Respiratory Pathologies

Obstructive vs. Restrictive Lung Diseases

Respiratory pathologies can be broadly classified into obstructive and restrictive disorders, each affecting lung function in distinct ways. Understanding these differences is crucial for interpreting spirometry and managing respiratory health.

• Obstructive Pathologies: These disorders primarily impede expiration due to airway narrowing or blockage. Examples include asthma, bronchitis, and chronic obstructive pulmonary disease (COPD).

• Restrictive Pathologies: These conditions reduce lung compliance, making inspiration difficult. Examples include pulmonary fibrosis, respiratory distress syndrome (due to insufficient surfactant), asbestosis, black lung, and cystic fibrosis.

Key Differences

• Obstructive: Problem is mainly during expiration; increased residual volume (RV); decreased expiratory capacity and vital capacity.

• Restrictive: Problem is mainly during inspiration; decreased inspiratory capacity and vital capacity; total lung capacity is reduced.

Example: Spirometry Changes

• Obstructive: Larger RV (e.g., >2,300 mL vs. normal ~1,200 mL), reduced slopes during expiration, lower vital capacity.

• Restrictive: Lowered inspiratory capacity and vital capacity, but RV may remain unchanged.

Clinical Application: Inhalers (e.g., albuterol) are bronchodilators used in obstructive diseases to relax airway smooth muscle and improve expiration.

Control of Ventilation

Mechanisms of Breathing Regulation

Ventilation is tightly regulated to maintain homeostasis of blood gases (O2, CO2) and pH. This involves both automatic (pacemaker) and sensory-driven control mechanisms.

• Normal Quiet Breathing: Set by autorhythmic cells in the pre-Bötzinger complex (medulla), which act as pacemakers for inspiration. Signals are sent via somatic motor neurons to the diaphragm, causing rhythmic contraction and relaxation.

• Active Regulation: When blood gas levels deviate from normal, sensory input from chemoreceptors modulates the activity of respiratory control centers, recruiting additional muscles (e.g., intercostals, rectus abdominis) for forceful breathing.

Key Terms

• Tidal Volume (TV): The volume of air moved in or out during normal breathing (~500 mL in adults).

• Alveolar Ventilation:

• Dead Space: Air in the conducting airways not involved in gas exchange.

Respiratory Control Centers

• Pre-Bötzinger Complex: Pacemaker for rhythmic inspiration.

• Dorsal Respiratory Group (DRG): Primarily controls inspiration; integrates sensory input.

• Ventral Respiratory Group (VRG): Primarily controls expiration; active during forceful breathing.

• Pons: Modulates rhythm and transitions between inspiration and expiration.

Chemoreceptor Regulation of Breathing

Peripheral Chemoreceptors

Located in the carotid bodies (at the bifurcation of the carotid arteries) and aortic bodies (in the aortic arch), these receptors sense changes in arterial PO2, PCO2, and pH.

• Stimulus: Low PO2, high PCO2, or low pH.

• Mechanism: Low O2 is detected by glomus cells, which close K+ channels, depolarize, open voltage-gated Ca2+ channels, and release neurotransmitters (e.g., adenosine) to sensory neurons.

• Effect: Increased afferent signaling to the medulla increases ventilation rate.

Central Chemoreceptors

Located in the medulla, these receptors are bathed in cerebrospinal fluid (CSF) and primarily respond to changes in arterial CO2 (not directly to H+ or O2).

• CO2 crosses the blood-brain barrier and forms carbonic acid in CSF, which dissociates to HCO3- and H+:

• Effect: Increased H+ (from CO2) stimulates central chemoreceptors to increase ventilation.

Note: Central chemoreceptors do not respond directly to blood pH or O2 because H+ and O2 do not cross the blood-brain barrier efficiently.

Relative Sensitivity to Blood Gases

• CO2 is the most reliable and sensitive signal for ventilation control. Small changes in PCO2 cause large changes in ventilation rate.

• O2: Ventilation rate increases only after significant drops in PO2 (e.g., at high altitude).

Table: Chemoreceptor Sensitivity

Adaptation to High Altitude

Effects of Altitude on Gas Exchange

At high altitude, atmospheric PO2 is lower, resulting in reduced alveolar and arterial PO2. The body must adapt to maintain adequate oxygenation.

• Immediate Response: Increase ventilation rate (hyperventilation) to raise alveolar PO2.

• Consequence: Hyperventilation lowers PCO2, which can cause respiratory alkalosis (increased blood pH).

• Long-term Adaptation: Increased red blood cell synthesis (stimulated by erythropoietin) to improve oxygen-carrying capacity.

Example: Acclimatization

• At 15,000 feet, alveolar PO2 may drop to ~50 mmHg (vs. 100 mmHg at sea level).

• Increased ventilation helps, but also lowers PCO2, creating a physiological challenge (balancing O2 and CO2 needs).

Clinical Applications and Disorders

Hyperventilation and Hypoventilation

• Hyperventilation: Excessive breathing rate lowers PCO2, leading to alkalosis (pH > 7.45). May cause fainting. Treated by rebreathing exhaled air (e.g., into a paper bag) to restore CO2 levels.

• Hypoventilation: Insufficient breathing increases PCO2, leading to acidosis (pH < 7.35).

Sleep Apnea

• Obstructive Sleep Apnea: Upper airway obstruction during sleep causes intermittent hypoxia and arousal from sleep.

• CPAP (Continuous Positive Airway Pressure): Device that maintains airway patency, improving sleep quality and oxygenation.

Summary Table: Effects of Blood Gases on Ventilation

Key Equations

• Alveolar Ventilation:

• CO2 Buffering Reaction:

• pH Calculation:

Summary Points

• Obstructive lung diseases impair expiration; restrictive diseases impair inspiration.

• Ventilation is regulated by the pre-Bötzinger complex (pacemaker), respiratory groups, and chemoreceptors.

• CO2 is the most sensitive and reliable signal for ventilation control.

• Peripheral chemoreceptors respond to O2, CO2, and pH; central chemoreceptors respond only to CO2 (via CSF).

• High altitude challenges the balance between O2 uptake and CO2 removal, requiring both immediate and long-term physiological adaptations.

• Clinical conditions such as hyperventilation, sleep apnea, and altitude sickness illustrate the importance of these regulatory mechanisms.

Additional info: Some explanations and clinical examples were expanded for clarity and completeness.