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.