Difficulty: Intermediate | Prerequisites: Basic tissue types, blood vessel structure (Ch. 20), thoracic cavity orientation
Tags: respiratory system, conducting zone, respiratory zone, nasal cavity, pharynx, larynx, trachea, bronchi, alveoli, lungs, pleura, respiratory membrane, APK2100c, University of Florida
The respiratory system is the body's gas-exchange interface with the outside world. This chapter walks through every structure from the nostrils down to the alveoli, distinguishing the "conducting zone" (which moves air but does not exchange gas) from the "respiratory zone" (where actual O₂/CO₂ exchange occurs). You will need to know the specific tissues, cartilages, and mucosae at each level, and how the structure of the alveolar wall makes gas exchange possible. This material builds directly on your knowledge of pulmonary blood vessels from Ch. 20.
Air passes from the nose through the pharynx, larynx, trachea, and a branching series of bronchi before reaching the alveoli, where gas exchange takes place across a very thin respiratory membrane. Everything from the nose to the terminal bronchioles is conducting zone (air conditioning and transport). The respiratory bronchioles, alveolar ducts, and alveoli make up the respiratory zone. The lungs are divided into lobes (three on the right, two on the left) and wrapped in a double-layered pleural membrane.
Conducting zone
All airways from the nose/mouth through the terminal bronchioles. These passages warm, humidify, and filter air but do not participate in gas exchange. Sometimes called "anatomical dead space."
Respiratory zone
The gas-exchanging portions of the lungs: respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli.
Respiratory mucosa
Pseudostratified ciliated columnar epithelium with goblet cells. Lines most of the conducting zone. The cilia beat in coordinated waves to move mucus (and trapped particles) toward the pharynx.
Olfactory mucosa
Specialised epithelium in the superior portion of the nasal cavity containing olfactory receptor neurons for the sense of smell. It is not ciliated in the respiratory sense and occupies a small patch on the superior nasal concha and adjacent septum.
Larynx
The voice box, made of nine cartilages (three paired, three unpaired). It connects the pharynx to the trachea and houses the vocal folds.
True vocal cords (vocal folds)
The inferior pair of mucosal folds in the larynx, containing the vocalis muscle and vocal ligament. They vibrate to produce sound.
False vocal cords (vestibular folds)
The superior pair of mucosal folds. They do not produce sound but help close the airway during swallowing and during the Valsalva manoeuvre (bearing down).
Glottis (rima glottidis)
The opening between the true vocal folds. This is the narrowest point of the larynx and the actual space through which air (and sound) passes.
Trachealis
The smooth muscle that spans the open, posterior gap of the C-shaped tracheal cartilage rings. It allows the oesophagus to expand into the tracheal space during swallowing and can contract to narrow the trachea during coughing.
Alveolus
A tiny, thin-walled air sac where gas exchange occurs. Walls are mostly Type I alveolar cells (simple squamous, for diffusion) with Type II alveolar cells (cuboidal, secrete surfactant) and alveolar macrophages (dust cells).
Respiratory membrane
The thin barrier across which gas exchange occurs, consisting of (from air to blood): a thin layer of surfactant, the Type I alveolar cell, fused basement membranes of the alveolar and capillary walls, and the capillary endothelium. Total thickness is about 0.5 µm.
Pleura
A double-layered serous membrane surrounding each lung: the visceral pleura (on the lung surface) and the parietal pleura (lining the thoracic wall). The pleural cavity between them contains a thin film of serous fluid that reduces friction and creates surface tension holding the lungs against the chest wall.
Hilum (hilus)
The medial indentation on each lung where the bronchi, pulmonary vessels, lymphatic vessels, and nerves enter and exit. The structures passing through the hilum collectively form the lung root.
External nose → nasal cavity
Pharynx (nasopharynx → oropharynx → laryngopharynx)
Larynx
Trachea
Primary (main) bronchi (right and left)
Secondary (lobar) bronchi
Tertiary (segmental) bronchi
Bronchioles → terminal bronchioles (end of conducting zone)
Respiratory bronchioles (start of respiratory zone)
Alveolar ducts
Alveolar sacs / alveoli
Conducting zone (nose through terminal bronchioles):
Functions: warm, humidify, and filter incoming air
Lined mostly with respiratory mucosa (pseudostratified ciliated columnar epithelium with goblet cells)
Contains cartilage support (from trachea through bronchi; bronchioles lack cartilage)
Smooth muscle increases proportionally as cartilage decreases from bronchi to bronchioles
No gas exchange here
Respiratory zone (respiratory bronchioles through alveoli):
Functions: gas exchange between air and blood
Epithelium transitions to simple squamous (Type I alveolar cells) to minimise diffusion distance
Surrounded by dense pulmonary capillary networks
Supported superiorly by nasal bones (bony framework) and inferiorly by hyaline cartilage (flexible framework)
The bridge is bone; the tip and sides are cartilage
External nares (nostrils) are the entry points
Divided by the nasal septum (perpendicular plate of the ethmoid, vomer, and septal cartilage)
Three nasal conchae (superior, middle, inferior) project from the lateral walls, creating turbulent airflow
The conchae increase surface area for warming and humidifying air and direct airflow over the mucosa
Hard palate (bone) and soft palate form the floor, separating the nasal cavity from the oral cavity
Paranasal sinuses (frontal, maxillary, ethmoid, sphenoid) drain into the nasal cavity and lighten the skull
Olfactory mucosa:
Located on the superior nasal concha and adjacent nasal septum (roof region)
Contains olfactory receptor neurons (bipolar neurons), supporting cells, and basal cells
Function: smell
Respiratory mucosa:
Lines the rest of the nasal cavity and most of the conducting airways
Pseudostratified ciliated columnar epithelium with goblet cells
Mucus traps particles; cilia sweep the mucus toward the pharynx (mucociliary escalator)
Nasopharynx:
Posterior to the nasal cavity, superior to the soft palate
Pseudostratified ciliated columnar epithelium (respiratory)
Contains the pharyngeal tonsil (adenoid) and openings of the pharyngotympanic (Eustachian/auditory) tubes
Strictly an air passageway
Oropharynx:
Posterior to the oral cavity, from the soft palate to the epiglottis
Stratified squamous epithelium (abrasion protection from food)
Contains the palatine and lingual tonsils
Shared passageway for air and food
Laryngopharynx:
From the epiglottis to the opening of the larynx (anteriorly) and oesophagus (posteriorly)
Stratified squamous epithelium
Shared passageway for air and food; food is directed posteriorly into the oesophagus
Framework of nine cartilages:
Unpaired: thyroid cartilage (Adam's apple, largest), cricoid cartilage (complete ring, inferior), epiglottis (elastic cartilage flap)
Paired: arytenoid, corniculate, cuneiform
The epiglottis folds over the laryngeal inlet during swallowing to prevent food from entering the airway
Mostly hyaline cartilage, except the epiglottis (elastic cartilage) for flexibility
True vocal cords (folds): inferior pair; vibrate to produce sound; contain the vocal ligaments and vocalis muscles
False vocal cords (vestibular folds): superior pair; do not vibrate; protect the true cords and close during swallowing
Glottis/rima glottidis: the opening between the true vocal folds; it is the actual passage for air, not the folds themselves
Approximately 10–12 cm long, extending from the larynx to the carina (where it divides)
Supported by 16–20 C-shaped rings of hyaline cartilage, open posteriorly
Trachealis muscle bridges the posterior gap, allowing oesophageal expansion during swallowing and forceful narrowing during coughing
Lined with respiratory mucosa (pseudostratified ciliated columnar epithelium with goblet cells)
Submucosa contains seromucous glands
Trachea → right and left primary (main) bronchi at the carina
Right primary bronchus is wider, shorter, and more vertical (inhaled objects more likely to lodge here)
Primary bronchi → secondary (lobar) bronchi (three on the right, two on the left, matching the lung lobes)
Secondary bronchi → tertiary (segmental) bronchi (each supplies one bronchopulmonary segment)
Further branching into smaller and smaller bronchioles
As branching progresses: cartilage decreases, smooth muscle proportion increases, epithelium transitions from pseudostratified columnar to simple columnar to simple cuboidal
Terminal bronchioles are the smallest conducting airways (no cartilage, simple cuboidal epithelium)
Right lung: three lobes (superior, middle, inferior) separated by the oblique and horizontal fissures; shorter and wider (liver pushes it up)
Left lung: two lobes (superior, inferior) separated by the oblique fissure; has a cardiac notch on the medial surface to accommodate the heart
Each lobe is divided into bronchopulmonary segments (each served by a tertiary bronchus and its own blood supply; can be surgically removed independently)
Segments are further divided into lobules, each wrapped in elastic connective tissue and served by a single terminal bronchiole
Terminal bronchioles lead to respiratory bronchioles, which have scattered alveoli budding from their walls
Respiratory bronchioles lead to alveolar ducts, lined almost entirely with alveoli
Alveolar ducts terminate in alveolar sacs: clusters of alveoli arranged around a common central opening (duct)
Approximately 300 million alveoli in total, providing an enormous surface area (roughly 70 m²)
Type I alveolar cells (squamous pneumocytes): simple squamous cells forming most of the alveolar wall; extremely thin for diffusion
Type II alveolar cells (great alveolar cells / septal cells): cuboidal cells that secrete pulmonary surfactant, reducing surface tension and preventing alveolar collapse
Alveolar macrophages (dust cells): phagocytic cells that remove debris, bacteria, and particles from the alveolar surface
Elastic fibres in the alveolar walls allow recoil during exhalation
Respiratory membrane layers (air side to blood side):
Surfactant film
Type I alveolar cell (with its basement membrane)
Fused basement membranes of the alveolar wall and capillary
Capillary endothelial cell
This membrane is approximately 0.5 µm thick, allowing rapid diffusion of O₂ and CO₂
Apex: superior tip, extends slightly above the clavicle into the root of the neck
Base: inferior surface, sits on the diaphragm
Hilum (hilus): medial indentation where the bronchi, pulmonary vessels, nerves, and lymphatics enter/exit
Lung root: the collection of structures passing through the hilum (primary bronchus, pulmonary artery, two pulmonary veins, bronchial vessels, nerves, lymphatics)
The hilum is the "doorway"; the root is "everything passing through that doorway"
Visceral pleura: serous membrane directly on the lung surface
Parietal pleura: serous membrane lining the thoracic wall, mediastinum, and diaphragm
Pleural cavity: the potential space between them, containing a small amount of serous fluid
The fluid creates surface tension that keeps the lungs expanded against the chest wall and reduces friction during breathing
Tissue: mesothelium (simple squamous epithelium) on a thin connective tissue layer
Damage to the larynx or vocal folds → voice changes, hoarseness, or inability to speak; potentially compromised airway protection during swallowing
Damage to the trachea → difficulty breathing, stridor (abnormal breathing sounds)
Damage to a primary bronchus → collapse or impaired ventilation of an entire lung
Damage to alveoli (e.g., emphysema) → reduced gas exchange surface area → shortness of breath, hypoxia
Damage to the pleura (e.g., pneumothorax, air in the pleural cavity) → loss of surface tension → lung collapse on the affected side
Damage to the phrenic nerve or diaphragm → impaired ventilation of the lung on that side
The C-shaped tracheal cartilages are the reason a tracheostomy is performed from the front: the posterior gap (where the trachealis muscle sits) faces the oesophagus and would be dangerous to cut through. Surfactant produced by Type II cells is critically important in premature infants; its absence causes neonatal respiratory distress syndrome because the alveoli collapse with each breath. This is why premature babies may receive artificial surfactant therapy.
Students often confuse the glottis with the epiglottis. The glottis (rima glottidis) is the opening between the vocal folds. The epiglottis is the cartilage flap that covers the laryngeal inlet during swallowing. They are in the same neighbourhood but are different structures with different functions.
The false vocal cords do not produce sound. Students sometimes mix up "true" and "false" on exams; the true (inferior) cords are the ones that vibrate.
The right lung has three lobes, the left has two. Students sometimes reverse this. The left lung is smaller because the heart occupies space on the left side (cardiac notch).
Bronchioles do not have cartilage. Students sometimes describe bronchioles as having cartilage rings; the cartilage disappears at the bronchiole level.
⚠️ Know the full sequence of air passages from nose to alveoli and where the conducting zone ends and the respiratory zone begins (the transition is at the respiratory bronchioles).
⚠️ Be able to distinguish olfactory mucosa from respiratory mucosa by location, tissue type, and function.
⚠️ Know the three regions of the pharynx, their boundaries, epithelium, and structures within each.
⚠️ True vs. false vocal cords, and what the glottis actually is.
⚠️ The respiratory membrane layers, in order, from air to blood.
⚠️ Right lung (3 lobes) vs. left lung (2 lobes), and the structural reason for the difference.
⚠️ Hilum vs. root: what each term means.
True or False: The respiratory zone begins at the terminal bronchioles.
Fill in the blank: The posterior gap in the tracheal cartilage rings is spanned by the __________ muscle.
True or False: The right lung has two lobes.
Fill in the blank: Type II alveolar cells secrete __________.
True or False: The glottis is a cartilage flap that covers the airway during swallowing.
Answers: 1. False (it begins at the respiratory bronchioles). 2. Trachealis. 3. False (three lobes). 4. Surfactant. 5. False (the glottis is the opening between the vocal folds; the epiglottis is the cartilage flap).
Q: List the respiratory passageways in order from the nose to the alveoli.
A: Nasal cavity → pharynx (naso-, oro-, laryngopharynx) → larynx → trachea → primary bronchi → secondary bronchi → tertiary bronchi → bronchioles → terminal bronchioles → respiratory bronchioles → alveolar ducts → alveolar sacs/alveoli.
Q: How do the conducting zone and respiratory zone differ in structure and function?
A: The conducting zone (nose through terminal bronchioles) warms, humidifies, and filters air but does not exchange gases. It is lined mainly with pseudostratified ciliated columnar epithelium and supported by cartilage in the larger airways. The respiratory zone (respiratory bronchioles through alveoli) is the site of gas exchange, lined with thin simple squamous epithelium (Type I cells), and surrounded by dense capillary networks.
Q: Name the layers of the respiratory membrane from air to blood.
A: Surfactant film → Type I alveolar cell and its basement membrane → fused basement membranes → capillary endothelial cell.
Q: Distinguish between the hilum and the root of the lung.
A: The hilum (hilus) is the medial indentation on the lung surface where structures enter and exit. The root is the collection of structures that pass through the hilum: the primary bronchus, pulmonary artery, pulmonary veins, bronchial vessels, lymphatics, and nerves.
Q: What are the three types of cells found in the alveolar wall, and what does each do?
A: Type I alveolar cells (squamous pneumocytes) form the thin diffusion surface. Type II alveolar cells (great alveolar cells) secrete surfactant to reduce surface tension. Alveolar macrophages (dust cells) phagocytise debris and pathogens.
Q: A patient has a puncture wound that introduces air into the pleural cavity. What happens and why?
A: Air in the pleural cavity (pneumothorax) breaks the surface tension created by the serous fluid between the visceral and parietal pleurae. Without that surface tension holding the lung against the chest wall, the lung's elastic fibres cause it to recoil and collapse on the affected side.
This connects to Blood Vessels (Ch. 20) because the pulmonary capillaries surrounding the alveoli are the site of gas exchange, and understanding capillary types (continuous capillaries in the lungs) matters here. It also connects to the Heart (Ch. 19) through the pulmonary circuit: the right ventricle sends deoxygenated blood to the lungs, and the left atrium receives oxygenated blood returning from them. The pharynx is shared with the Digestive System (Ch. 23), since the oropharynx and laryngopharynx are common passageways for air and food.
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