4.1 Respiratory System

Respiratory System

Respiratory system anatomy is 11% of General Anatomy on the NBCE Part I test plan (General Anatomy itself is 20% of the 255-item exam). The official bullets are nose and sinuses; larynx, trachea, and bronchi; lungs, pleura, and mediastinum; development; and histology. Respiratory physiology (compliance, spirometry, chemoreceptors, hemoglobin) is a separate Physiology topic—keep this section on structure.

Nose and Sinuses

The external nose is nasal bones, frontal processes of the maxilla, and hyaline cartilages (septal, lateral, greater and lesser alar). The nasal cavity extends from the nares to the choanae. The vestibule is lined by keratinized stratified squamous epithelium with vibrissae. Behind the limen nasi, most of the cavity is respiratory mucosa; the roof (cribriform plate region) is olfactory mucosa.

The nasal septum is the perpendicular plate of the ethmoid, the vomer, and septal cartilage. The lateral wall carries three conchae (turbinates). Superior and middle conchae are parts of the ethmoid; the inferior concha is its own bone. Beneath each concha is a meatus.

The maxillary sinus (antrum of Highmore) is the largest paranasal sinus. Its ostium sits high, so gravity does not empty it in the upright head—a structural reason maxillary sinusitis is common. Tooth roots of the upper molars can project into its floor. The frontal sinus is absent or rudimentary at birth and expands after the second year. Ethmoid cells sit between the orbit and the nasal cavity; the thin lamina papyracea is the medial orbital wall. The sphenoid sinus lies in the body of the sphenoid, inferior to the sella, so its lateral wall neighbors the cavernous sinus and internal carotid artery.

Arterial supply of the septum is concentrated in Kiesselbach plexus (Little area) on the anterior septum: sphenopalatine (from maxillary), greater palatine, anterior ethmoidal (from ophthalmic), and superior labial (from facial). Most epistaxis is anterior and arterial from this anastomosis. Woodruff plexus is a posterior venous plexus near the sphenopalatine foramen. General sensation is V1 (anterior/superior: anterior ethmoidal) and V2 (posterior/inferior: nasopalatine, greater palatine, posterior superior nasal). Special smell is CN I through the cribriform plate.

Larynx, Trachea, and Bronchi

The larynx spans about C3–C6. Unpaired cartilages: thyroid (laryngeal prominence), cricoid (the only complete ring of the airway), epiglottis (elastic cartilage). Paired cartilages: arytenoid (vocal and muscular processes), corniculate, cuneiform. The hyoid is not a laryngeal cartilage but is slung to the thyroid by the thyrohyoid membrane (pierced by the internal laryngeal nerve and superior laryngeal vessels).

Two fibroelastic membranes organize the interior. The quadrangular membrane runs from epiglottis to arytenoid; its free inferior border is the vestibular (false vocal) ligament, and its superior border helps form the aryepiglottic fold. The conus elasticus (cricovocal membrane) rises from the cricoid; its free superior border is the vocal ligament. The rima glottidis is the opening between the true vocal folds and the arytenoids. The laryngeal ventricle sits between false and true folds; its anterior extension is the saccule.

All intrinsic muscles are CN X. Cricothyroid is the exception to recurrent-laryngeal innervation. Internal laryngeal nerve (also from superior laryngeal) is sensory to mucosa above the vocal folds, including the piriform recess. Recurrent laryngeal nerve is motor to the remaining intrinsics and sensory below the vocal folds. The right recurrent loops the right subclavian artery; the left recurrent loops the aortic arch (ligamentum arteriosum) and ascends in the tracheoesophageal groove. Bilateral recurrent injury can leave the folds near the midline and compromise the airway because abduction is lost.

Valleculae lie between tongue base and epiglottis (median and lateral glossoepiglottic folds). Piriform recesses flank the laryngeal inlet; a foreign body or internal-laryngeal-nerve lesion here dulls the cough reflex on that side.

The trachea begins at the inferior border of the cricoid (C6) and bifurcates at the sternal angle (T4/T5) as the carina. It has 15–20 C-shaped hyaline rings; trachealis smooth muscle closes the ring posteriorly against the esophagus. The right main (primary) bronchus is shorter, wider, and more vertical; the left is longer, more horizontal, passes under the aortic arch, and crosses over the left atrium. Aspirated material therefore prefers the right bronchial tree. Secondary (lobar) bronchi: three on the right, two on the left. Tertiary bronchi supply bronchopulmonary segments—typically 10 on the right and 8–10 on the left (the left medial basal segment is often fused, and the lingula is the left-sided analogue of the right middle lobe).

Lungs, Pleura, and Mediastinum

Each lung has an apex into the neck (above the anterior first rib, behind sternocleidomastoid—cervical pleura / cupola), a base on the diaphragm, costal and mediastinal surfaces, and a hilum. At the hilum, from anterior to posterior on both sides, think vein–artery–bronchus at the upper part of the root, with the pulmonary ligament hanging inferiorly (a sleeve of pleura that lets the root vessels descend with inspiration).

Right lung: superior, middle, and inferior lobes; oblique and horizontal fissures. The horizontal fissure follows the right 4th rib and costal cartilage; the oblique fissure runs from about the T3 spinous process to the 6th costochondral junction. Left lung: superior and inferior lobes; oblique fissure only; lingula and cardiac notch on the superior lobe. The cardiac impression and groove for the aorta are left-sided surface features; the right mediastinal surface shows grooves for the SVC, azygos arch, and esophagus.

Bronchopulmonary segments are independent units with a tertiary bronchus and a segmental pulmonary artery in the center; pulmonary veins run in intersegmental septa. That is why a surgeon can remove one segment. Classic named segments to recite: right upper (apical, posterior, anterior), right middle (lateral, medial), right lower (superior / apical-basal plus four basal), left upper (apicoposterior, anterior, superior lingular, inferior lingular), left lower (superior plus basal).

Pulmonary arteries carry deoxygenated blood and follow bronchi. Bronchial arteries (usually two left from the thoracic aorta, one right often from the third posterior intercostal or a common trunk) nourish the bronchial tree and visceral pleura. Pulmonary veins (two per lung) return oxygenated blood to the left atrium. Bronchial veins drain to the azygos / hemiazygos system; a small right-to-left shunt is therefore normal.

Visceral pleura is insensitive to ordinary pain (autonomic). Parietal pleura is exquisitely sensitive: costal and peripheral diaphragmatic pleura via intercostal nerves; mediastinal and central diaphragmatic pleura via phrenic nerves (C3–C5). Central diaphragmatic inflammation can therefore refer to the shoulder (C3–C5 dermatomes). The costodiaphragmatic recess is the lowest pleural space (midclavicular ~7th rib lung / 9th rib pleura; midaxillary ~8th / 10th; paravertebral ~10th / 12th)—the usual site of a thoracentesis, performed above a rib to miss the neurovascular bundle.

The mediastinum is the midline thoracic viscera between the pleural sacs. A plane through the sternal angle and T4/T5 disc divides superior from inferior. Inferior mediastinum is anterior, middle, and posterior.

The thoracic duct ascends on the vertebral bodies, crosses from right to left at about T4–T5, and empties at the left venous angle. The right recurrent laryngeal is in the root of the neck, not the mediastinum; a mediastinal lesion that hoarsens the voice is pointing at the left recurrent nerve.

Development

The respiratory diverticulum (laryngotracheal groove) appears in the ventral foregut in week 4. A tracheoesophageal septum partitions trachea from esophagus. Incomplete partitioning produces tracheoesophageal fistula, most often esophageal atresia with a distal TEF. Endoderm lines the airway; splanchnic mesoderm supplies cartilage, muscle, and visceral pleura. The lung buds split into three right and two left secondary buds, then into segmental buds.

Surfactant is a phospholipid film (chiefly dipalmitoyl phosphatidylcholine) from type II pneumocytes, stored in lamellar bodies. It lowers surface tension and is the anatomic reason the canalicular–saccular transition matters. Congenital diaphragmatic hernia (usually left posterolateral, foramen of Bochdalek) lets abdominal viscera occupy the thorax and produces pulmonary hypoplasia—a developmental, not merely mechanical, lung problem.

Laryngeal cartilages and muscles arise from pharyngeal arches 4 and 6 (vagus), which is why laryngeal innervation is CN X rather than a spinal nerve. The epiglottis has a contribution from arch 4 mesenchyme.

Histology

The airway is divided into conducting (nasal cavity through terminal bronchioles) and respiratory (respiratory bronchioles, alveolar ducts, sacs, alveoli) zones. Respiratory epithelium is pseudostratified ciliated columnar with goblet cells. Olfactory epithelium adds bipolar olfactory neurons, supporting cells, basal cells, and Bowman serous glands.

True vocal folds are covered by stratified squamous epithelium (mechanical stress); the rest of the larynx is mostly respiratory epithelium. Trachea: respiratory epithelium, thick basement membrane, submucosal seromucous glands, C-shaped hyaline cartilage, adventitia.

Type I pneumocytes are thin squamous cells that cover about 95% of the alveolar surface (they are not the majority by cell number). Type II pneumocytes are cuboidal, often in corners, and both secrete surfactant and serve as progenitors that replace type I cells. Alveolar macrophages sit in the air space and in septa. Pores of Kohn connect adjacent alveoli. Elastic fibers in the interstitium recoil the lung; collagen excess is the histologic substrate of restrictive disease, but Part I wants the normal fiber mix.

The blood–air barrier, from air to erythrocyte, is the surfactant film, type I cytoplasm, the fused basal laminae of epithelium and endothelium, and the endothelial cytoplasm. Capillaries are continuous. This is the structure that canalicular remodeling first approximates and that saccular/alveolar stages thin to adult dimensions.

After the named meatuses, the posterior cricoarytenoid, the right main bronchus, mediastinal compartments, and the canalicular period are automatic, work mixed items on /practice/nbce-part1. Part I will pair a named sinus with a named meatus far more often than it will ask an isolated muscle origin.