Diffusing capacity

Diffusing capacity of the lung (D_L) (also known as transfer factor) measures the transfer of gas from air in the lung, to the red blood cells in lung blood vessels. 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. D_L, especially D_LCO, is reduced in certain diseases of the lung and heart. D_LCO measurement has been standardized according to a position paper^[1] by a task force of the European Respiratory and American Thoracic Societies.

In respiratory physiology, the diffusing capacity has a long history of great utility, representing conductance of gas across the alveolar-capillary membrane and also takes into account factors affecting the behaviour of a given gas with hemoglobin.^{[citation needed]}

The term may be considered a misnomer as it represents neither diffusion nor a capacity (as it is typically measured under submaximal conditions) nor capacitance. In addition, gas transport is only diffusion limited in extreme cases, such as for oxygen uptake at very low ambient oxygen or very high pulmonary blood flow.^{[citation needed]}

The diffusing capacity does not directly measure the primary cause of hypoxemia, or low blood oxygen, namely mismatch of ventilation to perfusion:^[2]

• Not all pulmonary arterial blood goes to areas of the lung where gas exchange can occur (the anatomic or physiologic shunts), and this poorly oxygenated blood rejoins the well oxygenated blood from healthy lung in the pulmonary vein. Together, the mixture has less oxygen than that blood from the healthy lung alone, and so is hypoxemic.
• Similarly, not all inspired air goes to areas of the lung where gas exchange can occur (the anatomic and the physiological dead spaces), and so is wasted.