Why This Matters

Clinical Importance of the Respiratory System

Understanding the anatomy and function of the respiratory system is essential for assessing and treating patients with respiratory disorders. The respiratory system is closely linked to the circulatory system and plays a vital role in cellular respiration, speech, and olfaction.

• Major functions: Supply body with O2 for cellular respiration and dispose of CO2, a waste product.

• Coupling: Respiratory and circulatory systems are closely coupled.

• Additional functions: Circulation and speech.

Respiratory System Overview

Processes of Respiration

The respiratory system involves four main processes that facilitate gas exchange and homeostasis.

1. Pulmonary ventilation: Movement of air into and out of lungs.

2. External respiration: Exchange of O2 and CO2 between lungs and blood.

3. Transport: Movement of O2 and CO2 in blood.

4. Internal respiration: Exchange of O2 and CO2 between systemic blood vessels and tissues.

Functional Anatomy of the Respiratory System

Major Organs

• Upper respiratory tract:

  • Nose and nasal cavity

  • Paranasal sinuses

  • Pharynx

• Lower respiratory tract:

  • Larynx

  • Trachea

  • Bronchi and branches

  • Lungs and alveoli

Upper Respiratory System

The Nose and Paranasal Sinuses

The nose is the only external portion of the respiratory system and plays a key role in respiration, filtration, and speech.

• Functions of the nose:

  • Provides airway for respiration

  • Moistens and warms entering air

  • Filters and cleans inspired air

  • Serves as resonating chamber for speech

  • Houses olfactory receptors

• Anatomical features:

  • Root, bridge, dorsum nasi, apex

  • Nostrils (nares), external nose features

  • Bounded by nasal and frontal bones, maxillary bones, plates of hyaline cartilage

Nasal Cavity

The nasal cavity is divided by the midline nasal septum and contains several important structures for air filtration and conditioning.

• Divisions: Septum formed by septal cartilage, vomer bone, perpendicular plate of ethmoid bone.

• Posterior nasal apertures (choanae): Open into nasopharynx.

• Floor: Hard palate (bone) and soft palate (muscle).

Nasal Vestibule and Mucosa

• Nasal vestibule: Superior to nostrils, lined with vibrissae (hairs) that filter particles.

• Mucous membranes:

  • Olfactory mucosa: Lines superior region, contains olfactory epithelium.

  • Respiratory mucosa: Pseudostratified ciliated columnar epithelium with goblet cells and seromucous nasal glands.

• Functions:

  • Mucus and serous secretions contain lysozyme and defensins.

  • Cilia sweep contaminated mucus posteriorly.

  • Inspired air is warmed by capillaries and veins.

  • Many sensory nerve endings trigger sneezing to expel particles.

Nasal Conchae

• Three sections: Superior, middle, and inferior conchae.

• Functions:

  • During inhalation: filter, heat, and moisten air.

  • During exhalation: reclaim heat and moisture.

  • Enhance air turbulence.

Paranasal Sinuses

Paranasal sinuses surround the nasal cavity and are located in frontal, sphenoid, ethmoid, and maxillary bones.

• Functions:

  • Lighten skull

  • Secrete mucus

  • Help warm and moisten air

Clinical – Homeostatic Imbalance 22.1: Rhinitis

• Inflammation of nasal mucosa, often spreading to throat and chest.

• Can block sinus passageways, causing vacuum and sinus headache.

The Pharynx

Structure and Regions

The pharynx is a funnel-shaped muscular tube connecting the nasal cavity and mouth to the larynx and esophagus.

• Regions:

  • Nasopharynx: Air passageway, contains pharyngeal tonsil and pharyngotympanic tubes.

  • Oropharynx: Passageway for food and air, contains palatine and lingual tonsils.

  • Laryngopharynx: Passageway for food and air, posterior to upright epiglottis.

Clinical – Homeostatic Imbalance 22.2

• Enlarged adenoids can block air passage, causing mouth breathing and sleep disturbance.

Lower Respiratory System

Major Structures

• Larynx

• Trachea

• Bronchi and branches

• Lungs and alveoli

Zones of the Lower Respiratory System

• Respiratory zone: Site of gas exchange; includes respiratory bronchioles, alveolar ducts, and alveoli.

• Conducting zone: Conduits for air to and from gas exchange sites; cleanses, warms, and humidifies air.

The Larynx

Functions

• Provides patent airway

• Routes air and food into proper channels

• Voice production (houses vocal folds)

Anatomy of the Larynx

• Framework of nine hyaline cartilages (except epiglottis):

  • Thyroid cartilage (Adam's apple)

  • Cricoid cartilage

  • Paired arytenoid, cuneiform, and corniculate cartilages

  • Epiglottis (elastic cartilage)

Epiglottis

• Covers laryngeal inlet during swallowing

• Contains taste bud-containing mucosa

Vocal Folds

• Vocal ligaments: Form core of vocal folds (true vocal cords)

• Glottis: Opening between vocal folds

• Folds vibrate to produce sound as air rushes up from lungs

Vestibular Folds

• False vocal cords, superior to vocal folds

• No part in sound production

• Help close glottis during swallowing

Voice Production

• Speech: Intermittent release of expired air during opening and closing of glottis

• Pitch: Determined by length and tension of vocal cords

• Loudness: Depends on force of air

• Sound shaped into language by muscles of pharynx, tongue, soft palate, and lips

Sphincter Functions of the Larynx

• Vocal folds may act as sphincter to prevent air passage (e.g., Valsalva's maneuver)

• Glottis closes to prevent exhalation

• Abdominal muscles contract, increasing intra-abdominal pressure

Clinical – Homeostatic Imbalance 22.3: Laryngitis

• Inflammation of vocal folds, interfering with vibrations and causing hoarseness

• May be caused by viral infections, overuse, tumors, or inhalation of irritants

The Trachea

Structure

• Extends from larynx into mediastinum, dividing into two main bronchi

• About 4 inches long, 3/4 inch in diameter, very flexible

• Wall composed of three layers:

  • Mucosa: Ciliated pseudostratified epithelium with goblet cells

  • Submucosa: Connective tissue with seromucous glands, supported by 16–20 C-shaped cartilage rings

  • Adventitia: Outermost layer made of connective tissue

Carina

• Last tracheal cartilage, expanded and found at point where trachea branches into two main bronchi

• Mucosa of carina is highly sensitive; violent coughing triggered by foreign objects

Clinical – Homeostatic Imbalance 22.4 & 22.5

• Smoking inhibits and destroys cilia, increasing risk of respiratory complications

• Tracheal obstruction is life-threatening; Heimlich maneuver may be required

The Bronchi and Subdivisions

Conducting Zone Structures

• Trachea divides to form right and left main (primary) bronchi

• Each main bronchus branches into lobar (secondary) bronchi, then segmental (tertiary) bronchi

• Bronchi divide repeatedly, forming a bronchial tree

• Support structure changes: cartilage rings become irregular plates, elastic fibers replace cartilage, epithelium changes from pseudostratified columnar to cuboidal, smooth muscle increases

Respiratory Zone Structures

• Terminal bronchioles lead into respiratory bronchioles, which lead into alveolar ducts and alveolar sacs

• Alveolar sacs contain clusters of alveoli (~300 million alveoli make up most of lung volume)

• Site of actual gas exchange

Respiratory Membrane

• Blood air barrier consists of alveolar and capillary walls along with fused basement membranes

• Very thin (~0.5 μm), allows gas exchange by simple diffusion

• Alveolar walls consist of:

  • Single layer of squamous epithelium (type I alveolar cells)

  • Scattered type II alveolar cells secrete surfactant and antimicrobial proteins

• Alveolar macrophages: Keep alveolar surfaces sterile; 2 million dead macrophages/hour carried by cilia to throat and swallowed

Table: Comparison of Conducting Zone vs. Respiratory Zone Structures

Blood Supply and Innervation of the Lungs

Pulmonary and Systemic Circulation

• Pulmonary circulation: Low pressure, high volume; pulmonary arteries deliver deoxygenated blood to lungs, pulmonary veins return oxygenated blood to heart.

• Systemic circulation: High pressure, low volume; bronchial arteries provide oxygenated blood to lung tissue.

Additional info:

• Gas exchange across the respiratory membrane occurs by simple diffusion, described by Fick's Law:

• Where is the partial pressure difference, is the surface area, is the diffusion coefficient, and is the thickness of the membrane.