Respiratory LOs

Introduction to Structure & Function

Outline the gross features and regions of the respiratory system. 

The respiratory system conducts air that is inhaled from the environment to the lungs, to be processed and used by the body.

Components from proximal to distal:

• Nose/nasal cavity and Mouth/oral cavity

• Pharynx - throat

  • Shared with digestive pathway

• Larynx - ‘voice box’

• Trachea - ‘windpipe,’ and Bronchial tree

Nose, Nasal Cavity, Mouth, and Oral Cavity

Define the boundaries, spaces, structures and functions of the nose, nasal cavity, mouth, and oral cavity.  

Nose

The nose is an external structure of the face that is formed by the nasal bones and cartilage that project anteriorly to form the bridge of the nose. Two openings, the nares, or nostrils, serve as the entrance into the nose. Just past the nares, there is a cavity on each side about the size of a fingertip, called the nasal vestibules. The skin flaps forming the lateral walls of the nasal vestibules are the ala (= wings) of the nose. The nares are divided from one another by the nasal columella, a skin covered column that runs vertically between the upper lip and the tip of the nose. The nasal columella is an anterior extension of the nasal septum. 

Nasal cavity

Boundaries: 

• Anterior: nasal vestibules

• Lateral: lateral nasal wall, conchae and meatuses 

• Posterior: choanae 

• Inferior: palate 

The nasal cavity is divided at the midline by the nasal septum, which forms a right and left nasal cavity. Each side of the nasal cavity has a lateral wall and a shared medial wall, the nasal septum. The nasal septum is made up of three parts:

• Anterior: septal cartilage 

• Superior posterior: perpendicular plate of the ethmoid bone

• Inferior posterior: vomer 

Associated with the lateral wall are three conchae (singular: concha), superior, middle and inferior, which project from the wall into the nasal cavity. Also known as the turbinates, the nasal conchae are mucosa-covered, scroll-shaped bony elements that act to increase surface area of the nasal cavity, protect the openings of the sinuses that drain inferior to them, and direct airflow to the choanae and nasopharynx. The superior and middle conchae are of the ethmoid bone, while the inferior nasal concha is its own bone. 

Inferior to each concha is a meatus (superior, middle, and inferior), which is a space that contains openings for various sinuses.

The mucosa covering the walls of the nasal cavity serves to warm and moisten the air that is destined for the lungs, as well as trap particles before they reach the rest of the respiratory tract. 

There is a choana (plural: choanae) on each side of the posterior aspect of the septum, which separates the nasal cavities from the nasopharynx. As air passes through the nasal cavity, it travels through the choanae to enter the nasopharynx. 

Mouth

The mouth is an oral fissure of the face surrounded by two labia (lips) and it is the entrance into the oral cavity.

Oral Cavity

Boundaries:

• Anterior: lips

• Lateral: cheeks

• Posterior: palatoglossal arch

• Superior: hard palate

• Inferior: mylohyoid muscle

Subdivisions:

• Oral vestibule

• Oral cavity proper

• Sublingual space

The oral vestibule occupies the space between the teeth and the deep surfaces of the lips and cheeks. 

The oral cavity proper is the largest division of the oral cavity and is home to the body (anterior two-thirds) of the tongue. This space is also the location of several salivary gland openings. 

The sublingual space is a fascial space that sits deep to the oral floor mucosa and superior to the belly of the mylohyoid muscle. Neurovasculature serving the tongue is found in this space.

Explain the communication between the nasal meatuses, paranasal sinuses and nasolacrimal duct. 

Sphenoethmoid recess

• Location: anterior to body of sphenoid in superior nasal cavity

• Communication: sphenoid sinus 

Superior nasal meatus

• Location: deep to superior nasal concha

• Communication: posterior ethmoid air cells

Middle nasal meatus:

• Location: deep to middle nasal concha

• Communication:

  • Frontal sinus

  • Maxillary sinus 

  • Anterior and middle ethmoid air cells near ethmoid bulla and semilunar hiatus

Inferior nasal meatus

• Location: deep to inferior nasal concha

Communication: orbit via nasolacrimal duct

Pharynx

Describe the boundaries, contents, muscles, and innervation of the pharynx.

Boundaries: 

The pharynx is a common space for the conductive pathways of the respiratory and digestive systems. As such, the pharynx shares borders with the nasal cavity (choanae), oral cavity (palatoglossal folds), larynx (laryngeal inlet), and esophagus (entrance to esophagus).

The pharynx is divided into three regions, which reflect the above borders. They are the:

• Nasopharynx (choanae; soft palate)

• Oropharynx (soft palate; palatoglossal arch; epiglottis & pharyngo-epiglottic folds)

• Laryngopharynx (epiglottis & pharyngo-epiglottic folds; inferior margin of cricoid cartilage)

Contents:

The pharyngeal regions each contain a number of significant anatomical elements. The highlights of each region are as follows:

• Nasopharynx:

  • Pharyngeal tonsil (adenoid)

  • Pharyngotympanic (auditory) tubes & associated tubal tonsils

• Oropharynx:

  • Uvula

  • Fauces (=throat) - the space bounded by the palatoglossal folds (anteriorly) and palatopharyngeal folds (posteriorly)

    • Palatine tonsillar fossae & associated palatine tonsils

  • Root of the tongue (pharyngeal part of tongue) & associated lingual tonsil

  • Epiglottic valleculae

• Laryngopharynx:

  • Epiglottis

  • Laryngeal inlet

  • Piriform recesses

  • Opening to esophagus

Muscles

The muscular wall of the pharynx consists of two layers: an outer layer of predominantly circular-oriented constrictors, and an inner layer of longitudinally oriented muscles that shorten and widen the pharynx, and elevate the larynx.

The three pharyngeal constrictor muscles in the external layer of the pharynx surround the pharynx, and meet along a posterior midline pharyngeal raphe. When activated, the pharyngeal constrictor mm. serially constrict the lumen of the pharynx.

The superior constrictor m. attaches from the cranium and pharyngeal raphe to portions of the skull and a small part of the tongue. 

The middle constrictor m. originates from the pharyngeal raphe and attaches to the hyoid bone. 

And the inferior constrictor m. attaches from the raphe to the larynx. 

The pharyngeal constrictor mm. are innervated by the pharyngeal plexus, which receives input from:

• Vagus nn. (efferent; somatic motor)

• Glossopharyngeal nn. (afferent; somatic sensory). 

The three longitudinal muscles of the inner layer have a less coordinated anatomical arrangement than the constrictor muscles:

Stylopharyngeus m.

• Proximal attachment: styloid process of temporal bone

• Distal attachment: thyroid cartilage of larynx

• Innervation: Glossopharyngeal n. (CN IX)

• Actions: elevates the larynx and pharynx, widens and shortens the pharynx during deglutition (swallowing) 

Palatopharyngeus m.

• Proximal attachment: palatine aponeurosis 

• Distal attachment: superior margin of thyroid cartilage

• Innervation: Pharyngeal plexus

• Actions: elevates the larynx and pharynx, widens and shortens the pharynx during deglutition (swallowing) 

Salpingopharyngeus m.

• Proximal attachment: distal pharyngotympanic (auditory, Eustachian) tube 

• Distal attachment: superior margin of thyroid cartilage

• Innervation: Pharyngeal plexus

• Actions: elevates the larynx and pharynx, widens and shortens the pharynx during deglutition (swallowing), can aid in opening the distal pharyngotympanic tube to equalize pressure in the middle ear

Laryngoskeleton

Define the larynx and explain its location and functions.

The larynx is colloquially known as the voicebox. It’s a region of the respiratory system that is exclusively for the passage of air, and as the vocal folds approximate and vibrate, sound is produced. 

The larynx is suspended inferiorly from the hyoid bone by the thyrohyoid membrane, and the trachea is suspended from the inferior margin of the larynx by the cricotracheal ligament. Air within the larynx passes inferiorly into the trachea. 

The larynx sits anterior to the laryngopharynx, and as food or beverages pass from the oropharynx into the laryngopharynx toward the esophagus, the larynx elevates and epiglottis folds posteriorly to cover the laryngeal inlet.

Describe the parts of the laryngoskeleton.

The laryngoskeleton is the cartilaginous skeleton on the larynx that provides structural support and a site for laryngeal muscle attachment.   

There are three major and unpaired cartilages of the larynx:

• Thyroid cartilage

  • Largest

  • Made up of two laminae that join at the midline anteriorly 

    • Laryngeal prominence “Adam’s apple”

  • Superior and inferior horns

  • Incomplete posteriorly 

  • Articulates with:

    • Hyoid bone superiorly - via the thyrohyoid membrane

    • Cricoid cartilage inferiorly - via the cricothyroid joints

      • Action at this joint: thyroid cartilage tips anteriorly to increase tension on vocal folds 

• Cricoid cartilage

  • Inferior to the thyroid cartilage

  • Articulates with the thyroid cartilage at the cricothyroid joints 

  • Forms a complete ring around airway 

• Epiglottic cartilage 

  • Articulates with and is encompassed by the thyroid cartilage

  • Shaped like a leaf with a stem 

  • Mobile during deglutition (swallowing) 

There are three paired, much smaller cartilages of the larynx:

• Arytenoid cartilages

  • Sit directly atop the cricoid cartilage posteriorly 

  • Vocal process (anterior) and muscular process (posterolateral) 

  • Attachment site for the vocal ligaments 

  • Pivot and glide to position vocal ligaments 

• Corniculate cartilages

  • Sit atop arytenoid cartilages 

  • Act to extend attachment surface for arytenoid cartilages 

• Cuneiform cartilages 

  • Embedded within the ary-epiglottic folds that span the distance between the arytenoid and epiglottic cartilages 

  • Provide structural support for the walls of the laryngeal inlet

Explain the effects of the arytenoid cartilages and thyroid cartilage on the vocal folds. 

The vocal folds (vocal ligaments + overlying mucosa) vibrate with exhaled air (in a slightly adducted position), and they can be manipulated by the arytenoid cartilages and thyroid cartilage to have different effects on pitch and tone during phonation. 

Controlled by muscles attached to muscular processes, the arytenoid cartilages are mobile. As the arytenoids pivot and glide, the vocal ligaments (attached to vocal processes of the arytenoids) may either adduct of abduct. When the vocal ligaments are abducted (lateral movement), the space between the folds (rima glottidis/rima glottis) widens. This makes the airway more patent and allows for increased airflow. When the vocal ligaments are abducted (brought closer together), the space between them narrows. This action narrows the airway and approximates the vocal folds for phonation (speech). 

When the thyroid cartilage tips anteriorly at the cricothyroid joints, tension on the vocal ligaments increases. More tension increases pitch of sound. As the thyroid cartilage returns posteriorly, the decreased tension on the vocal ligaments lowers pitch. 

Laryngeal Membranes & Cavities

Describe the location, boundaries, and functions of the quadrangular membranes and conus elasticus. 

Two sheets of fibro-elastic connective tissue help to give shape to the walls of the larynx and laryngeal features: the quadrangular membrane (superiorly) and the conus elasticus (inferiorly).

The quadrangular membrane spans the distance between the lateral margins of the epiglottis and the arytenoid cartilages (ary-epiglottic ligaments). The inferior-most extent of the quadrangular membrane forms the vestibular ligaments. The vestibular ligaments are covered by mucosa to form the vestibular (false vocal) folds. The vestibular folds play an important role in protecting the airway, and may also be used to produce harmonic tone in singing. The quadrangular membrane is an important funnel for air that is inhaled.

The conus elasticus shapes the walls of the larynx as a funnel between the thyroid and cricoid cartilages. The superior-most aspects of the conus elasticus are the vocal ligaments. The anteromedial aspect of the conus elasticus is the (median) cricothyroid ligament, the connective tissue pierced during a cricothyrotomy. 

Understand the cavities of the larynx, including their locations and functions. 

Associated with the various membranes and folds of the larynx are different cavities (air spaces). As air passes through the larynx towards the lungs, it passes through these space in the follow order from superior to inferior:

• Laryngeal inlet - opening into larynx

• Laryngeal vestibule - transitional area between the inlet and deeper larynx 

• Rima vestibuli - space between the vestibular folds

• Laryngeal ventricle - space between the vestibular and vocal folds

  • Separates the quadrangular membranes from the conus elasticus

• Rima glottidis - space between the vocal folds 

  • Glottis = vocal folds + rima glottidis

    • Affects voice modulation through expansion or contraction

• Infraglottic cavity 

  • Narrows and continues inferiorly as the trachea

Discuss the location, attachments, and functions of the vestibular and vocal folds. 

The vestibular folds (ventricular folds; false vocal folds) are superior to the vocal folds and they attach from the arytenoid cartilages to the thyroid cartilage. The folds are formed by the vestibular ligament (free inferior edge of the quadrangular membrane) with the overlying mucosa. Their function is to protect the airway and vocal folds. 

The vocal ligaments are formed by the free superior edge of the conus elasticus in the larynx. They originate on the posterior aspect of the thyroid cartilage and run posteriorly to attach to the vocal processes of the arytenoid cartilages. The vocal ligaments are covered by mucosa, creating a fold (ridge) that forms the vocal cords. The vocal folds vibrate as air is exhaled, and create varying sounds based on their position.

Explain different means of airway access through the larynx for both emergent and long term care. 

A cricothyrotomy is an emergency procedure done to obtain access to the airway. A small passage is incised through the skin and (median) cricothyroid ligament and a cannula (small tube) is inserted to keep the opening patent and allow air flow. 

To locate the (median) cricothyroid ligament, palpate the thyroid and cricoid cartilages at the midline of the anterior neck. Between these two rigid structures is a divot where one can palpate the cricothyroid membrane. 

Define the attachments, actions, and innervation of the intrinsic muscles of the larynx. Discuss the effects of the intrinsic laryngeal muscles on the vocal ligaments and phonation.

The attachments of each intrinsic laryngeal muscle is indicated by its name and suggest the action it provides based on those attachments. 

Explain the arteries and nerves that supply the larynx. 

The larynx is exclusively innervated by the vagus n., specifically by means of the:

• Superior laryngeal n.:

• External br.

  • Efferent/somatic motor to cricothyroid m. (intrinsic laryngeal muscle)

• Internal br.

  • Afferent/somatic sensory from (and secretomotor/visceral motor to) laryngeal mucosa proximal to vocal folds

• Recurrent laryngeal n.:

  • Efferent/somatic motor to all intrinsic laryngeal mm. (except cricothyroid m.)

  • Afferent/somatic sensory from (and secretomotor/visceral motor to) laryngeal mucosa distal to vocal folds

Blood supply to the larynx comes from two major sources:

• Superior laryngeal a. (External carotid a. → Superior thyroid a. → Superior laryngeal a.)

  • Accompanied by the internal br. of the superior laryngeal n. through the thyrohyoid membrane.

• Inferior laryngeal a. (1st part of SCA → Thyrocervical trunk → Inferior thyroid a. → Inferior laryngeal a.)

  • Accompanied by the recurrent laryngeal n. in the tracheoesophageal groove

Trachea

Describe the structure, features, and function of the trachea.

The trachea (windpipe) is a hollow, tube-like structure that is a major component of the respiratory tract. It’s suspended from the larynx by the cricotracheal ligament and distally bifurcates into the left and right main bronchi at the level of the sternal angle. At the distal most end of the trachea, between the main bronchi, is a v-shaped ridge called the carina. The carina is a region of highly innervate mucosa that detects a foreign object and protects the airway by inducing a cough when stimulated. 

The walls of the trachea are composed of 16-20 incomplete rings of hyaline cartilage, which structurally maintain the patency of the airway. The rings are incomplete on the posterior aspect of the trachea, which allows room for the esophagus to expand as a food bolus passes through. 

The trachealis m. is smooth muscle within the mucosa of the posterior wall of the trachea, between the ends of the tracheal rings. It functions under autonomic control to constrict the trachea and forcefully expel air (coughing). During sympathetic control, the trachealis muscle is fully relaxed to allow for maximum patency of the trachea, and therefore maximum airflow with the lungs. 

Pleurae & Lungs

Describe the pleural membranes.

The pulmonary cavities contain the lungs and pleurae, serous membranes that line each of the pulmonary cavities and lungs. The outer margins of the pulmonary cavities are lined with parietal pleurae, whereas visceral pleurae adhere to all surfaces of the lungs. The parietal pleurae and visceral pleurae are contiguous as the pleurae reflects from the margins of the cavity onto the lungs. The space between parietal and visceral pleurae are the pleural cavities. 

The parietal pleurae each have 3-4 parts:

• Costal pleura, which covers the ribs and intercostal spaces

  • innervated be intercostal nn.

• Diaphragmatic pleura, which covers the superior surface of the diaphragm

  • innervated by intercostal nn. peripherally and phrenic nn. centrally

• Mediastinal pleura. which covers the mediastinal borders of the pulmonary cavities

  • innervated by phrenic nn.

  • Cervical pleura (extensions of the mediastinal and costal parts) which forms domes over the apices of each lung.

Bronchial Tree

Diagram the tracheobronchial tree. How does the histology of the bronchial tree change according to region?

The spaces through which air is conducted inferior to the larynx is the tracheobronchial tree. The larynx transitions into the trachea (trunk of tracheobronchial tree) at the inferior border of the cricoid cartilage (approximately C6-level). The bifurcation of the trachea into a right and left main bronchus occurs at the level of the sternal angle. There are important differences between the R. and L. main bronchi, including:

• Right main bronchus: more vertical in orientation, wider, and shorter

  • Aspirated materials are more likely to enter into the R. main bronchus (and certain branches) due to its orientation.

• Left main bronchus: more horizontal in orientation and longer

The R. and L. main bronchi enter the hila of the lungs. Further branching occurs within the parenchyma of the lungs.

The primary bronchi branch into secondary (lobar) bronchi. The secondary bronchi supply the lobes of each lung, with one secondary bronchus per lobe (3 secondary bronchi in the right lung, 2 secondary bronchi in the left lung)

Secondary bronchi branch into tertiary (segmental) bronchi, which serve the bronchopulmonary segments of the lungs. 

Tertiary bronchi give rise to bronchioles. Within the walls of bronchioles, hyaline cartilage (rich within the walls of the bronchi) begins to diminish as the bronchioles continue to divide and narrow. Cartilage is replaced with smooth muscle. The proximal conducting bronchioles quickly divide into the terminal bronchioles. The terminal bronchioles mark the end of the conducting zone of the respiratory pathway. 

Club cells (specialized cells within the terminal bronchioles) protect the small tubes of the airway. These cells have an immune component, secrete surfactant (to reduce surface tension), and act as stem cells for airway repair.  

All elements distal to the terminal bronchioles are part of the respiratory zone where gas exchange occurs.

Terminal bronchioles give rise to respiratory bronchioles, devoid of cartilage and predominantly smooth muscle, which is under autonomic control. 

Alveolar ducts arise from the respiratory bronchioles and contain alveolar sacs, the distal-most portion of the tracheobronchial tree where gas exchange occurs. 

What happens to the bronchial tree during a sympathetic response?

Autonomics regulate the diameter, or patency of the airway. During a sympathetic response, the smooth muscle completely relaxes, which opens the airway to allow for increased ventilation. 

Why is the superior segment of the inferior lobe clinically significant?

The superior segment of the inferior lobe is the only segment that has a posteriorly oriented tertiary bronchi. This can be problematic for individuals who are confined to their bed for long periods of time, if they are supine and aren’t being turned. This position can promote the accumulation of fluid in this particular segment, which increases the risk of pneumonia. 

Discuss the muscles involved with ventilation. 

Relaxed breathing mainly involves the diaphragm alone. 

Other muscles are able to assist with expansion of the thoracic cavity when forceful breathing: 

• External intercostal mm.

• Sternocleidomastoid mm. 

• Scalene mm.

• Pectoralis minor m.

During exhalation, the diaphragm passively relaxes back into its normal position. When the diaphragm projects back into the thoracic cavity, the cavity decreases in volume, and pressure increases, forcing air out of the lungs. One can engage their diaphragm for a more forceful exhalation, if need be, such as to cough. 

There are accessory respiratory muscles that assist with forceful exhalation:

• Internal and innermost intercostal mm.

• Superior fibers of abdominal mm.

Describe the effects that smoking has on the respiratory system.

• Tar deposits throughout the lungs (from nicotine) restricts the terminal bronchioles

  • Limits gas exchange

• Carbon monoxide is produced from the burning cigarette and is inhaled

  • CO is a competitive inhibitor for the binding of oxygen to hemoglobin

• Smoking increases the production of bronchial mucous

  • Clogs bronchioles and restricts airflow 

• Irritation of mucosal lining, causing mucosal edema

  • Restricts airflow 

• Converts respiratory epithelium (ciliated epithelium cells) to non-ciliated stratified squamous epithelium

  • Cilia sweep mucus out of the respiratory zone 

  • Without cilia, mucus builds up and leads to “smoker’s cough”

• Destruction of the elastic fibers in the lung tissue

  • Prevents the recoil of lung tissue back to resting position during exhalation

• Leading cause of cancer related deaths in the US