Pharm 111 Week 1 Class Notes 3

Breathing Mechanics

• Inspiration (Breathing In):

  • Diaphragm descends (moves down from a concave shape).

  • Ribs move up and out.

  • Volume of the thoracic cavity increases.

  • Resulting pressure in the thoracic cavity decreases.

  • Oxygen and other trace gases flow from the atmosphere into the lungs.

• Expiration (Breathing Out):

  • Diaphragm relaxes and moves up.

  • Rib cage moves down and inward.

  • Volume of the thoracic cavity decreases.

  • Resulting pressure in the thoracic cavity increases.

  • Air flows out of the lungs due to pressure differentials.

Hypoxemia vs. Hypoxia

• Hypoxemia: Decreased oxygen (O2) levels in the blood.

• Hypoxia: Decreased oxygen levels in tissues.

• PaO2 (Partial Pressure of Arterial Oxygen): Diagnostic measure for hypoxemia.

• Normal Range for PaO2: 80 to 100 mmHg.

• Levels of Hypoxemia:

  • Mild Hypoxemia: 60-79 mmHg.

  • Moderate Hypoxemia: 40-59 mmHg.

  • Severe Hypoxemia: < 40 mmHg.

Neural Control of Breathing

• Medulla Oblongata: Contains inspiratory and expiratory centers that send signals to the respiratory system.

• Inspiratory Neurons fire during inhalation, causing diaphragm contraction (via phrenic nerve) and intercostal muscle contraction, allowing air in.

• Expiratory Neurons fire to initiate passive exhalation, indicating that inspiration and expiration are opposite processes.

• Pneumotoxic Center: Modifies breathing pattern based on various physical states (e.g., exercise).

  • Controls the rhythm and depth of breath.

Hering-Breuer Reflex

• Reflex triggered by lung inflation (stretch receptors) to inhibit further inspiration to prevent lung damage.

• Activated with large tidal volumes > 800-1000 ml.

• Less significant during quiet breathing but vital during vigorous activities like exercise.

Chemical Control of Respiration

• Central Chemoreceptors: Located in the CNS (medulla); respond to changes in CO2 and H+ levels in cerebrospinal fluid.

  • Increased CO2/ H+ levels → Increased rate and depth of breathing.

• Peripheral Chemoreceptors: Located in carotid and aortic bodies; sensitive to low O2 and high H+ (hydration levels).

  • Must detect PaO2 < 60 mmHg to trigger hypoxic drive (drive to breathe).

CO2 as Major Regulator of Respiration

• Ventilation controls CO2 levels.

• High CO2 indicates inadequate ventilation, prompting the body to breathe more.

• Low CO2 may indicate hyperventilation, requiring a reduction in breathing rate.

Hypoxic Drive Theory

• Patients with chronic obstructive pulmonary disease (COPD) may rely on hypoxic drive.

• Normal CO2 levels: 35-45 mmHg. COPD patients may function with higher levels (e.g., 70 mmHg).

• Peripheral chemoreceptors adapt to elevated CO2 in COPD, leading to reduced sensitivity to high CO2 levels.

• Patients at risk of losing their drive to breathe if given excess oxygen.

• Importance of monitoring patient history and oxygen supplementation to avoid respiratory failure.

Overall Understanding and Importance

• A comprehensive understanding of breathing mechanics, control mechanisms, and distinguishing signs of conditions like hypoxemia and hypoxia is critical to patient care.

• Continuous study and retention of this information are crucial for mastering respiratory physiology.