Viscerocranium, Pharynx, Larynx, Trachea & Respiratory System Anatomy

Viscerocranium & Pharynx Anatomy

Viscerocranium

  • The viscerocranium includes structures such as the:
    • Frontal sinus.
    • Nasal cavity with superior, middle, and inferior conchae.
    • Nasopharynx.
    • Hard palate.
    • Oral cavity proper.
    • Oropharynx.
    • Laryngopharynx.
    • Hyoid bone.
    • Esophagus.
    • Sphenoidal sinus.

Pharynx

  • Location: Connects the nasal and oral cavities to the larynx and esophagus.
  • Regions:
    • Nasopharynx: Extends from the base of the skull to the soft palate; primarily respiratory function; contains the pharyngeal tonsil (adenoids when enlarged), opening of the pharyngotympanic tube, salpingopharyngeal fold, and tubal tonsil.
      • Communicates with the nasal cavity via the choanae (posterior nasal apertures).
      • Pharyngeal tonsil: lymphoid tissue located in the roof of the nasopharynx. Enlarged pharyngeal tonsils are known as adenoids.
      • Pharyngeal opening of auditory tube: Located in the lateral wall of the nasopharynx. Cartilage of the auditory tube elevates the mucosa, forming the torus tubarius.
      • Salpingopharyngeal fold: A mucosal fold extending downward from the posterior limb of the torus tubarius, covering the salpingopharyngeus muscle.
      • Tubal tonsil: Lymphoid tissue around the pharyngeal opening of the auditory tube.
    • Oropharynx: Extends from the soft palate to the superior border of the epiglottis; involved in swallowing; contains the palatine tonsil in the tonsillar fossa between the palatoglossal and palatopharyngeal arches; epiglottic valleculae are located between the epiglottis and the base of the tongue.
      • Communicates with the oral cavity via the oropharyngeal isthmus.
      • Palatine tonsil: Located in the lateral wall between the palatoglossal and palatopharyngeal arches, within the tonsillar fossa/sinus.
      • Glossoepiglottic folds: Three folds (1 median and 2 lateral) that connect the anterior surface of the epiglottis to the back of the tongue.
      • Epiglottic valleculae: Two small depressions located between the epiglottis and the back of the tongue, one on either side of the median glossoepiglottic fold.
    • Laryngopharynx (Hypopharynx): Extends from the superior border of the epiglottis to the inferior border of the cricoid cartilage; connects to the esophagus; contains the piriform fossa (recess) where the internal laryngeal nerve is located under the mucosa.
      • Extends from the superior border of the epiglottis to the inferior border of the cricoid cartilage, where it becomes continuous with the esophagus.
      • Communicates with the laryngeal cavity via the laryngeal inlet.
      • Piriform fossa (recess): A depression located inferolateral to the laryngeal inlet. The internal laryngeal nerve is located under the mucosa of the piriform fossa.
  • Pharyngeal Lymphoid Ring (Waldeyer’s Ring):
    • Accumulations of lymphoid tissue at the entrance of the pharynx from the nasal and oral cavities.
    • Components: pharyngeal, tubal, lingual, and palatine tonsils.

Muscles of the Pharynx

  • Constrictor Muscles:
    • Fibers are mostly circularly oriented.
    • Three constrictor muscles: superior, middle, and inferior.
    • Arrangement: like a stack of three glasses, with the superior constrictor partly overlapped by the middle constrictor, and the middle constrictor partly overlapped by the inferior constrictor.
  • Longitudinal Muscles:
    • Stylopharyngeus
    • Palatopharyngeus
    • Salpingopharyngeus

Pharynx Muscle Details

  • Superior Constrictor:
    • Parts: pterygopharyngeal, buccopharyngeal, mylopharyngeal, glossopharyngeal, pterygomandibular raphe.
  • Middle Constrictor.
  • Inferior Constrictor:
    • Parts: thyropharyngeal, cricopharyngeal.

Nerve Supply of the Pharynx

  • Motor Nerve Supply:
    • All muscles are supplied by the vagus nerve (CN X), except the stylopharyngeus (supplied by the glossopharyngeal nerve (CN IX)).
  • Sensory Nerve Supply:
    • Glossopharyngeal nerve (CN IX) is the main source of sensory innervation to the pharyngeal mucosa.
    • Maxillary nerve (V2) supplies the mucosa of the upper part of the nasopharynx via the pharyngeal branch of the pterygopalatine ganglion.
    • Vagus nerve supplies mucosa around the laryngeal inlet.

Blood Supply of the Pharynx

  • Arteries:
    • Ascending pharyngeal artery
    • Facial artery
    • Maxillary artery
    • Lingual artery
    • Superior thyroid artery
  • Veins:
    • Form a plexus on the outer surface of the pharynx.
    • Drains above into the pterygoid venous plexus and below into the facial vein and directly into the internal jugular vein.

Larynx Anatomy

  • A complex organ of voice production composed of nine cartilages connected by membranes and ligaments and containing vocal folds.

  • Location: Anterior neck at the level of C3-C6 vertebrae.

  • Connects the inferior part of the pharynx with the trachea.

  • Functions:

    • Phonation (voice production).
    • Guards the air passages, especially during swallowing, acting as a sphincter or valve of the lower respiratory tract to maintain a patent airway.

  • Laryngeal Skeleton: Consists of nine cartilages:

    • Three single cartilages: thyroid, cricoid, and epiglottic.
    • Three paired cartilages: arytenoid, corniculate, and cuneiform.

Interior of Larynx:

  • Laryngeal Vestibule: lies between the laryngeal inlet and the vestibular folds.
  • Middle Part of the Laryngeal Cavity: The central airway between the vestibular and vocal folds.
  • Laryngeal Ventricle: Recesses extending laterally from the middle part of the laryngeal cavity between the vestibular and vocal folds.
  • Laryngeal Saccule: A blind pocket opening into each ventricle.
  • Infraglottic Cavity: The inferior cavity of the larynx between the vocal folds and the inferior border of the cricoid cartilage.
  • Vocal Folds:
    • Function: control sound production.
    • Each vocal fold contains:
      • Vocal ligament: thickened elastic tissue.
      • Vocalis muscle: composed of exceptionally fine muscle.

Arteries of the Larynx

  • Superior Laryngeal Artery: Supplies the internal surface of the larynx.
  • Cricothyroid Artery: Supplies the cricothyroid muscle.
  • Inferior Laryngeal Artery: Supplies the mucous membrane and muscles in the inferior part of the larynx.

Nerves of the Larynx

  • The nerves of the larynx are the superior and inferior laryngeal branches of the vagus nerve (CN X).
  • Superior Laryngeal Nerve:
    • Divides into two terminal branches within the carotid sheath:
      • Internal laryngeal nerve: sensory and autonomic functions.
      • External laryngeal nerve: motor function.
  • Inferior Laryngeal Nerve:
    • Continuation of the recurrent laryngeal nerve.
    • Divides into anterior and posterior branches:
      • Anterior branch supplies: lateral cricoarytenoid, thyroarytenoid, vocalis, aryepiglottic, and thyroepiglottic muscles.
      • Posterior branch supplies: posterior cricoarytenoid, and transverse and oblique arytenoid muscles.
    • The inferior laryngeal nerve supplies all intrinsic muscles of the larynx, except the cricothyroid muscle.

Trachea

  • Extends from the larynx into the thorax, terminating inferiorly as it divides into right and left main bronchi.
  • Function: Transports air to and from the lungs; its epithelium propels debris-laden mucus toward the pharynx for expulsion from the mouth.
  • Fibrocartilaginous tube, supported by incomplete cartilaginous tracheal cartilages (rings) that keep the trachea patent.
  • Location: Occupies a median position in the neck.
  • The posterior gaps in the tracheal rings are spanned by the involuntary trachealis muscle.
  • Dimensions:
    • Adults: approximately 2.5 cm in diameter.
    • Infants: approximately the diameter of a pencil.
  • Extends from the inferior end of the larynx at the level of the C6 vertebra to the sternal angle (T4/T5 disc level), where it bifurcates into the main bronchi.

Respiratory System Overview

  • The larynx is the superior end of the lower respiratory tract, modified to regulate entry into or close off the lower respiratory tract. It also modifies the exit of air from the tract to produce tone for vocalization.
  • With the diaphragm, it regulates intra-abdominal pressure through air retention and the force and suddenness by which air exits the tract (e.g., exhaling versus coughing or sneezing).
  • The larynx consists of a cartilaginous articulating skeleton joined by ligaments, membranes, and muscles, lined with mucous membrane.
  • Nerve Supply:
    • **Internal laryngeal nerve: sensory nerve.
    • **Recurrent laryngeal nerve: motor nerve to all muscles of the larynx except one.
      • Inferior laryngeal nerve: terminal branch of the recurrent laryngeal nerve.
    • External laryngeal nerve: supplies the cricothyroid muscle.
  • The trachea is the median fibrocartilaginous tube extending between the cricoid cartilage at the C6 vertebral level and its bifurcation into main bronchi at the T4–T5 IV disc level (level of sternal angle).

Thoracic Cavity

  • Divided into three compartments:
    • Right and left pulmonary cavities: contain the lungs and pleurae.
    • Mediastinum: a central compartment between the pulmonary cavities, containing the heart, great vessels, trachea, esophagus, thymus, and lymph nodes.
      • Extends vertically from the superior thoracic aperture to the diaphragm and anteroposteriorly from the sternum to the thoracic vertebrae.

Pleurae

  • Each lung is invested by a serous pleural sac of two continuous membranes:
    • Visceral pleura: invests all surfaces of the lungs.
    • Parietal pleura: lines the pulmonary cavities.
  • Pleural Cavity:
    • Potential space between the layers of pleura.
    • Contains a capillary layer of serous pleural fluid for lubrication during respiration.
  • Parts of the Parietal Pleura:
    • Mediastinal pleura: covers the lateral aspects of the mediastinum.
    • Diaphragmatic pleura: covers the superior surface of the diaphragm on each side of the mediastinum, except along its costal attachments and where it is fused to the pericardium.
    • Cervical pleura: covers the apex of the lung, extending superiorly into the root of the neck.

General Lung Anatomy

  • Each lung has:
    • An apex: extending above the 1st rib into the root of the neck, covered by cervical pleura.
    • A base: concave inferior surface resting on the diaphragm.
    • Two or three lobes, created by one or two fissures.
    • Three surfaces: costal, mediastinal, and diaphragmatic.
    • Three borders: anterior, inferior, and posterior.
  • Hilum of the Lung: a wedge-shaped area on the mediastinal surface through which structures enter and exit.

Lungs

  • The lungs are the vital organs of respiration in which venous blood exchanges oxygen and carbon dioxide with a tidal airflow.
  • Air and blood are delivered to each lung via its root, consisting of a pulmonary artery and vein and a main bronchus and their branches/tributaries that enter the lung at its hilum.
  • Both lungs are pyramidal, having an apex, a base, three surfaces, and three borders.
  • The right lung has three lobes that are separated by horizontal and oblique fissures.
  • The left lung has two lobes, separated by an oblique fissure, and features a marked cardiac notch in its anterior border owing to the asymmetrical placement of the heart.

Tracheobronchial Tree

  • The trachea bifurcates at the level of the transverse thoracic plane (or sternal angle) into main bronchi, one to each lung.
  • The right main bronchus is wider, shorter, and runs more vertically than the left main bronchus.
  • The left main bronchus passes inferolaterally, inferior to the arch of the aorta and anterior to the esophagus and thoracic aorta.
Bronchopulmonary Segments:
  • Largest subdivisions of a lobe.
  • Pyramidal-shaped segments of the lung, with their apices facing the lung root and their bases at the pleural surface.
  • Separated from adjacent segments by connective tissue septa.
  • Supplied independently by a segmental bronchus and a tertiary branch of the pulmonary artery.
  • Named according to the segmental bronchi supplying them.
  • Drained by intersegmental parts of the pulmonary veins that lie in the connective tissue between and drain adjacent segments.
  • Usually 18–20 in number (10 in the right lung; 8–10 in the left lung, depending on the combining of segments).
  • Surgically resectable.

Tracheobronchial Tree

  • The tracheobronchial tree is distinguished grossly by cartilage in its walls.
  • The bifurcation of the trachea (at the level of the sternal angle) is asymmetrical: the right main bronchus is more vertical and of greater caliber than the left.
  • The bronchi and pulmonary arteries course and branch together: the main bronchi/arteries each serve a lung, second-order lobar branches supply two left and three right lobes, and third-order segmental branches supply the 8–10 bronchopulmonary segments of each lung.
  • The bronchopulmonary segment is the smallest resectable division of the lung.
  • The pulmonary veins run independent intersegmental courses, draining adjacent bronchopulmonary segments.
  • The structures of the root of the lung and supporting tissues (and part of the esophagus) are supplied by bronchial arteries.
Vasculature of Lungs and Pleurae
  • Bronchial Arteries:
    • Supply blood for nutrition of the structures making up the root of the lungs, the supporting tissues of the lungs, and the visceral pleura.
    • The two left bronchial arteries usually arise directly from the thoracic aorta.
    • The single right bronchial artery may also arise directly from the aorta; however, it commonly arises indirectly, either by way of the proximal part of one of the upper posterior intercostal arteries (usually the right 3rd posterior intercostal artery) or from a common trunk with the arterial supply
  • Bronchial Veins:
    • Drain only part of the blood supplied to the lungs by the bronchial arteries.
    • The remainder of the blood is drained by the pulmonary veins, specifically the blood returning from the visceral pleura, the peripheral regions of the lung, and the distal components of the root of the lung.
    • The right bronchial vein drains into the azygos vein.
    • The left bronchial vein drains into the accessory hemi-azygos vein or the left superior intercostal vein.