I have heard that "air in the GI tract" results in flatulence and burping etc, however I initially assumed that the "empty" GI tract was a hollow tube filled with air anyway.
So on an empty stomach (or "empty" GI tract rather), does the GI tract collapse due to the pressure in the surrounding cavities, or is the GI tract normally a hollow tube filled with air, and flatulence merely a result of "excess" air?
Under normal circumstances the stomach and GI tract have very little "gas" in them - whether it be flatus, air, CO2, or anything else - and so are essentially collapsed with the walls against each other. However, gas is continually being produced by the microorganisms that inhabit our gut, and can also come from swallowing small amounts of air, fizzy drinks, etc., so the environment is never 100% gas-free. A typical person produces on the order of 500-1500 ml (0.5-1.5 liters) of gas per day.
If you're interested, the Wikipedia article on flatulence is a good introduction.
If you look below you can see two X rays. One with bowel obstruction on the right and a normal on the left. On an X ray air bubbles are black as indicated. Under normal circumstances as you can see there is very little air in the stomach and GI tract, although more frequently there will be some in the stomach due to fizzy drinks or reactions involving stomach acid producing gas in our stomach. That air later on in our GI tract tends to be a mix of carbon dioxide, methane and other waste products that bacteria in our gut have made. As Matt said, we produce quite a substantial amount of gas (500-1500 ml) per day. In an obstruction all of this can collect and further increase as microogranisms are in contact with the food for a longer duration as well as more enzymatic breakdown. And this can be seen in Figure 2.
