Medical School · Year 1 · Respiratory · includes a quiz and discussion video

Lecture 1: Respiratory Anatomy

Unit 1.8: Respiratory System

Learning Objectives

By the end of this lecture, students will be able to:

1. Describe the anatomy of the upper and lower respiratory tract
2. Identify the structural features of the conducting and respiratory zones
3. Describe the bronchopulmonary segments and their clinical significance
4. Explain the anatomy of the thoracic cage and respiratory muscles
5. Describe the pleura and pleural spaces
6. Describe the blood supply and innervation of the respiratory system

Lecture Content

Overview of the Respiratory System

The respiratory system can be conceptualized through two complementary classification schemes that illuminate both structure and function. Understanding these divisions provides a framework for comprehending how air moves from the environment to the gas exchange surfaces and how various pathological processes affect respiratory function.

The functional division separates the respiratory system into conducting and respiratory zones based on whether gas exchange occurs. The conducting zone encompasses all structures from the nose through the terminal bronchioles, serving to warm, humidify, and filter inspired air while directing it toward the gas exchange surfaces. No gas exchange occurs in these airways because they lack alveoli; this volume constitutes the anatomical dead space. The respiratory zone begins with the respiratory bronchioles and continues through the alveolar ducts to the alveoli themselves, representing the functional portion where oxygen and carbon dioxide transfer between air and blood.

The anatomical division distinguishes upper and lower respiratory tracts based on location relative to the larynx. The upper respiratory tract includes the nose, nasal cavity, paranasal sinuses, pharynx, and larynx. The lower respiratory tract comprises the trachea, bronchi, bronchioles, and lungs. This distinction has clinical relevance because infections and other pathological processes often affect the upper and lower tracts differently.

<image>Panel A: Sagittal section of head, neck, and thorax showing the conducting zone in blue from nose through terminal bronchioles with labeled structures. Panel B: Respiratory zone in pink showing respiratory bronchioles, alveolar ducts, and alveoli with magnified inset. Panel C: Anatomical division diagram with horizontal line at larynx separating upper and lower respiratory tracts. Panel D: Functional annotations indicating conducting zone warming/humidifying/filtering and respiratory zone gas exchange.</image>

Upper Respiratory Tract: Nasal Cavity

The nasal cavity serves as the primary entry point for inspired air, performing critical conditioning functions before air reaches the delicate lower airways. The vestibule, the anteriormost portion just inside the nostrils, contains vibrissae (nasal hairs) that filter large particulate matter. Beyond the vestibule, the nasal cavity proper is divided by the nasal septum and features paired lateral projections called conchae or turbinates.

The nasal septum comprises three components: the septal cartilage anteriorly, the perpendicular plate of the ethmoid bone superiorly, and the vomer bone posteroinferiorly. Deviation of the septum is common and may contribute to nasal obstruction when severe. The superior, middle, and inferior conchae project from the lateral nasal wall, creating corresponding meatuses beneath each. These structures dramatically increase the surface area available for air conditioning while creating turbulent airflow patterns that maximize contact between air and the mucosa.

The paranasal sinuses, air-filled extensions of the nasal cavity into surrounding bones, drain into specific meatuses: the maxillary, frontal, and anterior ethmoid sinuses drain into the middle meatus through the osteomeatal complex, while the posterior ethmoid sinuses drain into the superior meatus and the sphenoid sinus drains into the sphenoethmoidal recess. Obstruction of these drainage pathways leads to sinusitis.

The nasal mucosa contains pseudostratified ciliated columnar epithelium with abundant goblet cells producing mucus. The rich vascular supply beneath the epithelium warms inspired air, while the mucus layer humidifies the air and traps particles. The cilia beat in coordinated waves, propelling mucus toward the pharynx for swallowing (the mucociliary escalator).

Kiesselbach's plexus, located on the anterior nasal septum in Little's area, represents an anastomosis of branches from both external and internal carotid systems. This exposed vascular network is the most common site of anterior epistaxis. Posterior epistaxis, though less common, arises from branches of the sphenopalatine artery and is more difficult to control.

<image>Panel A: Coronal section of nasal cavity showing nasal septum components and three conchae with corresponding meatuses and turbulent airflow arrows. Panel B: Sagittal view of lateral nasal wall with color-coded paranasal sinus drainage pathways to their respective meatuses. Panel C: Kiesselbach's plexus on anterior septum showing anastomotic network of contributing arterial branches. Panel D: Magnified nasal mucosa histology displaying pseudostratified ciliated columnar epithelium with goblet cells and rich submucosal vasculature.</image>

Upper Respiratory Tract: Pharynx and Larynx

The pharynx is a muscular tube extending from the base of the skull to the level of the cricoid cartilage, serving as a shared conduit for both respiratory and digestive systems. It is conventionally divided into three regions based on anatomical relationships and distinct functions.

The nasopharynx lies posterior to the nasal cavity and superior to the soft palate. It contains the pharyngeal tonsil (adenoid) on its posterior wall and the openings of the Eustachian tubes laterally. Because it lies above the soft palate, the nasopharynx functions exclusively in respiration, receiving air from the nasal cavity. The oropharynx extends from the soft palate to the epiglottis, forming the posterior boundary of the oral cavity. It contains the palatine tonsils between the anterior and posterior tonsillar pillars and the lingual tonsil on the posterior tongue. The oropharynx participates in both respiration and deglutition. The laryngopharynx (hypopharynx) extends from the epiglottis to the cricoid cartilage, where it continues as the esophagus posteriorly while opening into the larynx anteriorly. This region is critical for directing food toward the esophagus while protecting the airway.

The larynx serves three primary functions: airway protection during swallowing, phonation, and participation in the cough reflex. Its framework consists of cartilages connected by membranes and ligaments. The thyroid cartilage is the largest, forming the laryngeal prominence (Adam's apple). The cricoid cartilage is the only complete cartilaginous ring in the airway, shaped like a signet ring with the wider lamina posterior. The paired arytenoid cartilages sit atop the cricoid lamina and provide attachment for the vocal ligaments. The epiglottis is a leaf-shaped elastic cartilage that folds over the laryngeal inlet during swallowing.

The vocal cords stretch between the arytenoid cartilages posteriorly and the thyroid cartilage anteriorly. The true vocal cords (vocal folds) contain the vocal ligaments and are responsible for phonation; the space between them is the rima glottidis. The false vocal cords (vestibular folds) lie superiorly and play minimal role in voice production.

Laryngeal innervation has critical clinical implications. The superior laryngeal nerve divides into external and internal branches: the external branch supplies the cricothyroid muscle (the only tensor of the vocal cords), while the internal branch provides sensation above the vocal cords. The recurrent laryngeal nerve supplies all other intrinsic laryngeal muscles and provides sensation below the vocal cords. The left recurrent laryngeal nerve loops under the aortic arch, making it vulnerable to compression by aortic aneurysms, mediastinal tumors, or during thoracic surgery.

<image>Panel A: Midsagittal section showing three pharyngeal divisions with nasopharynx, oropharynx, and laryngopharynx structures labeled. Panel B: Laryngeal cartilages in anterior, posterior, and lateral views showing thyroid, cricoid, arytenoid cartilages, and epiglottis. Panel C: Laryngeal innervation diagram with superior and recurrent laryngeal nerve branches and their targets. Panel D: Superior view of vocal cords showing true and false vocal folds with rima glottidis.</image>

Lower Respiratory Tract: Trachea and Bronchial Tree

The trachea begins at the lower border of the cricoid cartilage (approximately the C6 vertebral level) and extends to its bifurcation at the carina (approximately T4-5 level). This fibrocartilaginous tube measures ten to twelve centimeters in length and approximately 2.5 centimeters in diameter in adults. Sixteen to twenty C-shaped cartilage rings provide anterior and lateral support, maintaining patency even with negative intrathoracic pressure during inspiration. The posterior wall, which abuts the esophagus, lacks cartilage and instead contains the trachealis muscle, allowing flexibility during swallowing and coughing.

At the carina, the trachea divides into the right and left main bronchi. The right main bronchus is shorter, wider, and more vertical than the left, making it the more common destination for aspirated foreign bodies. The left main bronchus is longer and courses more horizontally beneath the aortic arch. Each main bronchus enters its respective lung at the hilum.

The bronchial tree undergoes approximately twenty-three generations of branching from the trachea to the alveoli. The main bronchi divide into lobar bronchi (three on the right, two on the left, corresponding to the lung lobes), which further divide into segmental bronchi supplying the bronchopulmonary segments. Progressive branching continues through subsegmental bronchi and smaller bronchi until the bronchioles are reached.

Bronchioles are distinguished from bronchi by the absence of cartilage in their walls; smooth muscle becomes the primary structural component determining airway caliber. Terminal bronchioles represent the final purely conducting airways, averaging 0.5 millimeters in diameter. Respiratory bronchioles, the first airways with alveoli budding from their walls, mark the beginning of the respiratory zone and lead to alveolar ducts and alveolar sacs.

As the airways progress distally, several structural changes occur: cartilage disappears, smooth muscle becomes proportionally more prominent, the epithelium transitions from pseudostratified ciliated columnar to simple cuboidal, and the total cross-sectional area increases dramatically despite decreasing individual airway diameter. This increasing cross-sectional area is clinically important because it means that small airways contribute relatively little to total airway resistance despite their narrow individual diameters.

<image>Panel A: Anterior view of tracheobronchial tree showing trachea, carina, and main bronchi with right being shorter and more vertical than left. Panel B: Lobar bronchial divisions for right lung (three lobes) and left lung (two lobes) with diameter annotations. Panel C: Magnified inset showing progressive structural changes from large bronchi through bronchioles with histological details. Panel D: Graph of airway generation versus total cross-sectional area demonstrating dramatic increase in respiratory zone.</image>

Lung Anatomy and Bronchopulmonary Segments

The lungs occupy the thoracic cavity on either side of the mediastinum, enclosed by the pleura and resting on the diaphragm inferiorly. Although similar in overall shape, the right and left lungs differ in ways that reflect their relationship to mediastinal structures.

The right lung is larger and heavier (approximately 600 grams) and has three lobes: superior, middle, and inferior. Two fissures separate these lobes: the oblique fissure separates the inferior lobe from the others, while the horizontal fissure separates the superior and middle lobes. The left lung (approximately 500 grams) has only two lobes, superior and inferior, separated by a single oblique fissure. The cardiac notch on the left lung's anteromedial surface accommodates the heart, and the lingula, a tongue-like projection of the superior lobe, represents the embryological equivalent of the right middle lobe.

Each lung has three surfaces: the costal surface contacts the ribs and intercostal spaces, the mediastinal surface faces the mediastinum and contains the hilum, and the diaphragmatic surface (base) rests on the diaphragm. The apex projects above the clavicle into the root of the neck. The borders include a sharp anterior border, a rounded posterior border along the vertebral column, and a sharp inferior border at the costophrenic angle.

The hilum is the root of the lung where structures enter and exit. The arrangement of hilar structures differs between sides: on the right, the bronchus is posterior (eparterial bronchus to upper lobe is superior), the pulmonary artery is anterior and superior, and the pulmonary veins are anterior and inferior. On the left, the pulmonary artery is most superior, the bronchus is posterior, and the pulmonary veins are inferior.

Bronchopulmonary segments are functionally independent units of lung tissue, each supplied by its own segmental bronchus and segmental artery. There are typically ten segments in each lung, though variations occur. Importantly, the pulmonary veins run in the intersegmental planes rather than within segments. This anatomical arrangement allows surgical resection of individual segments (segmentectomy) with minimal impact on adjacent tissue. Clinically, segmental anatomy explains the distribution patterns of aspiration pneumonia (favoring the posterior segment of the right upper lobe in supine patients) and helps localize pathology on imaging.

<image>Panel A: Lateral and medial views of both lungs showing lobes, fissures, cardiac notch, and lingula. Panel B: Cross-sectional views of right and left hila showing arrangement of bronchi, pulmonary arteries, and pulmonary veins. Panel C: Anterior and lateral views with color-coded bronchopulmonary segments for all lobes. Panel D: Demonstration of pulmonary veins running between segments while arteries travel with bronchi within segments.</image>

Respiratory Zone and Alveolar Structure

The respiratory zone comprises approximately ninety percent of total lung volume and is dedicated entirely to gas exchange. This zone begins with respiratory bronchioles, which have alveoli budding from their walls, and continues through alveolar ducts (completely lined with alveoli) to alveolar sacs (clusters of alveoli sharing a common opening).

The alveoli are the fundamental units of gas exchange, numbering approximately 300 to 500 million and providing a total surface area of 70 to 100 square meters for diffusion. Each alveolus measures 200 to 300 micrometers in diameter and has walls only 0.2 to 0.5 micrometers thick. This remarkable thinness minimizes the diffusion distance while the vast surface area maximizes diffusion capacity.

Three cell types populate the alveolar epithelium. Type I pneumocytes are extremely thin, squamous cells that cover approximately 95 percent of the alveolar surface area despite comprising only 40 percent of alveolar cells by number. Their thinness is essential for efficient gas diffusion. Type II pneumocytes are cuboidal cells that produce pulmonary surfactant, stored in lamellar bodies and secreted onto the alveolar surface. These cells also serve as progenitor cells that can differentiate into Type I cells following injury. Alveolar macrophages are mobile phagocytic cells derived from monocytes that patrol the alveolar surface, engulfing inhaled particles and pathogens, and representing the primary immune defense in the alveoli.

The blood-gas barrier through which oxygen and carbon dioxide must diffuse consists of four layers: the surfactant film on the alveolar surface, the Type I pneumocyte cytoplasm, the fused basement membranes of the alveolar epithelium and capillary endothelium, and the capillary endothelial cell. The total thickness is only 0.3 to 0.5 micrometers in healthy lung, allowing rapid equilibration of gases.

Pulmonary surfactant deserves special attention because of its critical role in respiratory physiology. Composed primarily of dipalmitoyl phosphatidylcholine (DPPC) along with surfactant proteins SP-A, SP-B, SP-C, and SP-D, surfactant reduces alveolar surface tension. Without surfactant, the surface tension at the air-liquid interface would cause alveolar collapse according to the Law of LaPlace. Surfactant production begins around 24 weeks of gestation, and prematurity before adequate surfactant production leads to neonatal respiratory distress syndrome.

<image>Panel A: Cross-section of alveolus showing Type I pneumocytes, Type II pneumocytes with lamellar bodies, and alveolar macrophages with surrounding capillaries. Panel B: Magnified blood-gas barrier layers with thickness measurements for each component totaling 0.3-0.5 micrometers. Panel C: Surfactant function diagram with LaPlace's Law equation showing prevention of small alveoli collapse. Panel D: Surfactant composition details including DPPC and surfactant proteins with Type II cell origin annotated.</image>

Thoracic Cage and Respiratory Muscles

The thoracic cage provides the bony framework that protects thoracic organs while enabling the mechanical ventilation process. The sternum forms the anterior midline, consisting of the manubrium (which articulates with the clavicles and first rib), the body (articulating with ribs two through seven), and the xiphoid process inferiorly. The twelve pairs of ribs articulate posteriorly with the thoracic vertebrae: the first seven pairs (true ribs) attach directly to the sternum via costal cartilages, ribs eight through ten (false ribs) attach to the costal cartilage of the rib above, and ribs eleven and twelve (floating ribs) have no anterior attachment.

The intercostal spaces between adjacent ribs contain three muscle layers. The external intercostals, with fibers running downward and forward, elevate the ribs during inspiration. The internal intercostals, with perpendicular fiber orientation, depress the ribs during forced expiration. The innermost intercostals are variable and poorly defined. The neurovascular bundle, containing the intercostal vein, artery, and nerve in that superior-to-inferior order, courses along the inferior border of each rib. This relationship is clinically important: procedures such as thoracentesis should enter just above a rib to avoid the neurovascular bundle.

Surface anatomy landmarks guide clinical examination and procedures. The sternal angle (angle of Louis) at the junction of the manubrium and body corresponds to the T4-5 vertebral level, marks the attachment of the second rib, and indicates the level of the tracheal bifurcation. The lung apex extends two to three centimeters above the clavicle. The lower lung border can be traced along a line from the sixth rib at the midclavicular line to the eighth rib at the midaxillary line to the tenth rib posteriorly.

The diaphragm is the primary muscle of inspiration, responsible for approximately 75 percent of tidal volume during quiet breathing. This dome-shaped musculotendinous structure separates the thoracic and abdominal cavities. Its muscular fibers arise from the xiphoid process, the lower six ribs, and the lumbar vertebrae (via the crura) and insert into the central tendon. Three major apertures transmit structures between thorax and abdomen: the caval opening at T8 (for the inferior vena cava), the esophageal hiatus at T10 (for the esophagus and vagus nerves), and the aortic hiatus at T12 (for the aorta and thoracic duct). The mnemonic "8, 10, 12 - Vena Cava, Esophagus, Aortic hiatus" helps recall these levels.

The phrenic nerve, derived from cervical roots C3, C4, and C5 ("C3, 4, 5 keeps the diaphragm alive"), provides motor innervation to the diaphragm and sensory innervation to the central diaphragm and mediastinal pleura. Diaphragmatic contraction causes descent of the dome, increasing thoracic volume and decreasing intrathoracic pressure, thereby drawing air into the lungs.

<image>Panel A: Anterior view of thoracic cage showing sternum parts, rib classifications, and sternal angle with anatomical correlates. Panel B: Cross-section through intercostal space showing three muscle layers and neurovascular bundle along inferior rib border. Panel C: Superior view of diaphragm with central tendon, muscular portions, and apertures at T8, T10, and T12. Panel D: Chest outline showing lung surface landmarks and phrenic nerve distribution from C3-4-5.</image>

Pleura and Pleural Spaces

The pleura is a serous membrane that envelops each lung and lines the thoracic cavity, creating the pleural space between its two layers. Understanding pleural anatomy is essential for interpreting chest imaging and performing thoracic procedures.

The visceral pleura directly covers the lung surface, following its contours including entry into the fissures. It is thin, transparent, and firmly adherent to the underlying lung, sharing its blood supply (bronchial arteries) and lymphatic drainage. Importantly, the visceral pleura lacks pain receptors, so diseases involving only this layer are typically painless.

The parietal pleura lines the inner surface of the chest wall and is described according to the surface it covers. The costal pleura lines the ribs and intercostal spaces. The diaphragmatic pleura covers the superior surface of the diaphragm. The mediastinal pleura covers the lateral mediastinum, reflecting over the lung root to become continuous with the visceral pleura. The cervical pleura (cupola) extends above the first rib into the root of the neck, where it is reinforced by the suprapleural membrane. Unlike the visceral pleura, the parietal pleura receives somatic sensory innervation and is exquisitely sensitive to pain; pleuritic pain typically indicates parietal pleural involvement.

The pleural cavity is a potential space between the visceral and parietal layers containing a thin film of serous fluid (normally 10 to 20 milliliters) that lubricates the pleural surfaces during respiration. The pleural recesses are areas where the parietal pleura reflects onto itself without intervening lung. The costodiaphragmatic recess, the largest, exists where the costal pleura reflects onto the diaphragmatic pleura; this recess fills with lung during deep inspiration but remains as a potential space at end-expiration and is the most dependent portion where pleural fluid accumulates in effusions. The costomediastinal recess exists where the costal pleura meets the mediastinal pleura, most prominent on the left where the cardiac notch creates a larger recess.

Pathological conditions affecting the pleural space include pleural effusion (excess fluid accumulation), pneumothorax (air in the pleural space causing lung collapse), empyema (purulent fluid from infection), and hemothorax (blood from trauma or other causes). These conditions are detected clinically and radiographically by their effects on the pleural space.

<image>Panel A: Transverse thorax section showing visceral and parietal pleura divisions with pleural cavity containing serous fluid. Panel B: Coronal section demonstrating costodiaphragmatic and costomediastinal recesses at different respiratory phases. Panel C: Hilum illustration showing reflection points where visceral becomes parietal pleura. Panel D: Clinical panel with pathological conditions including pleural effusion, pneumothorax, and hemothorax with corresponding chest X-ray appearances.</image>

Blood Supply and Innervation

The respiratory system receives blood from two distinct circulations serving different purposes. The pulmonary circulation carries deoxygenated blood from the right heart to the lungs for gas exchange, while the bronchial circulation supplies oxygenated blood to the conducting airways and supporting structures.

The pulmonary circulation is a low-pressure, high-flow system. The pulmonary trunk arises from the right ventricle and divides into the right and left pulmonary arteries, which carry deoxygenated blood to the respective lungs. These arteries branch alongside the airways, reaching the capillary networks surrounding the alveoli where gas exchange occurs. Normal pulmonary arterial pressure is approximately 25/10 mmHg, much lower than systemic pressure, reflecting the thin-walled pulmonary vessels and minimal distance blood must travel. The oxygenated blood returns via the pulmonary veins, typically four in number (two from each lung), which drain directly into the left atrium.

The bronchial circulation arises from the systemic circulation and supplies the conducting airways, visceral pleura, lymph nodes, nerves, and pulmonary vessels down to the level of the respiratory bronchioles. The bronchial arteries typically arise from the thoracic aorta on the left and from the intercostal arteries or a common trunk on the right. Bronchial veins drain the proximal airways into the azygos system (right side) or hemiazygos/accessory hemiazygos (left side). Importantly, some bronchial blood drains into the pulmonary veins, contributing to the normal physiological shunt.

Lymphatic drainage from the lungs follows two routes. The superficial (subpleural) lymphatics drain the visceral pleura and peripheral lung tissue, following the pleural surface to the bronchopulmonary (hilar) nodes. The deep lymphatics follow the bronchial tree from the alveolar level through intrapulmonary nodes to bronchopulmonary nodes, then to tracheobronchial nodes at the carina, and finally to mediastinal nodes. The lymphatic system is critical for clearance of particles, proteins, and fluid from the interstitium and for immune surveillance.

Autonomic innervation of the respiratory system occurs through the pulmonary plexuses located anterior and posterior to the lung roots. Parasympathetic fibers from the vagus nerve cause bronchoconstriction, increased glandular secretion, and vasodilation of pulmonary vessels. Sympathetic fibers from the upper thoracic sympathetic chain cause bronchodilation, decreased secretion, and vasoconstriction. Sensory afferents, primarily vagal, mediate reflexes including the cough reflex, bronchospasm in response to irritants, and the Hering-Breuer inflation reflex.

<image>Panel A: Pulmonary circulation pathway from right heart through pulmonary arteries and veins to left atrium with pressure values annotated. Panel B: Bronchial circulation showing arterial supply from thoracic aorta and venous drainage to azygos system with physiological shunt. Panel C: Lymphatic drainage pathways from superficial and deep systems converging on regional lymph nodes. Panel D: Autonomic innervation showing pulmonary plexus with parasympathetic and sympathetic fibers and their effects on airways.</image>

Summary

The respiratory system is divided functionally into conducting zone (nose to terminal bronchioles for air conditioning) and respiratory zone (respiratory bronchioles to alveoli for gas exchange), and anatomically into upper (nose to larynx) and lower (trachea to alveoli) respiratory tracts. The nasal cavity warms, humidifies, and filters air through its mucosal surface amplified by the conchae. The pharynx directs air toward the larynx, which protects the airway and produces voice through the vocal cords innervated by the recurrent laryngeal nerve. The trachea bifurcates at the carina into main bronchi, with the right being shorter, wider, and more vertical, making it the common destination for aspirated material. The bronchial tree undergoes approximately 23 generations of branching, transitioning from cartilage-supported bronchi to smooth muscle-dominated bronchioles. The lungs contain bronchopulmonary segments, functionally independent units supplied by individual segmental bronchi and arteries with veins running between segments. Approximately 300 to 500 million alveoli provide 70 to 100 square meters of surface area, with Type I pneumocytes covering 95 percent of the surface for gas exchange and Type II pneumocytes producing surfactant. The diaphragm, innervated by the phrenic nerve from C3-5, is the primary inspiratory muscle. The pleura creates a potential space allowing lung expansion, with the costodiaphragmatic recess being the most dependent area for fluid accumulation. Dual blood supply consists of pulmonary circulation for gas exchange and bronchial circulation for airway nutrition.

Key Terms

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