Mechanics of Breathing

Ventilation is the bulk flow of air between the atmosphere and the alveoli, and like all bulk flow of a fluid, it obeys one rule: gas moves from a region of higher pressure to a region of lower pressure. The lungs themselves have no skeletal muscle in their walls and cannot actively inflate or deflate. Instead, the respiratory system produces airflow indirectly, by using the intercostal muscles and diaphragm to change the volume of the thoracic cavity. Because the lungs are elastic and stuck to the inside of the thoracic cage via the pleural linings, whatever volume change the cage undergoes, the lungs are forced to follow — and a volume change in a gas-filled space instantly produces a pressure change, according to Boyle's Law: at constant temperature, pressure and volume of a fixed amount of gas are inversely related (P₁V₁ = P₂V₂). Three pressures matter in this system, and confusing them is the single most common source of error for students. Atmospheric pressure (P_atm) is the pressure of the air outside the body, taken as the reference zero (760 mmHg absolute, or 0 relative). Alveolar pressure (P_alv, also called intrapulmonary pressure) is the pressure inside the alveoli, and it rises above and falls below atmospheric pressure during breathing to drive airflow. Intrapleural pressure (P_ip, also called intrathoracic pressure) is the pressure inside the thin, fluid-filled pleural cavity between the visceral pleura (coating the lung) and parietal pleura (lining the chest wall); this pressure is always sub-atmospheric during normal breathing, roughly −4 mmHg at rest and dropping further during inspiration. The mechanical relationship that ties everything together is the transpulmonary pressure (P_tp = P_alv − P_ip), the pressure difference across the lung wall that keeps the lung inflated against its own tendency to collapse. As long as transpulmonary pressure stays positive, the lung remains open. This single concept — that the lung is a passive elastic balloon held open only by a pressure difference, never by direct muscular action — is the foundation for everything else in respiratory mechanics, and it connects directly to ideas covered under Lung Volumes & Compliance and Respiratory System Anatomy.

Step 1 — Resting state before a breath. Between breaths (at the end of a normal expiration, called the resting or functional residual capacity point), the inward elastic recoil of the lungs is exactly balanced by the outward elastic recoil of the chest wall. No air is flowing because alveolar pressure equals atmospheric pressure (P_alv = 0 relative). Intrapleural pressure at this point is about −4 mmHg, sub-atmospheric because the lung's inward pull and the chest wall's outward pull are literally pulling the two pleural membranes apart, and the thin layer of pleural fluid between them resists separation (like two wet glass slides), generating suction.

Step 2 — Inspiration begins with active muscle contraction. The diaphragm, a dome-shaped sheet of skeletal muscle innervated by the phrenic nerves (spinal roots C3–C5), contracts and flattens, moving downward into the abdominal cavity by roughly 1–10 cm depending on effort. Simultaneously the external intercostal muscles contract, lifting the rib cage up and outward (the "bucket-handle" and "pump-handle" movements of the ribs). Together these actions increase the volume of the thoracic cavity.

Step 3 — Volume change is transmitted to the lung. Because the pleural space is a sealed, fluid-coupled cavity, the expanding chest wall pulls the parietal pleura outward, and via the thin fluid film, drags the visceral pleura (and hence the lung surface) along with it. Intrapleural pressure falls further, from about −4 mmHg to about −8 mmHg. Since transpulmonary pressure (P_alv − P_ip) must increase as P_ip drops, the lungs are forced to expand along with the chest wall.

Step 4 — Boyle's Law produces the pressure gradient that drives flow. As the alveoli expand, the same quantity of gas is now distributed through a larger volume, so alveolar pressure drops below atmospheric pressure — typically to about −1 mmHg. Air at the mouth (0 mmHg, atmospheric) is now at higher pressure than air in the alveoli, so air flows down this small gradient into the lungs until P_alv returns to 0 and flow stops. A full quiet breath moves about 500 mL of air (the tidal volume) using a pressure gradient of only about 1 mmHg — a remarkably efficient system.

Step 5 — Expiration during quiet breathing is passive. The diaphragm and external intercostals simply relax. The chest wall and stretched lungs recoil elastically back toward their resting volume, driven by the same elastic forces described in Step 1 plus the aid of surfactant-modulated surface tension in the alveoli (surfactant reduces the work needed here, foreshadowed in Lung Volumes & Compliance). As thoracic volume decreases, alveolar pressure rises above atmospheric pressure (to about +1 mmHg), and air flows out down this gradient until P_alv again equals 0.

Step 6 — Forced breathing recruits additional muscles in both directions. During exercise, exertion, or respiratory distress, inspiration is deepened by contraction of the sternocleidomastoid muscles (elevating the sternum) and the scalene muscles (elevating the first two ribs), on top of a more forceful diaphragm and external intercostal contraction. Forced expiration becomes an active process, recruiting the internal intercostal muscles (pulling the ribs down and in) and the abdominal muscles (rectus abdominis, internal/external obliques, transversus abdominis), which contract to raise intra-abdominal pressure and push the diaphragm upward, compressing the thoracic cavity well below its resting volume.

Step 7 — Airway resistance shapes the whole process. Airflow through the tracheobronchial tree also depends on resistance, governed in simplified form by an equation analogous to Ohm's Law: Flow = ΔP / Resistance, and resistance is exquisitely sensitive to airway radius (resistance ∝ 1/r⁴ following Poiseuille-type relationships for laminar flow). This is why bronchoconstriction in asthma, or airway collapse in emphysema, dramatically increases the work of breathing even when the pressure-generating machinery (muscles, chest wall) is completely normal.

Establishing that airflow follows a pressure gradient, not muscular "suction" on the lung. Nineteenth-century physiologists debated whether the lungs were pulled open by direct attachment to the chest wall or by some other mechanism. The decisive demonstration came from experiments on the intact chest wall versus the opened chest wall. If the thoracic cavity is punctured — allowing atmospheric air to enter the pleural space — the lung on that side immediately collapses to a small fraction of its inflated volume, even though the diaphragm and rib cage continue to move normally. This is the physiological basis of traumatic pneumothorax, and it was documented clinically and reproduced experimentally (classic vivisection and later controlled animal studies through the early-to-mid 20th century) by inserting a needle or cannula into the pleural space of an anesthetized animal and measuring pressure directly with a manometer. The result: intrapleural pressure, measured to be sub-atmospheric (roughly −4 to −8 mmHg) in the intact chest, rises abruptly to atmospheric pressure the instant the pleural space is opened to air, and the lung collapses inward due to unopposed elastic recoil while the chest wall springs outward due to its own unopposed recoil. This single experiment proves three things simultaneously: that the lung has intrinsic elastic recoil tending to collapse it, that the chest wall has an opposing elastic recoil tending to spring it open, and that only a sealed, sub-atmospheric pleural space keeps these two opposing tendencies mechanically coupled so that lung volume tracks chest wall volume.

Direct manometric measurement of pleural pressure. Because directly cannulating the pleural space is invasive, physiologists including Julius H. Comroe and colleagues in the mid-20th century validated an indirect method: an esophageal balloon catheter. A thin balloon is passed through the nose into the esophagus, which runs through the thoracic cavity and is exposed to the same pressure changes as the surrounding pleural space (the esophagus is flexible and its wall pressure equilibrates with intrathoracic pressure). Pressure recorded by the esophageal balloon tracks pleural pressure recorded by direct pleural cannulation almost exactly, confirming the method and giving physiologists a safe, repeatable way to measure human intrapleural pressure during breathing, exercise, and disease. This technique, still used in pulmonary function laboratories today to measure lung compliance, directly confirmed the numeric values quoted above (about −4 mmHg at rest, falling to about −8 mmHg or lower during forceful inspiration).

Demonstrating the diaphragm's role via nerve section and electromyography. Selective section or block of the phrenic nerve in experimental animals (and clinical observation of patients with phrenic nerve injury or diaphragmatic paralysis, for instance from high cervical spinal cord injury) abolishes diaphragmatic contraction while leaving intercostal muscle function intact. Under these conditions, tidal volume drops substantially (the diaphragm normally contributes roughly 70–80% of the volume change during quiet resting breathing), and paradoxical inward movement of the paralyzed hemidiaphragm during inspiration can be observed by fluoroscopy — direct proof, from a loss-of-function experiment, that diaphragmatic descent (not just rib elevation) is the dominant driver of normal tidal breathing. Electromyographic recordings from the diaphragm and intercostal muscles, pioneered in respiratory physiology labs through the mid-20th century, further confirmed the precise temporal sequence: diaphragmatic and external intercostal electrical activity rises just before inspiratory airflow begins, and falls to baseline during quiet expiration, exactly as the passive-recoil model predicts.

Example 1 — Calculating airflow direction and pressure gradient during a forced inspiration. A patient with reduced lung compliance (pulmonary fibrosis) must generate a larger pressure change than normal to move the same tidal volume. Suppose at rest their intrapleural pressure is −5 mmHg, and during a forced inspiratory effort it falls to −12 mmHg (compared to a healthy person's typical −8 mmHg during a similar effort). Given: normal transpulmonary pressure at rest ≈ P_alv (0) − P_ip (−5) = 5 mmHg. During forced inspiration in this patient: transpulmonary pressure = 0 − (−12) = 12 mmHg, more than double the healthy resting value, needed just to achieve a normal tidal volume because the fibrotic lung resists stretching (low compliance means a larger pressure change is required per unit volume change). Interpretation: this patient's inspiratory muscles must do substantially more mechanical work per breath than a healthy person's, even though the pressure-gradient principle driving airflow (air flows from atmosphere, 0 mmHg, toward the lower alveolar pressure) is completely unchanged. This is exactly why patients with restrictive lung disease report dyspnea (breathlessness) on exertion long before their alveolar gas exchange itself is measurably impaired — the mechanical cost of breathing, not the gas exchange, is often the limiting factor early in disease. This calculation foreshadows the compliance curves discussed in Lung Volumes & Compliance.

Example 2 — Diagnosing a tension pneumothorax from clinical signs, reasoned through the mechanics. A 24-year-old man arrives after a motorcycle accident with a fractured rib, severe shortness of breath, and a trachea that on physical exam is deviated away from the injured (right) side. Blood pressure is falling. Reasoning through the mechanism: the fractured rib has torn the visceral pleura, creating a one-way valve that lets air enter the right pleural space during inspiration but not escape during expiration. Each breath adds more air to the right pleural space, so intrapleural pressure on that side rises from its normal −4 mmHg toward, and eventually above, atmospheric pressure (0 mmHg) — abolishing the transpulmonary pressure gradient and collapsing the right lung, exactly as in the classic pneumothorax experiments described above. Unlike a simple pneumothorax, the trapped air continues to accumulate because of the one-way valve, so pressure keeps rising and begins to push the entire mediastinum (heart, great vessels, trachea) toward the opposite side — explaining the tracheal deviation. The rising intrathoracic pressure also compresses the vena cava, reducing venous return to the heart, which explains the falling blood pressure. Prediction and action: this is a life-threatening tension pneumothorax, and the correct emergency intervention is immediate needle decompression (inserting a large-bore needle into the pleural space) to let the trapped air escape and re-establish sub-atmospheric pleural pressure, followed by chest tube placement — a direct clinical application of the pressure principles established by the classic pneumothorax experiments.

Much of the quantitative, instrumented understanding of respiratory mechanics — pressure-volume relationships, the esophageal balloon technique, and the mechanical work of breathing — was established through the mid-20th-century work of American physiologist Julius H. Comroe Jr., who founded the Cardiovascular Research Institute at the University of California, San Francisco, and became one of the principal architects of modern respiratory physiology as a rigorous, quantitative discipline built on direct pressure measurement rather than qualitative description.

Understanding the pressure mechanics of breathing underlies nearly every tool and intervention in respiratory and critical-care medicine. Mechanical ventilators used in intensive care units and during surgery work by directly manipulating alveolar and airway pressure (positive-pressure ventilation pushes air in, the reverse of natural negative-pressure breathing), and clinicians must understand transpulmonary pressure to avoid barotrauma (lung injury from excessive pressure) in ventilated patients, especially those with acute respiratory distress syndrome (ARDS). Chest tube and needle decompression protocols for pneumothorax and tension pneumothorax, used in emergency medicine and trauma surgery worldwide, are a direct clinical translation of the pleural-pressure experiments described above. Spirometry and pulmonary function testing, used to diagnose and monitor asthma, COPD, and pulmonary fibrosis, measure exactly the pressure-driven airflows and volumes this mechanism produces, and compliance measurements using esophageal balloon catheters remain a research and clinical tool for characterizing restrictive lung disease. In anesthesiology, understanding that opening the chest (as in thoracic surgery) collapses the ipsilateral lung by abolishing pleural sub-atmospheric pressure is essential for planning single-lung ventilation techniques. In sports and exercise physiology, the recruitment of accessory inspiratory and expiratory muscles during exertion — predictable directly from this mechanism — explains training adaptations in respiratory muscle endurance seen in elite athletes and in pulmonary rehabilitation programs for patients with chronic lung disease. Even high-altitude and aerospace physiology depend on this framework, since reduced atmospheric pressure at altitude changes the absolute pressure gradients (though not the underlying mechanics) driving ventilation, directly relevant to Control of Ventilation and to acclimatization strategies for mountaineers and pilots.

Ask the tutor: How does surfactant change the pressure-volume relationship of the lung, and why does its absence in premature infants cause respiratory distress?