The Respiratory System: Structure and Function

The Respiratory System

Overview

The respiratory system is essential for gas exchange, supplying oxygen to the body and removing carbon dioxide. It is divided into upper and lower regions, each with specialized structures and functions.

Functional Anatomy

Upper Respiratory System

• Nose and Paranasal Sinuses: Serve as the main entryway for air, filter, warm, and moisten incoming air, and house olfactory receptors.

• Pharynx (Throat): Connects the nasal cavity to the larynx and mouth to the esophagus; divided into nasopharynx, oropharynx, and laryngopharynx.

Lower Respiratory System

• Larynx: Also known as the voice box; involved in sound production and routing air and food.

• Trachea: Windpipe that conducts air to the bronchi.

• Bronchi and Bronchioles: Branching airways that distribute air to the lungs.

• Lungs and Alveoli: Main organs of respiration; alveoli are the sites of gas exchange.

Respiratory muscles (e.g., diaphragm, intercostals) are classified as part of the muscular system but are essential for ventilation.

Respiratory System: Functions

• Respiration: Provides oxygen to cells and removes carbon dioxide. Involves four processes:

  1. Pulmonary ventilation (air movement in and out of lungs)

  2. External respiration (gas exchange between lungs and blood)

  3. Transport of respiratory gases (in blood)

  4. Internal respiration (gas exchange between blood and tissues)

• Larynx: Produces speech and other vocalizations.

• Nasal cavity: Detects odors.

• Expulsion of abdominal contents: Pressure from the respiratory system aids in urination, defecation, and childbirth.

• Assists venous and lymphatic flow in thoracic and abdominopelvic cavities.

• Maintains acid-base homeostasis by regulating CO2 levels.

The Nose and Paranasal Sinuses

Nose

• Only externally visible part of the respiratory system.

• Functions:

  • Provides airway for respiration

  • Moistens and warms entering air

  • Filters and cleans inspired air

  • Serves as a resonating chamber for speech

  • Houses olfactory receptors

• Divided into external nose and nasal cavity.

Nasal Cavity

• Located within and posterior to the external nose.

• Divided by the nasal septum.

• Lined by respiratory mucosa (pseudostratified ciliated columnar epithelium with goblet cells) that produces mucus.

Paranasal Sinuses

• Hollow cavities (frontal, sphenoid, ethmoid, maxillary) connected to the nasal cavity.

• Functions:

  • Lighten the skull

  • Warm and moisten air

  • Enhance voice resonance

  • Secrete mucus

The Pharynx

Commonly called the throat, the pharynx connects the nasal cavity to the larynx and the mouth to the esophagus. It is divided into three regions:

• Nasopharynx: Airway only; lined with pseudostratified ciliated columnar epithelium. Contains pharyngeal tonsils and pharyngotympanic (auditory) tubes.

• Oropharynx: Passageway for food and air.

• Laryngopharynx: Posterior to the larynx; passageway for food and air; lined with nonkeratinized stratified squamous epithelium. Food has 'right of way' during swallowing.

The Larynx

• Located superior to the trachea; also called the voice box.

• Lined with protective nonkeratinized stratified epithelium and pseudostratified ciliated columnar epithelium.

• Functions:

  1. Provides a patent (open) airway

  2. Routes air and food into proper channels

  3. Voice production (houses vocal folds)

• Sound production: Air passes over vocal ligaments. Loudness is determined by force of air; pitch by tension and speed of vibration of vocal ligaments. Higher pitch: ligaments are tense (adducted). Lower pitch: ligaments are looser (abducted). Adult males have lower-pitched voices due to longer, thicker vocal ligaments and a wider larynx.

The Trachea

• Delivers air to lower respiratory structures.

• Supported by C-shaped rings of hyaline cartilage (anterior/lateral) to keep airway open and allow shape change during ventilation.

• Posterior surface has elastic connective tissue and smooth muscle to allow esophageal expansion during swallowing.

• Wall layers:

  • Mucosa: Inner layer; ciliated pseudostratified epithelium with goblet cells.

  • Submucosa: Middle layer; loose connective tissue with seromucous glands and cartilage rings.

  • Adventitia: Outermost layer; dense irregular connective tissue anchoring the trachea.

The Bronchi Tree

Conducting Zone Structures

• Trachea divides into right and left main (primary) bronchi (one per lung).

• Each main bronchus branches into lobar (secondary) bronchi (one per lobe: three in right lung, two in left).

• Secondary bronchi branch into segmental (tertiary) bronchi, which divide into smaller bronchioles (<1 mm diameter).

• Terminal bronchioles are less than 0.5 mm in diameter.

Respiratory Zone Structures

• Bronchioles surrounded by thin bands of smooth muscle.

• Terminal bronchioles feed into respiratory bronchioles, then alveolar ducts, and finally alveolar sacs (clusters of alveoli).

• Alveoli: Sites of gas exchange.

Alveoli

• Surrounded by elastic fibers, smooth muscle, and pulmonary capillaries.

• Three cell types:

  • Type I Alveolar Cells: Squamous cells (~90%); permit rapid gas diffusion.

  • Type II Alveolar Cells: Cuboidal cells (~10%); produce surfactant to reduce surface tension.

  • Alveolar Macrophages (Dust Cells): Phagocytes that clean debris and migrate to bronchioles for removal.

Respiratory Membrane

The thin barrier facilitating gas exchange between alveoli and blood, composed of:

• Type I alveolar cells

• Fused basal lamina of alveolar and capillary cells

• Capillary endothelial cells

The Lungs

• Left lung: Two lobes (superior, inferior) separated by the left oblique fissure.

• Right lung: Three lobes (superior, middle, inferior) separated by horizontal and right oblique fissures.

• Lobes divided into bronchopulmonary segments (each served by a tertiary bronchus), further subdivided into lobules.

Blood Circulation to Lungs

• Pulmonary artery: Brings deoxygenated blood from right ventricle to lungs.

• Pulmonary veins: Return oxygenated blood to left atrium.

• Bronchial arteries: Supply lung tissue with oxygenated blood (systemic circuit).

• Pleural cavity: Double-layered serous membrane (parietal and visceral pleura) filled with pleural fluid for lubrication.

Clinical Application Example

Case Study: A patient with recurrent sinus infections and difficulty breathing through one nostril may have a deviated nasal septum, which narrows one nasal passage and impairs airflow and mucus drainage.

Case Study: A patient with a history of smoking and reduced gas exchange efficiency likely has damage to Type I alveolar cells, which are responsible for rapid gas diffusion.

Key Equations

• Boyle's Law (Pressure-Volume Relationship):

$P_1V_1 = P_2V_2$

At constant temperature, the pressure and volume of a gas are inversely related.