# Lecture 10: The Respiratory System — Anatomy
## Anatomy and Physiology II
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## Learning Objectives
By the end of this lecture, students will be able to:
1. List the major functions of the respiratory system
2. Describe the structure and function of each component of the upper respiratory tract
3. Describe the structure and function of each component of the lower respiratory tract
4. Trace the pathway of air from the nose to the alveoli
5. Describe the histological changes along the respiratory tree
6. Explain the structure of the respiratory membrane and its role in gas exchange
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## Lecture Content
### I. Overview of the Respiratory System
The primary function of the respiratory system is gas exchange: delivering oxygen to the blood and removing carbon dioxide. Beyond this central role, the respiratory system also helps regulate blood pH through carbon dioxide elimination via the bicarbonate buffer system, enables voice production at the larynx, supports olfaction (the sense of smell), protects against inhaled pathogens and irritants through the mucociliary escalator and alveolar macrophages, and contributes to blood pressure regulation by converting angiotensin I to angiotensin II via ACE in the pulmonary capillaries.
Anatomically, the respiratory system is divided into the **upper respiratory tract** (nose, nasal cavity, pharynx, and larynx) and the **lower respiratory tract** (trachea, bronchi, bronchioles, alveolar ducts, and alveoli). Functionally, it is divided into the **conducting zone**, which spans from the nose through the terminal bronchioles and serves to transport, warm, humidify, and filter air without participating in gas exchange (this region constitutes the anatomical dead space of approximately 150 mL), and the **respiratory zone**, which includes the respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli where gas exchange occurs.
### II. Upper Respiratory Tract
#### Nose and Nasal Cavity
The **external nose** is supported by nasal bones and flexible cartilage and opens to the exterior through two nares (nostrils). The **nasal cavity** is divided by the nasal septum, formed by the perpendicular plate of the ethmoid bone, the vomer, and septal cartilage. Just inside the nostrils, the **nasal vestibule** is lined with skin and coarse hairs called vibrissae that filter large particles from inhaled air.
Three bony projections called **nasal conchae** (or turbinates) extend from the lateral walls of the nasal cavity: the superior, middle, and inferior conchae. These structures serve to increase the surface area of the nasal cavity and create turbulent airflow as air passes through the narrow passages (meatuses) between them. This turbulence enhances the warming, humidifying, and filtering of inspired air. The majority of the nasal cavity is lined with **respiratory epithelium** (pseudostratified ciliated columnar epithelium with goblet cells). Mucus produced by goblet cells traps particles, while cilia sweep the mucus toward the pharynx in the mucociliary escalator. A rich blood supply in the submucosa warms the inhaled air. The superior portion of the nasal cavity contains **olfactory epithelium** with olfactory receptors for the sense of smell.
The **paranasal sinuses** are air-filled spaces within the frontal, maxillary, ethmoid, and sphenoid bones. Lined with respiratory epithelium and draining into the nasal cavity, they lighten the skull, warm and humidify air, and provide resonance for the voice. When the sinus mucosa becomes inflamed and infected, the result is sinusitis.
#### Pharynx (Throat)
The pharynx is a muscular tube connecting the nasal cavity and mouth to the larynx and esophagus. It has three regions. The **nasopharynx**, posterior to the nasal cavity, receives air only and contains the pharyngeal tonsil (adenoid) and openings to the auditory tubes. It is lined with respiratory epithelium. The **oropharynx**, posterior to the oral cavity, serves as a passage for both air and food. It contains the palatine and lingual tonsils and is lined with stratified squamous epithelium to protect against abrasion from food. The **laryngopharynx** (hypopharynx) is the inferior portion, continuous with the esophagus posteriorly and the larynx anteriorly. It also serves as a passage for both air and food and is lined with stratified squamous epithelium.
#### Larynx (Voice Box)
The larynx connects the pharynx to the trachea and is built on a framework of nine cartilages. The **thyroid cartilage** is the largest and forms the anterior prominence known as the Adam's apple. The **cricoid cartilage** is ring-shaped and sits inferior to the thyroid cartilage as the only complete cartilage ring in the airway. The **epiglottis** is an elastic cartilage flap that covers the laryngeal inlet during swallowing to prevent aspiration of food into the trachea. The paired **arytenoid cartilages** anchor the vocal cords and move to open and close the glottis. The paired **corniculate and cuneiform cartilages** are small structures that support soft tissue.
The **vocal folds** (true vocal cords) are mucosal folds stretched across the larynx that vibrate as air passes through the glottis (the opening between them), producing sound. The tension and length of the vocal folds, controlled by laryngeal muscles, determine pitch. The **vestibular folds** (false vocal cords), located superior to the true cords, protect them but play no role in voice production. The larynx is lined with respiratory epithelium except over the vocal folds, which are covered by stratified squamous epithelium to resist friction.
A midsagittal section of the head and neck showing the complete upper respiratory tract. Panel A: The nasal cavity with labeled structures — nasal conchae (superior, middle, inferior), meatuses, nasal septum, vestibule with vibrissae, olfactory epithelium at the roof, paranasal sinuses (frontal and sphenoid shown), and hard and soft palate. The three regions of the pharynx (nasopharynx, oropharynx, laryngopharynx) are demarcated with their respective linings noted. Panel B: An anterior view of the larynx showing the thyroid cartilage, cricoid cartilage, epiglottis, and tracheal rings. Panel C: A superior view looking down into the larynx showing the vocal folds (true and false) in both abducted (open glottis, during breathing) and adducted (closed glottis, during phonation) positions, with the arytenoid cartilages labeled.
### III. Lower Respiratory Tract
#### Trachea (Windpipe)
The trachea is approximately 12 cm long and 2.5 cm in diameter, extending from the larynx at the level of C6 to the carina at T4-T5, where it bifurcates into the two main bronchi. It is supported by 16 to 20 C-shaped rings of hyaline cartilage whose open ends face posteriorly and are connected by the trachealis muscle (smooth muscle) and soft connective tissue. The cartilage rings prevent the airway from collapsing, while the open posterior design allows the esophagus to expand during swallowing and permits more forceful airway compression during coughing. The trachea is lined with respiratory epithelium, and its mucociliary escalator moves trapped debris upward toward the pharynx. The **carina**, an internal ridge at the tracheal bifurcation, is highly sensitive to touch and triggers the cough reflex.
#### Bronchial Tree
The bronchial tree branches progressively from the trachea to the alveoli through approximately 23 generations of branching.
The **primary (main) bronchi** enter the lungs at the hilum. The right main bronchus is wider, shorter, and more vertical than the left, which is why aspirated foreign objects more commonly lodge on the right side. Primary bronchi are supported by C-shaped cartilage rings and lined with respiratory epithelium. **Secondary (lobar) bronchi** branch off to supply each lobe (three on the right, two on the left), and their cartilage support transitions from C-shaped rings to irregular cartilage plates. **Tertiary (segmental) bronchi** supply the bronchopulmonary segments, of which there are 10 per lung.
**Bronchioles**, with diameters less than 1 mm, lack cartilage entirely and rely on smooth muscle and elastic fibers for structural support. The smooth muscle tone in bronchioles regulates airway diameter: sympathetic stimulation via epinephrine acting on beta-2 receptors causes bronchodilation, while parasympathetic stimulation via acetylcholine on muscarinic receptors causes bronchoconstriction. Histamine and leukotrienes also cause bronchoconstriction, which is the underlying mechanism in asthma. **Terminal bronchioles** are the last components of the conducting zone and are lined with simple cuboidal epithelium containing club cells (Clara cells), which secrete surfactant-like substances and detoxifying enzymes.
**Respiratory bronchioles** mark the beginning of the respiratory zone. They have scattered alveoli budding from their walls and lead to alveolar ducts and alveolar sacs.
#### Histological Changes Along the Respiratory Tree
As the airways branch and narrow, the epithelium transitions progressively from pseudostratified ciliated columnar to ciliated columnar to ciliated cuboidal to simple cuboidal and finally to simple squamous epithelium in the alveoli. Goblet cells decrease in number and disappear by the bronchiolar level. Cartilage support decreases from rings to plates and is entirely absent in bronchioles. Smooth muscle increases proportionally through the bronchioles, then decreases in the respiratory zone. Club cells appear beginning in the terminal bronchioles.
### IV. The Lungs and Pleura
#### Lungs
The lungs are paired organs that occupy most of the thoracic cavity. The **right lung** has three lobes (superior, middle, and inferior) separated by the oblique and horizontal fissures. The **left lung** has two lobes (superior and inferior) separated by the oblique fissure, and it features the cardiac notch and lingula to accommodate the heart. Each lung is divided into 10 bronchopulmonary segments, which are clinically important units for surgical resection. The **hilum** on the medial surface serves as the entry and exit point for the bronchi, pulmonary vessels, nerves, and lymphatics, collectively forming the root of the lung.
#### Pleura
Each lung is enclosed by a double-layered serous membrane. The **visceral pleura** is tightly adherent to the lung surface, while the **parietal pleura** lines the thoracic wall, mediastinum, and diaphragm. Between them lies the **pleural cavity**, a potential space containing a thin film of serous fluid that reduces friction during breathing. Surface tension between the layers holds the visceral and parietal pleurae together, much like two glass slides joined by a thin film of water. This arrangement, combined with the negative intrapleural pressure, is essential for keeping the lungs inflated. **Pneumothorax**, the entry of air into the pleural cavity, disrupts the negative pressure and causes the lung to collapse (atelectasis). **Pleural effusion**, an excess of fluid in the pleural cavity, can also compromise lung expansion.
### V. The Respiratory Membrane (Air-Blood Barrier)
The respiratory membrane is the barrier across which gas exchange occurs between alveolar air and pulmonary capillary blood. It is extremely thin, approximately 0.5 micrometers, to facilitate rapid diffusion. From air to blood, it consists of four layers: a thin layer of alveolar fluid containing surfactant, the alveolar epithelium composed of type I pneumocytes (simple squamous cells), the fused basement membranes of the alveolar epithelium and capillary endothelium, and the capillary endothelium itself. The total surface area available for gas exchange is approximately 70 square meters (about the size of a tennis court), distributed across roughly 300 million alveoli in both lungs.
#### Alveolar Cell Types
**Type I pneumocytes** (alveolar cells) are thin squamous cells that cover approximately 95% of the alveolar surface and are the primary cells through which gas exchange occurs. **Type II pneumocytes** (alveolar cells) are cuboidal cells that secrete **pulmonary surfactant**, a mixture of phospholipids (mainly dipalmitoylphosphatidylcholine) and proteins that reduces surface tension in the alveoli, preventing their collapse during expiration. Insufficient surfactant production, as occurs in premature infants, causes infant respiratory distress syndrome (IRDS). **Alveolar macrophages** (dust cells) reside within the alveoli, phagocytizing bacteria, dust, and debris as part of the innate immune defense.
A detailed diagram of alveolar structure and the respiratory membrane. Panel A: A cluster of alveoli at the end of an alveolar duct, showing the grape-like arrangement of alveolar sacs, with pulmonary capillaries wrapping around each alveolus. Type I cells, type II cells (with lamellar bodies secreting surfactant), and an alveolar macrophage are labeled within the alveolar wall. Panel B: A magnified cross-section of the respiratory membrane showing the four layers from the air-filled alveolus to the blood-filled capillary — alveolar fluid with surfactant, type I alveolar epithelium, fused basement membranes, and capillary endothelium. A red blood cell within the capillary is shown in close proximity to the alveolar air, with O2 and CO2 diffusion arrows. The total thickness of approximately 0.5 micrometers is noted.
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