Types of Hypoxia

Overview 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 low oxygen in the 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 Changes and Lung Inflation

The process of breathing relies on pressure differences between the atmosphere, alveoli, and pleural cavity. The pleural cavity acts as a vacuum, keeping the lungs inflated by maintaining a lower pressure than the lungs themselves.

• Intrapleural Pressure: Normally about 756 mmHg (lower than atmospheric and alveolar pressure).

• Pneumothorax: Loss of the vacuum in the pleural cavity (e.g., due to trauma) causes lung collapse.

Boyle's Law and Breathing

Boyle's Law describes the inverse relationship between pressure and volume in a closed system:

• During inspiration, thoracic volume increases, alveolar pressure decreases, and air flows in.

• During expiration, thoracic volume decreases, alveolar pressure increases, and air flows out.

Muscles of Respiration

• Inspiration (quiet): Diaphragm contracts (moves down), external intercostals assist for deeper breaths.

• Expiration (quiet): Passive relaxation of diaphragm.

• Forced Expiration: Internal intercostals and rectus abdominis contract to force air out.

Gas Laws Relevant to Respiration

Dalton's Law of Partial Pressures

Each gas in a mixture exerts its own pressure, called partial pressure. The sum of all partial pressures equals the total pressure.

• Atmospheric pressure at sea level: 760 mmHg

• Partial pressures: N2 ≈ 593 mmHg, O2 ≈ 160 mmHg, CO2 ≈ 0.3 mmHg

• Gas exchange depends on the gradient of partial pressures (e.g., O2 diffuses from high to low PO2).

Lung Structure and Function

Alveoli and Gas Exchange

Alveoli are the primary sites of gas exchange, featuring a thin barrier (one endothelial cell and one alveolar cell) and surrounded by capillaries. Elastic fibers allow expansion and recoil, essential for tidal breathing.

• Compliance: Refers to the lung's ability to expand. High compliance means the lungs are soft and easy to inflate; low compliance (e.g., in emphysema) means the lungs are stiff.

• Elasticity: The ability of the lungs to recoil after expansion, necessary for effective expiration.

Surface Tension and Surfactant

Surface tension in the alveoli tends to cause collapse, especially in smaller alveoli. Surfactant, produced by type II alveolar cells, reduces surface tension and prevents collapse.

• Law of Laplace: The inward pressure in an alveolus is given by:

• T = surface tension, r = radius of alveolus

• Smaller alveoli have higher inward pressure and are more prone to collapse without surfactant.

• Surfactant: Main component is dipalmitoyl phosphatidylcholine, which reduces surface tension, especially in smaller alveoli.

• Infant Respiratory Distress Syndrome: Caused by insufficient surfactant production in newborns, leading to alveolar collapse and impaired gas exchange.

Assessment of Lung Function

Anatomical Dead Space

Not all inhaled air reaches the alveoli; some remains in the conducting airways (trachea, bronchi, bronchioles), known as anatomical dead space (~150 mL).

• During each breath, only a portion of the tidal volume (about 350 mL out of 500 mL) reaches the alveoli for gas exchange.

• This mixing of fresh and stale air prevents alveolar PO2 from reaching atmospheric levels.

Alveolar Ventilation

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

Example:

Spirometry and Lung Volumes

Spirometry measures lung volumes and capacities, useful for diagnosing respiratory disorders.

Respiratory Pathologies

Obstructive Pulmonary Pathology

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

• During expiration, airway narrowing increases resistance, trapping air and increasing residual volume.

• Spirometry shows reduced expiratory capacity and increased residual volume.

Restrictive Pulmonary Pathology

Restrictive diseases (not detailed in this excerpt) typically reduce lung expansion, affecting inspiration more than expiration.

Summary Table: Key Respiratory Concepts

Additional info:

• Birds use unidirectional (circular) airflow, unlike mammals' tidal breathing.

• CO2 is more soluble than O2, facilitating its removal from the body.