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 clinical symptoms.

• 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, coal worker's pneumoconiosis ("black lung"), and cystic fibrosis.

Obstructive Disorders: Mechanisms and Spirometry

• Obstruction is present during both inspiration and expiration, but expiration is disproportionately affected because airway diameter decreases during exhalation, making the obstruction more significant.

• Spirometry Findings:

  • Increased residual volume (RV) (more air remains in the lungs after expiration; normal ~1200 mL, can increase to >2300 mL in severe cases).

  • Decreased expiratory capacity and vital capacity (VC).

  • Slower, more shallow expiratory slope on spirometry curves.

  • Common symptom: wheezing due to turbulent airflow.

• Treatment Example: Albuterol (a bronchodilator) relaxes airway smooth muscle, increasing airway diameter and easing expiration.

Restrictive Disorders: Mechanisms and Spirometry

• Characterized by stiff, non-compliant lungs due to fibrosis or thick mucus (as in cystic fibrosis).

• Spirometry Findings:

  • Decreased inspiratory capacity and vital capacity ("lower ceiling").

  • Total lung capacity is reduced.

  • Residual volume may be unchanged or slightly reduced.

• Examples: Pulmonary fibrosis (scar tissue), respiratory distress syndrome (low surfactant), cystic fibrosis (defective CFTR chloride channel leads to thick mucus), and emphysema (often due to smoking or particulate exposure).

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 (feedback) control systems.

• Normal Quiet Breathing: Set by autorhythmic cells in the pre-Bötzinger complex (medulla oblongata), which act as pacemakers for respiratory rhythm (~12 breaths/min).

• Somatic Motor Neurons: Transmit signals from the pre-Bötzinger complex to the diaphragm, causing rhythmic contraction (inspiration is active; expiration is passive and elastic).

• No sensory input is required for normal quiet breathing.

Modulation by Sensory Input

• When blood gas levels deviate from normal, sensory input modulates breathing via respiratory control centers:

• Dorsal Respiratory Group (DRG): Primarily controls inspiration (external intercostals, diaphragm).

• Ventral Respiratory Group (VRG): Primarily controls expiration (internal intercostals, rectus abdominis).

• These groups coordinate antagonistic muscle activity for forceful breathing (e.g., during exercise or respiratory distress).

Chemoreceptor Regulation of Ventilation

Peripheral and Central Chemoreceptors

Chemoreceptors detect changes in blood O2, CO2, and pH, providing feedback to respiratory centers to adjust ventilation.

• Peripheral Chemoreceptors:

  • Located in carotid bodies (at the bifurcation of the carotid arteries) and aortic bodies (in the aortic arch).

  • Detect changes in arterial PO2, PCO2, and pH.

  • Key cells: Glomus cells detect low O2 by closing K+ channels, leading to depolarization, opening voltage-gated Ca2+ channels, and neurotransmitter (adenosine) release, which increases afferent signaling to the medulla.

  • Most important for increasing ventilation in response to hypoxia (low O2), such as at high altitude.

• Central Chemoreceptors:

  • Located in the medulla oblongata (brainstem), bathed in cerebrospinal fluid (CSF).

  • Respond primarily to changes in CO2 (which diffuses across the blood-brain barrier), not directly to blood pH or O2.

  • CO2 combines with H2O to form carbonic acid, which dissociates into HCO3- and H+ (catalyzed by carbonic anhydrase):

  • Central chemoreceptors sense the resulting H+ (from CO2), increasing ventilation if CO2 is high.

Relative Sensitivity to O2 and CO2

• Ventilation rate is much more sensitive to changes in CO2 than O2:

• A 25% increase in CO2 can increase ventilation rate sevenfold, while a 25% decrease in O2 increases ventilation by only 50%.

• CO2 is the most reliable signal for respiratory control because both central and peripheral chemoreceptors respond to it.

Negative Feedback Regulation

• Both chemoreceptor systems operate via negative feedback loops: deviations in blood gases trigger increased or decreased ventilation to restore homeostasis.

• Example: Hyperventilation (rapid breathing) lowers CO2, causing alkalosis (high blood pH). Breathing into a bag helps restore CO2 levels and pH.

Adaptation to High Altitude

Physiological Challenges and Responses

At high altitude, atmospheric and alveolar PO2 are reduced, challenging oxygen delivery to tissues.

• Immediate Response: Increased ventilation rate to raise alveolar and arterial PO2.

• Consequences: Increased ventilation also lowers PCO2, risking respiratory alkalosis (high blood pH).

• Acclimatization: Over time, the body increases red blood cell synthesis (stimulated by erythropoietin) to improve oxygen-carrying capacity.

• Clinical Note: At rest (e.g., during sleep), the balance between O2 and CO2 regulation can cause periodic breathing and sleep disturbances at altitude.

Sleep Apnea and Ventilation Control

Sleep Apnea

• Definition: Sleep apnea is a disorder characterized by repeated episodes of airway obstruction during sleep, leading to intermittent hypoxia and arousal from sleep.

• Mechanism: Obstruction (often in the pharynx) prevents airflow; CO2 rises until it triggers arousal and resumption of breathing.

• Treatment: Continuous Positive Airway Pressure (CPAP) devices maintain airway patency by providing constant air pressure.

• Importance: Untreated sleep apnea leads to poor sleep quality, cognitive impairment, and increased risk of cardiovascular disease.

Summary Table: Chemoreceptor Functions

Key Equations

• Alveolar Ventilation Rate:

• CO2 and Bicarbonate Buffer System:

• pH Calculation:

Key Points for Exam Preparation

• Obstructive disorders impair expiration; restrictive disorders impair inspiration.

• Normal quiet breathing is set by the pre-Bötzinger complex without sensory input.

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

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

• At high altitude, increased ventilation is driven by peripheral chemoreceptors sensing low O2.

• Sleep apnea disrupts normal ventilation during sleep; CPAP devices are effective treatments.

Additional info: The notes above expand on the mechanisms of chemoreceptor signaling, the physiological basis for spirometry changes in lung disease, and the clinical relevance of sleep apnea and altitude adaptation, as inferred from the lecture transcript.