Respiratory System: Structure, Function, and Disorders – Study Notes

Respiratory System Overview

Conducting and Respiratory Zones

The respiratory system is divided into conducting and respiratory zones, each with distinct anatomical structures and functions.

• Conducting Zone: Includes the nose, pharynx, larynx, trachea, bronchi, and bronchioles. These structures transport air to the lungs but do not participate in gas exchange.

• Respiratory Zone: Comprises respiratory bronchioles, alveolar ducts, and alveoli. This is where gas exchange occurs between air and blood.

• Functions: The conducting zone filters, warms, and humidifies air, while the respiratory zone facilitates oxygen and carbon dioxide exchange.

Alveolar Structure and Function

Alveoli are the primary sites of gas exchange in the lungs, supported by specialized cells and structures.

• Alveolar Macrophages: Immune cells that remove debris and pathogens from the alveolar surface.

• Type I Alveolar Cells: Thin epithelial cells that form the alveolar wall and facilitate gas diffusion.

• Type II Alveolar Cells: Secrete surfactant, a substance that reduces surface tension and prevents alveolar collapse.

• Elastic Fibers: Provide structural support and elasticity to the alveoli, aiding in lung expansion and recoil.

• Cartilage: Found in larger airways (trachea, bronchi) to maintain airway patency.

Mechanisms of Pulmonary Ventilation

Pulmonary ventilation (breathing) involves the movement of air into and out of the lungs, regulated by pressure changes and muscle activity.

• Inspiration: Diaphragm and external intercostal muscles contract, increasing thoracic volume and decreasing intrapulmonary pressure, drawing air in.

• Expiration: Usually passive; diaphragm and intercostals relax, thoracic volume decreases, intrapulmonary pressure rises, and air is expelled.

• Key Pressures: Atmospheric pressure, intrapulmonary pressure, and intrapleural pressure.

• Regulation: Respiratory centers in the brainstem (medulla and pons) regulate rate and depth of breathing.

Airflow Resistance and Factors Affecting Ventilation

Airflow in the respiratory system can be influenced by resistance in the airways and other factors.

• Airway Resistance: Determined by airway diameter; constriction increases resistance and reduces airflow.

• Factors: Bronchoconstriction, mucus, inflammation, and external pressures can slow or block airflow.

Assessment of Ventilation and Homeostatic Components

Ventilation is assessed using various measurements and is closely linked to homeostatic regulation of blood gases.

• Minute Ventilation: Total volume of air entering or leaving the lungs per minute.

• Vital Capacity (VC): Maximum amount of air exhaled after a maximal inhalation.

• Forced Expiratory Volume (FEV1): Volume of air exhaled in the first second of a forced breath.

• Homeostatic Components: Include responses to hypoxia (low O2), hypercapnia (high CO2), and acid-base imbalances.

Gases in the Air and Their Relative Percentages

The air we breathe contains several gases, each with a specific percentage.

• Nitrogen (N2): ~78%

• Oxygen (O2): ~21%

• Carbon Dioxide (CO2): ~0.04%

• Other Gases: Argon, water vapor, and trace gases.

Transport of Gases in the Blood

Oxygen and carbon dioxide are transported in the blood via different mechanisms.

• Oxygen: Mostly bound to hemoglobin in red blood cells; a small amount is dissolved in plasma.

• Carbon Dioxide: Transported as bicarbonate ions (HCO3-), bound to hemoglobin, or dissolved in plasma.

Hemoglobin Structure and Gas Exchange

Hemoglobin is a protein in red blood cells that binds and transports oxygen.

• Structure: Four polypeptide chains, each with a heme group that binds one O2 molecule.

• Oxygen Loading: Occurs in the lungs; affected by partial pressure of O2, pH, temperature, and CO2 levels.

• Oxygen Unloading: Occurs in tissues; facilitated by lower O2 pressure, higher CO2, and lower pH (Bohr effect).

Diffusion Principles and Gas Exchange

Gas exchange relies on diffusion across respiratory membranes.

• Partial Pressure Gradient: Gases move from areas of higher to lower partial pressure.

• Pulmonary Capillaries: O2 diffuses from alveoli to blood; CO2 diffuses from blood to alveoli.

• Systemic Capillaries: O2 diffuses from blood to tissues; CO2 diffuses from tissues to blood.

Factors Affecting Diffusion and Gas Exchange

Several factors can impair gas exchange in the lungs.

• Respiratory Membrane Thickness: Increased thickness (e.g., in pulmonary edema) slows diffusion.

• Surface Area: Reduced in diseases like emphysema, limiting gas exchange.

• Obstructions: Blocked airways or blood vessels (e.g., pulmonary embolism) impair gas delivery.

Respiratory Disorders

Various disorders affect the respiratory system, each with unique causes, symptoms, and consequences.

• Pulmonary Diseases: Asthma, COPD (chronic obstructive pulmonary disease), sleep apnea, ARDS (acute respiratory distress syndrome), IRDS (infant respiratory distress syndrome).

• Obstructive Disorders: Characterized by increased airway resistance (e.g., asthma, COPD).

• Restrictive Disorders: Characterized by reduced lung compliance or volume (e.g., pulmonary fibrosis).

• Risk Factors: Smoking, environmental exposures, genetic predisposition.

Obstructive vs. Restrictive Respiratory Disorders

It is important to distinguish between obstructive and restrictive respiratory disorders.

• Obstructive Disorders: Airflow is impeded, especially during exhalation. Examples: asthma, chronic bronchitis, emphysema.

• Restrictive Disorders: Lung expansion is limited, reducing total lung capacity. Examples: pulmonary fibrosis, ARDS.

Important Medical Terms

Definitions

• Eupnea: Normal, unlabored breathing.

• Hyperpnea: Increased depth and rate of breathing, usually in response to increased metabolic demand.

• Dyspnea: Difficult or labored breathing; shortness of breath.

• Orthopnea: Difficulty breathing when lying flat; relieved by sitting or standing.

• Apnea: Temporary cessation of breathing.

• Respiratory Arrest: Complete cessation of breathing, requiring immediate intervention.

• Kussmaul Breathing: Deep, labored breathing pattern often associated with severe metabolic acidosis (e.g., diabetic ketoacidosis).

• Hyperventilation: Increased rate and depth of breathing, leading to decreased CO2 levels in the blood.

• Hypoventilation: Decreased rate and depth of breathing, leading to increased CO2 levels in the blood.

Table: Comparison of Obstructive and Restrictive Respiratory Disorders

Feature	Obstructive Disorders	Restrictive Disorders
Main Problem	Increased airway resistance	Decreased lung compliance/volume
Examples	Asthma, COPD, chronic bronchitis, emphysema	Pulmonary fibrosis, ARDS, IRDS
FEV1/FVC Ratio	Decreased	Normal or increased
Total Lung Capacity	Normal or increased	Decreased

Key Equations

• Minute Ventilation:

$\text{Minute Ventilation} = \text{Tidal Volume} \times \text{Respiratory Rate}$

• Partial Pressure of a Gas:

$P_{\text{gas}} = \text{Fraction of gas} \times \text{Total atmospheric pressure}$

• Oxygen Content in Blood:

$\text{O}_2 \text{ content} = (1.34 \times [\text{Hb}] \times \text{SaO}_2) + (0.003 \times \text{PaO}_2)$

Additional info: Equations and table content expanded for academic completeness.