muscles of the airways and the resulting diameter of these conducting tubes (and resistance to air flow within them).

Airways ↑

The airways resemble the branches of a tree that ultimately end in the leaves (alveoli) where gas exchange takes place. The airways serve as the conduit for the flow of gas from the mouth to the alveoli and back out.

Every time an airway branches and a single airway becomes two, we say we have moved to another "generation" of airways. The single trachea, or windpipe, bifurcates into two large tubes that lead to the left and right lungs and are called mainstem bronchi. The mainstem bronchi divide into smaller tubes, called lobar bronchi, which lead to the lobes of the lungs. The right lung has three lobes, and the left lung has two. A lobe is completely surrounded by pleura, so air cannot move from one lobe to another except through the airways. Within each lobe are segments, which receive air from branches of the lobar bronchi called segmental bronchi. Segments are divided into subsegments and receive air via subsegmental bronchi, which lead ultimately to the terminal bronchioles, the smallest airways before the alveoli appear. Bronchi contain cartilage, submucosal glands, ciliated epithelial cells, and goblet cells. The ciliated cells and goblet cells form part of the lungs' defense system. Goblet cells produce mucus. Foreign material and infectious agents may be trapped in the airway mucus, which is then moved toward the pharynx by the cilia, hairlike structures that have a rhythmic beat. When it reaches the pharynx, the mucus is either swallowed or coughed out. The bronchioles do not normally contain cartilage, glands, or goblet cells.

The sum of the cross-sectional area of the two new airways at each branch point is greater than that of the parent branch. Thus, as you move farther out toward the alveoli, the cross-sectional area of all of the small airways in parallel is quite large, far larger than the trachea (Fig. 2-6).

The number of branch points from the trachea to the alveolus ranges from 10 to 23. As you move from conducting airways, tubes through which air flows but no gas exchange takes place, to the alveoli or gas exchange units, you go through what is known as the transitional zone. This zone consists of airways called respiratory bronchioles, from which a few alveoli originate; thus, there is a transition between the conducting airways and alveoli. Finally, you arrive at the alveolar ducts, which are completely lined by alveoli and are the primary site of gas exchange. The portion of the lung composed of alveoli has been termed the respiratory zone.

Regions of lung that receive air but do not participate in gas exchange are termed dead space. The size of the conducting airways offers a tradeoff between two physiologic demands. On the one hand, the amount of wasted ventilation or dead space needs to be minimized. This would favor conducting airways with very small diameter and volume. On the other hand, the resistance in these airways must also be minimized (see Chapter 4), the need to minimize the work of breathing suggests that the system should have the largest airway possible. Hence, you can see the dilemma.

Moving from the single trachea to millions of respiratory bronchioles arranged in parallel, the cross-sectional area of the airways increases tremendously. Consequently, the velocity of the air moving through the airways decreases as it travels from the trachea to the periphery of the lung. Consider the rapidly moving water in a creek that empties into a large pond; the velocity of the water decreases markedly as it enters the broad pond. Because the total cross-sectional area is a key factor in determining resistance, defined as a pressure decrease divided by flow, most of the resistance in the lungs is in the central airways. The first six branches of the airways dominate the total resistance of the lung.

The bronchioles are generally less than 1 mm in diameter. As already noted, the bronchioles do not have cartilage in their walls. Thus, they are susceptible to collapse if the pressure in the pleural space is greater than the pressure in the airway. The bronchioles are located within the connective tissue structure of the lung and are supported by this tissue. As the lung increases in volume during inspiration, the diameter of the bronchioles increases as well. As the lung volume decreases during expiration, so does the diameter of the bronchioles. Emphysema, a disease associated with cigarette smoking, destroys much of the connective tissue support structure within the lung, thereby making these small airways even more susceptible to collapse.

Gas moves from the pharynx to the terminal bronchioles by bulk flow. The driving force during inspiration is the pressure differential between the pharynx and the alveolus (alveolar pressure is negative during inspiration because of the action of the inspiratory muscles). Beyond the terminal bronchioles, the cross-sectional area of the millions of respiratory bronchioles and alveolar ducts, all arranged in parallel, becomes enormous and the velocity of the gas entering the gas-exchanging units decreases precipitously (see Chapter 4). In this region of the airways, diffusion is the predominant means for moving gas. The very low velocity of gas in this region of the lung allows particles being carried in the inspired gas to settle out. The low velocity here likely accounts for the fact that this anatomic region is often the site of deposition of very small dust particles (and the site of diseases that are associated with these particles) (Fig. 2-7).

Because the respiratory bronchioles and alveolar ducts contribute little to the overall resistance of the lungs, the distribution of gas among these units is determined largely by their relative compliance. Compliance is a measure of the stiffness of an object and is equal to the change in volume that occurs in the object for a given change in the pressure across the wall of the object.

The more compliant units receive more ventilation. In the normal lung, the compliance of a terminal lung unit depends on the volume of the unit-the larger the unit at the beginning of a breath, the less compliant it is (see Chapter 3). At the end of exhalation, alveolar units at the apex of the lung are larger than at the bases and, as a result, are less compliant. This accounts partly for the finding that inspired gas is distributed preferentially to the bases of the lungs relative to the apices (see Chapter 5). The distribution of ventilation within the lungs is also alfected by microscopic passages called the pores of Kohn, which connect the alveoli within a lobe (Fig. 2-8). These connections permit transfer of gas between alveoli and function to minimize the collapse of lung units if a more central airway is obstructed. As noted previously, however, individual lobes of the lung are fully surrounded by pleura, and cross-ventilation between the lobes can only occur via the airways.

In disease states such as chronic bronchitis, excess production of mucus may reduce the functional diameter of the airways and increase resistance to airflow.