133 Conducting Zone

Nose and Paranasal Sinuses

The shape of the nose is determined by the ethmoid bone and the nasal septum.

The nose and paranasal sinuses form much of the upper respiratory tract, along with the pharynx. The upper respiratory tract is the entrance to the respiratory system, where air first enters the body. The overall function of the upper respiratory tract is to provide a pathway for air to reach the lower respiratory tract, where gas exchange occurs.

Anatomy and Physiology of the Nose

The external part of the human nose is the protruding part of the face that bears the nostrils. The shape of the nose is determined by the ethmoid bone and the nasal septum.

The ethmoid bone is the bone that separates the nose from the brain, and supports the shape and structure of the nasal and orbital cavities. The nasal septum is wall of cartilage that separates the right and left nostril chambers from each other. On average, the nose of a male is larger than that of a female, due to differences in facial bone structure between genders.

The interior of the nasal cavity is lined with mucous membranes, nasal hairs, and cilia (microscopic hairs), that perform many of the specialized functions of the nose. The macroscopic nasal hairs prevent large particles from reaching the lungs, while the cilia and mucus trap pathogens and dust to take them to the pharynx, where they can be destroyed by digestion.

Another function of the nose is the conditioning of inhaled air, warming it and making it more humid. Sneezing occurs from irritation of the nasal mucus, which expels foreign particles, but can also spread microbial and viral infections between humans.

Finally, the nose has an area of specialized cells that are responsible for smelling, which is considered a nervous system function rather than a respiratory system function.

Anatomy and Physiology of the Paranasal Sinuses

The paranasal sinuses are a group of four, paired, air-filled spaces, lined with respiratory epithelium (ciliated columnar epithelium). These are named according to the bones within which the sinuses lie: surrounding the nasal cavity (maxillary sinuses), above the eyes (frontal sinuses), between the eyes (ethmoid sinuses), and behind the ethmoid bone (sphenoid sinuses).

The functions of the sinuses are not fully understood, but there are many possible functions. The most important function is the sinuses’ role in draining mucus from the nasal cavity to the nasopharynx, which helps regulate pressure inside the nasal cavity. This may be a component of the barrier defenses of the innate immune system because of antimicrobial proteins found in the mucosa.

Other possible sinus functions include giving resonance to the voice, supporting the structure of the skull and facial bones, heating and humidifying inhaled air, and protecting the face from injury.

Pharynx

The human pharynx is part of the digestive system and also the respiratory system.

The Pharynx

The human pharynx (plural: pharynges) is the part of the throat situated immediately posterior to the mouth and nasal cavity, and superior to the esophagus and larynx.

The human pharynx is divided into three sections: the nasopharynx (epipharynx), the oropharynx (mesopharynx), and the laryngopharynx (hypopharynx), which are all innervated by the pharyngeal plexus.

The pharynx is part of both the digestive system and the respiratory system. As a component of the upper respiratory tract, the pharynx is part of the conducting zone for air into the lungs. Therefore, one of its primary functions is to warm and humidify air before it reaches the lungs.

The Nasopharynx

The nasopharynx is the upper region of the pharynx. It extends from the base of the skull to the upper surface of the soft palate above the oral cavity. The nasophaynx connects the nasal cavity with the throat.

The nasopharynx connects to the eustachian tubes of the middle ear, which allows the nasopharynx to help balance pressure within the ear. However, it also allows infections to spread easily between the nasopharynx and ear. The nasopharynx contains psuedo-stratified squamous cell epithelia tissue that is ciliated (covered in tiny hairs that move mucus).

The adenoids (pharyngeal tonsils) are a mass of lymphatic tissue found in the roof of the nasopharynx. The adenoids play a minor role in embyonic development and have a minor role in producing T-lymphocytes for the immune system after birth.

The adenoids are often removed in childhood due to infection or hypertrophy (enlargement of the cells in its tissues), which can obstruct the flow of air from the nose to the lung if left untreated. While loss of the adenoids does not make a significant difference in immune system function, the procedure occasionally has complications.

The lateral walls of the nasopharynx are made of the pharyngeal ostia (bone) of the auditory tube, and supported by the torus tubarus, a mound of cartilage tissue from the auditory tube. Two folds arise from the cartilaginous opening of the auditory tube.

The salpingopharyngeal fold is a vertical fold of mucous membrane extending from the inferior part of the torus and is made up of salpingopharyngeus muscle. The salpingopalatine fold is a smaller fold extending from the superior part of the torus to the palate; it contains the levator veli palatini muscle.

Behind the bone of the auditory tube is a deep recess, the pharyngeal recess. Above the adenoid, in the midline, is an irregular flask-shaped depression of the mucous membrane called the pharyngeal bursa.

The Oropharynx

The orpharnyx (mesopharynx) is the middle portion of the pharynx. It lies between the oral cavity, below the nasopharynx, and above the laryngopharynx, and has an opening to each of these other cavities. The anterior wall of the oropharynx consists of the base of the tongue and the superior wall consists of the bottom surface of the soft palate and the uvula.

The oropharynx is lined by non-keratinized squamous stratified epithelium, which is thicker than the epithelium found in other parts of the respiratory tract in order to prevent damage from food, but not as thick as skin as it lacks keratin.

The epiglottis lies between the oropharynx and the laryngopharynx, and it is a flap of elastic cartilage that closes during swallowing to ensure food enters the esophagus rather than the trachea.

The oropharynx contains the palatine tonsils, which are masses of lymphoid tissue found on the lateral walls of the oropharynx. Compared to the adenoids of the nasopharynx, the palatine tonsils contain many folds (called crypts), and aren’t ciliated like the adenoids are. These tonsils are also occasionally removed in people with infection or enlargement.

The Laryngopharynx

The laryngopharynx or hypopharynx is the caudal part of the pharynx; it is the part of the throat that connects to the esophagus and trachea. It lies inferior to the epiglottis and marks the division between the respiratory and digestive system pathways.

During swallowing, the epiglottis closes over the trachea and air passage temporarily stops. The laryngopharynx naturally continues into the esophagus tissue and is made up of a similar type of stratified squamous epithelium tissue.

The layngopharynx itself has a few important demarcations and regions. The formal superior boundary that separates the laryngopharynx from the oropharynx is at the level of the hyoid bone.

The laryngopharynx includes three major regions: the pyriform sinus, the postcricoid area, and the posterior pharyngeal wall, which are separated by small folds of cartilage. Unlike the nasopharynx and oropharynx, there are no tonsils in the laryngopharynx.

Larynx

The larynx is an organ in the neck involved in breathing, sound production, and protecting the trachea against food aspiration.

The larynx (plural: larynges), commonly called the voice box, is an organ in the neck of humans and most animals that is involved in breathing, sound production, coughing, and protecting the trachea against food aspiration during eating.

Anatomy of the Larynx

In adult humans, the larynx is found in the anterior neck at the level of the C3–C6 vertebrae in the backbone. It connects the inferior part of the pharynx (laryngopharynx) with the trachea. The laryngeal skeleton consists of three single cartilages (thyroid, epiglottic, and cricoid).

1. The thyroid cartilage is particularly notable for forming the Adam’s apple, the visible bulge made by the larynx when looking at the throat, and protects the larynx from injury.
2. The epiglottic cartilage is the body of the epiglottis itself that connects to the larynx from above.
3. The cricoid cartilage connects the larynx to the trachea from below.

There are also three sets of cartilages that are paired on either side of the larynx (arytenoid, corniculate, and cuneiform) that allow the position of the larynx to move during voice production.

The larynx connects to the hyoid bone (the bone that forms the floor of the mouth) from above. The larynx extends vertically from the tip of the epiglottis to the border of the cricoid cartilage that marks the formal beginning of the trachea.

The interior of the larynx consists of three regions, the supraglottis, glottis, and subglottis. The glottis is the midsection that contains the vocal folds (folds of muscular epithelium ), while the supraglottis and subglottis are the areas of the larynx that are above and below the glottis respectively. In newborn infants, the larynx is initially at the level of the C2–C3 vertebrae, but descends as the child grows.

The glottis consists of two pairs of mucosal folds. These folds are false vocal folds (vestibular folds) and true vocal folds (folds). The false vocal folds are covered by respiratory epithelium, while the true vocal folds are covered by stratified squamous epithelium.

The false vocal folds are not responsible for sound production, but rather for resonance. These false vocal folds do not contain muscle, while the true vocal folds do have skeletal muscle. The two sets of folds are separated by the vocal ligament, with the false vocal folds above, and the true vocal cords below the ligament. The true vocal folds are often referred to as the vocal cords, however the folds technically aren’t cords.

Physiology of the Larynx

The most notable and unique function of the larynx is phonation (voice production). The vocal folds of the larynx have two positions, open and closed. During breathing the folds remain open, but they close during swallowing or phonation.

When air from the lungs passes through closed folds during exhalation, the folds vibrate and create sound. The pitch produced depends on the length and tightness of the vocal folds.

The vagus nerves innervate the larynx and signal the muscles and paired cartilage (the arytenoid) of the larynx to work together to open and close the vocal folds as well as change their length and tension to alter pitch. Longer vocal folds have a lower pitch, which is part of the reason why men have deeper voices compared to women, because their larger larynxes have longer vocal folds.

Besides phonation, there are a few other important functions of the larynx. The folds of the larynx close and move upwards during swallowing, which causes the epiglottis to close off the trachea. This helps prevent aspiration of food into the lungs or choking from a blockage of food in the trachea.

The larynx closes off during coughing to help prevent harmful gasses from entering the lungs. During a cough reflex, air is forced out of the lungs, which can remove accumulated mucus, fluid, or blood from the lungs during injury, infection, or cancer of the lungs, as well as food or objects in the trachea during choking.

Finally, the larynx can be signaled to open its folds wider than usual to increase the flow of air into and out of the lungs during heavy breathing when the body requires more oxygen.

Structures Used in Voice Production

Voices produce sounds through a steady flow of air through the larynx, which causes vibrations and creates fluctuations in air pressure.

Voice production is a complex process with many different layers and intricacies. The three basic mechanisms of voice production are air supply, vibration, and resonance.

Passive and active articulation shapes and refines phonation (vocal sound production) into the sounds and words used in communication. Voice production is an important physiological process because it enables complex communication between humans.

While the brain is responsible for higher organization and understanding language, the structures of the respiratory system are largely responsible for the production of sound itself.

Basic Mechanisms of Voice Production

Sound is produced by a combination of different structures of the respiratory system working together to create and resonate a sound. There are three basic mechanisms by which the human body produces a voice.

1. Air Supply: In order for voice to be produced, air must flow through the vocal folds. The supply of air for phonation comes from the lungs, and the speed and pressure by which is flows through the vocal folds is determined by the diaphragm. The speed of air flow also determines the strength and loudness of the voice.
2. Vibration: The vocal folds in the glottis of the larynx vibrate as air passes through them. The vibration creates changes in air pressure that manifest as audible sound waves. They only vibrate if the vocal folds are in the closed position, when the folds are held together by the movement of ayrtenoid cartilage. The pitch of the vibration depends on the length and tension of the vocal folds, which can be altered by muscle action.
3. Resonance: The structures of the upper respiratory tract—particularly the soft palate of the mouth, the nasopharynx, and the paranasal sinuses —resonate and amplify the vibration of the vocal folds, making the sound louder and changing its tone. It works similarly to the way the sounding board of a guitar amplifies the vibration of the strings.

These basic mechanisms work together to create the voice. If they are altered, the produced voice will also be altered as well.

For example, during loud voice production, such as shouting or singing, a greater air supply and greater pressure for the flow of air through the vocal folds is required to produce the louder sound. The diaphragm must contract harder to support this greater flow of air compared to normal speech.

Similarly, whispering takes less air compared to normal speech, because the sound produced during whispering is much weaker in comparison.

Articulation

Articulation is the process by which phonation is refined into the specific consonants and vowels used to form words. The articulation of consonants occurs at a point of either active or passive articulation, which is a place in the vocal tract where an obstruction stops the sound.

After the sound is obstructed, the pressure from the air builds based on the shape of that obstruction, which changes the sound into the form it is vocalized as. Vowels are articulated sounds that do not come from obstruction, and instead come from an open vocal tract.

Passive Place of Articulation

The passive place of articulation is the place on the more stationary part of the vocal tract where the articulation occurs. It can be anywhere from the lips, upper teeth, gums, roof of the mouth, or the back of the throat. These areas are passive because no specific action or activity is involved within that area to pronounce the consonant.

Passive articulation is considered a continuum because the obstruction of many different places is needed to produce most of the consonants. There are also several different combinations of areas that can produce the same consonant; for example, many languages may distinguish consonants by articulating them in different areas. Passive places of articulation include:

• The upper lip (labial).
• The upper teeth, either on the edge of the teeth or inner surface (dental).
• The alveolar ridge, the gum line just behind the teeth (alveolar).
• The back of the alveolar ridge (post-alveolar).
• The hard palate on the roof of the mouth (palatal).
• The soft palate further back on the roof of the mouth (velar).
• The uvula hanging down at the entrance to the throat (uvular).
• The throat itself, also known as the pharynx (pharyngeal).
• The epiglottis at the entrance to the windpipe, above the voice box (epiglottal).

Active Place of Articulation

The articulatory gesture of the active place of articulation involves the more mobile part of the vocal tract. This is typically some part of the tongue or lips. It is considered active because these areas change the consonant pronounced by moving or changing.

The active places of articulation are not considered a continuum (unlike passive articulation) because they work independently of each other, but they have the capacity to work together for certain consonants. Active places of articulation include:

• The lower lip (labial).
• Various parts of the front of the tongue.
• The back of the tongue. The aryepiglottic folds at the entrance to the larynx (also epiglottal).
• The glottis (laryngeal).

Vowels

A vowel is a sound that comes from an open vocal tract, and does involve strict obstruction of the sound as with consonants. Therefore, there is more variation in the mechanisms used to create vowels compared to consonants.  Vowels are mainly articulated by the shape of the lips, the position of the tongue (both vertical and horizontal), and by the phonation of the larynx itself.

Trachea

The trachea, or windpipe, is a tube that connects the pharynx or larynx to the lungs, allowing the passage of air.

The trachea, or windpipe, is a tube that connects the pharynx or larynx to the lungs, allowing the passage of air. It is lined with pseudostratified ciliated columnar epithelium cells with goblet cells that produce mucus. The trachea is part of the conducting zone for air into and out of the lungs.

Anatomy of the Trachea

The trachea is a long tube that extends from the pharynx and larynx to the bronchi of the lungs. It typically has an inner diameter of about 25.4 millimeters (1.00 in) and a length of about 10 to 16 centimeters.

The trachea commences at the lower border of the larynx, level with the sixth cervical vertebra, and bifurcates into the primary bronchi at the vertebral level of thoracic vertebra T5, or up to two vertebrae lower or higher, depending on breathing.

At the top of the trachea and bottom of the larynx is the cricoid cartilage, the only complete ring of cartilage in the trachea. Extending downward throughout the length of the tube are about fifteen to 20 C-shaped cartilaginous rings that reinforce the outer structure and shape of the trachea—the open part of each C-shaped ring reveals a membranous wall on the inside of the trachea.

The cartilage of the trachea is considered hyaline cartilage: simple, transparent, and made primarily of collagen. The trachealis muscle connects the open ends of the C-shaped rings of cartilage and contracts during coughing, reducing the size of the lumen of the trachea to increase the air flow rate.

The esophagus lies behind the trachea. The C-shaped cartilaginous rings allow the trachea to collapse slightly at its opening, so food can pass down the esophagus after swallowing.

The epiglottis closes the opening to the larynx during swallowing to prevent swallowed matter from entering the trachea.

Physiology of the Trachea

This mucus and cilia of the trachea form the mucociliary escalator, which lines the cells of the trachea with mucus to trap inhaled foreign particles. The cilia then waft upward toward the larynx and the pharynx, where it can be either swallowed into the stomach (and destroyed by acid) or expelled as phlegm.

The mucociliary escalator is one of the most important functions of the trachea and is also considered a barrier component of the immune system due its role in preventing pathogens from entering the lungs. The epithelium and the mucociliary ladder can be damaged by smoking tobacco and alcohol consumption, which can make pneumonia (an infection of the alveoli of the lungs) from bacteria in the upper respiratory tract more likely to occur due to the loss of barrier function.

As a part of the conducting zone of the lungs, the trachea is important in warming and moistening air before it reaches the lungs. The trachea is also considered a part of normal anatomical dead space (space in the airway that isn’t involved in alveolar gas exchange) and its volume contributes to calculations of ventilation and physiological (total) dead space. It is not considered alveolar dead space, a term that refers to alveoli that don’t partake in gas exchange due to damage or lack of blood supply.