Respiratory System: Mechanics, Gas Exchange, and Pathology

Types of Hypoxia

Hypoxia refers to a deficiency in the amount of oxygen reaching the tissues. There are several types, each with distinct causes and physiological implications.

• Hypoxic Hypoxia: Caused by a low oxygen environment (e.g., high altitude, hypoventilation). Results in low arterial PO2.

• Anemic Hypoxia: Due to problems with oxygen delivery by the blood (e.g., anemia, low hemoglobin, carbon monoxide poisoning).

• Ischemic Hypoxia: Caused by reduced blood flow (ischemia) to tissues (e.g., heart attack, shock, thrombosis).

• Histotoxic Hypoxia: Occurs when cells cannot utilize oxygen, often due to poisoning (e.g., cyanide poisoning interfering with the electron transport chain).

Comparison Table of Hypoxia Types:

Mechanics of Breathing

Pressure Relationships and Laws

Breathing is driven by pressure changes in the thoracic cavity, governed by physical laws:

• Dalton's Law: The total pressure of a gas mixture is the sum of the partial pressures of each individual gas. Partial pressures determine the direction of gas diffusion.

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

Boyle's Law Equation:

As lung volume increases during inspiration, pressure decreases, drawing air in. During expiration, volume decreases and pressure increases, pushing air out.

Pleural Cavity and Lung Inflation

• The pleural cavity surrounds the lungs and maintains a lower pressure than the lungs themselves, acting as a vacuum to keep the lungs inflated.

• Intrapleural pressure is typically about 4 mmHg less than alveolar pressure (e.g., 756 mmHg vs. 760 mmHg at rest).

• Pneumothorax occurs when the pleural cavity is breached, eliminating the pressure difference and causing lung collapse.

Muscles of Respiration

• Inspiration (breathing in): Primarily the diaphragm contracts (moves down), with external intercostals assisting during deep breaths.

• Expiration (breathing out): Normally passive (diaphragm relaxes). Forced expiration uses internal intercostals and rectus abdominis.

Pressure and Volume Changes During Breathing

• During inspiration, thoracic volume increases, alveolar and intrapleural pressures decrease, and air flows in.

• During expiration, thoracic volume decreases, pressures increase, and air flows out.

• Normal tidal volume (amount of air moved per breath) is about 500 mL.

Summary Table: Pressures During Breathing

Alveolar Structure and Function

Alveoli and Gas Exchange

• Alveoli are the primary sites of gas exchange, highly vascularized and lined by a thin layer of epithelial cells and capillary endothelium.

• Elastic fibers around alveoli allow for expansion and recoil, aiding both inspiration and expiration.

Lung Compliance and Elasticity

• Lung compliance refers to the ease with which the lungs can expand. High compliance means the lungs are soft and easy to inflate.

• Loss of compliance (stiff lungs) occurs in diseases like emphysema or black lung, making breathing difficult.

• Elasticity is the ability of the lungs to recoil after expansion, essential for effective expiration.

Surface Tension and Surfactant

• Water lining the alveoli creates surface tension, which tends to collapse the alveoli, especially the smaller ones.

• The Law of Laplace describes the inward pressure due to surface tension:

• P = inward pressure, T = surface tension, r = radius of alveolus.

• Smaller alveoli have higher inward pressure and are more prone to collapse.

• Surfactant (produced by type II alveolar cells) reduces surface tension, preventing collapse of small alveoli and equalizing pressures.

• Main surfactant: dipalmitoyl phosphatidylcholine.

Clinical Note: Insufficient surfactant in newborns leads to Infant Respiratory Distress Syndrome (IRDS), causing alveolar collapse and impaired gas exchange.

Gas Exchange and Dead Space

Partial Pressures in the Lungs

• Atmospheric PO2 ≈ 160 mmHg, but alveolar PO2 ≈ 100 mmHg due to mixing with stale air in dead space.

• Alveolar PCO2 ≈ 40 mmHg, higher than atmospheric (≈ 0.3 mmHg) for the same reason.

Anatomical Dead Space

• Airways (trachea, bronchi, bronchioles) that do not participate in gas exchange constitute anatomical dead space (~150 mL).

• During each breath, some fresh air remains in dead space and does not reach alveoli; likewise, some stale air remains in dead space after expiration.

Alveolar Ventilation Equation

Alveolar ventilation is the volume of fresh air reaching the alveoli per minute:

Example:

Spirometry and Lung Volumes

Spirometry

• Spirometry measures the volume of air moved in and out of the lungs during quiet and maximal breathing.

• Normal tidal volume: ~500 mL per breath.

Lung Volumes and Capacities

Pulmonary Pathology

Obstructive Pulmonary Diseases

• Obstructive diseases (e.g., asthma, bronchitis, COPD) involve blockage or narrowing of airways, making expiration difficult.

• During inspiration, airway expansion minimizes obstruction; during expiration, airway narrowing exacerbates it.

• Results in increased residual volume and reduced expiratory capacity.

Restrictive Pulmonary Diseases

• Restrictive diseases (not detailed here) limit lung expansion, disproportionately affecting inspiration.

Summary Table: Obstructive vs. Restrictive Pathology

Key Equations and Concepts

• Boyle's Law:

• Law of Laplace:

• Alveolar Ventilation:

Summary

• Pleural cavity pressure keeps lungs inflated.

• Changes in thoracic volume drive inspiration and expiration via pressure gradients.

• Lung compliance and elasticity are essential for effective breathing.

• Surfactant prevents alveolar collapse, especially in small alveoli.

• Anatomical dead space limits the efficiency of gas exchange.

• Spirometry is used to assess lung function and diagnose pulmonary pathologies.

Additional info: Restrictive pulmonary diseases will be covered in more detail in subsequent lectures. The notes above include inferred academic context for clarity and completeness.