Anatomy and Physiology of the Lungs

Related Subjects: |AP of the Lungs |AP of the Heart |Coronary AP |Cardiac Embryology |Gastrointestinal tract Physiology |Autonomic Nervous System

🫁 AP of the Lungs - In Depth (Makindo)

The lungs are paired, highly vascular, elastic organs primarily responsible for gas exchange, maintaining acid–base balance by eliminating CO₂ (the major volatile acid), and contributing to metabolic functions (e.g., angiotensin conversion, arachidonic acid metabolism). They process ~8,000–10,000 L of air daily in adults at rest, achieving near-equilibrium oxygenation and CO₂ removal. Optimal function depends on precise integration of ventilation (air movement), perfusion (blood flow), diffusion (gas transfer), and neural/chemical control mechanisms. Disruptions lead to common pathologies like COPD, ARDS, pneumonia, and pulmonary embolism.

🏗️ 1) Gross Anatomy and Organisation

• Position and Shape: Lungs lie in pleural cavities flanking the mediastinum; cone-shaped with apex superior (extends 2–3 cm above 1st rib into supraclavicular fossa) and broad base resting on diaphragm. Right lung larger (~10% bigger) due to liver elevation; weighs ~625 g; left ~565 g. Cardiac notch on left anterior border accommodates heart.
• Surfaces: Costal (convex, rib impressions), mediastinal (concave with hilum impressions: heart, aorta, esophagus), diaphragmatic (concave, fits diaphragm dome), and sharp anterior/inferior borders.
• Hilum (Root): Medial gateway; contains main bronchus (posterior), pulmonary artery (superior), two pulmonary veins (inferior), bronchial arteries/veins, lymph nodes/vessels, vagus/sympathetic nerves. Right hilum slightly higher.
• Blood Supply Overview: Dual: pulmonary (deoxygenated, low-pressure) and bronchial (oxygenated, systemic, high-pressure, ~1–2% cardiac output).

🧩 Lobes, Fissures, and Bronchopulmonary Segments

• Right lung: 3 lobes (superior, middle, inferior) separated by horizontal fissure (between superior/middle) and oblique fissure (between superior/middle and inferior). 10 bronchopulmonary segments: superior (apical, posterior, anterior), middle (lateral, medial), inferior (superior, medial basal, anterior basal, lateral basal, posterior basal).
• Left lung: 2 lobes (superior with lingula, inferior) separated by oblique fissure only. Lingula (tongue-like projection) analogous to right middle lobe. 8–10 segments (superior: apicoposterior, anterior, superior lingular, inferior lingular; inferior similar to right but fused basal segments).
• Bronchopulmonary Segments: Independent functional/surgical units; each with own segmental bronchus (from lobar), pulmonary artery branch, and segmental vein (drains to pulmonary veins). Separated by connective tissue planes with minimal collateral circulation.
  📌 Clinical: Enables precise resections (segmentectomy for early lung cancer, bronchiectasis) while sparing healthy tissue. Segmental anatomy guides bronchoscopy and targeted therapies.

🫧 2) Pleura and Pleural Space

• Visceral Pleura: Thin serous layer tightly adherent to lung surface, fissures, interlobar surfaces; continuous with parietal at hilum; poor sensory innervation except near hilum (vagus/phrenic nerves).
• Parietal Pleura: Lines thoracic cavity: costal (ribs/intercostals), diaphragmatic (diaphragm), mediastinal (mediastinum/pericardium), cervical (neck). Rich sensory supply (phrenic for diaphragmatic/mediastinal → referred shoulder pain; intercostal for costal).
• Pleural Space: Potential space (~10–20 mL fluid total); negative pressure maintained by lymphatic drainage and Starling forces; fluid turnover ~0.5 mL/h.
• Intrapleural Pressure Dynamics: Resting -5 cmH₂O; inspiration -8 to -10 cmH₂O (diaphragm descent); forced expiration positive. Couples lung to chest wall expansion.

📌 Clinical: Pneumothorax → atmospheric pressure equilibration → lung collapse (atelectasis). Tension pneumothorax → progressive pressure rise → mediastinal shift, vena cava compression, hypotension/shock. Pleural effusion (>300 mL) → restrictive defect, reduced FRC, dyspnea.

🌳 3) The Airway Tree - Conducting vs Respiratory Zones

• Conducting Zone (0–16 generations): Trachea → main bronchi → lobar → segmental → bronchioles → terminal bronchioles. No gas exchange; warms/humidifies/filters air; anatomical dead space ~150 mL (2 mL/kg). Cartilage in larger airways prevents collapse; smooth muscle increases distally.
• Respiratory Zone (17–23 generations): Respiratory bronchioles (first alveoli) → alveolar ducts → alveolar sacs → alveoli. ~300–480 million alveoli; surface area 50–100 m²; primary gas exchange site.

🔎 Trachea and Main Bronchi

• Trachea: 10–12 cm long, 2–2.5 cm diameter; 16–20 C-shaped hyaline cartilage rings (open posteriorly); trachealis muscle (smooth) allows esophageal expansion; bifurcates at carina (T4/T5 vertebral level, descends ~2 cm on deep inspiration).
• Right Main Bronchus: Shorter (~2.5 cm), wider, steeper angle (~25° from vertical) → higher aspiration risk (foreign bodies, aspiration pneumonia often right-sided).
• Left Main Bronchus: Longer (~5 cm), narrower, more horizontal (~45° angle); crosses under aortic arch.

🧬 4) Microanatomy of the Alveoli

• Alveoli: Thin-walled polyhedral sacs (~250–300 μm diameter); ~300–480 million; total surface ~70–100 m² in adults; walls contain elastic fibers for recoil.
• Type I Pneumocytes: Flat squamous cells (cover ~95% surface area); thin cytoplasm (0.1–0.2 μm) for minimal diffusion barrier; tight junctions prevent fluid leak.
• Type II Pneumocytes: Cuboidal; ~5% surface; synthesize/store surfactant in lamellar bodies; can proliferate to repair type I damage (stem-like function); express aquaporins for fluid balance.
• Alveolar Macrophages: Mobile phagocytes derived from monocytes; clear debris, pathogens, surfactant; iron-laden in smokers ("smoker's macrophages").
• Alveolar–Capillary Membrane: ~0.2–0.5 μm thick (type I + basement membrane + endothelium); pores of Kohn allow collateral air flow; tight junctions limit protein leak.

🫧 Surfactant (Critical Stabilizer)

Surfactant (~90% phospholipids like dipalmitoylphosphatidylcholine, 10% proteins A–D) lowers surface tension dynamically (more in smaller alveoli per Laplace’s law: P = 2T/r), preventing atelectasis, reducing work of breathing by ~50–70%, and stabilizing alveoli during expiration. Type II cells produce ~10–20 mg/kg/day. Deficiency → neonatal RDS (prematurity), ARDS (inactivation by inflammation/protein leak) → stiff lungs, high ventilator pressures, barotrauma risk.

🩸 5) Pulmonary and Bronchial Circulations

• Pulmonary Circulation: Low-resistance, high-capacitance (~500–1000 mL blood volume); right ventricle → pulmonary trunk → arteries → alveolar capillary network (sheet flow) → veins → left atrium. Mean pressure ~15 mmHg; accommodates full cardiac output with minimal pressure rise.
• Bronchial Circulation: Systemic (aortic branches); supplies airways to terminal bronchioles, supporting structures, visceral pleura (~1–2% cardiac output); venous drainage mixes with pulmonary veins (physiological shunt ~1–2%).
• Hypoxic Pulmonary Vasoconstriction (HPV): Local response to low alveolar PO₂ (<70 mmHg) → vasoconstriction diverts blood to better-ventilated areas; optimizes V/Q; inhibited by volatiles, alkalosis.

📌 Clinical: Pulmonary embolism → acute dead space increase, hypoxemia, right heart strain. Chronic hypoxia → persistent HPV → pulmonary hypertension → cor pulmonale.

⚙️ 6) Mechanics of Breathing

• Inspiration: Active; diaphragm (primary, 75% tidal volume), external intercostals, scalenes/accessories → thoracic volume ↑ → intrapleural pressure ↓ → alveolar pressure negative → airflow in.
• Expiration: Passive (elastic recoil dominant at rest); active in forced breathing (internal intercostals, abdominals).
• Elastic Recoil: Lung inward (elastin + surface tension ~2/3); chest wall outward; balanced at FRC.
• Airway Resistance: ~80% in medium/large airways; Poiseuille’s law (R ∝ 1/r⁴) explains dramatic rise in asthma/COPD.

🧮 Lung Compliance - Detailed Physiology

Lung compliance quantifies distensibility: ability to expand with pressure change. C = ΔV / ΔP (mL/cmH₂O). Normal static lung compliance ~200 mL/cmH₂O; total respiratory system ~70–100 mL/cmH₂O.

• Types: Static (no flow, elastic only); Dynamic (includes resistance); Specific (per unit volume).
• Factors: Surfactant (major ↑); elastin/collagen; chest wall; volume (sigmoid P-V curve, peak mid-range); age (↓); posture (supine ↓ FRC/compliance).
• Clinical: ↑ in emphysema (loss recoil → hyperinflation); ↓ in fibrosis/ARDS/pneumonia (stiff → ↑ work, high pressures).
• Measurement: Ventilator: C_stat = TV / (P_plateau - PEEP); C_dyn = TV / (P_peak - PEEP). P-V loops show hysteresis (surfactant).

📦 7) Lung Volumes, Capacities, and Dead Space

• Tidal Volume (TV): 500 mL (~7 mL/kg).
• Inspiratory/Expiratory Reserve Volumes (IRV/ERV): ~3 L / ~1.2 L.
• Residual Volume (RV): ~1.2 L (prevents collapse).
• Vital Capacity (VC): ~4.8 L; Total Lung Capacity (TLC): ~6 L.
• Functional Residual Capacity (FRC): ~2.4 L (ERV + RV); buffer zone; ↓ in obesity/anesthesia → atelectasis.
• Dead Space: Anatomical ~150 mL; physiological (includes alveolar) ~30% TV normally; ↑ in PE, low cardiac output.

📌 Clinical: Obstructive disease ↑ RV/TLC ratio; restrictive ↓ VC/TLC. PEEP recruits alveoli, ↑ FRC, improves oxygenation.

🌬️ 8) Gas Exchange and V/Q Matching

• Diffusion: Fick’s law; O₂ limited by perfusion normally; CO₂ ~20× more soluble.
• V/Q Ratio: Ideal ~0.8; gravity creates apex high V/Q (~3), base low (~0.6).
• Zones (West): Zone 1 (dead space-like, rare); Zone 2 (intermittent flow); Zone 3 (continuous, base).

Shunt (V/Q=0): atelectasis/pneumonia → refractory hypoxemia. Dead space (V/Q=∞): PE → hyperventilation, normal/high PaO₂ but ↑ A-a gradient.

🩸 9) Oxygen and CO₂ Transport

🟦 Oxygen Transport

• ~97% Hb-bound (1.34 mL O₂/g Hb); 3% dissolved.
• Sigmoid curve (cooperative binding); P50 ~27 mmHg.
• Bohr effect: right shift (acidosis, hypercapnia, fever, 2,3-DPG) → tissue unloading.

🟩 Carbon Dioxide Transport

• ~70% bicarbonate (carbonic anhydrase in RBCs); 23% carbamino-Hb; 7% dissolved.
• Haldane effect: deoxy-Hb carries more CO₂.

🧠 10) Control of Ventilation

• Rhythm Generators: Pre-Bötzinger complex (medulla).
• Chemoreceptors: Central (CO₂/pH via CSF, 70–80% drive); Peripheral (carotid/aortic, hypoxic when PaO₂ <60 mmHg).
• Other: Stretch (Hering-Breuer), irritant, J-receptors, cortical/voluntary.

In chronic hypercapnia (COPD), hypoxic drive predominates - supplemental O₂ risks CO₂ retention via V/Q worsening + minor drive loss.

🛡️ 11) Lung Defence Mechanisms

• Mucociliary clearance (5–20 mm/min); cough; alveolar macrophages; IgA, defensins, collectins.

Impaired in smoking (cilia paralysis), CF (thick mucus), immunosuppression → infection risk.

🩺 12) Clinical Correlations (Physiology to Bedside)

• Asthma: ↑ resistance, reversible.
• COPD: Emphysema (↓ recoil, ↑ compliance); chronic bronchitis (mucus).
• Fibrosis/ARDS: ↓ compliance, diffusion barrier.
• PE: Dead space, V/Q mismatch.
• Pneumonia: Shunt.

📊 Summary (High-Yield Integration)

Lungs excel through matched ventilation-perfusion-diffusion, governed by mechanics (compliance/recoil/resistance) and precise control. Failures stem from disrupted components: obstruction (airflow), alveolar filling/collapse (diffusion/shunt), vascular occlusion (dead space), or control issues. Master V/Q as the unifying principle.

💡 Teaching Tip: Lungs = “pump (mechanics) + filter (alveoli/capillaries) + controller (chemoreflexes)”. Pathology = which breaks and how it skews V/Q/gas exchange.