How the Lungs Work: Gas Exchange and Breathing Mechanics

The lungs perform two inseparable mechanical and chemical tasks — moving air in and out of the body and transferring oxygen into the bloodstream while removing carbon dioxide. Understanding how these processes work is foundational to interpreting pulmonary function data, recognizing early disease patterns, and understanding the treatments pulmonologists prescribe. This page covers the anatomy of ventilation, the physiology of alveolar gas exchange, the pressure mechanics that drive breathing, and the clinical boundaries where normal function ends and pathology begins.

Definition and Scope

The respiratory system, covered in detail on The Respiratory System, spans two functional domains: ventilation (the mechanical movement of air) and gas exchange (the diffusion of oxygen and carbon dioxide across the alveolar-capillary membrane). The lungs themselves contain an estimated 300 million alveoli in a healthy adult — a figure cited by the American Thoracic Society (ATS) — creating a total gas-exchange surface area of approximately 70 square meters, roughly the size of a singles tennis court.

These two domains are physiologically distinct. Ventilation failure (e.g., in COPD or neuromuscular disease) involves impaired airflow mechanics. Gas exchange failure (e.g., in pulmonary fibrosis or pulmonary embolism) involves impaired diffusion or perfusion, even when airways remain patent. Regulatory standards for lung health — including occupational exposure thresholds and disability determinations — depend on distinguishing these two failure modes, as discussed in the regulatory context for pulmonary medicine.

How It Works

Ventilation: Pressure-Driven Airflow

Breathing operates on Boyle's Law: pressure and volume are inversely related in a closed system. The diaphragm, the primary muscle of respiration, contracts and flattens during inhalation, increasing thoracic volume. This expansion decreases intrapleural pressure from approximately −5 cmH₂O at rest to −8 to −10 cmH₂O during normal tidal breathing, creating a pressure gradient that draws air into the lungs. Exhalation at rest is passive — the diaphragm relaxes, elastic recoil reduces thoracic volume, and air is expelled.

Forced exhalation recruits accessory muscles including the internal intercostals and abdominal wall muscles. The distinction between passive and active exhalation is clinically significant: diseases that destroy alveolar elasticity (emphysema, a subtype of COPD) reduce recoil, trapping air and increasing functional residual capacity.

Key lung volumes defined by the National Heart, Lung, and Blood Institute (NHLBI) and standardized in pulmonary function tests:

  1. Tidal Volume (TV) — approximately 500 mL in a resting adult; the volume moved per normal breath
  2. Inspiratory Reserve Volume (IRV) — additional air inhalable beyond a normal breath (~3,000 mL)
  3. Expiratory Reserve Volume (ERV) — additional air exhaled beyond passive expiration (~1,200 mL)
  4. Residual Volume (RV) — air remaining after maximal exhalation (~1,200 mL); cannot be measured by spirometry alone
  5. Total Lung Capacity (TLC) — sum of all volumes; typically 6,000 mL in a healthy adult male
  6. FEV₁/FVC ratio — the fraction of forced vital capacity exhaled in one second; a ratio below 0.70 is the GOLD criteria threshold for airflow obstruction (Global Initiative for Chronic Obstructive Lung Disease, GOLD 2023)

Gas Exchange: Alveolar-Capillary Diffusion

At the alveolar level, gas exchange follows Fick's Law of Diffusion: the rate of transfer is proportional to the surface area and the partial pressure gradient, and inversely proportional to membrane thickness. Oxygen partial pressure in alveolar air is approximately 100 mmHg; in deoxygenated venous blood arriving at the capillary, it is approximately 40 mmHg. This 60 mmHg gradient drives oxygen across the membrane in under 0.25 seconds — faster than the 0.75 seconds a red blood cell typically spends transiting the capillary at rest.

Carbon dioxide diffuses approximately 20 times more readily than oxygen across the same membrane, meaning CO₂ elimination is rarely the rate-limiting step under resting conditions. During exercise or in conditions that thicken the alveolar membrane (interstitial lung disease, pulmonary edema), oxygen transfer is impaired before CO₂ retention becomes apparent.

The alveolar gas equation, standardized in respiratory physiology texts and applied in arterial blood gas interpretation, expresses expected alveolar oxygen as:

PAO₂ = FiO₂ × (Patm − PH₂O) − (PaCO₂ / R)

Where R (respiratory quotient) is typically 0.8, FiO₂ is fractional inspired oxygen (0.21 at room air), and PH₂O is 47 mmHg at body temperature.

Common Scenarios

Pulmonary clinicians encounter three primary physiological failure patterns:

Obstructive pattern: Airflow limitation due to airway narrowing or loss of elastic recoil. Seen in asthma, COPD, and bronchiectasis. FEV₁/FVC ratio falls below 0.70. Total lung capacity is preserved or elevated; residual volume increases.

Restrictive pattern: Reduced lung volumes without proportional airflow limitation. FVC and TLC are both reduced; FEV₁/FVC ratio is normal or elevated. Causes include pulmonary fibrosis, pleural disease, chest wall deformity, or obesity. TLC below 80% of predicted constitutes restriction by ATS/ERS criteria (American Thoracic Society/European Respiratory Society, 2005 standardization document).

Diffusion impairment: Reduced DLCO (diffusing capacity for carbon monoxide) with preserved or borderline lung volumes. Seen in early pulmonary fibrosis, pulmonary hypertension, and pulmonary vascular disease. DLCO below 60% of predicted indicates moderate impairment by ATS standards.

Mixed patterns — obstruction combined with restriction — occur in conditions like sarcoidosis or advanced occupational lung disease.

Decision Boundaries

The boundary between normal physiological variation and measurable pathology in lung function is not absolute, but established thresholds guide clinical and regulatory decisions. The ATS and European Respiratory Society (ERS) define the lower limit of normal (LLN) as the 5th percentile of a reference population — a statistically derived boundary that accounts for age, sex, height, and race, as detailed in the NHLBI-funded NHANES III reference equations widely used in U.S. clinical practice.

Applying fixed thresholds (e.g., FEV₁/FVC < 0.70 regardless of age) can overdiagnose obstruction in adults over 70 and underdiagnose it in younger patients — a recognized limitation discussed in GOLD and ATS guidelines. Age-matched LLN-based interpretation is preferred for diagnostic accuracy in clinical contexts, while fixed thresholds remain embedded in some occupational and disability frameworks administered by agencies including the Social Security Administration (SSA Bluebook, Respiratory Disorders, Section 3.00) and the Department of Labor under Black Lung Benefits regulations (20 CFR Part 718).

Oxygen therapy thresholds are similarly fixed by Medicare criteria: a resting PaO₂ at or below 55 mmHg, or SaO₂ at or below 88%, qualifies a patient for reimbursed supplemental oxygen under Centers for Medicare & Medicaid Services (CMS) Local Coverage Determinations. The pulmonary medicine overview contextualizes how these physiological parameters connect to specialty evaluation and treatment pathways.

References

The law belongs to the people. Georgia v. Public.Resource.Org, 590 U.S. (2020)