CorpusHuman Anatomy

Respiratory Tract

Head & neck in median sagittal section, thorax anterior view

A composite view of the whole respiratory tract. The head and neck are shown in median sagittal section facing left — nasal cavity with its conchae, frontal and sphenoid sinuses, the three parts of the pharynx and the larynx — while the thorax is shown from the front: trachea, carina, main, lobar and segmental bronchi branching into bronchioles and alveolar clusters, the three-lobed right lung and two-lobed left lung (with horizontal and oblique fissures, cardiac notch and lingula), the pleura and the diaphragm. The inset shows the laryngeal inlet as seen from above (as on laryngoscopy, anterior at the top).

The animation runs a 5-second breathing cycle: in inspiration the diaphragm contracts and flattens, the lungs and parietal pleura expand together, the vocal folds abduct to widen the rima glottidis, and the air column (blue particles) moves in; expiration reverses everything by passive recoil. Lower-lobe bronchi are drawn fainter because they lie posteriorly. Use the steps to follow a breath of inspired air from the nostrils to the alveoli.

Step-by-step

  1. Nostrils and vestibule

    Inspired air enters through the nares (nostrils) into the nasal vestibule, which is lined by skin bearing coarse hairs (vibrissae) that trap large particles.

  2. Nasal cavity and conchae

    The superior, middle and inferior conchae make the airflow turbulent and greatly increase the mucosal surface. The highly vascular respiratory mucosa warms and humidifies the air, and its mucus blanket traps particles larger than ≈10 µm, which cilia sweep back towards the pharynx. The paranasal sinuses drain into the meatuses beneath the conchae.

  3. Nasopharynx

    Air passes through the choanae into the nasopharynx, above the soft palate. It is purely respiratory, lined by pseudostratified ciliated columnar epithelium, and contains the pharyngeal tonsil and the openings of the auditory tubes. The soft palate elevates to close it off during swallowing.

  4. Oropharynx

    From the soft palate to the tip of the epiglottis, behind the oral cavity. A shared passage for air and food, lined by non-keratinised stratified squamous epithelium that withstands abrasion.

  5. Laryngopharynx

    From the epiglottis to the lower border of the cricoid cartilage (C6), where it becomes the oesophagus. Air turns forwards through the laryngeal inlet; swallowed food is channelled around it through the piriform fossae.

  6. Larynx and glottis

    The epiglottis guards the laryngeal inlet. Air passes the vestibular folds and then the rima glottidis between the vocal folds — the narrowest part of the adult airway. In inspiration the vocal folds are abducted by the posterior cricoarytenoid muscles (the only abductors), as animated in the inset.

  7. Trachea

    The trachea runs from C6 to the sternal angle: ≈10–12 cm long and ≈2–2.5 cm wide, held open by 16–20 C-shaped rings of hyaline cartilage that are open posteriorly (bridged by the trachealis muscle, next to the oesophagus). Its ciliated mucosa forms the mucociliary escalator.

  8. Carina

    At the level of the sternal angle (T4/T5) the trachea bifurcates. The carina, a keel-shaped ridge of the last tracheal ring, is the most sensitive trigger site for the cough reflex; its widening or distortion on bronchoscopy suggests subcarinal lymphadenopathy.

  9. Main (primary) bronchi

    The right main bronchus is wider, shorter (≈2.5 cm) and more vertical (≈25° from the midline) than the left (≈5 cm long, ≈45°), which passes beneath the aortic arch. Hence inhaled foreign bodies and aspirated material more often enter the right lung.

  10. Lobar (secondary) bronchi

    One lobar bronchus per lobe: three on the right (superior, middle, inferior — the superior arising before the hilum) and two on the left (superior, which gives the lingular division, and inferior).

  11. Segmental (tertiary) bronchi

    Each segmental bronchus supplies one bronchopulmonary segment — a pyramidal, surgically resectable unit with its own artery: 10 on the right and 8–10 on the left. Cartilage becomes irregular plates rather than rings. (Drawing shows representative branches; lower-lobe branches are fainter because they lie posteriorly.)

  12. Bronchioles

    After repeated dichotomous branching (≈23 generations in all), airways < 1 mm wide lose cartilage and glands: these bronchioles have a prominent smooth-muscle coat (the site of bronchoconstriction in asthma) and club cells. The terminal bronchioles end the conducting zone (anatomical dead space ≈150 mL).

  13. Alveoli

    Respiratory bronchioles lead into alveolar ducts and sacs. Around 300–500 million alveoli provide ≈70 m² of surface across which O₂ and CO₂ diffuse through a respiratory membrane ≈0.5 µm thick (see the Alveolus diagram).

  14. Pleura

    Each lung is wrapped in visceral pleura, which is continuous at the hilum with the parietal pleura lining the thoracic wall, mediastinum and diaphragm. Between them the pleural cavity holds only ≈10–20 mL of serous fluid; its pressure is sub-atmospheric (≈ −5 cmH₂O at rest), which keeps the lungs expanded. The costodiaphragmatic recesses extend about two ribs below the lungs.

  15. Diaphragm drives ventilation

    In quiet inspiration the diaphragm (phrenic nerve, C3–C5) contracts and its domes descend ≈1–2 cm (up to ≈10 cm in deep breathing), enlarging the thorax; intrapleural and alveolar pressures fall (Boyle's law) and air flows in. Quiet expiration is passive elastic recoil. The right dome sits higher (liver). Its apertures: IVC at T8, oesophagus at T10, aorta at T12 ("I 8 10 Eggs At 12").

Labelled parts