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Diffusing capacity of the lung (DL) 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 tests (pulmonary function testing) to determine the overall ability of the lung to transport gas into and out of the blood. DL, especially DLCO, is reduced in certain diseases of the lung and heart. DLCO measurement has been standardized according to a position paper 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. 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. The diffusing capacity does not directly measure the primary cause of hypoxemia, or low blood oxygen, namely mismatch of ventilation to perfusion:〔West, J. 2011. Respiratory Physiology: The Essentials. 9e. ISBN 978-1-60913-640-6〕 * 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. == Calculation == The diffusion capacity for oxygen can be calculated as the ratio of the volume of the oxygen taken up by the lung | }} ::(For , say "V dot". This is the notation of Isaac Newton for a first derivative (or rate) and is commonly used in respiratory physiology for this purpose.) :::: is the partial pressure of oxygen in the alveoli. :::: is the partial pressure of oxygen in the systemic veins (where it can actually be measured). Thus, the higher the diffusing capacity , the more gas will be transferred into the lung per unit time for a given gradient in partial pressure (or concentration) of the gas. Since it can be possible to know the alveolar oxygen concentration and the rate of oxygen uptake - but not the oxygen concentration in the pulmonary artery - it is the venous oxygen concentration that is generally employed as a useful approximation in a clinical setting. Sampling the oxygen concentration in the pulmonary artery is a highly invasive procedure, but fortunately another similar gas can be used instead that obviates this need (DLCO). Carbon monoxide (CO) is tightly and rapidly bound to hemoglobin in the blood, so the partial pressure of CO in the capillaries is negligible and the second term in the denominator can be ignored. For this reason, CO is generally the test gas used to measure the diffusing capacity and the equation simplifies to: = \frac } . | }} 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Diffusing capacity」の詳細全文を読む スポンサード リンク
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