Can someone explain why air rises to the highest point in the body cavities like peritoneal/pleural?
Although there could well be small amounts of gas in amongst our organs, they are likely to be dissolved eventually in the fluids. Any small amounts of gas can give severe discomfort, as in decompression sickness, which is suffered by divers and air embolisms. CO2 is not too much of a problem because it dissolves but tiny nitrogen bubbles which have come out of solution as the pressure reduces take a long time to dissolve, once they have joined together. Hence, divers decompress slowly enough to let the body get rid of the tiny bubbles.Buoyant force? I guess that that we can assume that gas in cavities is actually submerged in fluid? There is tiny amout of fluid in this cavities that keep organs and serous surface stick to each other by surface tension.