Respiratory System

Gas Exchange (Advanced)

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GAS TRANSPORT AND EXCHANGE

Gas exchange in the lungs involves the diffusion of oxygen and carbon dioxide between the lungs and peripheral tissues.

  • Partial pressure gradient is a key driver of diffusion;
  • In healthy lungs, oxygen and carbon dioxide diffuse rapidly and achieve equilibrium.

Dalton’s Law

  • Partial pressure of a gas is the pressure a gas would exert if it occupied the total volume of a mixture.
  • Example:
    Total pressure of the gases a mixture equals 7 mmHg; this is equal to the sum of:
    The partial pressure of “a,” which happens to be 4 mmHg, plus the partial pressure of “b,” which happens to be 3 mmHg.
  • Recognize that the partial pressure of a gas is not its concentration; however, Henry’ Law states that the concentration of a gas is dependent, in part, upon its partial pressure.

PARTIAL PRESSURE DRIVES DIFFUSION

Key Points:

Dry inspired air (P I)

  • Oxygen partial pressure = 160 mmHg
  • Carbon dioxide partial pressure = 0 mmHg.
  • As it moves through the moist trachea, the oxygen is “diluted” by water vapor, so,

Humidified tracheal air partial pressures:

  • Oxygen ~150 mm Hg
  • Carbon dioxide remains unchanged = 0 mmHg.

Partial pressures of oxygen and carbon dioxide within the mixed venous blood of the capillary:

  • Partial pressure of oxygen is ~ 40 mmHg; this relatively low value reflects the metabolic activity of the peripheral tissues, which have removed much of the oxygen from the blood.
  • Partial pressure of carbon dioxide is ~ 46 mmHg; this relatively high value reflects the production of carbon dioxide by the peripheral tissues.

As the pulmonary blood passes by the alveolus, gas exchange occurs and rapidly reaches equilibrium so that:

  • Oxygen diffuses from the alveolus to the capillary, so that both alveolar and systemic arterial blood partial pressure is 100 mmHg, and,
  • Carbon dioxide diffuses from the capillary to the alveolus, which increases the alveolar partial pressure of carbon dioxide to 40 mmHg (from 0), and, reduces the systemic arterial blood partial pressure of carbon dioxide to 40 mmHg.

Perfusion-limited Diffusion:

  • Notice that, because diffusion occurs rapidly and equilibrium is achieved, the partial pressure gradient is negated
  • The only way to increase gas exchange at this point would be to increase pulmonary blood perfusion and the rate at which mixed venous blood arrived at the alveoli.

Rate of diffusion of oxygen and carbon dioxide

Fick’s Law:

Vgas = (D*(P1-P2)*A)/ T

D = Diffusion coefficient (aka, constant) specific to that gas (which depends upon the solubility of the gas and the square root of the molecular weight)
P1-P2 = The partial pressure gradient, the driving force of diffusion
A = Surface area available for diffusion
T = Thickness of the membrane barrier

Clinical Correlation:

  • With the diffusion rate equation, we can predict that respiratory diseases will negatively affect diffusion rates:
  • Emphysema is characterized by alveolar destruction, and, therefore, decreased surface area available for diffusion.
  • Fibrosis causes thickening of the alveolar wall, and, therefore, increases the barrier to diffusion.

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