15.2 Mechanics of Breathing, Pressures & Lung Volumes

15.2 Mechanics of Breathing, Pressures & Lung Volumes

Pulmonary ventilation, commonly referred to as breathing, is the cyclical mechanical process by which atmospheric air is inhaled into the alveoli and waste gases are exhaled into the environment. Ventilation consists of two discrete phases: inspiration (inhalation), during which atmospheric air flows into the pulmonary airways, and expiration (exhalation), during which gases exit the lungs. This continuous physical movement is not driven by cellular pumps or active molecular transport; rather, it is dictated purely by the laws of physics—specifically, alternating bulk flow down hydrostatic pressure gradients established by rhythmic alterations in thoracic cavity volume.

---

The Biophysical Foundation: Boyle's Law & Pressure Gradients

Bulk air movement into and out of the lungs is governed directly by Boyle's Law, an ideal gas law that describes the behavior of a gas contained within a closed space:

$P_1V_1 = P_2V_2$

Boyle's law dictates that at a constant temperature, the pressure exerted by a gas in a closed container is inversely proportional to the volume of that container.

• If the volume of a container increases, the gas molecules within it disperse across a larger space, reducing their collision frequency against the container walls; consequently, internal gas pressure drops.
• Conversely, if the volume of the container decreases, the gas molecules are compressed into a smaller space, increasing collision frequency; consequently, internal gas pressure rises.

In the human body, the thoracic cage and lungs serve as the variable-volume container. Contraction and relaxation of respiratory muscles cyclically expand and compress thoracic volume. These volume changes produce corresponding pressure differentials between the interior of the lungs and the outside atmosphere. Gases always flow down a pressure gradient from an area of higher pressure to an area of lower pressure until equilibrium is reached.

---

Respiratory Pressures & Transpulmonary Pressure

All respiratory pressures are expressed relative to Atmospheric Pressure ($P_{atm}$), which is the pressure exerted by ambient air surrounding the body:

• At sea level, $P_{atm} = 760\text{ mmHg}$ ($1\text{ atm}$).
• In respiratory physiology, $P_{atm}$ is designated as the zero reference point ($0\text{ mmHg}$). Therefore, a "negative pressure" (e.g., $-1\text{ mmHg}$) represents a pressure below $760\text{ mmHg}$ ($759\text{ mmHg}$), whereas a "positive pressure" (e.g., $+1\text{ mmHg}$) denotes a pressure above $760\text{ mmHg}$ ($761\text{ mmHg}$).

Dynamic Pressure Fluctuations During Quiet Breathing

     Phase         Intrapulmonary (P_pul)     Intrapleural (P_ip)     Transpulmonary (P_tp)
────────────────────────────────────────────────────────────────────────────────────────
End-Expiration         0 mmHg (760 mmHg)       -4 mmHg (756 mmHg)      +4 mmHg [Resting]
Inspiration (Mid)     -1 mmHg (759 mmHg)       -6 mmHg (754 mmHg)      +5 mmHg [Expanding]
End-Inspiration        0 mmHg (760 mmHg)       -6 mmHg (754 mmHg)      +6 mmHg [Peak Volume]
Expiration (Mid)      +1 mmHg (761 mmHg)       -4 mmHg (756 mmHg)      +5 mmHg [Recoiling]

1. Intrapulmonary (Intra-alveolar) Pressure ($P_{pul}$)

Intrapulmonary pressure is the pressure exerted by air within the alveoli of the lungs. As thoracic dimensions change, $P_{pul}$ rises and falls relative to atmospheric pressure. However, because the respiratory tree communicates freely with the outside atmosphere, $P_{pul}$ always equalizes with $P_{atm}$ ($0\text{ mmHg}$) between breaths (at end-inspiration and end-expiration).

2. Intrapleural Pressure ($P_{ip}$)

Intrapleural pressure is the hydrostatic pressure existing within the sealed pleural cavity between the visceral and parietal pleurae. During normal, quiet, healthy breathing, intrapleural pressure is ALWAYS NEGATIVE (subatmospheric) relative to both $P_{pul}$ and $P_{atm}$. At rest, $P_{ip}$ measures approximately $-4\text{ mmHg}$ (756 mmHg); during deep inspiration, it drops further to $-6\text{ to } -8\text{ mmHg}$ (752 to 754 mmHg).

The Opposing Vector Forces That Generate Negative $P_{ip}$: Negative intrapleural pressure is maintained by the dynamic equilibrium between two opposing sets of physical forces that pull the two pleural membranes in opposite directions:

1. Inward Forces Promoting Lung Collapse:
   • Elastic Recoil of the Lungs: The pulmonary parenchyma is densely interwoven with elastic connective tissue fibers that are continuously stretched during inflation. These fibers possess an innate physical tendency to snap back and recoil inward toward the hilum.
   • Surface Tension of Alveolar Fluid: The thin film of aqueous fluid lining the luminal surface of the alveoli exerts an inward surface tension that continually acts to collapse the alveoli to their smallest possible diameter.
2. Outward Force Expanding the Thoracic Wall:
   • Elasticity of the Thoracic Cage: The ribs and intercostal cartilages naturally spring outward, pulling the parietal pleura laterally and away from the lung surface.

Because the parietal and visceral pleurae are bonded together by the strong cohesive surface tension of the intervening pleural fluid, the lungs are physically held against the chest wall. However, the opposing inward and outward mechanical pulls act like two suction cups being pulled apart: they create a slight vacuum within the pleural space, establishing a persistent negative pressure of $-4\text{ mmHg}$.

3. Transpulmonary Pressure ($P_{tp}$)

Transpulmonary pressure is the pressure differential across the lung wall, calculated as intrapulmonary pressure minus intrapleural pressure:

$P_{tp} = P_{pul} - P_{ip}$ $\text{At resting end-expiration: } P_{tp} = 0\text{ mmHg} - (-4\text{ mmHg}) = +4\text{ mmHg}$ $\text{At peak inspiration: } P_{tp} = 0\text{ mmHg} - (-6\text{ mmHg}) = +6\text{ mmHg}$

Transpulmonary pressure is the distending pressure that keeps the airways and alveoli patent. The greater the transpulmonary pressure, the larger the lung volume.

• Atelectasis & Pneumothorax: If a penetrating stab wound punctures the parietal pleura, or if an emphysematous bulla ruptures the visceral pleura, atmospheric air rushes down its pressure gradient into the negative pleural space. Intrapleural pressure rises until it equalizes with atmospheric pressure ($P_{ip} \rightarrow 0\text{ mmHg}$). Consequently, transpulmonary pressure plummets to zero ($P_{tp} = 0 - 0 = 0\text{ mmHg}$). Stripped of its outward distending force, the lung's unopposed inward elastic recoil causes immediate, total collapse (atelectasis).

---

Mechanics of Inspiration: Quiet vs. Forced

Muscular and Pressure Sequence of Quiet Inspiration

[Brainstem fires Action Potentials]
               │
     Phrenic & Intercostal Nerves
               │
               ▼
1. Diaphragm contracts (flattens inferiorly)
2. External Intercostals contract (elevate ribs/sternum)
               │
               ▼
[Thoracic Cavity Volume Increases (~500 mL)]
               │
               ▼
[Parietal Pleura pulls Visceral Pleura Outward]
               │
               ▼
[Intrapulmonary Volume Increases]
               │
               ▼
[Intrapulmonary Pressure Drops to -1 mmHg (759 mmHg)]  <-- Boyle's Law!
               │
               ▼
[Air rushes into Lungs down gradient until P_pul = P_atm]

Quiet Inspiration (Eupnea)

Quiet inspiration is an active process requiring muscular work and metabolic energy (ATP). It involves two primary muscle groups:

1. The Diaphragm: The primary inspiratory muscle, accounting for approximately 75% of total air movement during quiet breathing. The diaphragm is a dome-shaped sheet of skeletal muscle that partitions the thoracic and abdominal cavities, innervated by the paired phrenic nerves (cervical spinal roots C3, C4, C5). Upon contraction, the diaphragm flattens inferiorly toward the abdominal cavity by approximately 1 to 2 cm. This downward excursion expands the superior-to-inferior (vertical) dimension of the thoracic cavity.
2. External Intercostal Muscles: Situated between adjacent ribs, innervated by somatic intercostal nerves (T1-T11). When they contract, they pull the ribs superiorly and anteriorly, pushing the sternum forward. This movement mimics a "pump handle" (elevating the sternum and expanding the anteroposterior diameter) and a "bucket handle" (lifting the curved ribs upward and outward, expanding the lateral transverse diameter).

The Biophysical Cascade: As the thoracic cavity expands, its volume increases by approximately 500 mL. The cohesive attraction of pleural fluid pulls the visceral pleura along with the parietal pleura, expanding pulmonary parenchymal volume. In accordance with Boyle's law, this intrapulmonary volume expansion causes intrapulmonary pressure to drop approximately $1\text{ mmHg}$ below atmospheric pressure ($760 \rightarrow 759\text{ mmHg}$, or $-1\text{ mmHg}$). Because the atmosphere is at $760\text{ mmHg}$, air rushes into the airways down this pressure gradient until intrapulmonary pressure equalizes with atmospheric pressure ($0\text{ mmHg}$). At this point, approximately 500 mL of fresh air (the Tidal Volume) has entered the lungs.

Deep or Forced Inspiration

During strenuous exercise, emotional stress, or chronic airway obstruction, the body demands significantly larger tidal volumes. To expand thoracic cavity volume further, the body recruits accessory muscles of inspiration:

• Scalenes: Elevate the first two ribs.
• Sternocleidomastoid (SCM): Forcefully lifts the sternum.
• Pectoralis Minor: Elevates ribs 3 through 5.
• Erector Spinae: Straightens the thoracic spine, maximizing thoracic diameter.

These accessory contractions drop intrapulmonary pressure to $-2\text{ to } -4\text{ mmHg}$ (or even lower), pulling up to 2,000 to 3,000 mL of additional air into the pulmonary tree.

---

Mechanics of Expiration: Quiet vs. Forced

Quiet Expiration

In healthy individuals, quiet expiration is an entirely passive process that requires no active muscular contractions and zero ATP expenditure. Instead, it depends completely on the natural elastic recoil of stretched pulmonary tissues and the surface tension of the alveoli.

The Biophysical Sequence:

1. The inspiratory drive terminates; motor nerve impulses to the diaphragm and external intercostals cease.
2. The diaphragm relaxes, springing passively upward into its resting dome-shaped position. The external intercostal muscles relax, allowing the ribs and sternum to descend under the influence of gravity.
3. The stretched elastic fibers of the pulmonary parenchyma snap back inward, and alveolar surface tension draws the alveoli toward a smaller radius.
4. Consequently, thoracic cavity and intrapulmonary volumes decrease.
5. According to Boyle's law, this compression of pulmonary gas forces intrapulmonary pressure to rise $1\text{ mmHg}$ above atmospheric pressure ($760 \rightarrow 761\text{ mmHg}$, or $+1\text{ mmHg}$).
6. Because alveolar pressure exceeds ambient atmospheric pressure, air flows rapidly out of the lungs down the pressure gradient until intrapulmonary pressure once again equalizes with atmospheric pressure ($0\text{ mmHg}$). The 500 mL tidal volume is evacuated.

Forced or Active Expiration

Forced expiration is an active, energy-consuming process that occurs during vigorous physical exercise, coughing, sneezing, balloon blowing, or in patients suffering from obstructive pulmonary diseases like asthma or chronic obstructive pulmonary disease (COPD).

Forced expiration recruits two major muscle groups:

1. Internal Intercostal Muscles: Situated deep to the external intercostals with fibers running obliquely downward and backward. When they contract, they forcefully pull the ribs inferiorly and posteriorly, compressing the thoracic cage.
2. Abdominal Wall Muscles: Including the rectus abdominis, external and internal obliques, and transversus abdominis. Powerful contraction of the anterior abdominal wall dramatically increases intra-abdominal pressure, forcing the abdominal viscera (liver, stomach, intestines) superiorly against the inferior surface of the relaxed diaphragm. This drives the diaphragm deep into the thoracic cavity.

Together, these active contractions decrease thoracic volume precipitously, driving intrapulmonary pressure up to $+20\text{ to } +30\text{ mmHg}$ (and exceeding $+100\text{ mmHg}$ during an explosive cough), forcefully blowing air out of the lungs.

Comprehensive Comparison of Breathing Mechanics

---

Physical Factors Influencing Pulmonary Ventilation

Three physical factors determine the efficiency of pulmonary ventilation and the mechanical work required to breathe:

1. Airway Resistance ($F = \Delta P / R$)

Airflow ($F$) through the respiratory passages is directly proportional to the pressure gradient ($\Delta P = P_{atm} - P_{pul}$) and inversely proportional to airway resistance ($R$):

$F = \frac{\Delta P}{R}$

In a healthy respiratory system, resistance is remarkably low because conducting conduits (trachea, bronchi) are wide. The greatest physical resistance occurs in the medium-sized bronchi, not the terminal bronchioles (because millions of tiny bronchioles branch in parallel, creating an enormous cumulative cross-sectional area). Resistance is governed by Poiseuille's law, where resistance is inversely proportional to the fourth power of the airway radius ($R \propto 1/r^4$). A 50% reduction in bronchiole caliber increases resistance 16-fold!

• In acute asthma attacks, inflammatory mucosal edema and violent smooth muscle bronchospasm dramatically narrow bronchiolar radii, sky-rocketing resistance and requiring immense muscular work to inhale and exhale.

2. Alveolar Surface Tension & Surfactant

Water molecules lining the microscopic alveoli are strongly polar, forming hydrogen bonds with one another. This produces an inward surface tension that draws the alveolar walls together. If uncorrected, surface tension would cause alveoli to collapse into tiny droplets, demanding immense inspiratory pressures to reinflate. As detailed in Section 15.1, pulmonary surfactant secreted by Type II alveolar cells disrupts this intermolecular cohesion, dramatically lowering surface tension, preventing atelectasis, and reducing the work of breathing.

3. Lung Compliance (Distensibility)

Lung compliance is a measure of the distensibility or "stretchability" of the lungs and chest wall—specifically, the volume change ($\Delta V$) resulting from a given change in transpulmonary pressure ($\Delta P_{tp}$):

$\text{Compliance} = \frac{\Delta V}{\Delta P_{tp}}$

Healthy lungs exhibit high compliance, expanding easily with small pressure shifts. Compliance is determined primarily by lung tissue elasticity and adequate pulmonary surfactant.

• Decreased Compliance (Stiff Lungs): Seen in pulmonary fibrosis (replacement of delicate elastin with non-yielding collagenous scar tissue), severe pulmonary edema, or infant respiratory distress syndrome. Patients must expend massive muscular energy simply to expand the stiff lungs, leading to rapid, shallow breathing.
• Increased Compliance with Lost Elastic Recoil: Seen in pulmonary emphysema, where cigarette-induced elastase enzymes destroy alveolar septa and elastin fibers. Although the lungs stretch easily during inspiration, the loss of elastic recoil impairs passive expiration. Patients suffer severe air trapping, dynamic airway collapse, and develop a barrel-chested appearance.

---

Respiratory Volumes and Capacities (Spirometry)

Pulmonary ventilation is assessed clinically using a spirometer, a diagnostic instrument that records the volume of air inhaled and exhaled over time. The tracing obtained is a spirogram.

Spirometry Volumes and Capacities Breakdown

  Volume / Capacity          Male Adult Normal    Female Adult Normal
┌───────────────────────────┬────────────────────┬────────────────────┐
│ Tidal Volume (TV)         │      500 mL        │      500 mL        │
│ Inspiratory Reserve (IRV) │    3,100 mL        │    1,900 mL        │
│ Expiratory Reserve (ERV)  │    1,200 mL        │      700 mL        │
│ Residual Volume (RV)      │    1,200 mL        │    1,100 mL        │
├───────────────────────────┼────────────────────┼────────────────────┤
│ Inspiratory Capacity (IC) │    3,600 mL (TV+IRV)│   2,400 mL (TV+IRV)│
│ Functional Residual (FRC) │    2,400 mL (ERV+RV)│  1,800 mL (ERV+RV) │
│ Vital Capacity (VC)       │ 4,800 mL (TV+IRV+ERV)│3,100 mL (TV+IRV+ERV)│
│ Total Lung Capacity (TLC) │    6,000 mL (VC+RV)│    4,200 mL (VC+RV)│
└───────────────────────────┴────────────────────┴────────────────────┘

The Four Primary Respiratory Volumes (Non-Overlapping)

1. Tidal Volume (TV): The volume of air inspired or expired with each normal, resting quiet breath. In healthy adult males and females, resting TV averages approximately 500 mL.
2. Inspiratory Reserve Volume (IRV): The maximal volume of air that can be forcefully inspired above and beyond a normal quiet tidal inhalation. It averages approximately 3,100 mL in adult males and 1,900 mL in adult females.
3. Expiratory Reserve Volume (ERV): The maximal volume of air that can be forcefully exhaled following a normal, resting tidal exhalation. It averages approximately 1,200 mL in adult males and 700 mL in adult females.
4. Residual Volume (RV): The volume of air remaining inside the pulmonary airspaces following a maximal, forceful exhalation. It averages approximately 1,200 mL in adult males and 1,100 mL in adult females.
   • Key Exam Rule: Residual volume cannot be measured with a standard spirometer (it requires specialized gas-dilution or body plethysmography techniques). RV is essential: it prevents the alveoli from collapsing completely between forced breaths and ensures continuous gas exchange across the capillary beds even between ventilatory cycles.

The Four Respiratory Capacities (Combinations of Two or More Volumes)

1. Inspiratory Capacity (IC): The total volume of air that can be inhaled following a normal quiet tidal exhalation: $IC = TV + IRV = 500\text{ mL} + 3,100\text{ mL} = 3,600\text{ mL (Males)} \quad [2,400\text{ mL Females}]$
2. Functional Residual Capacity (FRC): The volume of air remaining in the lungs at the end of a normal, quiet tidal expiration: $FRC = ERV + RV = 1,200\text{ mL} + 1,200\text{ mL} = 2,400\text{ mL (Males)} \quad [1,800\text{ mL Females}]$
3. Vital Capacity (VC): The maximum volume of air that can be forcefully exhaled following a maximal, deep inhalation. This is the single most important diagnostic index of pulmonary ventilatory capability: $VC = TV + IRV + ERV = 500\text{ mL} + 3,100\text{ mL} + 1,200\text{ mL} = 4,800\text{ mL (Males)} \quad [3,100\text{ mL Females}]$
4. Total Lung Capacity (TLC): The total maximum volume of air contained within the lungs following a maximal, deep inspiration: $TLC = TV + IRV + ERV + RV = VC + RV = 4,800\text{ mL} + 1,200\text{ mL} = 6,000\text{ mL (Males)} \quad [4,200\text{ mL Females}]$

Comprehensive Spirometry Reference Table

---

Dead Space & Ventilation Rates: Minute Ventilation vs. AVR

Not all atmospheric air entering the respiratory tract reaches the gas-exchanging alveoli.

• Anatomical Dead Space: The volume of air contained within the conducting airways (nose, pharynx, larynx, trachea, bronchi, and terminal bronchioles). In a healthy adult, anatomical dead space averages approximately 150 mL (roughly $1\text{ mL per pound}$ of ideal body weight). This air fills structural conduits where no gas exchange occurs.
• Alveolar Dead Space: Air that reaches non-functional, damaged, or unperfused alveoli (e.g., in pulmonary embolism or alveolar collapse).
• Physiological (Total) Dead Space: The sum of anatomical dead space and alveolar dead space. In healthy individuals, alveolar dead space is negligible, so physiological dead space equals anatomical dead space.

1. Minute Ventilation (Total Pulmonary Ventilation)

Minute ventilation ($V_E$) is the total volume of air inspired or expired per minute:

$\text{Minute Ventilation} = \text{Tidal Volume (TV)} \times \text{Respiratory Rate (RR)}$ $\text{At rest: } 500\text{ mL} \times 12\text{ breaths/min} = 6,000\text{ mL/min} = 6.0\text{ L/min}$

2. Alveolar Ventilation Rate (AVR)

Minute ventilation is a misleading clinical index of gas exchange because it fails to account for anatomical dead space. The Alveolar Ventilation Rate (AVR) is the true measure of effective ventilation: it calculates the volume of fresh atmospheric air that actually enters the respiratory zone each minute:

$\text{AVR} = (\text{Tidal Volume} - \text{Anatomical Dead Space}) \times \text{Respiratory Rate}$ $\text{At rest: } (500\text{ mL} - 150\text{ mL}) \times 12\text{ breaths/min} = 350\text{ mL} \times 12 = 4,200\text{ mL/min} = 4.2\text{ L/min}$

Clinical Pearl: The Danger of Rapid, Shallow Breathing

Consider a patient experiencing panic or pleuritic pain who breathes with a shallow tidal volume of 200 mL at a rapid rate of 30 breaths per minute:

• Minute Ventilation: $200\text{ mL} \times 30\text{ bpm} = 6,000\text{ mL/min}$ (appears completely "normal" on paper!).
• True Alveolar Ventilation (AVR): $(200\text{ mL} - 150\text{ mL}) \times 30 = 50\text{ mL} \times 30 = 1,500\text{ mL/min}$.

Despite having an identical minute ventilation of 6.0 L/min, the patient's effective alveolar ventilation plummets by more than 60%, resulting in profound alveolar hypoventilation, hypercapnia, and hypoxemia. Conversely, slow, deep breathing (e.g., $TV = 1,000\text{ mL}, RR = 6$) produces an AVR of $(1,000 - 150) \times 6 = 5,100\text{ mL/min}$, maximizing alveolar oxygen delivery.