Respiratory System: Structure and Function – Anatomy & Physiology Study Notes

Respiratory System Overview

Introduction

The respiratory system is responsible for the exchange of gases (oxygen and carbon dioxide) between the body and the environment. It consists of a series of organs and structures that facilitate breathing, protect against pathogens, and contribute to speech and olfaction.

• Respiratory tract (before entering lung): Nose & nasal cavity, Paranasal sinuses, Pharynx, Larynx, Trachea

• Lungs and pleurae: Bronchial tree, Alveoli

• Ventilation: The process of moving air in and out of the lungs

Nose and Nasal Cavity

External Nose

The external nose forms the entrance to the respiratory system and is composed of skin, bone, and cartilage.

• Skin extends into the vestibule; contains many sebaceous glands

• Bone: Frontal, Nasal bones, Maxillary bone (frontal process)

• Cartilage: Hyaline cartilage (Alar & Septal cartilages)

• Size variation due to differences in nasal cartilages

Nasal Cavity

The nasal cavity is divided by the nasal septum and lined with mucous membrane. It is continuous with the nasopharynx and contains structures that filter, warm, and moisten incoming air.

• External nares (nostrils)

• Nasal septum: Ethmoid, Vomer, Septal cartilage

• Nasal conchae and meatuses: Superior, Middle, Inferior

• Nasal vestibule: Entryway lined with skin, hair, sebaceous glands, sweat glands

Nasal Mucosa

The nasal mucosa lines most of the nasal cavity and paranasal sinuses, playing a key role in air filtration and humidification.

• Respiratory epithelium: Pseudostratified ciliated columnar epithelium

• Cilia: Whiplike, motile extensions that move mucus

• Goblet cells: Produce mucus

• Lamina propria: Loose connective tissue with mucous and serous cells

• Rich plexus of blood vessels warms cold air

Functions of Nasal Structures

• Moistens and warms air

• Filters inhaled air

• Resonating chamber for speech

• Houses olfactory receptors

Paranasal Sinuses

Structure and Function

Paranasal sinuses are air-filled spaces in the skull that open into the nasal cavity and are lined with mucous membrane.

• Frontal sinus (frontal bone)

• Ethmoidal air cells (ethmoid bone)

• Sphenoidal sinus (sphenoid bone)

• Maxillary sinus (maxillae)

• Warm and moisten inhaled air

• Most sinuses open into the middle nasal meatus

Pharynx

General Structure

The pharynx is a funnel-shaped passageway for both air and food, connecting the nasal cavity and mouth to the larynx and esophagus.

• Divided into three sections by location and function:

  • Nasopharynx

  • Oropharynx

  • Laryngopharynx

Nasopharynx

• Continuous with nasal cavity (posterior nasal aperture = choanae)

• Only an air passageway

• Closed off during swallowing (uvula reflects superiorly)

• Respiratory epithelium propels mucus downward

• Pharyngeal tonsil (adenoids) destroys pathogens in inhaled air

• Opening of pharyngotympanic tube (auditory tube) and tubal tonsil provide protection from infection

Oropharynx

• Passageway for both food and air

• Superior boundary: soft palate; Inferior boundary: epiglottis

• Archlike entranceway = fauces

• Stratified squamous epithelium

• Palatine tonsils (lateral walls of fauces)

• Lingual tonsils (posterior surface of tongue)

Laryngopharynx

• Passageway for both food and air

• Stratified squamous epithelium

• Continuous with esophagus and larynx

• Extends to inferior boundary of cricoid cartilage

Larynx

Structure and Function

The larynx routes air and food into the proper channels and is involved in sound production.

• Extends from 4th to 6th cervical vertebrae

• Attaches to hyoid bone superiorly, opens into laryngopharynx

• Continuous with trachea inferiorly

• Superior opening closed during swallowing, open during breathing

Laryngeal Cartilages

• Epiglottis: Leaf-shaped, elastic cartilage, tips inferiorly during swallowing

• Thyroid cartilage: Shield-shaped, forms laryngeal prominence (Adam’s apple)

• Cricoid cartilage

• Three pairs of small cartilages: Arytenoid, Corniculate, Cuneiform

Vocal Ligaments

• Vocal folds (true vocal cords): Elastic fibers, sound production

• Vestibular folds (false vocal cords): No role in sound production, enhance high-frequency sounds

• Rima glottidis: Medial opening between vocal folds

• Glottis: Rima glottidis and vocal folds together

Laryngeal Epithelium

• Stratified squamous (superior to vocal cord)

• Pseudostratified ciliated columnar (inferior to vocal cord)

• Cough reflex expels substances other than air

Trachea

Structure and Function

The trachea is a flexible tube that descends from the larynx into the mediastinum, conducting air to the lungs.

• C-shaped hyaline cartilage rings keep airway open

• Trachealis: Muscle located between open ends of cartilage rings

• Carina: Marks division into two primary bronchi; mucosa highly sensitive to irritants (cough reflex)

Tracheal Wall Layers

• Mucosa: Respiratory epithelium (cilia propel superiorly), lamina propria

• Submucosa: Seromucous gland (serous and mucous cells)

• Adventitia

• Fibromusculocartilaginous layer: Fibroelastic connective tissue + cartilaginous rings + trachealis

Bronchial Tree

Branching Structure

The bronchial tree consists of extensively branching respiratory passageways that conduct air to the lungs.

• Primary bronchi (main bronchi): First branches of trachea

• Secondary (lobar) bronchi: Three on right, two on left; each supplies a lobe

• Tertiary (segmental) bronchi: Branch into each lung segment

• Smaller bronchi: Divide repeatedly, about 23 (20–25) orders

• Bronchioles: Less than 1 mm in diameter, usually no cartilage

• Terminal bronchioles: Less than 0.5 mm in diameter, last part of conducting zone

Bronchial Structures in the Conducting Zone

• Supportive connective tissues change: Cartilage plates reduce, disappear by bronchioles

• Epithelium changes: Pseudostratified ciliated columnar → simple columnar → simple cuboidal (no cilia, no mucus-producing cells)

• Smooth muscle becomes important: Airways widen with sympathetic stimulation, constrict under parasympathetic direction

Respiratory Zone

Respiratory Bronchioles and Alveoli

The respiratory zone is where gas exchange occurs, beginning at the respiratory bronchioles and ending at the alveoli.

• Respiratory bronchioles: Branch from terminal bronchioles

• Alveolar ducts: Lead to alveolar sacs

• Alveolar sacs: Clusters of alveoli

• ~400 million alveoli create a surface area of ~1500 square feet (140 square meters) for gas exchange

Respiratory Membrane

• Thin, delicate membrane (~0.4 μm)

• Made of alveolar and capillary walls plus their fused basal lamina

• Site of O2 and CO2 exchange

Structure of Alveoli

• Surrounded by elastic fibers

• Interconnected by alveolar pores

• Type I alveolar cells: Simple squamous epithelial cells, surrounded by basal lamina

• Type II alveolar cells: Cuboidal epithelial cells, secrete surfactant (reduces surface tension)

• Alveolar macrophages: Remove tiniest inhaled particles, migrate into bronchi, ciliary action moves them to pharynx

Lungs

Gross Anatomy

• Apex: Superior tip

• Base: Concave inferior surface

• Costal surface: Faces ribs

• Diaphragmatic surface: Faces diaphragm

• Mediastinal surface: Faces mediastinum

• Cardiac notch: Depression in left lung for heart

Hilum and Root

• Hilum: Indentation on mediastinal surface where blood vessels, bronchi, and nerves enter/exit

• Root: Structures entering/leaving at hilum (2 pulmonary veins, 1 pulmonary artery, bronchi, nerves)

Lobes and Segments

• Left lung: Superior and inferior lobes, oblique fissure, cardiac notch

• Right lung: Superior, middle, and inferior lobes, oblique & horizontal fissures

• Each segment supplied independently by tertiary bronchi and pulmonary artery branch; drained by intersegmental pulmonary veins; surgically resectable

Summary Table: Respiratory Tract Structures

Structure	Epithelium Type	Main Function
Nasal cavity	Pseudostratified ciliated columnar	Filter, warm, moisten air
Nasopharynx	Pseudostratified ciliated columnar	Air passageway, mucus propulsion
Oropharynx	Stratified squamous	Passageway for food & air
Laryngopharynx	Stratified squamous	Passageway for food & air
Larynx (superior)	Stratified squamous	Protection
Larynx (inferior)	Pseudostratified ciliated columnar	Mucus movement
Trachea	Pseudostratified ciliated columnar	Conduct air, trap particles
Bronchi	Pseudostratified ciliated columnar → simple columnar/cuboidal	Conduct air
Alveoli	Simple squamous (Type I), Cuboidal (Type II)	Gas exchange, surfactant secretion

Key Terms and Concepts

• Surfactant: Substance secreted by Type II alveolar cells to reduce surface tension in alveoli

• Conducting zone: Respiratory passages that conduct air to the respiratory zone (no gas exchange)

• Respiratory zone: Site of gas exchange (respiratory bronchioles, alveolar ducts, alveoli)

• Ventilation: Movement of air into and out of the lungs

Example: Gas Exchange Equation

Gas exchange across the respiratory membrane is governed by Fick's Law:

$J = -D \frac{\Delta C}{\Delta x}$

• J: Rate of diffusion

• D: Diffusion coefficient

• ΔC: Difference in concentration

• Δx: Thickness of membrane

Additional info:

• These notes cover the major anatomical structures and histological features of the respiratory system, as outlined in standard Anatomy & Physiology textbooks (Ch. 22 The Respiratory System).