Lung Biology and Pulmonary Physiology

Introduction to Lung Biology

The lung is a uniquely exposed organ—its 70 m2 gas exchange surface contacts the external environment with every breath, extracting oxygen and expelling CO2 while simultaneously protecting the host from inhaled pathogens, particles, and toxic chemicals. The human lung contains over 40 distinct cell types organised into conducting airways, respiratory bronchioles, alveolar ducts, and alveoli—each region with specialised cell biology. The alveolar-capillary interface is exquisitely thin (0.2-2 micrometres) enabling efficient gas diffusion driven by partial pressure differences, while maintaining barrier function against fluid leakage under the lung's microvascular hydrostatic pressure.

Respiratory diseases are among the most prevalent globally: COPD affects 380 million people; asthma 350 million; pneumonia kills over 2.5 million annually; lung cancer has the highest cancer mortality; idiopathic pulmonary fibrosis (IPF) median survival is 3-5 years. The COVID-19 pandemic caused acute respiratory distress syndrome (ARDS) in severe cases, highlighting the lung's central role in systemic inflammation and multi-organ failure. Understanding lung biology—from surfactant physics to mucus clearance, from alveolar stem cells to airway smooth muscle—drives development of targeted therapies for this panoply of lung diseases.

Lung Cell Biology

Alveolar Epithelium

The alveolar epithelium consists of type I (ATI) and type II (ATII) pneumocytes. ATI cells cover 90-95% of alveolar surface despite being a minority of cells—they are large, flat, thin cells optimised for gas exchange, with specialised aquaporin water channels (AQP5) controlling fluid balance. ATII cells are cuboidal secretory cells that produce and recycle pulmonary surfactant (SFTPB, SFTPC, SFTPD, ABCA3), regulate alveolar fluid composition, and function as alveolar progenitor cells after injury. SFTPB and SFTPC mutations cause familial interstitial pneumonia and neonatal respiratory distress syndrome. ATII cells serve as the alveolar stem cell population—activating Krt8+ transitional states during regeneration—essential for recovery from influenza, SARS-CoV-2, and other ARDS-causing infections.

Airway Epithelium and Mucociliary Clearance

The proximal airway epithelium contains ciliated cells, mucus-secreting goblet cells, club cells (secretory, formerly Clara cells), basal stem cells, ionocytes, and rare neuroendocrine cells. Ciliated cells bear ~200 cilia per cell beating at 8-15 Hz in a coordinated metachronal wave propelling the mucus blanket (mucociliary escalator) carrying trapped particles, pathogens, and debris toward the pharynx for clearance. Defective cilia in primary ciliary dyskinesia (PCD) cause bronchiectasis, sinusitis, and situs inversus. CFTR mutation in cystic fibrosis depletes airway surface liquid, stops mucociliary clearance, and causes recurrent bacterial infection, bronchiectasis, and pancreatic insufficiency. CFTR modulators (ivacaftor, lumacaftor, elexacaftor-tezacaftor-ivacaftor) correct CFTR dysfunction transforming CF outcomes.

Surfactant Biology

Pulmonary surfactant is a lipoprotein film produced by ATII cells that reduces alveolar surface tension—without it, small alveoli would collapse (Laplace's law: lower radius = higher pressure, favourable for collapse unless surface tension is reduced proportionally). Surfactant is 90% lipid (70% DPPC—the surface-active phospholipid), with 4 surfactant proteins (SP-A, SP-B, SP-C, SP-D) regulating surface film formation and innate immune defence. SP-B deficiency causes fatal neonatal respiratory failure—the most severe surfactant protein deficiency. Neonatal respiratory distress syndrome (NRDS) from surfactant immaturity in premature infants is treated with exogenous surfactant replacement—one of the most successful respiratory interventions saving 30,000+ premature infant lives annually in the US alone.

Chronic Lung Diseases

COPD and Emphysema

COPD (chronic obstructive pulmonary disease) combines emphysema (alveolar destruction by protease-antiprotease imbalance) and chronic bronchitis (mucus hypersecretion from goblet cell metaplasia). Cigarette smoke activates alveolar macrophages and recruits neutrophils; elastase and matrix metalloproteinases degrade elastin and collagen; alpha-1-antitrypsin deficiency creates genetic susceptibility with early-onset emphysema. Oxidative stress damages HDAC2, impairing glucocorticoid anti-inflammatory function and explaining relative steroid insensitivity in COPD versus asthma. Air trapping, hyperinflation, and impaired gas exchange progressively reduce exercise tolerance. Bronchodilators (SABA, LABA, LAMA) relieve obstruction; anti-IL-5 biologics help eosinophilic COPD; alpha-1-antitrypsin augmentation slows emphysema in deficient patients.

Idiopathic Pulmonary Fibrosis

IPF is a progressive fibrosing interstitial pneumonia with typical UIP pattern on CT and histology—honeycombing reflecting irreversible architectural destruction. Pathogenesis involves repeated microinjury to ageing ATII cells (telomere shortening genes MUC5B, TERT, TERC are major genetic risk factors); aberrant repair responses with TGF-beta-driven fibroblast activation and myofibroblast differentiation producing excessive collagen. Nintedanib (RTK inhibitor blocking PDGFR, VEGFR, FGFR) and pirfenidone (anti-fibrotic with unclear mechanism) slow FVC decline by approximately 50% but do not halt progression. Defining the senescent ATII cell as the initiating event has spurred senolytic trials; anti-autotaxin and anti-lysophosphatidic acid signalling are additional anti-fibrotic approaches in development.

Examples and Applications

Example 1: CFTR Modulator Therapy

Cystic fibrosis mutations classify as protein absence (class I, nonsense/splice), misfolding/degradation (class II, F508del affecting 70% of CF alleles), gating dysfunction (class III), conductance (class IV), reduced synthesis (class V), or reduced stability (class VI). CFTR modulators target specific defects: ivacaftor (VX-770) opens the mutant CFTR gate (class III G551D); lumacaftor/tezacaftor/elexacaftor correct F508del protein folding trafficking it to the cell surface (class II correctors) where it is potentiated by ivacaftor. Trikafta (elexacaftor-tezacaftor-ivacaftor) treats 90% of CF patients with F508del allele, reducing exacerbations by 63%, improving FEV1 by 14 percentage points, and improving quality of life dramatically—the closest approach to a cure for this disease.

Example 2: Asthma Biologics

Severe (type 2) asthma driven by IL-4, IL-5, IL-13, and IgE-dependent eosinophilic inflammation is targeted by biologics approved since 2016. Dupilumab (anti-IL-4Ralpha blocking IL-4 and IL-13) reduces exacerbations 50% and improves FEV1 in moderate-severe asthmatics. Anti-IL-5 (mepolizumab, reslizumab) and anti-IL-5Ralpha (benralizumab) reduce eosinophil counts and exacerbations. Tezepelumab (anti-TSLP—an upstream epithelial cytokine) reduces exacerbations across all T2 and non-T2 asthma patients. These biologics transformed severe asthma management—patients previously requiring oral corticosteroids (with systemic side effects) now achieve control without steroids. Biomarker-guided use (blood eosinophilis, FeNO, IgE, periostin) personalises biologic selection.

Example 3: COVID-19 ARDS Biology

Severe COVID-19 causes ARDS—diffuse alveolar damage with inflammatory cell infiltration, hyaline membrane formation, and impaired gas exchange requiring mechanical ventilation. SARS-CoV-2 infects ATII cells (highest ACE2 expression), killing them and triggering excessive innate immune activation (cytokine storm—IL-6, TNF, IL-1beta, type I interferon dysregulation). Lung pathology shows diffuse alveolar damage, microthrombi in pulmonary vasculature, and impaired alveolar regeneration. Dexamethasone reduced mortality by 30% in patients requiring oxygen—validating anti-inflammatory treatment after initial viral replication phase. Tocilizumab (anti-IL-6R) and baricitinib (JAK inhibitor) further reduce mortality in combination with dexamethasone, illustrating the layered cytokine biology underlying COVID-19 severity.

Example 4: Lung Cancer Driver Mutations

Lung adenocarcinoma driver mutations include EGFR (15% Western, 50% Asian never-smokers), KRAS (33%), ALK fusions (5%), ROS1 fusions (2%), BRAF (2%), MET exon 14 (3%), NTRK, RET fusions. Smoking-associated mutations create C-to-A transversions in a characteristic mutational signature; never-smoker lung cancer carries different signatures. Specific driver-targeted therapies: EGFR inhibitors (erlotinib, osimertinib for T790M-resistant EGFR); ALK inhibitors (crizotinib, alectinib, lorlatinib); RET inhibitors (selpercatinib, pralsetinib); MET inhibitors (capmatinib, tepotinib); KRAS G12C (sotorasib, adagrasib). Upfront molecular profiling is standard in advanced NSCLC enabling optimal first-line targeted therapy selection before empirical chemotherapy was replaced.

Example 5: Alveolar Regeneration Biology

After influenza or SARS-CoV-2 infection destroying ATII cells, regeneration involves activation of surviving ATII stem cells and putative airway progenitors. Single cell RNA-seq identified Krt8+ transitional cell state (aberrant basaloid cells) as transient intermediate during ATI cell regeneration from ATII precursors—highly expressing TGF-beta, collagen, and fibrotic mediators. Failure to resolve this transitional state correlates with fibrosis in post-COVID lung disease. Restoration of ATII identity from persistent Krt8+ cells requires WNT activation, and EGFR signalling resolution is required for ATI differentiation completion. Understanding alveolar regeneration provides potential therapeutic targets for post-viral fibrosis and IPF by promoting normal regenerative trajectories over aberrant fibrotic programmes.

Example 6: Primary Ciliary Dyskinesia

PCD arises from mutations in genes encoding axonemal dynein components (DNAI1, DNAI2, DNAH5, DNAH11), radial spoke proteins (RSPH9, RSPH4A), outer dynein arm docking complex proteins, or central apparatus components—each causing specific ultrastructural and motility defects. Approximately 50% of PCD patients have situs inversus or laterality defects from defective nodal cilia during embryogenesis. Clinical features: chronic wet cough from birth, bronchiectasis, chronic sinusitis, and male infertility from immotile spermatozoa. Nasal nitric oxide is a sensitive PCD screening marker (very low in PCD). Management focuses on regular physiotherapy and early antibiotic treatment of exacerbations; targeted therapies based on genomic stratification (similar to CF modulators) are in development.

Example 7: Lung Development Biology

Lung development proceeds through 5 stages: embryonic (foregut budding, branching initiation), pseudoglandular (branching morphogenesis—~15-25 generations), canalicular (vascularisation, ATII differentiation), saccular (subdivision into primitive alveoli), and alveolar (secondary septation forming mature alveoli—completes postnatally in humans through age 2-3 years). Branching morphogenesis is directed by FGF10 from mesenchyme signalling to FGFR2b on epithelium; Wnt, Shh, BMP4, and VEGF drive patterning and vascularisation. Premature birth interrupts alveolarisation; supplemental oxygen essential for preterm survival paradoxically impairs alveolar development (bronchopulmonary dysplasia) through ROS damage to alveolar progenitors. Understanding normal lung development guides interventions preventing BPD.

Example 8: CRISPR Gene Therapy for Lung Diseases

Inhaled delivery of CRISPR components (Cas9 protein + sgRNA in lipid nanoparticles, or AAV vectors) to airway and alveolar cells enables therapeutic gene editing in the lung. Alpha-1-antitrypsin deficiency (SERPINA1 mutations in liver) can be corrected by hepatic base editing delivered via LNPs—clinical trial of hepatocyte SERPINA1 correction is testing liver-targeted base editing. AE1 AAV delivering CFTR to airway cells was limited by episomal dilution from cell turnover; integration-capable vectors under investigation. Aerosol delivery of CRISPR base editors to correct the ∆F508 mutation directly in airway cells would theoretically substitute for modulator drugs, providing a permanent correction. Key challenges include delivery efficiency to both large airways and small airways, immune responses, and off-target edits in vulnerable lung cells.

Try it live

Everything above runs in your browser — open Lung Physiology Simulator: Breathing & Gas Exchange and change the parameters while it is running. Nothing is installed, nothing is uploaded, the whole model lives in one tab.