Respiratory System: Anatomy of the Lungs and Airways
Complete anatomy of the respiratory system - nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles, alveoli, and lungs. Pleura, blood supply, and mechanics of breathing.
This content is for informational purposes only. Always consult a healthcare professional.
Overview
Function
Mechanism
Gas exchange
O₂ and CO₂ diffusion across alveolar-capillary membrane
Air conduction
Conducting passages to alveoli
Filtration
Nasal hairs, mucosa, cilia
Vocalization
Larynx (vocal cords)
Olfaction
Olfactory epithelium
pH regulation
CO₂ elimination
Upper Respiratory Tract
Nasal Cavity
Structure
Description
External nares (nostrils)
Anterior openings
Nasal vestibule
Lined by skin (vibrissae)
Nasal cavity proper
Divided by nasal septum
Nasal conchae (turbinates)
Superior, middle, inferior (increase surface area)
Meatuses
Spaces below each concha
Choanae (posterior nares)
Openings into nasopharynx
Functions: Warm, humidify, filter inspired air; olfaction; resonance
Paranasal Sinuses
Frontal, maxillary, ethmoid (anterior/posterior), sphenoid
Lighten skull, warm/humidify air, resonance
Pharynx
Section
Location
Boundaries
Contents
Nasopharynx
Posterior to nasal cavity
Choanae → soft palate
Pharyngeal tonsil, Eustachian tube (pharyngeal) orifice
Oropharynx
Posterior to oral cavity
Soft palate → epiglottis
Palatine tonsils, lingual tonsil
Laryngopharynx
Posterior to larynx
Epiglottis → cricoid cartilage
Piriform recesses
Larynx
Cartilage
Type
Function
Thyroid
Hyaline (paired laminae)
Anterior prominence (Adam apple)
Cricoid
Hyaline (signet ring)
Complete ring below thyroid
Epiglottis
Elastic
Covers laryngeal inlet during swallowing
Arytenoid (paired)
Hyaline
Vocal process, muscular process
Corniculate (paired)
Hyaline
Above arytenoids
Cuneiform (paired)
Elastic
Within aryepiglottic folds
Vocal cords:
True vocal cords: Vibrate for phonation (vocal ligament + vocalis muscle)False vocal cords (vestibular folds): Above true cords, protection
Laryngeal innervation:
Nerve
Motor
Sensory
Superior laryngeal (CN X)
Cricothyroid
Above vocal cords
Recurrent laryngeal (CN X)
All other laryngeal muscles
Below vocal cords
Lower Respiratory Tract
Trachea
Feature
Description
Length
10-12 cm
Diameter
2-2.5 cm
Cartilage
16-20 C-shaped rings
Carina
Bifurcation (T4 level)
Membranous wall
Posterior (trachealis muscle)
Bronchial Tree
Generation
Structure
Cartilage
Epithelium
Smooth Muscle
1
Main (primary) bronchi
Yes (C-shaped)
Pseudostratified columnar
Spiral bands
2-3
Lobar (secondary) bronchi
Yes (irregular)
Pseudostratified columnar
Spiral bands
4-9
Segmental (tertiary) bronchi
Yes (irregular)
Pseudostratified columnar
Increasing
10-16
Bronchioles
No
Ciliated columnar → cuboidal
Prominent
17-23
Terminal bronchioles
No
Cuboidal
Present
14-16
Respiratory bronchioles
No
Cuboidal (some cilia)
Decreasing
17-19
Alveolar ducts
No
Squamous
Absent
20-23
Alveolar sacs
No
Squamous (alveoli)
Absent
Bronchopulmonary Segments
Each lung has 10 bronchopulmonary segments (left has 8-10, some fuse):
Right lung: Apical, posterior, anterior (upper); medial, lateral (middle); superior, medial basal, anterior basal, lateral basal, posterior basal (lower)
Left lung: Apicoposterior, anterior, superior lingular, inferior lingular (upper); superior, anteromedial basal, lateral basal, posterior basal (lower)
Alveoli
Feature
Description
Number
300-500 million
Diameter
200-300 microns
Surface area
70-100 m²
Wall thickness
0.2-0.5 microns
Alveolar cells:
Cell
Function
Type I pneumocyte
Gas exchange (95% of surface area)
Type II pneumocyte
Surfactant production (pulmonary surfactant)
Alveolar macrophages
Immune defense (dust cells)
Surfactant:
Produced by Type II cells (fetal ~week 26)
Reduces surface tension (prevents alveolar collapse)
Components: Dipalmitoylphosphatidylcholine (DPPC), SP-A, SP-B, SP-C, SP-D
Lungs
Gross Features
Feature
Right Lung
Left Lung
Lobes
3 (upper, middle, lower)
2 (upper, lower)
Fissures
Horizontal, oblique
Oblique (cardiac notch)
Weight
~600 g
~500 g
Hilum level
T5-T6
T5-T6
Lingula
Absent
Present (homolog of right middle lobe)
Pleura
Layer
Description
Visceral pleura
Covers lung surface (includes fissures)
Parietal pleura
Lines thoracic cavity, mediastinum, diaphragm
Pleural cavity
Potential space between layers (5-15 mL fluid)
Recesses:
Costodiaphragmatic recess (deepest, between costal and diaphragmatic pleura)
Costomediastinal recess (behind sternum)
Parietal pleura divisions: Cervical (cupula), costal, mediastinal, diaphragmatic
Blood Supply
Pulmonary Circulation
Pulmonary arteries: Deoxygenated blood to lungs (low pressure)
Pulmonary capillaries: Surround alveoli for gas exchange
Pulmonary veins: Oxygenated blood to left atrium (4 total)
Bronchial Circulation
Artery
Origin
Territory
Right bronchial
Third intercostal or left bronchial
Bronchi to respiratory bronchioles
Left bronchial
Descending thoracic aorta
Bronchi to respiratory bronchioles
Note: Bronchial veins drain to azygos/hemiazygos; some blood mixes with pulmonary veins (anatomic shunt)
Mechanics of Breathing
Muscle
Action
Innervation
Diaphragm
Inspiration (main)
Phrenic (C3-C5)
External intercostals
Inspiration (elevates ribs)
Intercostal nerves (T1-T11)
Internal intercostals
Forced expiration
Intercostal nerves
Accessory (SCM, scalenes)
Forced inspiration
CN XI, cervical nerves
Quiet breathing: Inspiration active (diaphragm), expiration passive (elastic recoil)
Pressure
Relationship
Value
Atmospheric
Reference
760 mmHg (0 cmH₂O)
Intrapleural
Negative (subatmospheric)
-4 to -8 cmH₂O
Intrapulmonary (alveolar)
Equals atmospheric at rest
0 cmH₂O
Transpulmonary
Alveolar - intrapleural
4-8 cmH₂O
Lung Volumes
Volume
Value (mL)
Description
Tidal volume (TV)
500
Normal breath
Inspiratory reserve (IRV)
3000
Maximal inspiration above TV
Expiratory reserve (ERV)
1100
Maximal expiration below TV
Residual volume (RV)
1200
Air remaining after maximal expiration
Vital capacity (VC)
4600
IRV + TV + ERV
Total lung capacity (TLC)
5800
VC + RV
Dead space (anatomical)
150
Conducting airways
Clinical Anatomy
Structure
Clinical Relevance
Carina
Most sensitive area for cough reflex
Right main bronchus
Wider, more vertical → aspiration more common
Pleural recesses
Thoracocentesis (9th intercostal space, midaxillary)
Cupola (cervical pleura)
Injury risk in supraclavicular approaches
Pulmonary ligament
Stabilizes lower lobe
Costodiaphragmatic recess
Common site of pleural effusion
Horizontal fissure
Right lung, 4th rib level
Oblique fissure
6th rib midaxillary line (spinose of T3 to 6th costochondral)