Module 2: The Respiratory System

Overview

The respiratory system is essential for gas exchange, supplying oxygen to the body and removing carbon dioxide. This module covers the anatomy, physiology, and clinical aspects of the respiratory system, including its subdivisions, cellular components, and related diseases.

Anatomy of the Respiratory System

Upper Respiratory System

The upper respiratory system consists of structures that filter, warm, and humidify air before it reaches the lungs.

• Nasal Cavity: Entry point for air; filters and humidifies.

• Pharynx: Passageway for air and food.

• Oral Cavity: Secondary entry for air.

• Vocal Cords: Produce sound; located in the larynx.

• Larynx: Connects pharynx to trachea; involved in sound production and airway protection.

Lower Respiratory System

The lower respiratory system is responsible for conducting air to the lungs and facilitating gas exchange.

• Thoracic Cavity: Houses the lungs and heart.

• Chest Wall: Provides protection and aids in breathing movements.

• Diaphragm: Major muscle of respiration; separates thoracic and abdominal cavities.

• Trachea: Windpipe; conducts air to bronchi.

• Bronchi: Left and right branches leading to each lung.

• Right Lung: Three lobes.

• Left Lung: Two lobes (to accommodate the heart).

Respiratory Tract: Conducting Zone vs. Respiratory Zone

Conducting Zone

The conducting zone transports air to the respiratory zone but does not participate in gas exchange.

• Non-collapsible airways supported by cartilaginous rings.

• Hyaline cartilage maintains airway patency.

• No gas exchange occurs in this zone.

• Includes: Nasal cavity, nasopharynx, larynx, trachea, primary/secondary/tertiary bronchi, bronchioles, terminal bronchioles.

Respiratory Zone

The respiratory zone is where gas exchange occurs between air and blood.

• Collapsible airways (lacking cartilage).

• Includes: Respiratory bronchioles, alveolar ducts, alveoli.

• Gas exchange occurs here.

Sequence of the Respiratory Tract

Air Conduction Portion

• Nasal cavity

• Nasopharynx, larynx

• Trachea

• Primary bronchi (left and right)

• Secondary bronchi

• Tertiary bronchi

• Bronchioles

• Terminal bronchioles

Respiratory Portion

• Respiratory bronchioles

• Alveolar ducts

• Alveoli (alveolus)

Alveoli and Gas Exchange

Structure and Function

Alveoli are tiny air sacs in the lungs where gas exchange occurs. Their large surface area and thin walls maximize the efficiency of oxygen and carbon dioxide transfer.

• Elastic fibers: Surround alveoli, aiding in recoil during exhalation.

• Capillaries: Cover about 90% of alveolar surface, facilitating gas exchange.

• Pulmonary artery and vein: Supply and drain blood from alveolar capillaries.

Major Cell Types in Alveoli

• Type I alveolar cells: Thin cells for gas exchange.

• Type II alveolar cells: Secrete surfactant to reduce surface tension.

• Alveolar macrophages: Remove debris and pathogens.

• Endothelial cells: Line capillaries.

Gas Exchange Mechanism

Gas exchange occurs by diffusion across the alveolar and capillary membranes.

• Oxygen diffuses from alveoli into blood; carbon dioxide diffuses from blood into alveoli.

• Short diffusion distance and large surface area optimize exchange.

The Pleural Cavity

Pleural Membranes and Fluid

The pleural cavity surrounds each lung and contains pleural fluid, which reduces friction and allows smooth lung movement.

• Parietal pleura: Lines the chest wall.

• Visceral pleura: Covers the lungs.

• Pleural fluid: Lubricates and creates surface tension for lung expansion.

Muscles of Ventilation

Primary Muscles

Ventilation is driven by muscle contraction and relaxation, changing thoracic volume.

• Diaphragm: Main muscle for inspiration.

• Intercostal muscles: Assist in expanding and contracting the rib cage.

Epithelia and Mucociliary Clearance

Airway Epithelium

The airway lining consists of epithelial cells that separate compartments and protect against pathogens.

• Pseudostratified ciliated epithelium: Moves mucus and trapped particles out of the airways.

• Mucus: Contains glycoproteins and immunoglobulins for defense.

Mucociliary Clearance

Mucociliary clearance is a defense mechanism that removes inhaled particles and pathogens from the respiratory tract.

• Cilia: Hair-like structures that move mucus upward toward the throat.

• Saline (periciliary) layer: Produced by epithelial chloride secretion; allows cilia to beat effectively.

• Importance: Prevents infection and maintains airway patency.

• Clinical note: Cigarette smoke paralyzes cilia, impairing clearance.

Cystic Fibrosis

Pathophysiology

Cystic fibrosis is a genetic disorder affecting chloride transport in epithelial cells, leading to thick, sticky mucus in the airways.

• Defective CFTR protein: Impairs chloride secretion and saline layer formation.

• Consequences: Mucus buildup, bacterial infections, respiratory damage.

• Prevalence: ~30,000 people in the US; no cure.

Pulmonary Circulation

Characteristics

Pulmonary circulation is a low-pressure, low-resistance, high-flow system that delivers deoxygenated blood to the lungs and returns oxygenated blood to the heart.

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

• Pulmonary vein: Returns oxygenated blood to the left atrium.

Summary Table: Conducting Zone vs. Respiratory Zone

Key Equations

• Gas Exchange (Fick's Law):

Where: J = rate of diffusion D = diffusion coefficient ΔC = concentration gradient Δx = thickness of membrane

Clinical Applications and Points to Consider

• Identify major anatomical sites of the upper and lower respiratory tract.

• Describe the gross anatomy of the lungs.

• Explain the role of the pleural space and pleural fluid.

• Compare and contrast the conducting and respiratory zones.

• Locate the terminal bronchiole and describe its function.

• List the cell types found in the airway and their functions.

• Define mucociliary clearance and its importance.

• Describe cystic fibrosis and its impact on respiratory function.

• Explain how the periciliary fluid is generated.

• Trace the path of an oxygen molecule from an alveolus to a red blood cell.

Additional info: Academic context and definitions have been expanded for clarity and completeness.