12.3 Pulmonary Ventilation

Pulmonary ventilation is the act of breathing, which is the movement of air into and out of the lungs. Pulmonary ventilation comprises two major steps: inspiration and expiration. Inspiration (inhalation) is the process that causes air to enter the lungs, and expiration (exhalation) is the process that causes air to leave the lungs. A respiratory cycle is one sequence of inspiration and expiration.

The main determinant of air flow into and out of the lungs is a difference in air pressure between the atmosphere (atmospheric pressure, P_atm) and inside the alveoli (intra-alveolar pressure, P_alv). The pressure within the pleural cavity, intrapleural pressure (P_ip), is another pressure important to ventilation. The difference in pressures drives pulmonary ventilation because air flows down a pressure gradient, from an area of higher pressure to an area of lower pressure. Air flows into the lungs when intra-alveolar pressure drops below atmospheric pressure. Air flows out of the lungs during expiration when intra-alveolar pressure becomes greater than the atmospheric pressure. These respiratory pressures (P_atm, P_alv, and P_ip) will be discussed in more detail later in this section.

Rhythmic contraction and relaxation of the respiratory muscles enable inspiration and expiration. In order to fully understand how these activities of the respiratory muscles lead to ventilation, it is essential to understand the relationship between the volume of an pressure of a gas at a constant temperature.

Pressure-Volume Relationships

When referring to a gas, pressure is a force created by the movement of gas molecules that are confined. Increasing the number of gas molecules in a confined space will increase the pressure. At a constant temperature, changing the volume occupied by the gas changes the pressure, as does changing the number of gas molecules. Consider Figure 12.3.1, which shows two containers of different volume, each containing 13 molecules of a gas. The force exerted by the movement of the gas molecules against the walls of the larger container on the left is lower than the force exerted by the gas molecules in the container on the right. Therefore, the pressure is lower in the larger container and higher in smaller container.

Boyle’s law describes the relationship between the volume and pressure of a gas in a confined space. Specifically, the Law states that for a given mass of a gas in a confined space at a constant temperature, the product of the pressure and volume is also constant (“k” in the equation in Figure 12.3.1). Boyle’s Law is expressed by the following formula:

P₁V₁ = P₂V₂

In this formula, P₁ represents the initial pressure and V₁ represents the initial volume, whereas the final pressure and volume are represented by P₂ and V_2, respectively. In short, pressure and volume are inversely related.

Respiratory Pressures

Pulmonary ventilation is dependent on three pressures: atmospheric, intra-alveolar, and intrapleural. Atmospheric pressure is the amount of force that is exerted by gases in the air surrounding any given surface, such as the body. Atmospheric pressure can be expressed in terms of the unit atmosphere, abbreviated atm, or in millimeters of mercury (mm Hg). One atm is equal to 760 mm Hg, which is the atmospheric pressure at sea level. Typically, for ventilation, other pressure values are discussed in relation to atmospheric pressure. Therefore, negative pressure is pressure lower than the atmospheric pressure, whereas positive pressure is pressure that it is greater than the atmospheric pressure. A pressure that is equal to the atmospheric pressure is expressed as zero.

Intra-alveolar pressure is the pressure of the air within the alveoli, which changes during the different phases of breathing (Figure 12.3.2). Because the alveoli are connected to the atmosphere via the airways, intra-alveolar pressure always equalizes with atmospheric pressure.

Intrapleural pressure (P_ip) is the pressure within the fluid-filled pleural cavity. Similar to intra-alveolar pressure, intrapleural pressure also changes during the different phases of breathing. Although it fluctuates during inspiration and expiration, intrapleural pressure is always less than atmospheric pressure and intra-alveolar pressure. P_ip remains approximately –4 mm Hg throughout the breathing cycle. P_ip results from the surface tension between the visceral and parietal pleura and the parietal pleura’s adhesion to the body wall and diaphragm. Competing forces within the thorax cause the formation of the negative intrapleural pressure. One of these forces relates to the elasticity of the lungs themselves—elastic tissue pulls the lungs inward, away from the thoracic wall. Alveolar surface tension also creates an inward pull of the lung tissue. This inward tension from the lungs is countered by opposing forces from the pleural fluid and thoracic wall: surface tension within the pleural cavity pulls the lungs outward and the natural elasticity of the chest wall opposes the inward pull of the lungs. Ultimately, the outward pull is slightly greater than the inward pull, creating the –4 mm Hg intrapleural pressure relative to the intra-alveolar pressure.

Transpulmonary pressure is the difference between the intrapleural and intra-alveolar pressures. Transpulmonary pressure is always positive in a healthy respiratory system; this is essential in allowing the lungs to stay open.

In summary, two main forces keeping the lungs open are pleural fluid surface tension and positive transpulmonary pressure.

Respiratory Muscles and Pulmonary Ventilation

Recall that the main determinant of ventilation is a difference in air pressure between the atmosphere (atmospheric pressure, P_atm) and inside the alveoli (intra-alveolar pressure, P_alv). The regular contraction and relaxation of the respiratory muscles cause this air pressure difference; specifically, P_alv changes as the respiratory muscles contract and relax.

Inspiration: Normal, Quiet Breathing

 In general, two muscle groups are used during normal, quiet inspiration: the diaphragm and the external intercostal muscles. Inspiration is active: it involves muscle contraction. When the diaphragm contracts, it moves inferiorly toward the abdominal cavity, creating a larger thoracic cavity. At the same time, contraction of the external intercostal muscles moves the ribs upward and outward, causing the rib cage to expand, which increases the volume of the thoracic cavity. Due to the surface tension of the pleural fluid, the expansion of the thoracic cavity forces the lungs to stretch and expand as well. This increase in volume leads to a decrease in intra-alveolar pressure, creating a pressure lower than atmospheric pressure. As a result, a pressure gradient is created that drives air into the lungs.

Expiration: Normal, Quiet Breathing

The process of normal, quiet expiration is passive, meaning that energy is not required to push air out of the lungs. Instead, the elasticity of the lung tissue causes the lung to recoil, as the diaphragm and intercostal muscles relax following inspiration. In turn, the thoracic cavity and lungs decrease in volume, causing an increase in intra-alveolar pressure. The intra-alveolar pressure rises above atmospheric pressure, creating a pressure gradient that causes air to leave the lungs.

Inspiration and Expiration During Forced Breathing

During forced breathing, inspiration and expiration both occur due to muscle contractions. During periods of high ventilatory need (or drive), additional muscles can contribute to expansion of the rib cage during inspiration (Figure 12.3.4a). These “accessory” muscles assist the external intercostals and include the sternocleidomastoids, the scalenes, and the pectoralis minor. All of these groups allow for a greater thoracic expansion and thus a greater lung volume. Recognizing that a patient is using these muscles to breathe is a useful clinical sign; use of these muscles during rest is highly indicative of a raised respiratory effort to cope with an underlying and probably significant problem.

During forced expiration, accessory muscles of the abdomen, including the obliques, contract, forcing abdominal organs upward against the diaphragm (Figure 12.3.4b). This helps to push the diaphragm further into the thorax, pushing more air out. In addition, accessory muscles (primarily the internal intercostals) help to compress the rib cage, which also reduces the volume of the thoracic cavity.

Resistance, Compliance, and Ventilation

Resistance

While pressure differences between the atmosphere and alveoli are the main determinants of ventilation, other factors influence the effort that must be expended to ventilate. Resistance (R) is the friction encountered by air as it passes through the airways; resistance slows the flow of gases. The greater the resistance, the greater the effort that is required to ventilate.

The diameter of the airway is the primary factor affecting resistance. A small tubular diameter forces air through a smaller space, causing more collisions of air molecules with the walls of the airways. To better understand this, consider the types of airflow that can occur in the airways. The most efficient form of flow is laminar flow; laminar flow takes the lowest pressure differential for flow to occur. In laminar flow the molecules are moving in an orderly manner, with those at the side of the tube moving a little slower due to contact with tube walls and those in the middle moving fastest (Figure 12.3.5).

When velocity increases or tube radius decreases then this organization is lost. Collisions between molecules and with the tube wall are now more frequent and movement is more chaotic, and the flow becomes turbulent (Figure 12.3.6). At this point some molecules are at times moving against the pressure gradient due to these collisions. Consequently, to generate the same amount of molecule movement (i.e., flow) from one end of the tube to another, a greater pressure differential is needed when flow becomes turbulent. Turbulent flow is more common in the large airways where velocity and airway radius are high.

 

In reality, the vast majority of the airways are branching small tubes, so we see a mixture of the two above—mostly laminar flow but some turbulence generated at the branch (or transitional) points (Figure 12.3.7)

The following formula helps to describe the relationship between airway resistance and pressure changes or differences between two areas in the respiratory tract:

F = ∆P / R

Where: F = gas flow; ∆P = pressure difference between atmosphere and alveoli; R = resistance

With diameter having such a powerful effect on airway resistance, we would expect that airway resistance would be a significant force to overcome during ventilation. However, in a person free of respiratory disease, airway resistance is insignificant. This is due to two main factors:

• In the first part of the conducting zone, airway diameters are huge relative to the low air viscosity.
• Although the airways get progressively smaller as we move through the bronchial tree, there are also progressively more airways; therefore despite the tiny diameters of bronchioles, there is an enormous number of them occurring in parallel which results in a large total cross-sectional area.

The greatest resistance to flow is actually in the medium-sized segmental bronchi (Figure 12.3.8).

There are a couple of clinically important points to make here:

1. If radius is reduced by disease, such as during inflammation of the airway wall or contraction of airway smooth muscle causing bronchoconstriction, then resistance is markedly increased, and to maintain flow, the pressure differential must be increased. Increasing this pressure differential means increasing the work of ventilation.
2. The vast total cross-sectional area of the lower airways means that a significant amount of damage can be done before symptoms arise. This “silent zone” means that a disease may be significantly established and be in a relatively late stage before the patient becomes symptomatic and goes to the physician.

Compliance

Compliance is the ability of the lungs to stretch, and therefore how easy they are to inflate. Compliance in healthy lungs is high. One factor that could make the lungs harder to inflate, i.e. less compliant, is the strong inward pull of the alveolar surface tension. However, recall that surfactant made by type II alveolar cells reduces the alveolar surface tension (see Section 12.1). In this way, surfactant production is one important factor keeping lungs compliant.

Compliance of the thoracic cavity is also important in facilitating proper ventilation. In order for inspiration to occur, the thoracic cavity must expand. The expansion of the thoracic cavity directly influences the capacity of the lungs to expand. If the tissues of the thoracic wall are not very compliant, more effort will be necessary to expand the thorax during breathing.

Respiratory volumes are volumes of air moved by or associated with the lungs at a given point in the respiratory cycle. The four major respiratory volumes—tidal, expiratory reserve volume, inspiratory reserve volume, and residual volume (Figure 12.3.9)—are described further as follows:

• Tidal volume (TV) is the amount of air that normally enters or exits the lungs during quiet breathing; average TV: 500 mL.
• Expiratory reserve volume (ERV) is the amount of air you can forcefully expire past a normal tidal expiration; average ERV: 700 to 1200 mL.
• Inspiratory reserve volume (IRV) is the amount of air you can forcefully inspire past a normal tidal inspiration; average IRV: 1,900 to 3,100 mL
• Residual volume (RV) is the air left in the lungs after a forced expiration. RV is typically 1,100 to 1,200 mL. The residual volume helps keep alveoli open after expiration.

The volumes presented above are typical volumes measured in healthy individuals. Respiratory volumes are dependent upon a variety of factors, including age, sex, and body size. In general, with the exception of tidal volume, respiratory volumes tend to be smaller in females than in males due to body size differences.

Since respiratory diseases such as asthma and COPD can greatly affect respiratory volumes, measuring these volumes can provide important clues about a person’s respiratory health.

• Total lung capacity (TLC) is the sum of all of the volumes (TV + ERV + IRV + RV); TLC represents the total amount of air a person can hold in the lungs after a forceful inhalation. TLC is about 6,000 mL for males and about 4,200 mL for females.
• Vital capacity (VC) is the sum of all of the volumes except residual volume (TV + ERV + IRV); VC is the total amount of exchangeable of air (the volume of air a person can move into or out of the lungs). VC in a healthy individual is between 3,100 mL and 4,800 mL, with females typically having lower VC values.
• Inspiratory capacity (IC) is the sum of the tidal volume and inspiratory reserve volume (TV + IRV); IC is the maximum amount of air that can be inhaled past a normal tidal expiration. IC in a healthy individual is between 2,400 mL and 3,600 mL, with females typically having lower IC values.
• Functional residual capacity (FRC) is the sum of expiratory reserve volume and residual volume (ERV + RV); FRC is the amount of air that remains in the lung after a normal tidal expiration; FRC in a healthy individual is between 1,800 mL and 2,400 mL, with females typically having lower FRC values.

Image Descriptions

Figure 12.3.1. This diagram illustrates Boyle’s Law, demonstrating the inverse relationship between pressure and volume in a gas. The image shows two cylindrical containers with movable pistons. The left cylinder has the piston positioned higher, creating a larger volume with red spheres (representing gas particles) shown with arrows indicating their random motion in various directions throughout the spacious container. Above this cylinder, a yellow text box states “As volume increases, pressure decreases” with a black arrow pointing upward to indicate the piston’s elevated position. The right cylinder shows the piston pushed down, creating a smaller volume with the same number of red spheres now more densely packed and their directional arrows showing increased collisions in the confined space. Above this cylinder, a yellow text box states “As volume decreases, pressure increases” with a black arrow pointing downward to indicate the piston’s lowered position. At the bottom of the diagram, a yellow text box contains the mathematical expressions for Boyle’s Law: “pV = k”, “p₁V₁ = p₂V₂”, and “p = k/V”, where p represents pressure, V represents volume, and k is a constant. [Return to Figure 12.3.1]

Figure 12.3.2.This anatomical diagram illustrates the pressure relationships within the respiratory system during breathing. The image shows a frontal cross-section of the thoracic cavity with the trachea at the top (depicted in light blue with cartilage rings), both lungs (shown in mauve/purple), and the diaphragm at the bottom (shown in coral). The structural layers are clearly labeled, including the parietal pleura (the outer coral-colored layer lining the chest wall), the visceral pleura (the inner light blue layer directly covering the lung surface), the pleural cavity (the thin space between the two pleural layers), the thoracic wall (the outermost coral layer containing ribs), and the lung tissue itself. At the top, an arrow labeled “Atmospheric pressure” points downward toward the trachea. The diagram includes three key pressure measurements with arrows and text boxes: “Transpulmonary pressure: 760 mm Hg – 756 mm Hg = 4 mm Hg” (indicated by a double-headed arrow spanning from the lung interior to the pleural space), “Intrapleural pressure: 756 mm Hg (-4 mm Hg)” (shown with an arrow in the pleural cavity space), and “Intra-alveolar pressure: 760 mm Hg (0 mm Hg)” (labeled at the bottom near the diaphragm). These pressure values demonstrate the pressure gradient that allows for normal breathing mechanics. [Return to Figure 12.3.2]

Figure 12.3.3. This comparative illustration shows two side-by-side diagrams of the human upper body in profile, demonstrating the mechanics of breathing during inspiration (left) and expiration (right). Both diagrams display the head, neck, rib cage, and upper torso with the respiratory passages and diaphragm clearly visible. In the inspiration diagram labeled “Inspiration” at the top, gray arrows point into the nose and down the airway (shown in light blue transitioning from the nasal cavity through the throat), indicating airflow direction. The nasal and oral cavities are depicted in coral-pink. Labels identify key components: “Thoracic cavity expands” points to the chest area, “External intercostal muscles contract” indicates the muscles between the ribs (shown in purple), and “Diaphragm contracts” points to the dome-shaped muscle at the bottom of the rib cage, which is shown flattened and moving downward. The expiration diagram labeled “Expiration” at the top shows gray arrows pointing outward from the nose, indicating air leaving the body. A dashed line on the chest shows the reduced thoracic cavity volume compared to inspiration. Labels include “Thoracic cavity reduces,” “External intercostal muscles relax,” and “Diaphragm relaxes,” with the diaphragm shown returning to its relaxed, dome-shaped position moving upward. The rib cage (shown in beige) is visible in both diagrams, demonstrating how it expands during inspiration and contracts during expiration as part of the breathing cycle. [Return to Figure 12.3.3]

Figure 12.3.4. This anatomical illustration compares the muscles involved in inspiration (breathing in) and expiration (breathing out) through two side-by-side images of the human rib cage and torso showing the muscular anatomy. The left side, titled “Muscles of inspiration” with a blue background, displays the anterior view of the chest and neck muscles during inhalation. The muscles are labeled in two categories: “Accessory” muscles including the Sternocleidomastoid (elevates sternum) and Scalenes Group (elevate upper ribs), with a note that Pectoralis minor is not shown; and “Principal” muscles including External intercostals, Interchondral part of internal intercostals (also elevates ribs), and the Diaphragm (dome descends, thus increasing vertical dimension of thoracic cavity; also elevates lower ribs). Blue arrows on the muscle diagram indicate the direction of muscle action during inspiration. The right side, titled “Muscles of expiration” with a pink background, shows the same anatomical view but highlights different muscles. It is divided into “Quiet breathing” which explains that expiration results from passive, elastic recoil of the lungs, rib cage and diaphragm, and “Active breathing” which labels the Internal intercostals, except interchondral part (pull ribs down) and Abdominals (pull ribs down, compress abdominal contents thus pushing diaphragm up), with a note showing Quadratus lumborum (pulls ribs down). Red arrows on this diagram indicate the direction of muscle action during expiration. Both diagrams show the rib cage structure in beige with muscle tissue in coral-red tones and connective tissue in light blue-gray. [Return to Figure 12.3.4]

Figure 12.3.5. This diagram illustrates the concept of laminar flow in fluid dynamics. At the top, the word “Laminar” appears in bold text. Below this, a series of horizontal brown layers represents fluid flowing in parallel streams, with black arrows of varying lengths pointing to the right, indicating the velocity profile across the layers – the arrows in the middle layers are longer, showing faster flow in the center compared to the edges. The bottom portion shows a U-shaped tube manometer used to measure pressure differential. The left and right chambers of the U-tube are labeled P₁ and P₂ respectively, with fluid levels at different heights. A vertical double-headed arrow labeled ΔP indicates the pressure difference between the two columns, shown by a dashed horizontal line connecting the fluid levels in the U-tube. The diagram demonstrates how pressure changes in laminar flow can be measured using this simple pressure measurement device. [Return to Figure 12.3.5]

Figure 12.3.6. This diagram illustrates the concept of turbulent flow in fluid dynamics. At the top, the word “Turbulent” appears in bold text. Below this, a horizontal brown rectangular channel contains numerous black curved arrows pointing in various chaotic directions – up, down, left, right, and in circular or swirling patterns – representing the disorderly and unpredictable nature of turbulent fluid flow. Unlike the organized parallel layers seen in laminar flow, these arrows show fluid particles moving in random, intersecting paths with eddies and vortices throughout the channel. The bottom of the diagram shows two vertical measurement points labeled P₁ on the left and P₂ on the right, indicating where pressure measurements would be taken at different points along the turbulent flow channel. [Return to Figure 12.3.6]

Figure 12.3.7. This diagram illustrates the concept of transitional flow in fluid dynamics, labeled “Transitional” at the top. The image shows a brown channel that splits into two branches forming a Y-shape. The incoming flow from the left shows parallel horizontal layers (indicated by straight black lines) representing laminar characteristics. At the branching point where the channel divides, the flow pattern becomes mixed and complex. In the upper branch extending diagonally upward to the right, black arrows show both straight directional flow and curved swirling patterns, indicating the presence of both laminar and turbulent characteristics. Similarly, the lower branch extending diagonally downward to the right also displays arrows showing straight flow along with circular eddy patterns, representing the coexistence of organized and chaotic flow. Two measurement points are labeled P₁ – one on the left side of the main channel and another at the bottom right branch. This diagram effectively demonstrates how transitional flow exhibits features of both laminar (orderly, parallel) and turbulent (chaotic, swirling) flow patterns simultaneously, particularly at points where the geometry changes such as at branch points or bifurcations. [Return to Figure 12.3.7]

Figure 12.3.8. This scatter plot displays the relationship between airway generation and resistance in the respiratory system. The x-axis shows “Airway generation” ranging from 0 to 20, while the y-axis shows “Resistance (cm water/L/sec)” ranging from 0 to 0.08. Dark red circular data points show a clear downward trend from left to right. The highest resistance values (around 0.08-0.09) occur at the earliest airway generations (0-4), labeled as “Segmental bronchi” in the upper left portion of the graph. The resistance decreases progressively through intermediate generations, with values dropping to around 0.04-0.06 at generations 6-8. The lowest resistance values (approaching 0) occur at the highest airway generations (13-17), labeled as “Terminal bronchioles” in the lower right portion of the graph. The data demonstrates that despite their smaller individual diameter, the terminal bronchioles collectively offer less resistance to airflow due to their vast number, while the larger but fewer segmental bronchi contribute more significantly to total airway resistance. [Return to Figure 12.3.8]

Figure 12.3.9. This comprehensive respiratory physiology diagram consists of two parts labeled (a) and (b). Part (a) on the left shows a spirometry trace over time, with a black curved line representing the breathing cycle plotted against volume in milliliters (mL) on the y-axis ranging from 0 to 6000 mL. The graph shows two complete breathing cycles with peaks reaching approximately 6000 mL and troughs dropping to about 1400 mL. The breathing curve is overlaid on colored horizontal bands representing different lung volumes: at the bottom is Residual volume (purple, approximately 0-1200 mL), above that is Expiratory reserve volume (green, approximately 1200-2500 mL), then Tidal volume (yellow, approximately 2500-3000 mL, shown oscillating with the breathing cycle), and at the top is Inspiratory reserve volume (peach/orange, approximately 3000-6000 mL). Part (b) on the right displays the same information as stacked bar charts showing lung capacities. From left to right, it shows three bars: the first labeled “Inspiratory capacity” (combining green inspiratory reserve volume at top with functional residual capacity in tan below), the middle bar showing “Vital capacity” (the largest combination of volumes excluding residual volume), and the rightmost bar labeled “Total lung capacity” (showing all volumes stacked together, with residual volume in purple at bottom, vital capacity in pale yellow in the middle, and additional capacity in green at top). Each capacity represents a combination of two or more lung volumes, demonstrating how these measurements relate to overall pulmonary function. [Return to Figure 12.3.9]

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