QS Study

Diffusion Capacity is a measure of how well oxygen and carbon dioxide are transferred (diffused) between the lungs and the blood. It is directly related to lung volume and so maximal at total lung capacity. It is the volume of a gas that diffuses through the respiratory membrane each minute for a pressure difference of 1 mm Hg.

Diffusion capacity for Oxygen – 21 ml/min/mmHg

Diffusion capacity for CO2 = 400 – 500 ml/ min /mmHg.

The average total lung capacity of an adult human male is about 6 liters of air. Diffusing capacity may be low if a lung disease is present that causes the membrane to be thicker, for example, in diseases such as pulmonary fibrosis and sarcoidosis.

Factors affecting diffusion capacity of the lungs for Oxygen –

It is part of a comprehensive series of pulmonary function tests to determine the overall ability of the lung to transport gas into and out of the blood.

  • A pressure difference between the two sides of the membrane.
  • The thickness of the membrane.
  • The surface area of the membrane.
  • The diffusion coefficient of the gas in the substance of the membrane.
  • The molecular weight of the gas.
  • The solubility of the gas in the fluid.