Respiratory Tract – Anatomy, Nerve Supply, Function

The Respiratory Tract is the subdivision of the respiratory system involved with the process of respiration in mammals. The respiratory tract is lined with respiratory mucosa or respiratory epithelium.^[rx]

The 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.

Types of Respiratory Tract

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:

• 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
• 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
• 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.

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

• Conducting bronchioles conduct airflow but do not contain any mucous glands or seromucous glands
• Terminal bronchioles are the last division of the airway without respiratory surfaces
• Respiratory bronchioles contain occasional alveoli and have surface surfactant-producing They each give rise to between two and 11 alveolar ducts.

The 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.

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

Respiratory Tree or Tracheobronchial Tree

The lower respiratory tract is also called the respiratory tree or tracheobronchial tree, to describe the branching structure of airways supplying air to the lungs, and includes the trachea, bronchi and bronchioles.^[rx]

• trachea
  • main bronchus (diameter approximately 1 – 1.4 cm in adults)
    • lobar bronchus (diameter approximately 1 cm)
      • segmental bronchus (diameter 4.5 to 13 mm)
        • subsegmental bronchus (diameter 1 to 6 mm)
          • conducting bronchiole
            • terminal bronchiole
              • respiratory bronchiole
                • alveolar duct
                  • alveolar sac
                    • alveolus

At each division point or generation, one airway branches into two or smaller airways. The human respiratory tree may consist on an average of 23 generations, while the respiratory tree of the mouse has up to 13 generations. Proximal divisions (those closest to the top of the tree, such as the bronchi) mainly function to transmit air to the lower airways. Later divisions including the respiratory bronchiole, alveolar ducts, and alveoli, are specialized for gas exchange.

Structure of Respiratory Tract

Airways allow airflow in ventilation from the external environment to the respiratory surfaces where gas exchange for respiratory processes can occur.[rx][rx]

To allow this and to maintain homeostasis and adequate protection from the external environment they must also perform other barrier functions.

• Moisture barrier is the mucous lining of the airway that provides a barrier to prevent loss of excessive moisture during ventilation by increasing the humidity of the air in the upper airway
• The temperature barrier is relative to body temperature as the external environment is nearly always colder, and the increased vasculature and structures such as nasal turbinates warm air as it enters the airways
• A barrier to infection as the airways are lined with a rich lymphatic system including mucosa-associated lymphoid tissue (MALT) that prevents early access to any invading pathogens. Macrophages also patrol the respiratory surfaces providing an important component of the “air-blood barrier.”

Blood Supply of Respiratory Tract

The upper airways receive blood supply from various branches of the external carotid artery and drain into the internal jugular. The naso and oropharynx also receive blood supply from the facial artery branch of the external carotid via the tonsillar artery. The venous drainage of these structures is via the pharyngeal plexus into the internal jugular vein. The lymphatic drainage is through various lymphatic plexuses of the neck surrounding the internal jugular vessels.

The lower airways receive blood flow from two sources: the pulmonary circulation and the bronchial circulation.

The pulmonary circulation provides blood from the heart for oxygenation through the right and left pulmonary arteries which follow a branching structure similar to that of the airways themselves. This blood returns as oxygenated blood through the pulmonary veins which follow an independently branching structure to return to the right ventricle.

Bronchial circulation provides oxygenated blood to the airway structures themselves. These arteries arise independently from the systemic circulation. The two left bronchial arteries to emerge from the thoracic aorta; whereas, the right bronchial artery arises either from one of the superior posterior intercostal arteries or a common trunk with the left superior bronchial artery. These provide nutrition and oxygen to tissues as far as the end of the conducting airways where they anastomose with the pulmonary circulation.

The bronchial veins are only present near the lung hilum which drain blood from the trachea, and bronchi drain into the azygos vein on the right and either the accessory hemiazygos veins or the intercostal vessels on the left. Pulmonary veins drain the more distal circulation where a small amount of deoxygenated blood makes a minimal impact on the saturation of the returning blood.

Lymphatic drainage of the lower airways is through the deep lymphatic plexuses of the pulmonary lymphatic plexuses. These drain to the superior and inferior tracheobronchial lymph nodes bilaterally and then to the right and left ducts connecting to the venous angles, usually directly but on the left, this may converge with the thoracic duct first.

Paratracheal nodes drain lymph from the trachea directly into the right and left lymphatic ducts.

Nerves Supply of Respiratory Tract

Innervation of the pharynx is via cranial nerves VII, IX, X, and XII. The larynx is supplied by the vagus (cranial nerve X) by the superior laryngeal branch directly and the clinically important recurrent laryngeal branch.

The lower airways receive parasympathetic fibers from the vagus, some of which are afferent sensory nerves that transmit cough sensations from specialized J receptors in the mucosa as well as stretch receptors from the bronchial muscles and inter-alveolar connective tissues. The efferent fibers of the vagus cause broncho-constriction and secretion from the glandular tissues in the airways. The efferent sympathetic fibers cause bronchodilation by inhibiting the activity of the smooth muscles of the airways.

Muscles

The muscles of the pharynx and larynx provide the structure of the upper airways and form from striated muscles under visceral and somatic control. They relate to the action of swallowing.

The lower airways have a layer of smooth muscle within their walls. It is present along all of the conducting airways and allows for visceral control of bronchoconstriction

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