The Hagen-Poiseuille law states that dividing the diameter in half increases the resistance by a factor of 16, a decrease to 1/3 causes a factor higher than 80!

In the course of COPD the inner diameter of the airways [lumen] drops considerably due to the swelling and the phlegm. This requires a highly elevated driving pressure on the inside and results in a tremendous increase of the exertion of force by the respiratory muscles on the outside in order to move the lungs. Breathing work is increased drastically and distinctly adds to the fact that overtime, more energy is consumed than absorbed (keyword: cachexia). Furthermore there is crucial impact on the respiratory muscles: over the years, fatigue (possibly right up to exhaustion) of the respiratory pump develops. For this reason, the lung function and, if possible, respiratory muscle function should be determined on a regular basis.

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