Comprehensive Study Guide to the Human Respiratory System and Pharyngeal Anatomy

Functional Classification of the Respiratory System

The respiratory system is anatomically and functionally divided into two primary sections: the conducting portion and the respiratory portion. The conducting portion is responsible for transporting, filtering, warming, and humidifying air. This process begins at the nostrils and extends down to the terminal bronchioles ($bronchiole \, terminal$). The respiratory portion is primarily responsible for gas exchange, a process termed oxygenation, which involves the exchange of gases between the alveoli ($Alveolus$) and the blood capillaries. This exchange begins at the point where the first alveoli appear, specifically within the respiratory bronchioles ($bronchiole \, Respiratory$). The four major components of the respiratory system passing through the head and neck are the nose (including the external nose and nasal cavity), the paranasal sinuses, the larynx, and the trachea. From a superior to inferior perspective, the system includes the nose, pharynx, larynx, trachea, and lungs.

Anatomy of the External Nose and Nasal Framework

The nose ($Nose$) is divided into the external nose ($nose \, External$), which is the visible portion on the face, and the deeper nasal cavity ($cavity \, Nasal$). The external nose framework consists of both bony and cartilaginous components. The bony portion includes the nasal bones ($bones \, Nasal$), parts of the maxilla (upper jaw), and the frontal bone (forehead). The two nasal bones sit side-by-side, forming the bridge of the nose where eyeglasses typically rest. Below these bones lies the cartilaginous structure. Nasal fractures commonly involve the nasal bone; mild fractures may be treated simply, but severe cases, classified as Grade 4, involve depression ($Depression$) or a fractured nasal septum ($septum$) and require surgical intervention.

The nasal cartilages are categorized into several types: the lateral nasal cartilages ($cartilage \, nasal \, Lateral$) located on the sides, and the septal cartilage ($cartilage \, Septal$) in the midline, which forms the nasal wall and possesses two lateral processes. The wings of the nose, or $Ala \, of \, nose$, contain the major alar cartilages (also known as $cartilage \, alar \, Major$ or $Great \, alar \, cartilage$) and several smaller cartilages behind them called lesser alar cartilages ($Lesser \, alar \, cartilage$ or $Minor \, alar \, cartilage$), usually numbering around three. The $ala$ is primarily fibrocartilaginous and covered by muscle, allowing for voluntary movement. Plastic surgery often focuses on these cartilaginous structures. Blood supply to the external nose is provided by branches of the ophthalmic and maxillary arteries. Specifically, the skin of the $ala$ and the lower septum is supplied by the facial artery via the angular and lateral nasal arteries. The facial and maxillary arteries are branches of the external carotid, while the ophthalmic artery is a branch of the internal carotid.

The Ethmoid Bone and Nasal Cavity Compartments

The ethmoid bone ($bone \, Ethmoid$) is a cube-shaped skull bone often described as sieve-like or "cribriform." It forms the anterior part of the skull base and the lateral walls of the nasal cavity. Key features include the cribriform plate ($plate \, Cribriform$), which contains numerous perforations for the passage of olfactory nerves from the nose to the brain. It features the $Crista \, galli$, a bony prominence in the midline, and the perpendicular plate ($Perpendicular \, plate$), which descends to form the superior part of the nasal septum. The ethmoid also contains two ethmoidal labyrinths ($Ethmoidal \, labyrinths$) which house ethmoidal air cells and form part of the medial wall of the orbit ($Orbit$). The bone contributes to the superior concha ($concha \, Superior$) and middle concha ($concha \, Middle$), while the inferior nasal concha ($concha \, nasal \, Inferior$) is a separate, independent bone.

The nasal cavity features radiator-like plates called conchae or turbinates. These structures serve two vital roles: first, they create air turbulence (vortexes) to trap dust and debris; second, they increase the surface area to warm and humidify inhaled air, protecting the delicate respiratory tissues from cold, dry air. The central septum, which divides the cavity, is composed of the septal cartilage anteriorly, the perpendicular plate of the ethmoid posteriorly-superiorly, and the vomer bone ($Vomer \, bone$) posteriorly-inferiorly. The presence of three conchae creates four distinct spaces: the sphenoethmoidal recess ($recess \, spheno-ethmoidal$) located between the superior concha and the roof, the superior nasal meatus ($meatus \, nasal \, Superior$) between the superior and middle conchae, the middle nasal meatus ($meatus \, nasal \, Middle$) between the middle and inferior conchae, and the inferior nasal meatus ($meatus \, nasal \, Inferior$) between the inferior concha and the floor. The middle meatus contains a bump called the ethmoidal bulla ($bulla \, ethmoidalis$).

Drainage and Boundaries of the Nasal Cavity

The roof of the nasal cavity is formed from anterior to posterior by the nasal, frontal, ethmoid, and sphenoid ($Sphenoid$) bones, creating a curved surface. The floor is the hard palate ($plate \, Hard$), which is also the roof of the mouth; it is composed of the maxillary bone ($3/4$ anteriorly) and the palatine bone ($1/4$ posteriorly). The lacrimal bone ($Lacrimal$) also contributes to the lateral wall, and the pterygoid processes ($Pterygoid$) of the sphenoid bone are visible on the internal and external views. The nasal cavity is divided into three regions: the vestibular region (the dilated area behind the nostril containing hairs or $vibrissae$), the respiratory region (the largest part, lined with ciliated columnar epithelium and rich in vessels), and the olfactory region (the yellow-tinted uppermost part containing bipolar neurons for smell).

The paranasal sinuses are air-filled cavities in the skull bones that drain into the nasal cavity. These include the frontal sinuses, which are often asymmetrical; the ethmoidal sinuses; the sphenoid sinuses; and the maxillary sinuses, which are pyramidal with the apex pointing upward. The drainage pattern is specific: the sphenoid sinus drains into the sphenoethmoidal recess; the posterior ethmoidal sinuses drain into the superior meatus; the maxillary, frontal, and anterior/middle ethmoidal sinuses drain into the middle meatus (the maxillary via the semilunar hiatus, $hiatus \, semi-lunar$); and the nasolacrimal duct ($duct \, nasolacrimal$) drains into the inferior meatus. Clinical examination for sinusitis involves palpating for tenderness ($Tenderness$): frontal sinuses are checked by pressing above the inner orbit, and maxillary sinuses by pressing under the cheekbones.

Blood Supply, Innervation, and Lymphatics of the Nose

The nasal cavity receives a dual blood supply from both the internal and external carotid arteries. The external carotid provides the sphenopalatine, greater palatine, superior labial, and lateral nasal arteries. The internal carotid provides the anterior and posterior ethmoidal arteries (via the ophthalmic artery). A critical clinical area in the anterior septum is the Kiesselbach ($Kiesselbach$) plexus, an area of extensive vascular anastomosis highly prone to nosebleeds ($Epistaxis$). These can usually be managed with a tamponade (saturated cotton or gauze). Venous drainage occurs via the ophthalmic vein (draining to the cavernous sinus), the pterygoid plexus ($plexus \, pterygoid$), and the facial vein anteriorly.

Sensory innervation is provided by the olfactory nerve ($nerve \, olfactory$, CN I) for smell, which passes through the cribriform plate to the olfactory bulb ($olfactory \, bulb$). This bulb is a target for the COVID-19 virus, leading to $Anosmia$ (loss of smell). General sensation is provided by the trigeminal nerve (CN V), specifically the ophthalmic ($V_1$) and maxillary ($V_2$) branches. Lymphatic drainage is split: the anterior part drains to the submandibular nodes ($nodes \, submandibular$), while the posterior parts and paranasal sinuses drain to the upper deep cervical lymph nodes ($nodes \, cervical \, deep \, Upper$), sometimes passing through retropharyngeal nodes.

Structure and Muscles of the Pharynx

The pharynx ($Pharynx$) is a conical intersection for air and food, leading to the nose, mouth, esophagus, and larynx. It is approximately $12$ to $14\,cm$ long, starting at the skull base and ending at the level of the $C_6$ vertebra. Its width tapers from $3.5\,cm$ at the top to $1.5\,cm$ at the bottom. It is divided into three parts: the nasopharynx ($nasopharynx$), the oropharynx ($oropharynx$), and the laryngopharynx ($laryngopharynx$), which is the narrowest part. The nasopharynx contains the Eustachian tube ($tube \, Eustachian$), the torus tubarius ($torus \, tubarius$), the torus levatorius ($torus \, levatorius$), and the pharyngeal tonsils (adenoids, $tonsil \, Pharyngeal$). If the adenoids enlarge, they can block the Eustachian tube, causing middle ear infections ($otit \, medial$), or block nasal breathing. The oropharynx contains the palatine tonsils ($tonsil \, palatine$) located in the tonsillar fossa ($fossa \, tonsillar$) between the palatoglossal and palatopharyngeal arches ($arches \, Palatoglossal$).

The pharyngeal muscles are divided into circular constrictors and longitudinal elevators. The three constrictors—superior, middle, and inferior—overlap and meet posteriorly at the pharyngeal raphe ($raphe \, Pharyngeal$). All three are innervated by the vagus nerve (CN X). Longitudinal muscles include the stylopharyngeus ($stylopharyngeus$, CN IX), salpingopharyngeus (from the tube, CN X), and palatopharyngeus (from the palate, CN X); these elevate the pharynx during swallowing. Between the muscles, various structures pass: the glossopharyngeal nerve (CN IX) and stylopharyngeus muscle pass between the superior and middle constrictors, while the internal laryngeal nerve and superior laryngeal artery pass between the middle and inferior constrictors. The recurrent laryngeal nerve ($nerve \, laryngeal \, Recurrent$) passes below the inferior constrictor.

Anatomy of the Larynx and Cartilaginous Framework

The larynx ($Larynx$) is a hollow, muscular-ligamentous structure functioning as a valve for sound production and airway protection. It is located at the levels of $C_4$, $C_5$, and $C_6$ vertebrae. It consists of three unpaired cartilages: the thyroid (large, forming the Adam's apple or $prominence \, Laryngeal$), the cricoid (a complete ring resembling a signet ring, with an $arch$ and $lamina$), and the epiglottis (a leaf-shaped cartilage that acts as a lid for the trachea). It also has three paired cartilages: the arytenoid ($Arytenoid$), which has vocal and muscular processes; the corniculate ($corniculate$); and the cuneiform ($cuneiform$), which are small and embedded in the aryepiglottic folds. The larynx is suspended from the hyoid bone ($bone \, Hyoid$) by the thyrohyoid membrane ($thyrohyoid$).

The internal larynx contains several membranes and ligaments. The cricothyroid membrane ($membrane \, Cricothyroid$) tenses at its upper border to form the true vocal cords (ligament $vocal$), while the quadrangular membrane ($membrane \, Quadriangular$) forms the false vocal cords (vestibular folds). The laryngeal cavity is divided into the vestibule (above the folds), the glottis ($glote$ or $glottis$, containing the cords and the space between them), and the infraglottic space ($infraglotic$). The laryngeal ventricle ($ventricle$) and saccule house mucus glands to keep the vocal cords moist. The space between the true cords is the $rima \, glottidis$, and the space between the vestibular folds is the $rima \, vestibuladis$. Deep to the piriform fossa ($fossa \, Preformiss$), the internal laryngeal nerve is located.

Muscles and Innervation of the Larynx

Intrinsic muscles of the larynx control sound and the airway. The cricothyroid muscle tenses the vocal cords to increase pitch; it is unique because it is the only one supplied by the external laryngeal nerve (a branch of the superior laryngeal nerve from CN X). The posterior cricoarytenoid muscle is the sole abductor (it opens the vocal cords). The lateral cricoarytenoid, transverse arytenoid, and oblique arytenoid muscles are adductors (closing the cords). The $vocalis$ muscle helps regulate cord tension. During normal inhalation, the cords are apart; during forced inspiration, they open wider. For phonation, they must be close together.

Nerve supply is entirely from the vagus nerve (CN X). The superior laryngeal nerve divides into the internal laryngeal (sensory above the cords) and external laryngeal (motor to the cricothyroid). The recurrent laryngeal nerve provides sensory coverage below the cords (down to $C_6$) and motor supply to all other intrinsic laryngeal muscles. Blood supply involves the superior laryngeal artery (from the superior thyroid artery) and the inferior laryngeal artery (from the inferior thyroid artery/thyrocervical trunk). Lymphatics drain to the deep cervical nodes ($nodes \, cervical \, Deep$).

The Trachea and Lung Structures

The trachea is a $12\,cm$ tube extending from $C_6$ to the $T_4$-$T_5$ level, where it bifurcates. It consists of C-shaped cartilaginous rings, closed posteriorly by the trachealis muscle ($Trachealis$). This flexible posterior wall allows for esophageal movement. The bifurcation point is the carina ($angle \, Carnia$), which normally measures $60$ degrees. The right main bronchus ($bronchus \, Main$) is wider, shorter, and more vertical than the left, making it a more common site for inhaled foreign bodies and aspiration pneumonia. Segmental bronchi ($bronchiole \, Segmental$) divide about 11 times into bronchioles, then terminal bronchioles, and finally respiratory bronchioles where alveoli appear. The total surface area of all alveoli is approximately $70\,m^2$.

Lungs are conical organs, pinkish at birth but darkening with age due to contaminants. The right lung is larger, wider, and shorter (due to the liver), with three lobes (upper, middle, lower) and two fissures (oblique and horizontal). The left lung is narrower and longer (due to the heart), with two lobes and an oblique fissure. The left lung also features the lingula ($Lingula$). The hilum ($hilar$) is the point of entry for the root of the lung ($Root \, of \, lung$), where the pulmonary artery is superior, bronchi are posterior, and pulmonary veins are antero-inferior. For the right lung, the eparterial bronchus (upper lobe bronchus) appears most superior. Nerves for the lungs come from the anterior and posterior pulmonary plexuses ($plexus \, pulmonary$), with sympathetic nerves causing bronchodilation and parasympathetic nerves causing bronchoconstriction.

The Pleura and Clinical Perspectives

The pleura is a serous membrane with a mesothelium layer secreting $5$ to $10\,cc$ of fluid to facilitate movement. The visceral pleura covers the lung directly, while the parietal pleura has costal, diaphragmatic, mediastinal, and cervical ($cupula$) parts. The cervical pleura extends $2.5\,cm$ above the clavicle. Important pleural recesses (spaces where fluid can collect) include the costodiaphragmatic recess ($recess \, Costodiaphragmatic$) and the costomediastinal recess. Surface anatomy levels for the pleura and lung are as follows:

  • Midclavicular line: pleura at 8th rib, lung at 6th rib.
  • Midaxillary line: pleura at 10th rib, lung at 8th rib.
  • Paravertebral line: pleura at 12th rib, lung at 10th rib.

Clinical notes include transnasal hypophysectomy for pituitary removal, and the risk of recurrent laryngeal nerve damage during thyroid surgery (causing voice weakness or breathing difficulty). During intubation, a laryngoscope is used in the "sniffing position" ($sniffing$) to align the oral, pharyngeal, and tracheal axes. Excessive accumulation of air ($Pneumothorax$), blood ($hemothorax$), or fluid ($hydrothorax$) in the pleural cavity can lead to lung collapse or mediastinal shift ($shift$).

Questions & Discussion

Sample Questions on Head and Neck Respiratory Anatomy:

  1. Which bone does not contribute to the nasal roof? (1: Nasal, 2: Palatine, 3: Sphenoid, 4: Ethmoid) - Answer: 2
  2. Which nerve provides sensation to the laryngopharynx? (1: Vagus, 2: Glossopharyngeal, 3: Maxillary, 4: Accessory) - Answer: 1
  3. Where does the nasolacrimal duct open? (1: Inferior meatus, 2: Middle meatus, 3: Superior meatus, 4: Sphenoethmoidal recess) - Answer: 1
  4. Which soft palate muscle is not innervated by the accessory nerve? (1: Palatoglossus, 2: Levator veli palatini, 3: Palatopharyngeus, 4: Tensor veli palatini) - Answer: 4
  5. Mandibular fracture affects which pharyngeal muscle? (1: Inferior constrictor, 2: Middle constrictor, 3: Stylopharyngeus, 4: Superior constrictor) - Answer: 4
  6. Which tonsil is more active in children? (1: Palatine, 2: Tubal, 3: Pharyngeal, 4: Lingual) - Answer: 3
  7. What limits the inferior opening of the Eustachian tube? (1: Vocal fold, 2: Torus tubarius, 3: Salpingopharyngeal fold, 4: Torus levatorius) - Answer: 4
  8. Where do swallowed foreign bodies most commonly get stuck? (1: Oropharynx, 2: Piriform recess, 3: Pharyngeal recess, 4: Nasopharynx) - Answer: 2
  9. To which nasal space does the $Ethmoidal \, bulla$ belong? (1: Inferior, 2: Superior, 3: Sphenoethmoidal, 4: Middle Meatus) - Answer: 4
  10. Which paranasal sinus is most commonly infected? (1: Frontal, 2: Maxillary, 3: Sphenoid, 4: Ethmoid) - Answer: 2
  11. What is the lowest boundary of the larynx? (1: Thyroid, 2: Epiglottis, 3: Arytenoid, 4: Cricoid) - Answer: 4
  12. Which nerve supplies sensation below the vocal cords? (1: Superior Laryngeal, 2: Recurrent Laryngeal, 3: Internal Laryngeal, 4: Glossopharyngeal) - Answer: 2
  13. Which muscle tenses the vocal cords? (1: Cricothyroid, 2: Lateral cricoarytenoid, 3: Posterior cricoarytenoid, 4: Thyroepiglottic) - Answer: 1
  14. Where is the bifurcation of the trachea? (1: $T_3$, 2: $T_4$, 3: $T_6$, 4: $T_7$) - Answer: 2
  15. Which nerve supplies the diaphragmatic pleura? (1: Vagus, 2: Intercostal, 3: Sympathetic, 4: Phrenic) - Answer: 4