„The changes in atmospheric pressure can only influence the gases inside the body that are not in connection with the exterior air, such gases are mainly found inside the colon. At a height of 5000 meter, this means at half the atmospheric pressure, the gases of the colon would take twice of the volume on earth, if it canned be achieved to release these gases through the natural exit. This results in an expansion of the abdominal cavity and the midriff, which separates the abdomen from the abdominal cavity, is pushed up and the volume of the lungs is diminished. Without a doubt this is a disadvantage for breathing, even though only a small one.“
Source: Bundesarchiv-Militärarchiv-Freiburg RM 3/8970 Reichsmarineamt, Dienstvorschrift, Einflüsse des Fliegens auf den menschlichen Körper und medizinische Ratschläge für Flieger, Januar 1918
Annotation: The described phenomenon is of a certain relevance concerning fling in the First World War, because the standard height varied between 1000 and 3000 meters. In the course of the War and the technological progress this height reached 5000 meters and more.