Anatomy of the Lower Respiratory Tract

Learning Outcomes

  • State major anatomical parts of lower respiratory system.

  • Relate function of each part to gross & microscopic structure.

  • Identify & describe muscles of respiration and their actions during each phase.

Functional Divisions of the Respiratory System

  • Conducting division

    • Nose ➔ major bronchioles (nostrils through terminal bronchioles).

    • Functions: airflow only; no gas exchange.

  • Respiratory division

    • Alveoli + all microscopic regions where gas exchange occurs.

  • Anatomical delimitation

    • Upper tract: nose ➔ larynx (head & neck).

    • Lower tract: trachea ➔ lungs (thorax).

Trachea (Windpipe)

  • Rigid tube, length \approx 12\,\text{cm}, diameter \approx 2.5\,\text{cm}.

  • Position: anterior to oesophagus; extends from larynx to sternal angle \text{(T4)} where it bifurcates.

  • Support

    • 16–20 C-shaped hyaline cartilage rings; opening faces posteriorly.

    • Trachealis muscle spans gap ➔ allows oesophageal expansion & adjusts airflow (contracts ⬇ lumen, relaxes ⬆ lumen).

  • Histology

    • Mucosa: pseudostratified ciliated columnar epithelium (goblet, ciliated, stem cells).

    • Mucociliary escalator: mucus traps debris; cilia beat upward toward pharynx for swallowing.

    • Middle layer: connective tissue containing mucous & serous glands, lymphoid nodules, cartilage.

    • Adventitia: outer fibrous CT continuous with mediastinal structures.

  • Carina: internal ridge at bifurcation directing airflow into R & L main bronchi; sensitive cough reflex area.

Main (Primary) Bronchi

  • Right main bronchus

    • Length 2–3\,\text{cm}, wider, more vertical ➔ aspirated objects lodge here more often.

  • Left main bronchus

    • Length \approx 5\,\text{cm}, narrower, more horizontal.

  • Both supported by C-shaped cartilage rings; enter lungs at hilum.

The Bronchial Tree

  • Hierarchy: Main (primary) ➔ Lobar (secondary) ➔ Segmental (tertiary) ➔ smaller bronchi ➔ bronchioles ➔ terminal bronchioles ➔ respiratory bronchioles ➔ alveolar ducts ➔ alveolar sacs ➔ alveoli.

  • About 65\,000 terminal bronchioles per lung.

Lobar (Secondary) Bronchi

  • Right lung: 3 lobar bronchi (superior, middle, inferior).

  • Left lung: 2 lobar bronchi (superior, inferior).

  • Supported by crescent-shaped cartilage plates.

Segmental (Tertiary) Bronchi

  • Right lung: 10 segments; Left lung: 8 segments.

  • Each supplies a bronchopulmonary segment = functionally independent lung unit (clinical relevance: segmental resections).

Histological Features (All Bronchi)

  • Lining: pseudostratified ciliated columnar epithelium; cell height ↓ distally.

  • Lamina propria rich in mucous glands & BALT ➔ immune defense.

  • Abundant elastic CT throughout tree ➔ recoil for expiration.

  • Muscularis mucosae (smooth muscle): regulates airway diameter.

  • Blood supply

    • Pulmonary arteries course with bronchi to alveoli (deoxygenated blood for gas exchange).

    • Bronchial arteries from aorta nourish bronchial walls (systemic, oxygenated).

Bronchioles

  • Diameter < 1\,\text{mm}; no cartilage.

  • Thick smooth-muscle layer; ciliated cuboidal epithelium.

  • Each bronchiole ventilates a pulmonary lobule.

  • Terminal bronchioles (50–80/lobule)

    • Diameter ≤ 0.5\,\text{mm}.

    • No goblet cells; still possess cilia ➔ mucus clearance via escalator.

    • Branch into 2+ respiratory bronchioles.

Respiratory Bronchioles & Alveolar Ducts

  • Respiratory bronchioles: first airways with alveoli budding from walls ➔ begin gas-exchange region.

  • Divide into 2–10 alveolar ducts ending in alveolar sacs (grape-like clusters around an atrium).

Alveoli

  • Number: 1.5\times10^8 (150 million) per lung; surface area \approx70\,\text{m}^2.

  • Cell types

    • Type I (squamous) cells ➔ 95 % surface; very thin for rapid diffusion.

    • Type II (great/septal) cells ➔ 5 %; cuboidal; functions:

    • Secrete pulmonary surfactant (phospholipid-protein mix) to reduce surface tension & prevent collapse during exhalation.

    • Repair epithelium after damage.

    • Clinical: deficiency causes Hyaline Membrane Disease (infant respiratory distress syndrome).

    • Alveolar macrophages (dust cells)

    • Most numerous; phagocytose debris & pathogens.

    • ~10^8 die daily, cleared by mucociliary escalator & swallowed.

  • Respiratory membrane (barrier for diffusion)

    1. Type I alveolar cell.

    2. Shared basement membrane.

    3. Capillary endothelial cell.

  • Fluid balance

    • Alveoli kept dry by capillary absorption & extensive lymphatic drainage.

    • Low pulmonary capillary pressure prevents membrane rupture.

Gross Anatomy of the Lungs

  • Shape: conical; base rests on diaphragm; apex extends above clavicle.

  • Surfaces

    • Costal (against ribs).

    • Mediastinal (faces heart) ➔ hilum where bronchi, vessels, lymphatics, nerves form root.

  • Right Lung

    • Shorter (large liver below).

    • 3 lobes: superior, middle, inferior.

    • Fissures: horizontal & oblique.

  • Left Lung

    • Taller, narrower (cardiac impression & notch).

    • 2 lobes: superior, inferior.

    • Single oblique fissure; lingula = tongue-like part of superior lobe.

  • Lungs do not entirely fill rib cage; asymmetrical due to heart & liver.

Bronchopulmonary Segments (Key Clinical Map)

  • Right (10): Apical (S1), Posterior (S2), Anterior (S3), Lateral (S4), Medial (S5) in middle lobe; Superior (S6), Medial basal (S7/S8), Anterior basal (S8), Lateral basal (S9), Posterior basal (S10) in lower lobe.

  • Left (8): Apico-posterior (S1+S2), Anterior (S3), Superior lingular (S4), Inferior lingular (S5), Superior (S6), Antero-medial basal (S7+S8), Lateral basal (S9), Posterior basal (S10).

Pleurae & Pleural Cavity

  • Visceral pleura: adheres to lung surface.

  • Parietal pleura: lines thoracic wall, diaphragm, mediastinum.

  • Pleural cavity: potential space with thin film of fluid.

  • Functions

    • Lubrication (↓ friction).

    • Pressure gradient (intrapleural pressure < atmospheric) assists lung inflation.

    • Compartmentalisation limits spread of infection.

Respiratory Muscles

Primary Muscle

  • Diaphragm (innervation: phrenic nerve C3–C5, chiefly C5 stated):

    • Contraction ➔ flattens, ⬆ thoracic vertical dimension ➔ inspiration (~2/3 of quiet airflow).

    • Relaxation ➔ domes upward, recoil aids passive expiration.

Intercostal Muscles

  • External & internal intercostals (between ribs)

    • Synergists to diaphragm.

    • Stiffen thoracic cage; prevent inward collapse; add ≈1/3 of ventilatory volume.

Scalenes

  • Fix/elevate ribs 1–2 during quiet inspiration (synergists).

Accessory Muscles (forced inspiration)

  • Erector spinae, sternocleidomastoid, pectoralis major & minor, serratus anterior, scalenes ➔ greatly enlarge thoracic cavity.

Forced Expiration Muscles

  • Internal intercostals (interosseous part), rectus abdominis, external abdominal oblique, lumbar & pelvic muscles ➔ increase abdominal & thoracic pressure to expel air.

Respiratory Mechanics

  • Quiet inspiration: active; diaphragm + external intercostals contract.

  • Quiet expiration: passive; elastic recoil of lungs & cage.

  • Forced inspiration/expiration use accessory groups above.

Valsalva Manoeuvre

  • Steps: deep breath ➔ close glottis ➔ contract abdominal muscles.

  • Raises abdominal pressure to expel contents.

  • Applications: childbirth, urination, defaecation, vomiting, heavy lifting.

Intrapulmonary & Pleural Blood Flow

  • Pulmonary arteries follow airways to alveolar capillaries.

  • Pulmonary veins run intersegmentally toward hilum.

  • Bronchial arteries (systemic) supply bronchial walls.

Clinical & Physiological Notes

  • Conducting zone cleansing: mucociliary escalator crucial; smoking or Kartagener’s syndrome impairs clearance ➔ infections.

  • Right main bronchus orientation explains object aspiration & oopneumonia distribution.

  • Surfactant deficiency in premature neonates ➔ Hyaline Membrane Disease; treated with exogenous surfactant.

  • Bronchopulmonary segment independence allows segmental lung resections without affecting neighbours.

  • Pleural pressure gradient essential; pneumothorax abolishes gradient ➔ lung collapse.

  • Valsalva manoeuvre alters intrathoracic pressure; affects venous return & can be used diagnostically (e.g., heart murmurs).