Anatomy, Airway

Excerpt

The airway, or respiratory tract, describes the organs of the respiratory tract that allow airflow during ventilation. They reach from the nares and buccal opening to the blind end of the alveolar sacs. They are subdivided into different regions with various organs and tissues to perform specific functions. The airway can be subdivided into the upper and lower airway, each of which has numerous subdivisions as follows.

Upper Airway

The pharynx is the mucous membrane-lined portion of the airway between the base of the skull and the esophagus and is subdivided as follows:

1. Nasopharynx, also known as the rhino-pharynx, post-nasal space, is the muscular tube from the nares, including the posterior nasal cavity, divide from the oropharynx by the palate and lining the skull base superiorly

2. The oro-pharynx connects the naso and hypopharynx. It is the region between the palate and the hyoid bone, anteriorly divided from the oral cavity by the tonsillar arch

3. The hypopharynx connects the oropharynx to the esophagus and the larynx, the region of pharynx below the hyoid bone.

The larynx is the portion of the airway between the pharynx and the trachea, contains the organs for the production of speech. Formed of a cartilaginous skeleton of nine cartilages, it includes the important organs of the epiglottis and the vocal folds (vocal cords) which are the opening to the glottis.

Lower Airway

The trachea is a ciliated pseudostratified columnar epithelium-lined tubular structure supported by C-shaped rings of hyaline cartilage. The flat open surface of these C rings opposes the esophagus to allow its expansion during swallowing. The trachea bifurcates and therefore terminates, superior to the heart at the level of the sternal angle.

The bronchi, the main bifurcation of the trachea, are similar in structure but have complete circular cartilage rings.

1. Main bronchi: There are two supplying ventilation to each lung. The right main bronchus has a larger diameter and is aligned more vertically than the left

2. Lobar bronchi: Two on the left and three on the right supply each of the main lobes of the lung

3. Segmental bronchi supply individual bronchopulmonary segments of the lungs.

Bronchioles lack supporting cartilage skeletons and have a diameter of around 1 mm. They are initially ciliated and graduate to the simple columnar epithelium and their lining cells no longer contain mucous producing cells.

1. Conducting bronchioles conduct airflow but do not contain any mucous glands or seromucous glands

2. Terminal bronchioles are the last division of the airway without respiratory surfaces

3. Respiratory bronchioles contain occasional alveoli and have surface surfactant-producing They each give rise to between two and 11 alveolar ducts.

Alveolar is the final portion of the airway and is lined with a single-cell layer of pneumocytes and in proximity to capillaries. They contain surfactant producing type II pneumocytes and Clara cells.

1. Alveolar ducts are tubular portions with respiratory surfaces from which the alveolar sacs bud.

2. Alveolar sacs are the blind-ended spaces from which the alveoli clusters are formed and to where they connect. These are connected by pores which allow air pressure to equalize between them. Together, with the capillaries, they form the air-blood barrier.