Respiratory Gases

Partial Pressure

Partial pressure is the pressure exerted by an individual gas in a mixture of gases.

  • Each gas behaves independently.
  • The sum of partial pressures equals the total pressure.
  • In the gas phase, partial pressure and concentration are proportional.
  • Gases diffuse from high to low partial pressure.

Gases in Aqueous Phase

At equilibrium, the partial pressure of a dissolved gas equals the partial pressure in the gas phase. However, concentration depends on partial pressure and solubility.

  • Partial pressure of oxygen at equilibrium is equal in air and water.
  • Concentration of oxygen is not equal.
  • Gases diffuse from high to low partial pressure, not necessarily high to low concentration.

Properties of Gases in Aqueous Phase

  1. Solubility: Different gases have different solubilities.

    • Absorption coefficient measures gas solubility.
    • Carbon dioxide is more soluble than oxygen or nitrogen.
    • Absorption coefficients:
      • Carbon dioxide: 77
      • Oxygen: 2.2
      • Nitrogen: 1.1
    • At one atmosphere in cold water:
      • Carbon dioxide: 77 millimoles per liter
      • Oxygen: 2.2 millimoles per liter
      • Nitrogen: 1.1 millimoles per liter

  2. Temperature: Gas solubility increases with decreasing temperature.

    • Colder water has more dissolved oxygen.

  3. Salinity: Gas solubility decreases with increasing salinity.

    • Higher salt concentration means lower oxygen concentration.

Diffusion

Gases diffuse from high to low partial pressure in gas phase, aqueous phase, and across the gas-water interface.

  • Diffusion is not always along the concentration gradient.

Diving Beetle Example

Diving beetles use an air bubble as a gill.

  • Beetles acquire air at the surface and dive with it.
  • Gases diffuse according to partial pressure, not concentration.
  • At equilibrium, partial pressure of oxygen is about 0.21 atmospheres in both air and water.
  • If a beetle consumes half the oxygen in the bubble, the partial pressure drops to about 0.1 atmospheres, and oxygen diffuses into the bubble from the water.
  • Concentration of oxygen in the atmosphere is about 8.6 millimoles per liter, whereas in the water, it's about 0.3 millimoles per liter.
  • If the beetle consumes oxygen, the bubble concentration drops to about 4.3 millimoles per liter, still higher than in the water. However, diffusion follows the partial pressure gradient.

Additional Points

  • Diffusion occurs more readily through air than water.
  • Chemically combined gas does not contribute to partial pressure.

Convection

Convection is the bulk flow of a gas.

  • Examples include blood flow and movement of gases in lungs.
  • Unidirectional flow: gases flow in one direction.
  • Tidal flow: back-and-forth movement of gases.

Diffusion vs Convection

  • Small animals: diffusion alone is sufficient.
  • Larger animals: rely on convective gas transport.
  • Oxygen diffuses into the blood, moves by convection, and then diffuses from blood into cells.