The Aerodigestive Tract
The aerodigestive tract is the term given to the anatomic regions and organs that contribute to voice production, swallowing, and breathing. These regions all have important inter-relating tasks aimed at maintaining each of these primary functions. The following section briefly describes the anatomy of the aerodigestive tract and the physiology of speech, swallowing, and breathing.
Nasal Cavity: Region starting at the nostrils and opening into the nasopharynx through 2 openings called choanae. The nasal cavity is divided into two chambers by a midline structure called the nasal septum and is continuous with the paranasal sinuses through small openings called ostia.
Oral Cavity: Region starting at the vermillion lips and ending at the soft palate, which includes the teeth, hard palate, buccal mucosa (cheeks), alveoli (gums), and anterior tongue.
Pharynx: Region starting just behind the nasal cavity at the level of the base of skull ending at the esophagus and larynx. There are 3 subdivisions of the pharynx:
- Nasopharynx: Portion behind the nasal cavity ending at the level of the soft palate, which includes the adenoids and the eustachian tubes (structures that connect the middle ear to the nasopharynx).
- Oropharynx: Portion visible through the mouth starting at the soft palate and ending at the level of the larynx, which includes the tonsils and base of tongue.
- Hypopharynx: Also called the laryngopharynx, this portion starts at the level of the larynx and ends at the esophagus.
Larynx (Voice Box): Organ starting at the level of the oropharynx and hypopharynx and ending at the trachea. The larynx is responsible for airway protection, breathing, and voice production. There are 3 subdivisions of the larynx:
- Supraglottis: The portion starts at the tip of the epiglottis and ends in the ventricle, just above the vocal cords. Subsites include the epiglottis, the aryepiglottic folds, the false vocal folds, and the ventricles.
- Glottis: The portion that is comprised by the vocal cords (true vocal folds). Normally functioning vocal cords open for breathing (abduction) and close for swallowing, coughing, voice production, or valsalva (adduction). The vocal cords have a delicate layered microstructure responsible for voice production; the superficial lamina propria is the most critical layer (see below).
- Subglottis: The portion starts just below the true vocal folds (vocal cords) and ends at the trachea.
The cartilages that form the framework around the larynx include the epiglottic, arytenoid, corniculate, cuneiform, thyroid, and cricoid cartilages. The muscles of the larynx include the thyroarytenoid, lateral cricoarytenoid, posterior cricoarytenoid, interarytenoid, and cricothyroid muscles. The nerves that supply motor (movement) and sensory function include the recurrent laryngeal and superior laryngeal nerves.
Trachea: Component of the respiratory system that starts just below the subglottic larynx and ends at the bifurcation of the airway into 2 mainstem bronchi. The trachea is primarily involved in ventilation, or moving air into the lower airway, including the distal bronchi and alveoli, allowing for oxygen and carbon dioxide exchange at the alveolar level of the lungs.
Esophagus: Organ of the digestive tract starting just below the hypopharynx at the upper esophageal sphincter, or cricopharyngeus, and ending at the stomach. The esophagus carries food and liquids from the pharynx to the stomach.
Speech is the expression of thought through articulation of sound. Speech is generated through a number of complex processes starting with airflow coming from the lungs and ending in the words that are spoken. The primary elements of speech production include:
- Air movement from the lungs, which are the power source driving speech
- Conversion of airflow powered by the lungs into sound by the vocal cords
- Modification of sound by the resonating chambers of the pharynx and nasal cavity
- Articulation of sound into words by the palate, tongue, and lips
In order for the the vocal cords to produce sound, pulses of air move past their surfaces and generate an oscillating wave that transforms the air into sound pulses. The vocal cords are unique in that they have a multi-layered microstructure that allows for the generation of this “mucosal wave” responsible for sound production. In particular, the layer just below the surface of the vocal cords, called the superficial lamina propria, is a soft layer that is critical for normal mucosal wave propagation. Processes that alter the layered microstructure of the vocal cords, particularly the superficial lamina propria, also alter the ability to produce sound. For example, even a small cyst that rests just under the surface of the vocal cords will not allow the normal propagation of the mucosal wave and produce a raspy quality to the voice.
Figure 6: Topical Anatomy of the Larynx in: a) Animation and b) A Patient
Video 1: Normal Phonating Vocal Cords
Swallowing (or deglutition) is a complex motor and sensory function aimed at moving food and/or liquids from the mouth to the stomach. The complexity lies in the fact that food and liquid must travel through a common pathway for breathing and swallowing (aerodigestive tract), while avoiding going into the purely breathing passageway (trachea down to the lungs). Swallowing is broken down into the following component stages:
1. Oral Preparatory Stage: Food is chewed (or masticated), mixed with saliva, and formed into a cohesive ball (or bolus)
2. Oral Stage: The bolus is moved backward through the oral cavity with a front-to-back movement of the the tongue toward the oropharynx
3. Pharyngeal Stage: The bolus is moved through the oropharynx to the esophagus. The following events occur:
- As the bolus enters the oropharynx the pharyngeal musculature contracts circumferentially while the base of tongue pushes against the posterior pharyngeal wall. This pushes the bolus toward the esophagus.
- The soft palate elevates and closes off the passageway to the nasopharynx and nasal cavity. This prevents reflux of food and liquid through the nose.
- The larynx elevates, the epiglottis closes over the larynx, and the vocal cords close completely. This protects the airway as food and liquid traverses the larynx on its way to the esophagus.
- The upper esophageal sphincter, or cricopharyngeus, relaxes so that food can enter into the esophagus.
4. Esophageal Stage: The bolus enters the esophagus through the upper esophageal sphincter and is transported to the stomach by involuntary muscle contractions called peristalsis.
In addition to the complex motor function required to achieve each of these processes, the coordination and timing of each is greatly affected by sensation. For example, in vocal cord paralysis, the vocal cords may not close completely during swallowing, which leads to poor airway protection. However, there may also be a lack of sensation by the larynx. This means that liquid may go into the trachea during swallowing (aspiration), while the normal cough reflex that is supposed to expel the liquid does not correctly trigger.
Respiration is the process of transporting oxygen from the atmosphere into the body and carbon dioxide in the opposite direction. Breathing, or ventilation, is the process of getting air into the lungs so that these molecules can exchange at the level of the alveoli of the lungs. Breathing consists of two primary processes:
- Inhalation is an active process initiated by a muscle below and adjacent to the lungs called the diaphragm and supported by muscles that span the ribs in the chest called the external intercostals. During quiet inhalation, the diaphragm and external intercostals contract, leading to ribcage expansion and downward movement of the abdominal contents. This process results in an increase in chest cavity volume and negative pressure that draws air into the airway. In order to breathe more rapidly and/or deeply, accessory muscles are recruited that lead to a greater expansion of the thoracic cavity so that a greater volume of air can move through the respiratory system.
- Exhalation is more typically passive, but can be active as well. For passive exhalation, the natural elasticity of the lungs allow them to recoil after inhalation, resulting in air flowing back out of the airway until the pressure in the chest cavity is the same as the pressure in the atmosphere. During active exhalation, another set of muscles spanning the ribs in the chest, the internal intercostals, combine with abdominal muscles to generate abdominal and thoracic pressure that forces air out.
Breathing problems can be due to a flaw in the muscles necessary to drive inhalation or exhalation or from blockage along the airway. For example, in bilateral vocal cord paralysis, the vocal cords may be fixed in a near-closed position. Because of this, there is restricted movement of airflow at the level of the vocal cords, leading patients to feel dyspneic, or short of breath, particularly if they try to exert themselves. Obstruction at the vocal cord level produces a typical abnormal breathing sound called stridor.
Breathing during sleep is dependent on the nasal cavities and the pharynx (or throat, see the top picture). When sleeping, the tissue in the pharynx relaxes and the airway gets smaller. If breathing through the nose is poor, it leads to either mouth breathing or having to “pull” harder to get air through the nose; both of these increase the collapse of tissues in the pharynx during sleep. Snoring and sleep apnea are a spectrum of obstructive breathing problems during sleep. When the tissues of the pharynx collapse enough so that they touch, the mildest problem is snoring, where the tissues vibrate and make sound. As the collapse becomes more significant, airflow can slow down enough that oxygen levels decrease in the blood (hypopnea) or airflow can stop completely (apnea). When this occurs regularly throughout the night, it is considered obstructive sleep apnea, which can lead to a number of health hazards.
Inspire Sleep has more info on snoring and sleep apnea!