Chapter 22: The Respiratory System

Functions of the Respiratory System

The respiratory system is essential for gas exchange, supplying oxygen to the body and removing carbon dioxide. It works closely with the circulatory system to transport gases to and from tissues.

• Gas Exchange: Oxygen is absorbed into the blood, and carbon dioxide is expelled.

• Regulation of Blood pH: By controlling CO2 levels, the respiratory system helps maintain acid-base balance.

• Voice Production: The larynx (voice box) enables speech.

• Olfaction: The nasal cavity contains receptors for the sense of smell.

• Protection: The respiratory tract filters, warms, and humidifies incoming air.

Processes of Respiration

Respiration involves four main processes, each utilizing different organ systems:

1. Pulmonary Ventilation: Movement of air into and out of the lungs (respiratory system).

2. External Respiration: Gas exchange between lungs and blood (respiratory system).

3. Transport of Respiratory Gases: Movement of gases in the blood (circulatory system).

4. Internal Respiration: Gas exchange between blood and tissues (circulatory system).

Pathway of Air Through the Respiratory System

Air enters and exits the respiratory system in a specific order:

1. Nasal cavity or oral cavity

2. Pharynx

3. Larynx

4. Trachea

5. Bronchi (primary, secondary, tertiary)

6. Bronchioles

7. Alveolar ducts

8. Alveoli

Divisions of the Respiratory System

• Upper Respiratory Tract: Nose, nasal cavity, pharynx, and larynx.

• Lower Respiratory Tract: Trachea, bronchi, bronchioles, and lungs.

• Conducting Division: Passages that conduct air (nose to terminal bronchioles).

• Respiratory Division: Sites of gas exchange (respiratory bronchioles, alveolar ducts, alveoli).

Functions and Locations of Respiratory Organs

• Nasal Cavity: Filters, warms, and moistens air; houses olfactory receptors.

• Pharynx: Passageway for air and food.

• Larynx: Air passage, prevents food from entering trachea, voice production.

• Trachea: Air passage to bronchi; lined with ciliated mucosa.

• Bronchi/Bronchioles: Distribute air to lungs; bronchioles control airflow resistance.

• Alveoli: Main site of gas exchange.

Nasal Cavity Conditioning of Air

• Warming: Blood vessels in the nasal mucosa warm incoming air.

• Humidifying: Mucous membranes add moisture to air.

• Filtering: Hairs and mucus trap particles and pathogens.

Laryngeal Cartilages

• Thyroid Cartilage: Largest, forms Adam's apple.

• Cricoid Cartilage: Ring-shaped, below thyroid cartilage.

• Epiglottis: Elastic cartilage, covers laryngeal inlet during swallowing.

• Arytenoid, Corniculate, Cuneiform Cartilages: Smaller, involved in voice production.

Linings of the Respiratory Tract

• Olfactory Mucosa: Roof of nasal cavity; contains olfactory receptors.

• Respiratory Mucosa: Pseudostratified ciliated columnar epithelium with goblet cells; lines most of nasal cavity, trachea, bronchi.

• Stratified Squamous Epithelium: Lines oropharynx, laryngopharynx, and superior larynx (areas subject to abrasion).

• Simple Cuboidal/Simple Squamous Epithelium: Lines smaller bronchioles and alveoli (site of gas exchange).

Compositional Changes Along the Respiratory Tract

• Cartilage: Decreases from trachea (rings) to bronchioles (none).

• Smooth Muscle: Increases as airways become smaller, allowing regulation of airflow.

• Epithelium: Changes from pseudostratified ciliated columnar (trachea/bronchi) to simple cuboidal (bronchioles) to simple squamous (alveoli).

• Cilia and Goblet Cells: Present in larger airways, decrease and disappear in smaller bronchioles and alveoli.

Alveolar Wall Cells and Functions

• Type I Alveolar Cells: Simple squamous cells; form most of alveolar wall; site of gas exchange.

• Type II Alveolar Cells: Secrete surfactant to reduce surface tension.

• Alveolar Macrophages: Remove debris and pathogens.

• Alveolar Pores: Equalize air pressure between alveoli.

Structure of the Alveolar Wall and Respiratory Membrane

• Alveolar Wall: Composed of Type I and II cells and macrophages.

• Respiratory Membrane: Thin barrier (0.5 μm) consisting of alveolar epithelium, fused basement membrane, and capillary endothelium; site of gas exchange.

Bronchopulmonary Segments

Each lung is divided into bronchopulmonary segments, each supplied by its own bronchus and artery. This allows for localized disease treatment and removal.

Lung Stroma

The main component of lung stroma is elastic connective tissue, which allows the lungs to recoil after expansion.

Pulmonary vs. Bronchial Circulation

• Pulmonary Circulation: Carries deoxygenated blood from the heart to the lungs and returns oxygenated blood to the heart.

• Bronchial Circulation: Supplies oxygenated blood to lung tissue (except alveoli).

Innervation of the Lungs

• Parasympathetic Fibers: Constrict airways.

• Sympathetic Fibers: Dilate airways.

• Pulmonary Plexus: Network of nerves entering the lungs at the hilum.

Pleura and Its Significance

• Visceral Pleura: Covers the lung surface.

• Parietal Pleura: Lines the thoracic cavity.

• Pleural Cavity: Space between layers; contains fluid to reduce friction and assist lung expansion and recoil.

Pressure Relationships in the Thoracic Cavity

• Intrapulmonary Pressure (Ppul): Pressure in alveoli; fluctuates with breathing, equalizes with atmospheric pressure.

• Intrapleural Pressure (Pip): Pressure in pleural cavity; always negative relative to intrapulmonary and atmospheric pressure.

• Transpulmonary Pressure: Difference between Ppul and Pip; keeps lungs inflated.

Forces Acting on the Lungs

• Inward Forces: Elastic recoil of lung tissue and surface tension of alveolar fluid.

• Outward Force: Elasticity of chest wall pulls thorax outward.

If transpulmonary pressure equals intrapulmonary or atmospheric pressure, the lungs collapse (atelectasis).

Pulmonary Ventilation and Boyle's Law

Pulmonary ventilation is driven by mechanical processes that change thoracic volume, leading to pressure changes and airflow.

• Boyle's Law: The pressure of a gas is inversely proportional to its volume at constant temperature.

$P_1V_1 = P_2V_2$

Muscles of Respiration

• Inspiration (Restful): Diaphragm and external intercostals contract.

• Inspiration (Forced): Sternocleidomastoid, scalenes, pectoralis minor assist.

• Expiration (Passive): Relaxation of inspiratory muscles.

• Expiration (Forced): Internal intercostals and abdominal muscles contract.

Physical Factors Influencing Pulmonary Ventilation

• Airway Resistance: Friction in airways; major source is medium-sized bronchi. Inverse relationship: as resistance increases, airflow decreases. Usually insignificant due to large airway diameters and parallel branching; greatest resistance in bronchioles.

• Alveolar Surface Tension: Tendency of liquid molecules to stick together; can cause alveoli to collapse. Surfactant (produced by Type II cells) reduces surface tension.

• Lung Compliance: Measure of lung expandability; normally high due to elastic fibers and surfactant. Decreased by fibrosis, reduced surfactant, or decreased thoracic flexibility.

Alveolar Ventilation Rate (AVR)

• AVR: Volume of fresh air reaching alveoli per minute; better indicator of effective ventilation than respiratory rate or tidal volume alone.

• Breathing Depth: Increasing depth (tidal volume) is more effective at raising AVR than increasing rate, because dead space volume remains constant.

Gas Laws Relevant to Respiration

• Dalton’s Law: Total pressure of a gas mixture is the sum of the partial pressures of each component gas.

$P_{total} = P_1 + P_2 + P_3 + ... + P_n$

• Partial Pressure: At higher altitudes, total atmospheric pressure and partial pressures decrease; at lower altitudes, they increase.

• Henry’s Law: The amount of gas dissolved in a liquid is proportional to its partial pressure and solubility.

$C = kP$

• Solubility: CO2 is more soluble in plasma than O2.

Alveolar Air vs. Atmospheric Air

• Alveolar Air: Contains more CO2 and water vapor, less O2 than atmospheric air.

• Causes: Gas exchange, humidification, and mixing of inspired and residual air.

Influences on External Respiration

• Partial Pressure Gradients and Gas Solubilities: O2 and CO2 move down their gradients; CO2 diffuses more readily due to higher solubility.

• Thickness and Surface Area of Respiratory Membrane: Thicker membranes or reduced surface area (e.g., in disease) impair gas exchange.

• Ventilation-Perfusion Coupling: Matching of airflow (ventilation) and blood flow (perfusion) for optimal gas exchange.

Additional info: For a more comprehensive understanding, students should also review acid-base balance and Chapter 25 content as referenced in the study guide.