Ventilation Control and Carbon Dioxide's Role

Henry's Law of Diffusion

• Describes gas diffusion into a liquid based on partial pressure, from high to low concentrations.
• Relevant at the alveoli-blood interface where gases transition between gaseous and liquid states.
• Diffusion is dependent on:
  • Pressure differential (gradient) between the gas above the fluid and the gas dissolved within it.
  • Inherent gas solubility: Each gas has its own solubility coefficient.

Illustration of Oxygen Diffusion

• Panel A: Pure water (no oxygen). Oxygen moves from the atmosphere into the water due to a large pressure gradient.
• Panel B: Net movement of oxygen into the fluid until saturation is reached.
• Panel C: Equilibrium: No net movement of oxygen in or out of the water.
• In humans, the pressure difference between alveolar and pulmonary blood gases drives gas exchange across the pulmonary membrane.

Carbon Dioxide Solubility

• Carbon dioxide dissolves more rapidly than oxygen due to its higher solubility coefficient.
  • For each unit of pressure, carbon dioxide is 25 times more soluble than oxygen.
• Solubility is influenced by atmospheric pressure and temperature.
• High solubility explains why carbon dioxide is used to carbonate beverages.
  • When a carbonated bottle is opened, carbon dioxide quickly escapes from the liquid into the atmosphere.

Carbon Dioxide in the Body

• Carbon dioxide is a byproduct of muscle metabolism.
• It readily goes into solution due to its high solubility.
• Partial pressure of carbon dioxide in muscle and venous blood is approximately 46 mmHg.
• Carbon dioxide moves from the blood into the alveoli due to a partial pressure gradient.
• Atmospheric air contains about 0.3 mmHg of carbon dioxide.
• The body maintains a background level of carbon dioxide to provide a chemical basis for ventilatory control.

Carbon Dioxide During Exercise

• Diffusion of both oxygen and carbon dioxide increases across the lung-blood and blood-muscle interfaces.
• This is driven by increased diffusion gradients.
  • Oxygen partial pressures in muscles can drop to near zero during exercise.
  • Arterial blood has a partial pressure of oxygen of about 100 mmHg, creating a significant gradient.
  • Carbon dioxide partial pressures in the muscle can almost double during intense exercise.
  • This creates a large driving gradient from 90 mmHg in the muscle to 40 mmHg in the alveoli.

Carbon Dioxide Transport in the Blood

• Carbon dioxide leaves the body via three methods:
  • Dissolved in plasma (5%)
    • This dissolved carbon dioxide establishes the partial pressure of carbon dioxide in the blood.
    • It travels to the lungs where the partial pressure of carbon dioxide is 40 mmHg and is then exhaled.
  • Bound to hemoglobin (20%), forming carbaminohemoglobin.
    • Carbon dioxide binds to amino acid molecules on hemoglobin.
    • The process is reversible, depending on the partial pressure of carbon dioxide.
    • Binding occurs easily when the partial pressure is high (e.g., in the muscle during exercise).
    • Carbon dioxide is released quickly when the partial pressure is low (e.g., in the lungs at 40 mmHg).
  • As plasma bicarbonate (the largest proportion).
    • Carbon dioxide combines with water in the muscles to make carbonic acid (H2CO3).
    • This reaction is sped up by the enzyme carbonic anhydrase, found in red blood cells.

Carbonic Anhydrase Equation

• Equation: CO2 + H2O
  leftrightarrow H2CO3
  leftrightarrow H^+ + HCO_3^-
• Carbonic anhydrase speeds up the process about 5,000 fold.
• Carbonic acid diffuses into the plasma and is exchanged for chloride to maintain ionic equilibrium, known as the chloride shift.
• In the lungs where partial pressure of carbon dioxide is low (40 mmHg), the reaction reverses.
  • Hydrogen ions recombine, carbonic acid reforms, and disassociates into carbon dioxide and water.
  • The carbon dioxide is then exhaled.

Exercise and Acidity

• Relationship between the partial pressure of carbon dioxide and the pH of the blood.
• Carbonic anhydrase catalyzes the formation of carbonic acid.
• Increased partial pressure of carbon dioxide causes a decrease in pH, which increases the oxyhemoglobin dissociation.
• More oxygen is liberated into the tissues, facilitating energy production.
• This synergistic effect helps deliver more oxygen to active tissues.