Anatomy of Respiratory Organs and Their Functions

Introduction

The respiratory system is a vital organ system responsible for gas exchange—taking in oxygen and expelling carbon dioxide. For pharmacy students, a thorough understanding of respiratory anatomy and physiology is crucial for comprehending respiratory diseases, the mechanisms of action of respiratory drugs, and the impact of various medications on lung function.

Overview of the Respiratory System

The respiratory system can be divided into two main parts:

Upper Respiratory System: Includes the nose, pharynx, and associated structures.
Lower Respiratory System: Includes the larynx, trachea, bronchi, and lungs.

Functionally, the respiratory system also has two zones:

Conducting Zone: Filters, warms, and moistens air and conducts it to the lungs. Includes the nose, pharynx, larynx, trachea, bronchi, bronchioles, and terminal bronchioles.
Respiratory Zone: Site of gas exchange. Includes the respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli.

Upper Respiratory System

1. Nose

The nose is the primary entry point for air into the respiratory system.

Anatomy

External Nose: Visible portion, composed of bone and hyaline cartilage.
Internal Nose (Nasal Cavity): Lies posterior to the external nose, divided by the nasal septum into right and left sides.
- Nostrils (External Nares): Openings to the exterior.
- Internal Nares: Openings into the pharynx.
- Nasal Vestibule: Anterior part of the nasal cavity, lined with skin and coarse hairs.
- Nasal Conchae (Superior, Middle, Inferior): Three scroll-like bony projections that increase surface area and create turbulence.
- Meatuses: Passageways inferior to each concha.
- Olfactory Epithelium: Located in the superior part of the nasal cavity, contains olfactory receptors.
- Paranasal Sinuses: Air-filled cavities in frontal, sphenoid, ethmoid, and maxillary bones that drain into the nasal cavity.

Functions

Warming, Moistening, and Filtering Air:
- Blood capillaries in the nasal mucosa warm incoming air.
- Mucus secreted by goblet cells moistens air and traps dust particles.
- Cilia move trapped particles towards the pharynx for swallowing.
- Vibrissae in the vestibule filter large particles.
Olfaction (Sense of Smell): Olfactory receptors detect odors.
Resonating Chamber: Modifies speech sounds.

2. Pharynx (Throat)

The pharynx is a funnel-shaped tube that extends from the internal nares to the larynx and esophagus. It serves as a passageway for both air and food.

Anatomy

Composed of skeletal muscle and lined with mucous membrane.
Divided into three regions:

a) Nasopharynx
- Location: Posterior to the nasal cavity, extends from internal nares to soft palate.
- Lining: Pseudostratified ciliated columnar epithelium.
- Contains: Pharyngeal tonsil (adenoid) and openings of the auditory (Eustachian) tubes.
- Function: Strictly a respiratory passageway.
b) Oropharynx
- Location: Posterior to the oral cavity, extends from soft palate to hyoid bone.
- Lining: Non-keratinized stratified squamous epithelium (to withstand abrasion from food).
- Contains: Palatine tonsils and lingual tonsils.
- Function: Common passageway for air and food.
c) Laryngopharynx
- Location: Extends from hyoid bone to the esophagus (posteriorly) and larynx (anteriorly).
- Lining: Non-keratinized stratified squamous epithelium.
- Function: Common passageway for air and food; opens into the esophagus and larynx.

Functions

Passageway for Air and Food: Depending on the region.
Resonating Chamber: For speech sounds.
Houses Tonsils: Participate in immune responses.

Lower Respiratory System

1. Larynx (Voice Box)

The larynx is a short passageway that connects the laryngopharynx with the trachea.

Anatomy

Composed of nine pieces of cartilage (three paired, three unpaired).
- Thyroid Cartilage: Largest, forms the "Adam's apple."
- Epiglottis: Large, leaf-shaped elastic cartilage; covers the glottis during swallowing.
- Cricoid Cartilage: Ring of hyaline cartilage, inferior to thyroid cartilage; attaches to trachea.
- Arytenoid Cartilages (paired): Influence tension of vocal folds.
- Corniculate Cartilages (paired): On apices of arytenoids.
- Cuneiform Cartilages (paired): Support vocal folds and lateral epiglottis.
Glottis: Consists of the vocal folds (true vocal cords) and the rima glottidis (space between them).
Vestibular Folds (False Vocal Cords): Superior to vocal folds; protect true vocal cords.

Functions

Passageway for Air: Always open except during swallowing.
Voice Production (Phonation): Vocal folds vibrate to produce sound.
Prevents Food/Liquid Entry into Trachea: Epiglottis closes over glottis during swallowing.

2. Trachea (Windpipe)

The trachea is a tubular passageway for air, extending from the larynx to the primary bronchi.

Anatomy

Length: Approximately 12 cm (4.5 inches)
Diameter: Approximately 2.5 cm (1 inch)
Location: Anterior to the esophagus.
Composed of 16-20 C-shaped rings of hyaline cartilage (open posteriorly).
- Tracheal Cartilages: Provide structural support to prevent collapse.
- Trachealis Muscle: Smooth muscle connecting the ends of the C-rings posteriorly; allows esophagus to expand during swallowing.
Lining: Pseudostratified ciliated columnar epithelium with goblet cells.
Carina: Point where the trachea divides into the right and left primary bronchi; highly sensitive area for triggering cough reflex.

Functions

Air Passageway: Conducts air to and from the lungs.
Filtering and Warming Air: Cilia and mucus continue to trap particles and warm air.

3. Bronchi

The trachea divides into primary bronchi, which then branch extensively within the lungs.

Anatomy

Primary (Main) Bronchi: One for each lung (right and left).
- Right Primary Bronchus: Shorter, wider, and more vertical than the left; more common site for aspirated foreign objects.
- Left Primary Bronchus: Longer and more horizontal.
Secondary (Lobar) Bronchi: Branch from primary bronchi, one for each lobe of the lung.
- Right lung: 3 secondary bronchi (superior, middle, inferior).
- Left lung: 2 secondary bronchi (superior, inferior).
Tertiary (Segmental) Bronchi: Branch from secondary bronchi, supplying bronchopulmonary segments.
Bronchioles: Smaller branches (less than 1 mm in diameter).
Terminal Bronchioles: Smallest bronchioles, mark the end of the conducting zone.

Structural Changes in Bronchial Tree

As the airways branch and become smaller:

Cartilage rings gradually disappear and are replaced by cartilage plates, then disappear completely in bronchioles.
Amount of smooth muscle increases, allowing for significant changes in airway diameter (bronchoconstriction/bronchodilation).
Epithelium changes from pseudostratified ciliated columnar to simple columnar, then cuboidal, and finally simple cuboidal in terminal bronchioles.
Goblet cells and cilia decrease.

Functions

Air Passageway: Conducts air to and from the respiratory zone.
Regulation of Airflow: Smooth muscle in bronchioles regulates airflow to alveoli.

4. Lungs

The lungs are the primary organs of respiration, located in the thoracic cavity.

Anatomy

Location: Occupy most of the thoracic cavity, separated by the mediastinum.
Pleural Membrane: Double-layered serous membrane enclosing each lung.
- Parietal Pleura: Lines the thoracic cavity.
- Visceral Pleura: Covers the surface of the lungs.
- Pleural Cavity: Space between the two pleurae, contains pleural fluid.
Apex: Superior portion, extends above the clavicle.
Base: Broad inferior portion, rests on the diaphragm.
Hilum: Medial indentation where bronchi, pulmonary blood vessels, lymphatic vessels, and nerves enter/exit the lung.
Lobes:
- Right Lung: Three lobes (superior, middle, inferior) separated by horizontal and oblique fissures.
- Left Lung: Two lobes (superior, inferior) separated by an oblique fissure; contains the cardiac notch (indentation for the heart).
Bronchopulmonary Segments: Each lobe is divided into smaller segments supplied by tertiary bronchi.
Lobules: Smallest visible divisions of the lung, supplied by a terminal bronchiole, arteriole, venule, and lymphatic vessel.

Functions

Gas Exchange: Primary site for oxygen and carbon dioxide exchange.
Protection: Pleural fluid reduces friction during breathing.
Pulmonary Ventilation: Facilitates breathing movements.

5. Respiratory Zone Structures

These are the microscopic structures where gas exchange actually occurs.

a) Respiratory Bronchioles

Branch from terminal bronchioles.
Contain scattered alveoli in their walls.
Mark the beginning of the respiratory zone.

b) Alveolar Ducts

Branch from respiratory bronchioles.
Lined with simple squamous epithelium.
Lead into alveolar sacs.

c) Alveolar Sacs

Clusters of alveoli.
Resemble a bunch of grapes.

d) Alveoli

Microscopic air sacs (approximately 300 million per lung).
Primary site of gas exchange.
Alveolar Wall: Composed of two types of alveolar cells:
- Type I Alveolar Cells (Squamous Alveolar Cells): Simple squamous epithelial cells, form the main part of the alveolar wall; site of gas exchange.
- Type II Alveolar Cells (Septal Cells): Cuboidal cells, secrete surfactant (a fluid that reduces surface tension, preventing alveolar collapse).
Alveolar Macrophages (Dust Cells): Phagocytes that remove dust particles and debris from alveolar spaces.
Respiratory Membrane: The thin barrier across which gas exchange occurs. Composed of:
1. Alveolar wall (Type I and Type II cells)
2. Basement membrane of alveolar wall
3. Basement membrane of capillary wall
4. Capillary endothelial cell wall
Pulmonary Capillaries: Surround each alveolus, forming a dense network.

Functions

Gas Exchange: Efficient diffusion of O₂ into blood and CO₂ out of blood due to:
- Large total surface area (approximately 70 m²).
- Extremely thin respiratory membrane (0.5 µm).
- Rich blood supply.
Surfactant Production: Prevents alveolar collapse.
Protection: Alveolar macrophages remove debris.

Respiratory Muscles

Breathing (pulmonary ventilation) involves the contraction and relaxation of respiratory muscles.

1. Diaphragm

Primary muscle of inspiration.
Dome-shaped muscle that separates the thoracic and abdominal cavities.
Contraction: Flattens, increasing vertical dimension of thoracic cavity.
Relaxation: Becomes dome-shaped, decreasing vertical dimension.

2. External Intercostals

Located between ribs.
Contraction: Pulls ribs upward and outward, increasing anterior-posterior and lateral dimensions of thoracic cavity.
Relaxation: Passive.

3. Internal Intercostals

Located between ribs, deep to external intercostals.
Contraction: Pulls ribs downward and inward (primarily during forced expiration).

4. Accessory Muscles of Respiration

Used during forced inspiration (e.g., sternocleidomastoid, scalenes, pectoralis minor) and forced expiration (e.g., abdominal muscles).

Clinical Correlations for Pharmacy Students

1. Respiratory Diseases

Asthma: Chronic inflammatory disease characterized by airway hyperresponsiveness, bronchoconstriction, and mucus production.
- Pharmacology: Bronchodilators (beta-2 agonists, anticholinergics) target smooth muscle in bronchioles; anti-inflammatory drugs (corticosteroids) reduce inflammation.
Chronic Obstructive Pulmonary Disease (COPD): Group of progressive lung diseases (emphysema, chronic bronchitis) characterized by airflow obstruction.
- Pharmacology: Similar to asthma, but often requires long-acting agents.
Cystic Fibrosis: Genetic disorder causing thick, sticky mucus that clogs airways and leads to recurrent infections.
- Pharmacology: Mucolytics, antibiotics, bronchodilators.
Pneumonia: Infection of the lung parenchyma, leading to inflammation and fluid accumulation in alveoli.
- Pharmacology: Antibiotics (bacterial), antivirals (viral), antifungals (fungal).

2. Drug Delivery to the Lungs

Inhalers (MDIs, DPIs, Nebulizers): Deliver drugs directly to the airways and lungs.
- Advantages: Rapid onset of action, reduced systemic side effects, lower doses.
- Anatomical considerations: Particle size is crucial for deposition in specific airway regions (larger particles in upper airways, smaller particles in alveoli).
Systemic Drugs Affecting Respiration:
- Opioids: Can cause respiratory depression by acting on the respiratory center in the brainstem.
- Neuromuscular blockers: Paralyze respiratory muscles.
- Beta-blockers: Can cause bronchoconstriction in susceptible individuals (e.g., asthmatics) by blocking beta-2 receptors in airway smooth muscle.

3. Diagnostic Procedures

Spirometry: Measures lung volumes and capacities to assess lung function.
Bronchoscopy: Visual examination of the airways using a flexible tube.
Chest X-ray/CT scan: Imaging techniques to visualize lung structures and pathology.

Summary Tables for Quick Revision

Table 1: Respiratory System Divisions and Components

Division	Components	Functional Zone
Upper Respiratory System	Nose, Pharynx	Conducting
Lower Respiratory System	Larynx, Trachea, Bronchi, Lungs	Conducting & Respiratory

Table 2: Functional Zones of the Respiratory System

Functional Zone	Components	Primary Function
Conducting Zone	Nose, Pharynx, Larynx, Trachea, Bronchi, Bronchioles, Terminal Bronchioles	Filter, warm, moisten air; conduct air to lungs
Respiratory Zone	Respiratory Bronchioles, Alveolar Ducts, Alveolar Sacs, Alveoli	Gas exchange

Table 3: Key Structures and Their Functions

Structure	Primary Function(s)	Key Anatomical Feature(s)
Nose	Filter, warm, moisten air; olfaction	Nasal conchae, vibrissae, olfactory epithelium
Pharynx	Passageway for air/food; immune defense	Naso-, Oro-, Laryngopharynx; tonsils
Larynx	Air passageway; voice production; prevent aspiration	Epiglottis, vocal folds, thyroid cartilage
Trachea	Air passageway; filter/warm air	C-shaped hyaline cartilage rings, carina
Bronchi	Air passageway; airflow regulation	Primary, secondary, tertiary branches; increasing smooth muscle
Lungs	Gas exchange; ventilation	Lobes, pleura, hilum, bronchopulmonary segments
Alveoli	Primary site of gas exchange	Type I & II alveolar cells, surfactant, respiratory membrane
Diaphragm	Primary muscle of inspiration	Dome-shaped, separates thoracic/abdominal cavities

Key Points for Exam Preparation

Must-Know Concepts

Divisions of the respiratory system: Upper vs. Lower, Conducting vs. Respiratory zones.
Functions of each organ: Be specific (e.g., nose warms, moistens, filters; larynx produces voice).
Structural changes in the bronchial tree: Cartilage decreases, smooth muscle increases, epithelium changes.
Alveolar structure: Type I (gas exchange), Type II (surfactant), macrophages.
Respiratory membrane: Components and its role in gas exchange efficiency.
Pleural membranes and fluid: Importance for lung movement.
Muscles of respiration: Diaphragm and external intercostals for inspiration; internal intercostals and abdominals for forced expiration.

Common Exam Questions

Q: What is the role of the epiglottis? A: To cover the glottis during swallowing, preventing food and liquid from entering the trachea.

Q: Why is the right primary bronchus a common site for aspirated foreign objects? A: It is shorter, wider, and more vertical than the left primary bronchus.

Q: What is the function of surfactant and which cells produce it? A: Surfactant reduces surface tension in the alveoli, preventing their collapse; produced by Type II alveolar cells (septal cells).

Q: Describe the components of the respiratory membrane. A: It consists of the alveolar wall (Type I and Type II cells), the basement membrane of the alveolar wall, the basement membrane of the capillary wall, and the capillary endothelial cell wall.

Q: How do bronchodilators work in asthma? A: They relax the smooth muscle in the walls of the bronchioles, widening the airways and reducing resistance to airflow.

Numerical Values to Remember

Trachea length: ~12 cm
Trachea diameter: ~2.5 cm
Number of C-shaped tracheal rings: 16-20
Right lung lobes: 3
Left lung lobes: 2
Number of alveoli: ~300 million per lung
Total surface area of alveoli: ~70 m²
Thickness of respiratory membrane: ~0.5 µm

Conclusion

The respiratory system is a complex and intricately designed system crucial for sustaining life. For pharmacy students, a solid grasp of its anatomy and functions is foundational. This knowledge not only aids in understanding the physiological processes of breathing and gas exchange but also provides the context for various respiratory diseases and the pharmacological interventions used to treat them.

By focusing on the detailed anatomy of each organ, their specific functions, and the clinical correlations, you will be well-equipped to excel in your studies and apply this knowledge in your future pharmacy practice.