Gas Exchange and Delivery Lecture

Gas Exchange and Delivery

• Key Factors for Increasing O₂ Delivery to Tissues:
  • Increase air pressure.
  • Increase partial pressure of O₂ (pO₂) in air.
  • Increase amount of hemoglobin.

Oxygen Transport in Blood

• Without Hemoglobin:

  • Alveolar pO₂ ≈ Arterial pO₂, since oxygen does not dissolve well in blood without hemoglobin.
  • Example: At pO₂ = 100 mm Hg, the amount of O₂ dissolved in plasma is limited (3 mL O₂/L blood).

• With Hemoglobin:

  • Total oxygen content depends on:
  • Partial pressure of O₂ (pO₂).
  • Amount of hemoglobin present.
  • Saturation of hemoglobin (% saturation).
  • Oxygen content of red blood cells at normal pO₂ (100 mm Hg):
  • Ranges from zero (if not bound) to up to approximately 197 mL O₂/L blood with maximum hemoglobin saturation.

Modifying Hemoglobin's Carrying Capacity

• Factors affecting hemoglobin affinity for O₂:
  • Increase in 2,3-BPG.
  • Increase in temperature.
  • Increase in CO₂ concentration.
  • Decrease in pH (Bohr effect).
  • Key Metrics:
    • At lower pH (7.2), hemoglobin releases more oxygen compared to a higher pH (7.4).
    • Higher temperature (37°C vs 20°C) increases O₂ release.
    • The impact of shifting pCO₂ levels on O₂ saturation.

Total Oxygen Content of Arterial Blood

• Total O₂ content depends on:
  • Amount of dissolved oxygen in plasma (influenced by inspired air composition, alveolar ventilation, diffusion, pulmonary edema, etc.)
  • O₂ saturation of hemoglobin is influenced by:
  • Alveolar pO₂.
  • Changes in physical conditions (e.g., pH and temperature).

CO₂ Transport in Blood

• Summary of CO₂ Handling:
  • Dry air has a total pressure of 760 mm Hg, with pO₂ = 160 mm Hg.
  • Major routes of CO₂ transport:
  1. Bicarbonate Ion (HCO₃¯) in plasma (70%).
  2. Carbaminohemoglobin (HbCO₂), where CO₂ binds to hemoglobin (23%).
  3. Dissolved CO₂ in plasma (7%).
  • Transport mechanism: CO₂ diffuses from muscle cells into systemic capillaries, where it is converted to bicarbonate and binds to hemoglobin.

Central Control of Respiration

• Mechanism of Breathing Control:
  • Activity of inspiratory neurons increases during inspiration and turns off abruptly leading to passive expiration.
  • Involvement of higher brain centers and reflex control from central nervous system (CN IX, X).
  • The Pre-Bötzinger complex in the medulla regulates rhythms of breathing.

Questions for Understanding

1. How does exercise affect oxygen release at lower pH levels?
2. What happens to oxygen release as muscle cells warm up?
3. Discuss the implications of low 2,3-BPG levels in blood stored in banks.
4. What is the pO₂ of fetal blood leaving the placenta, given it is 80% saturated with O₂?
5. What is the percent O₂ saturation of maternal hemoglobin at a fetal pO₂ of around 10 mm Hg?