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What is diffusion capacity?

The average adult lung contains an estimated 300 million alveoli. Each alveolus is supplied by many tiny blood vessels called capillaries. For oxygen and carbon dioxide to be exchanged, they must cross the alveolar capillary membrane. This exchange is promoted by diffusion — the passage of gas molecules through respiratory membranes. In diffusion, oxygen passes to the blood, and carbon dioxide, a byproduct of cellular metabolism, passes out of the blood and is channeled away through exhalation.”

Big Fat Reference Guide – AlphaNet

Individuals with Alpha-1 emphysema or COPD frequently have abnormal PFT results that indicate airway obstruction and air trapping. Similar changes can be seen in a variety of other lung conditions, such as asthma, chronic bronchitis, and smoking-related emphysema. If the lung function testing includes a diffusing capacity, or DLCO, then a low DLCO value is highly suggestive that these obstructive changes are due to emphysema.

Big Fat Reference Guide – AlphaNet

As you inhale, you are breathing a test mixture of gases that usually include an inert gas such as helium and a gas that mimics oxygen in the way it crosses into the blood and binds with the red blood cells (carbon monoxide or CO). You are then asked to exhale, and the machine takes a sample of the gases you exhale. By comparing the inhaled gas concentrations with those you exhale, the computed results give a reading of how well oxygen moves from your air sacs into your blood. Results of this test can provide information to your physician about the amount of damage or abnormality that is present where the air and the blood meet in your air sacs or alveoli.

Big Fat Reference Guide – AlphaNet