Chapter 22: The Respiratory System – Structure and Function

Chapter 22: The Respiratory System

I. Overview and Functions of the Respiratory System

The respiratory system is essential for supplying the body with oxygen and removing carbon dioxide. It consists of a series of organs and structures that facilitate gas exchange and protect the respiratory surfaces.

• Primary Functions:

  • Supplies body with oxygen (O2)

  • Removes carbon dioxide (CO2)

II. Conducting Zone

The conducting zone includes all respiratory passageways that convey air to the lungs. These structures filter, humidify, and warm incoming air.

• Respiratory Passageways: Nose, nasal cavity, paranasal sinuses, pharynx, larynx, trachea, bronchi, and bronchioles.

III. Organs of the Respiratory System

A. Nose, Nasal Cavity, and Paranasal Sinuses

• Functions:

  • Provides an airway for respiration

  • Moistens and warms air

  • Filters and cleans inhaled air

  • Resonating chamber for speech

  • Houses olfactory receptors

• Nasal Cavity:

  • Nares (nostrils) – external openings

  • Divided by nasal septum

  • Posterior nasal apertures – choanae

  • Vestibule – nasal cavity superior to nostrils

  • Olfactory mucosa – contains olfactory receptors

  • Respiratory mucosa – lines most of the nasal cavity

  • Goblet cells – secrete mucus

  • Cilia – move contaminated mucus posteriorly to pharynx

  • Richly supplied with sensory nerve endings (CN V)

• Nasal Conchae:

  • Superior and middle nasal conchae (part of ethmoid bone)

  • Inferior nasal conchae (separate bone)

  • Project medially from lateral wall of nasal cavity

• Paranasal Sinuses:

  • Located in frontal, maxillary, sphenoid, and ethmoid bones

  • Open into nasal cavity

B. Pharynx

The pharynx is a funnel-shaped passageway that connects the nasal cavity and mouth to the larynx and esophagus. It is divided into three regions:

• Nasopharynx:

  • Posterior to nasal cavity

  • Contains pharyngeal tonsil (adenoids)

  • Opens into the pharyngotympanic tube

• Oropharynx:

  • Posterior to oral cavity

  • Extends from soft palate to epiglottis

  • Contains palatine and lingual tonsils

  • Lined with stratified squamous epithelium

• Laryngopharynx:

  • Passageway for both food and air

  • Continuous with esophagus and larynx

C. Larynx

The larynx, or voice box, is located from the 4th to 6th cervical vertebrae and attaches to the hyoid bone. It opens into the laryngopharynx and is continuous with the trachea.

• Functions:

  • Provides an open airway

  • Routes air and food into proper channels

  • Voice production (houses vocal cords)

• Framework:

  • Formed by nine cartilages:

    • Thyroid cartilage (Adam’s apple)

    • Cricoid cartilage

    • Epiglottis (tips inferiorly during swallowing)

    • Other: Arytenoid, cuneiform, corniculate cartilages

• Vocal Ligaments:

  • Vocal folds (true vocal cords) – produce sound

  • Vestibular folds (false vocal cords) – no role in sound production

  • Rima glottidis – medial opening between vocal folds

  • Glottis – vocal folds and rima glottidis together

• Epithelium:

  • Superior portion: stratified squamous epithelium

  • Inferior portion: pseudostratified ciliated columnar epithelium

D. Trachea

The trachea, or windpipe, descends into the mediastinum and divides into the two primary bronchi at the carina. It is supported by C-shaped cartilage rings and lined with pseudostratified ciliated columnar epithelium.

E. Bronchi and Bronchioles

• Bronchial Tree:

  • Primary (main) bronchi – right and left

  • Secondary (lobar) bronchi – three on right, two on left

  • Tertiary (segmental) bronchi

  • Bronchioles – less than 1 mm in diameter

  • Terminal bronchioles – less than 0.5 mm in diameter

• Histological Changes:

  • Cartilage support changes from rings to plates, then disappears

  • Epithelium transitions from pseudostratified ciliated columnar to simple columnar, then simple cuboidal

  • Smooth muscle increases as airways become smaller

IV. Processes in Respiration

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

• Inspiration (Inhalation):

  • Diaphragm contracts and flattens

  • External intercostal muscles contract

  • Thoracic cavity volume increases, pressure decreases

• Expiration (Exhalation):

  • Usually a passive process

  • Inspiratory muscles relax, thoracic cavity volume decreases, pressure increases

  • Forced expiration uses abdominal and internal intercostal muscles

V. Respiratory Zone – Gas Exchange

• Structures:

  • Respiratory bronchioles

  • Alveolar ducts

  • Alveolar sacs

• Alveoli:

  • ~400 million alveoli provide a large surface area (~1500 sq ft or ~140 sq meters) for gas exchange

  • Type I alveolar cells – simple squamous epithelium

  • Type II alveolar cells – secrete surfactant

  • Alveolar macrophages – remove debris and pathogens

  • Alveolar pores – allow air pressure equalization and alternate air routes

VI. Lungs and Pleurae

• Lungs:

  • Apex – superior tip

  • Base – inferior surface resting on diaphragm

  • Hilum – site for entry/exit of blood vessels, bronchi, lymphatics, and nerves

  • Right lung – three lobes (superior, middle, inferior)

  • Left lung – two lobes (superior, inferior) and cardiac notch

• Blood Supply:

  • Pulmonary arteries – deliver oxygen-poor blood to lungs

  • Pulmonary veins – carry oxygenated blood to heart

• Innervation:

  • Sympathetic, parasympathetic, and visceral sensory fibers

  • Parasympathetic – bronchoconstriction

  • Sympathetic – bronchodilation

• Pleurae:

  • Double-layered serosa surrounding each lung

  • Parietal pleura – lines thoracic cavity

  • Visceral pleura – covers external lung surface

  • Pleural cavity – potential space between pleurae, contains lubricating fluid

VII. Summary Table: Key Structures and Functions of the Respiratory System

Structure	Main Function	Key Features
Nose/Nasal Cavity	Airway, filters, warms, moistens air	Olfactory receptors, conchae, mucosa
Pharynx	Passage for air and food	Three regions, tonsils, stratified squamous epithelium
Larynx	Voice production, airway protection	Vocal cords, cartilage framework
Trachea	Air passage to bronchi	C-shaped cartilage rings, ciliated epithelium
Bronchi/Bronchioles	Air distribution	Branching tree, smooth muscle, epithelium changes
Alveoli	Gas exchange	Type I/II cells, macrophages, pores
Lungs	House respiratory passages	Lobes, hilum, pleurae

Additional info: Academic context and definitions have been expanded for clarity and completeness. Table summarizes main structures and their functions for quick review.