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The Respiratory System SUMMARY OF KEY POINTS

  • The function of the respiratory system is to provide oxygen to the blood, with a secondary function of sound production in the larynx.

  • The respiratory system consists of an air conducting region (the upper respiratory tract in the head, as well as the larynx, trachea, bronchi, and most bronchioles) and a respiratory region with alveoli.

Nasal Cavities
  • The left and right nasal cavities of the upper respiratory tract have vestibules where air enters and three projections called conchae from their medial walls, which create turbulence in inspired air.

  • Moist vibrissae in the vestibular openings, the nares or nostrils, filter some material from inspired air.

  • Deeper areas of the vestibules and the floor, lateral walls, and most of the conchae of the nasal cavities themselves are lined by respiratory epithelium: pseudostratified ciliated columnar epithelium.

  • Respiratory epithelium includes goblet cells secreting mucus, ciliated columnar cells sweeping the mucus along the surface, chemosensory brush cells, scattered endocrine cells, and basal stem cells.

  • The roof and part of the superior concha in each nasal cavity are covered by olfactory epithelium, which is pseudostratified epithelium containing bipolar olfactory neurons, support cells, and stem cells.

  • The mucosa of the nasal cavities and nasopharynx also contains a rich vasculature and many seromucous glands, which help warm, humidify, and clean inspired air.

Larynx, Trachea, and Bronchial Tree
  • Within the lumen of the larynx, bilateral projecting vocal folds (or cords) can be placed under variable tension by the underlying vocalis muscles and caused to vibrate by expelled air, producing sounds.

  • The trachea is completely lined by respiratory epithelium and is supported by C-shaped rings of hyaline cartilage, with smooth trachealis muscles in the posterior opening of the rings.

  • Left and right primary bronchi enter the two lungs and bifurcate repeatedly as secondary, tertiary, and smaller segmental bronchi with the lung tissue as the bronchial tree.

  • Bronchi and their branches are lined by respiratory mucosa, with prominent spiraling bands of smooth muscle and increasingly smaller pieces of hyaline cartilage.

  • Branches of the bronchial tree with diameters of 1 mm or less are generally called bronchioles, which are lined by simple columnar or cuboidal ciliated cells, with circular smooth muscle but no cartilage.

  • Terminal bronchioles are the last branches to lack alveoli and are lined by simple cuboidal epithelium consisting mainly of club cells, which have innate immune and surfactant secretory functions.

Respiratory Region
  • Terminal bronchioles subdivide into two or three respiratory bronchioles, lined by simple cuboidal epithelium and interrupted by scattered squamous evaginations called alveoli, the sites of gas exchange.

  • A respiratory bronchiole leads to an alveolar duct, which is lined by a continuous series of alveoli and which ends in a cluster of alveoli called the alveolar sac.

  • All alveoli are surrounded ...

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