Respiratory Physiology and Gas Exchange

Overview of Respiratory Anatomy & Physiology

  • Transition from respiratory anatomy to respiratory physiology.
  • Focus on four processes of respiration.

Pressure Concepts in Respiration

  • Atmospheric Pressure:
    • Set to zero at sea level (760 mmHg).
    • Same as intrapulmonary pressure (pressure within the lungs).
  • Intrapleural Pressure: 4 mmHg lower than atmospheric and intrapulmonary pressures.
  • Key Diagram Review:
    • Visualize the thoracic cavity and pleural cavity to understand lung positioning.

Forces Acting on the Lungs

  • Forces Promoting Lung Collapse:
    • Elastic fibers in the lung cause recoil and promote deflation.
    • Surface tension within alveoli, especially without surfactant, complicates inflation.
  • Forces Promoting Lung Inflation:
    • Elasticity of thoracic wall.
    • Serous fluid in pleural cavity adheres lungs to thoracic cavity preventing collapse.
  • A constant balance between forces maintains lung expansion and prevents complete collapse.

Components of Pulmonary Ventilation

  • Definition: Pulmonary ventilation is synonymous with breathing (inhalation and exhalation).
  • Inhalation (Inspiration):
    • Muscles involved: diaphragm and external intercostals.
    • Diaphragm's dome shape lowers causing thoracic cavity volume to increase, leading to a decrease in lung pressure (Boyle's Law: P1V1 = P2V2).
    • Air flows in due to pressure gradient (air moves from high to low pressure).
    • Active process due to muscle contraction.
  • Exhalation (Expiration):
    • Two types: passive and active expiration.
    • Passive Expiration: Relaxation of diaphragm and external intercostals decreases volume and increases pressure, allowing air to flow out.
    • Active Expiration: Involves contraction of internal intercostals and abdominal muscles, further pushing air out of the lungs.

Gas Exchange and Dalton's Law of Partial Pressures

  • Gas Exchange: Occurs at both lungs and tissues.
  • Dalton's Law: Total pressure contributed by each gas in a mixture is proportional to its percentage in that mixture.
    • Example: Air consists of approximately 79% nitrogen, 21% oxygen, and 0.04% CO2.
  • Calculate partial pressures based on atmospheric pressure (e.g., pressure exerted by nitrogen is around 597 mmHg, oxygen is ~160 mmHg, CO2 is ~0.3 mmHg).

Gas Exchange at the Lungs

  • Inspired Air: New air has partial pressures of 160 mmHg for O2 and 0.3 mmHg for CO2.
  • Alveoli Partial Pressures:
    • O2: ~104 mmHg (increased due to gas exchange)
    • CO2: ~40 mmHg (increased as CO2 from blood enters alveoli).
  • Gas Exchange Process:
    • O2 moves from alveoli to blood.
    • CO2 moves from blood to alveoli.

Gas Exchange in Tissues

  • Blood to Tissues:
    • O2 decreases from ~100 mmHg in blood to 40 mmHg in tissues (drives diffusion into tissues).
    • CO2 rises from ~40 mmHg in blood to greater than 45 mmHg in tissues (drives diffusion into blood).

Summary of Gas Exchange Principles

  • Gas exchange relies on diffusion, driven by pressure gradients (high to low).
  • Oxygen and CO2 move in predictable patterns: oxygen enters tissues, CO2 exits to blood.
  • Understanding pressure dynamics aids in grasping respiratory functions during both internal and external respiration.

Important Takeaways

  • Recognize the physiological mechanisms behind inhalation and exhalation.
  • Understand how pressure differences influence respiratory dynamics.
  • Review Dalton’s Law to analyze gas mixtures and partial pressures.