Statics: Snapshots of the Ventilatory Pump

Lung Volumes and the Balance of Forces

Defining the Forces: Chest Wall and Lungs ↑

The chest wall, composed of the bones, muscles, and connective tissue of the thorax, has elastic properties. This means that when forces are applied to the chest wall and it is stressed or moved from its resting position, the chest wall resists the movement. When the stress is relieved, the chest wall returns to its resting or unstressed position (much as an elastic band resists being stretched and returns to its resting position when the force that is stretching it is removed). This resting position, also known as its equilibrium position, defines a volume because the chest wall is a three-dimensional structure. The chest wall resists deformation from this volume; if you apply a force to the chest wall to make it bigger or smaller and then release the force, the chest wall will return to its resting volume. Throughout much of the breathing that we do in our daily lives, the chest wall is at a volume that is smaller than its isolated, unstressed position (Fig. 3-1). Thus, at these volumes, it wants to spring out to resume its equilibrium configuration.

Similar to the chest wall, the lungs also have elastic properties. If you removed the lungs from the body, the lungs would like to collapse to their resting or unstressed position, a position that is equivalent to their minimum volume (see Fig. 3-1). In other words, the isolated lungs in their resting position are at their smallest possible volume; in a live human being, the lungs are always stretched above their equilibrium position and exert a collapsing force. To inflate the lungs, one must apply a force to overcome this elastic recoil of the lung tissue (and the surface forces active in the alveoli; more about this later). When the force is removed, the lung will recoil and gas will be expelled.

Play Animated Figure 3-1 to view the relationship between the equilibrium positions and the range of lung/chest wall volume in a live human being, including the range of resting breathing. Note that the chest wall tends to spring outward over most of the range of volume, and the lungs tend to recoil inward over the entire range of volume. Before reading on, think about how these forces would interact if we joined the chest wall and lungs together. At what volume do the lung and chest wall forces balance each other? We answer this question and address its significance shortly.

Of course, the chest wall and the lungs do not operate in isolation. The motion of one affects the other via the pleural space. As described in Chapter 2, the lungs are surrounded by a thin layer of tissue called the visceral pleura, and the chest wall is lined by a similar layer of tissue called the parietal pleura. Between these layers of tissue is the very small pleural space, which contains just a few milliliters of fluid. From a given unstressed position, if inspiration were initiated and the chest wall began to move outward to a larger volume, the pleural space would enlarge if the lungs did not also increase in volume. Similarly, during a passive exhalation, air exits the lungs, and the volume of the lungs decreases. If the chest wall volume did not change, the volume of the pleural space would enlarge during exhalation. The pleural space, however, is a closed space (i.e., no air or fluid moves into or out of it). The enlargement of the pleural space, therefore, results in the creation of a vacuum or negative pressure. This negative pressure is transmitted to the chest wall and lungs and intimately links the motion of one to the other. Consequently, the size of the pleural space never changes significantly as long as it remains a closed system. During normal breathing in a person at rest, the pleural pressure typically ranges during the respiratory cycle between -8 cm H₂O at end inspiration and -3 cm H₂O at end expiration.

Use Animated Figure 3-1A to show the changes in the lung and chest wall when air is traumatically introduced into the pleural space (pneumothorax). Note how small the lung can become, close to its equilibrium position or minimal volume, as well as how the chest wall expands on the affected side.

Pressure-Volume Characteristics of the Chest Wall, Lungs, and Respiratory System ↑

Because the chest wall and lungs have elastic properties and resist changes in volume from their resting positions, a force must be applied to them to produce a change in volume. These forces can be conceptualized as unequal pressures on either side of the walls of the chest cavity or the lung. We call the pressure across the wall of a structure the transmural pressure. Mathematically, transmural pressure is expressed as follows:

where P_TM is the transmural pressure, P_inside is the pressure on the inside of an enclosed structure, and P_outside is the pressure on the outside of an enclosed structure. By convention, the transmural pressure is always expressed as inside minus outside. Positive transmural pressures, therefore, are associated with forces that tend to expand or increase the volume of a structure. Alternatively, negative transmural pressures are associated with collapsing forces that tend to decrease the size or volume of a structure. The resting or unstressed position of the structure is the volume at which the transmural pressure is zero (Fig. 3-2).

A number of elements of Figure 3-2 must be examined closely. First, look at the axes. The x-axis is the transmural pressure. As noted, a positive transmural pressure indicates that the pressure inside the structure is greater than the pressure outside the structure (a positive transmural pressure is a distending pressure). A negative transmural pressure indicates that the pressure outside the structure is greater than inside (this leads to compression of the structure). The point at which the curve crosses the 0 pressure line is the resting or unstressed volume of the structure.

Now look at the y-axis, the volume axis. The volume axis is expressed in terms of the percent of total lung capacity (TLC), which is defined as the volume of air in the lungs at the end of a full inspiration. Residual volume (RV) is the volume remaining in the lung at the end of a forced exhalation. The difference between TLC and RV is the vital capacity (VC). Mathematically, the TLC is the sum of the RV and VC.

The resting position of the chest wall in isolation is at a volume that is approximately 75% to 80% of the TLC. Volumes below this level require a force to be applied to the chest wall to compress it (negative transmural pressure); volumes above this level require a force to be applied to expand the chest wall (positive transmural pressure). At the resting or unstressed position, the volume of the lung is near 0 (i.e., virtually no air in the alveoli). To expand the lung above this volume, a positive transmural force must be applied.

The line representing the respiratory system (lung + chest wall) in Figure 3-2 is the algebraic sum of the chest wall and lung pressure-volume curves. Note that the point at which the respiratory system crosses the 0 pressure line denotes the volume at which the force exerted by the tendency of the lung to recoil inward is equal to and opposite the force exerted by the chest wall to spring outward. This volume is referred to as functional residual capacity (FRC) and represents the volume of the lungs at the end of a normal, relaxed exhalation. At this volume, the inward or deflating force of the lungs is balanced by the outward or inflating force of the chest wall (recall that this balance of forces is mediated via the pleural space).

Use the slider in Animated Figure 3-2 to view the changes in the lung and chest wall forces (shown as force vectors directed inward or outward) over the range of lung volume. Pay special attention to the forces at FRC and to how the net force (i.e., sum of lung and chest wall forces) varies above or below this volume. You may find it helpful to reread the previous section while using the Animated Figure to view the associated forces at each volume.

The Forces: Additional Factors to Consider ↑

Elastic forces, although they explain much of the static properties of the lungs, do not provide all the information you need to understand lung volumes and the interactions of the chest wall and the lungs.

Muscles

We defined the resting position of the lungs, chest wall, and respiratory system as the volume that the structure would achieve when the transmural pressure is 0. What moves the system from the resting volume? How is the force generated to change the transmural pressure from 0?

Inspiration is an active phenomenon that requires the use of the inspiratory muscles (remember that the net force vector of the lungs plus the chest wall is directed inward above FRC). The tension generated in a muscle when stimulated by a neurological impulse depends on the length of the muscle at the moment at which the neurological stimulus reaches the muscle. The relationship between the length of a muscle and the tension generated in the muscle can be determined with in vitro experiments in which isolated muscle strips are examined (Fig. 3-3).

Figure 3-3 represents an isometric contraction in which the muscle generates force but is not allowed to shorten (analogous to pushing against an unmoving brick wall-your muscles generate tension despite staying the same length). A muscle can generate greater tension (force) when stimulated from a higher initial length, a relationship shown by the positive slope of the isometric contraction tension curve. The passive component is due to the elastic elements of the muscle being stretched. Based on this length-tension relationship, where do you think the diaphragm is in the most advantageous position for force generation—at end inspiration or at end expiration? Although this question may be difficult to answer if you do not yet have some familiarity with the anatomy of the diaphragm, think about it before reading on and finding out the answer.

The diaphragm, the primary inspiratory muscle, is dome-shaped at the end of expiration; the peak of the diaphragm comes to a point at a level near the xiphoid process. Laterally, the diaphragm is in close proximity to the rib cage, an area called the zone of apposition because the diaphragm and chest wall are literally apposed or up against one another here (Animated Figure 3-4). As the diaphragm contracts, it shortens and moves downward or caudally, leading to an increase in intra-abdominal pressure and outward movement of the rib cage.

Use Animated Figure 3-4 to view the movement of the rib cage and the change in length of the diaphragm over the range of breathing. At low thoracic volumes, for example, at the end of exhalation, the diaphragm along with the external intercostal muscles, the other major inspiratory muscles, are relatively long. Thus, the tension generated in the muscles is high when stimulated by a neurological impulse from the controller at FRC. This results in more effective pressure generation by the ventilatory pump, thereby creating appropriate inspiratory flow (by moving the chest wall outward, pleural pressure becomes more negative; this negative pressure is transmitted to the alveoli and produces flow into the lungs; more on this in Animated Fig. 3-9). In contrast, at high lung volumes, at the end of inspiration, the inspiratory muscles are shortened and are less effective at generating tension for a given neural stimulus. Consequently, inspiratory flow at high lung volumes is less than at low lung volumes for the same neurological stimulus to the muscles (more on this in Chapter 4).

The length-tension relationship shown in Figure 3-3 is a general property of skeletal muscles and therefore applies to the ventilatory muscles, as well as being characteristic of myocardial muscle cells (the length-tension relationship for the heart is an application of this principle to a three-dimensional structure, the ventricle, and is termed the Starling curve; if you have not yet studied cardiovascular physiology, remember this concept for Chapter 9, which provides some of the basics of cardiac function during exercise).

Changes in lung volumes occur in physiologic and pathologic states in ways that alter the length and, therefore, the performance of the inspiratory muscles. For example, during exercise, the system responds partly by exhaling more fully than normal. This has the effect of reducing the lung volume at the end of exhalation, thereby lengthening the diaphragm and enhancing its ability to contract just before the next inhalation. In contrast, some disease states, such as emphysema, are associated with a larger than normal lung volume at the end of exhalation, a finding that places the diaphragm in a shortened position and impairs its ability to generate tension during inspiration.

Unlike inspiration, expiration is usually a passive phenomenon during relaxed breathing. When the inspiratory muscles stop contracting at the end of inspiration, the normal elastic properties of the lung and, at high thoracic volumes, the recoil of the chest wall, lead to flow of gas out of the lung. During periods of increased ventilation, such as during exercise or when a person needs to generate high expiratory flow, as during a cough, expiration is the consequence of a combination of the passive recoil properties of the lung and chest wall as well as the active contraction of expiratory muscles. The primary expiratory muscles are the muscles of the abdominal wall and the internal intercostals. They are at their longest position and, hence, best able to generate tension, when the thoracic volume is high, as occurs after a full inhalation. Expiratory muscles are in their shortest position and are least able to generate tension at the end of exhalation.

Airway Closure

In children and young adults, residual volume (RV) is determined solely by the balance of forces exerted by the chest wall (outward), the lungs (inward), and the expiratory muscles (inward). In older individuals (often beginning after age 40), however, the elasticity of the lungs begins to diminish. The diameter of small airways in the lung is determined partly by the volume of the lung; the higher the lung volume, the bigger the diameter of the airways. During inhalation, the diameter of the airways increases. During exhalation, the stretch is reduced and the airways decrease in size. The residual "pulling" by the surrounding lung tissue, however, keeps the airway from collapsing completely as transmural pressure becomes negative. As elastic recoil of the lung diminishes with age, the small airways are more susceptible to collapse during exhalation.

During exhalation, the lung becomes progressively smaller and, if the person tries to force as much gas as possible out of the lungs (i.e., to exhale to RV), energy is needed to drive the chest wall below its resting position. Depending on the stiffness of the chest wall (which increases with age) and the elasticity of the lung (which decreases with age), pleural pressure may become positive as expiratory muscles are activated or recruited. As the lungs become smaller, the transmural pressure in the small airways becomes negative, predisposing some of these airways to collapse. When the airway collapses, gas distal to the point of collapse—in other words, the gas out toward the alveoli—cannot exit. The gas is described as trapped behind the collapsing or narrowed airway. The more air that is trapped in this manner, the greater the RV. In older adults, therefore, as well as in people with disease states such as emphysema that reduce the elastic recoil of the lungs, the balance of forces does not completely determine the RV. Air trapping must also be considered.

Compliance

As one tries to inflate or distend an object such as a balloon or a ball, one has to increase the pressure inside the object relative to the pressure outside it, that is, one has to create a positive transmural pressure. To go from one volume to a second, larger volume, one has to further increase the transmural pressure. The relationship between the change in volume and the change in the pressure needed to achieve that change in volume is called the compliance of the object.

An object with a low compliance is relatively stiff; it requires a large change in pressure to achieve a given change in volume. It is important to remember that the pressure that leads to a given volume is the pressure across the wall, the transmural pressure, of the object, and not the pressure within the object, to which one may refer as the intracavitary pressure (Animated Figure 3-5).

Use Animated Figure 3-5 to vary the transmural pressure for a floppy balloon, a normal balloon, and a stiff balloon and observe the resulting changes in volume. For a given change in transmural pressure, which of these balloons do you think undergoes the greatest change in volume? That balloon (the thin, floppy one) has the highest compliance of the three.

Because the pressure outside the object may be 0 (by convention, atmospheric pressure is taken as 0), the transmural pressure and the intracavitary pressure may be the same. In the context of the respiratory system, the pressure "outside" the lung is the pleural pressure which, under normal conditions, is approximately -3 to -5 cm H₂O at FRC. In individuals with obesity or those with respiratory failure who are being sustained on mechanical ventilators, however, the pleural pressure may be positive and quite different from 0. In these circumstances, it is critical to remember that changes in lung volume relate to changes in transmural pressure. Failure to determine pleural pressure as part of the calculation of transmural pressure in these circumstances can lead to errors in assessment of the patient's respiratory status. (Note: the concept of transmural pressure as a distending pressure is also critical in cardiovascular physiology when one assesses cardiac function in patients in whom pleural pressure is not close to 0.)

When one inflates the lungs, the change in transmural pressure needed to attain a given change in volume depends on the compliance of the respiratory system. Many disease states can affect the compliance of the lungs. Emphysema, for example, destroys lung tissue and reduces elastic recoil of the lung, leading to an increase in compliance. In contrast, some inflammatory conditions lead to scarring or fibrosis with deposition of stiff connective tissue in the lung and a decreased compliance (Fig. 3-6).

Diseases of the chest wall may also alter compliance. Curvature of the spine, or scoliosis, for example, as well as obesity (remember, the abdomen is the inferior border of the chest wall), are often associated with a decrease in chest wall compliance.

Lung Volumes: A Closer Look ↑

In Figure 3-2 and Animated Figure 3-2 we examined the pressure-volume characteristics of the lungs, chest wall, and respiratory system. Consider now the forces that are balanced to determine TLC, FRC, and RV. Let us start at FRC and ask an individual to breathe into a water spirometer, a device that collects expired gas under a drum that is partially suspended under water and transforms the motion of the drum as the patient inhales and exhales into a graphical representation of the change in lung volume as a function of time (Animated Fig. 3-7). You instruct the person, at the end of a relaxed exhalation, to take a deep breath in until she cannot get any more air into her lungs. The volume of the respiratory system at this point is the TLC and is determined by the balance of the forces exerted by the chest wall and lungs inward and the force of the inspiratory muscles to expand the system. The amount of air inhaled from FRC to TLC is termed the inspiratory capacity (not labeled in the figure).

Next you instruct the patient to exhale until she cannot get any more air out of her lungs. The volume of the respiratory system at this point is the RV and is determined by the balance of the force exerted by the chest wall to spring out and the force of the lungs and the expiratory muscles to make the system smaller (remember that collapse of small airways and the resulting air trapping may also play a small role in determining RV in the normal adult). The amount of air exhaled from TLC to RV is termed the vital capacity (VC). The difference in volume between FRC and RV is termed the expiratory reserve volume (ERV) (not labeled in the figure). The tidal volume (usually abbreviated as V_T or just VT) is the amount of air inhaled during a normal breath.

Use Animated Figure 3-7 along with Table 3-1 to understand the divisions of lung volume. Animated Figure 3-7 initially shows a person performing normal relaxed breathing into a water spirometer. The height of the waveform tracing represents tidal volume. The subject is then instructed to breathe in as fully as possible and then breathe out as fully as possible (the sequence discussed previously). The height of the resulting waveform tracing represents the vital capacity. Also note how the lung and chest wall force vectors vary over the breathing cycle. Based on your prior knowledge, what is the relationship between the forces exerted by the lungs and chest wall at FRC? If this question seems difficult, you may find it helpful to revisit Animated Figure 3-2.

When air is exhaled with as much force as the person can generate from TLC to RV, we term the volume the forced vital capacity, or FVC. During an FVC maneuver, we can also measure the amount of gas that is exhaled in the first second of the FVC. This volume is termed the forced expiratory volume in one second (FEV₁). Individuals with obstructive lung diseases such as asthma, emphysema, and chronic bronchitis typically have abnormally low FEV₁, because of high airway resistance and, in the case of emphysema, a reduced lung elastic recoil that results in abnormally low force generation during exhalation. The ratio of the FEV₁ to the FVC, which should be greater than 0.7 in most healthy people, is one of the measures used to diagnose airways disease.

Knowledge of the balance of forces that determine lung volumes is critical to your understanding of the patterns of abnormality seen with different disease states.