Inside your body, a fart looks like small bubbles and pockets of gas scattered through the folds of your intestines. On medical imaging, these gas pockets show up as dark patches against the lighter tissue of your gut walls. At any given moment, your digestive tract holds between 30 and 200 milliliters of gas, roughly a shot glass to a cup’s worth, distributed in pockets of varying size from your stomach all the way down to your rectum.
What Gas Actually Looks Like in There
If you could somehow peek inside your intestines, you’d see bubbles of gas mixed in with partially digested food and liquid. These aren’t perfectly round bubbles like you’d blow with soap. They’re irregular pockets that conform to the shape of whatever stretch of intestine they’re sitting in, pressed against the ridged, folded walls of your gut.
We know what this looks like because doctors see intestinal gas on imaging all the time. On an abdominal X-ray, gas appears as dark areas (radiologists call them “air densities”) because gas lets X-rays pass through easily while tissue and fluid block them. In the small intestine, you can see gas sitting in the center of the abdomen, outlining the tiny horizontal ridges that run across the inner wall. In the large intestine, gas collects along the outer edges of the abdomen, highlighting the larger pouch-like segments that give the colon its bumpy appearance. On a CT scan, the contrast is even clearer: gas shows up jet black against the gray of surrounding tissue.
The gas isn’t evenly spread out. It tends to pool in certain spots, especially at bends in the colon. One common collection point is the splenic flexure, a sharp turn in your colon tucked up near your left ribcage. Because gas rises, it naturally migrates upward and can get temporarily trapped at these high points before working its way through.
Where the Gas Comes From
Intestinal gas has two main sources. The first is swallowed air. Every time you eat, drink, or swallow saliva, small amounts of air travel down into your stomach and intestines. This accounts for most of the nitrogen and oxygen in your gut gas.
The second source is bacterial fermentation. Trillions of bacteria in your large intestine feed on carbohydrates your body couldn’t fully digest higher up in the digestive tract. As they break down these leftovers, they produce hydrogen, carbon dioxide, and in some people, methane. The specific mix varies widely from person to person. Nitrogen can make up anywhere from 26% to 88% of a fart, hydrogen from less than 1% to 49%, carbon dioxide from under 1% to 27%, and methane from 0% to about 30%. Only about a third of people produce significant methane at all, which depends on which bacterial species have colonized their gut.
The tiny fraction that makes some farts smell, the sulfur-containing gases, makes up less than 1% of the total volume. So the vast majority of what’s inside those dark pockets on an X-ray is completely odorless.
How Gas Moves Through Your Intestines
Gas doesn’t just sit still. Your intestines are constantly contracting in rhythmic waves called peristalsis. When a pocket of gas (or food, or liquid) stretches the intestinal wall, nerve sensors detect that stretch and trigger a coordinated muscle response. The circular muscles just behind the gas pocket squeeze tight, pushing it forward. At the same time, the muscles just ahead of it relax to make room. Each wave moves the gas a few centimeters at a time.
In the colon, this process is deliberately slow. The colon’s job is to absorb water, so it mixes and stores contents rather than rushing them through. Gas can spend hours working its way through the five or so feet of large intestine, negotiating bends, mixing with other contents, and occasionally merging with other gas pockets. Sometimes a stronger contraction called a “mass movement” sweeps gas and stool forward more quickly, which is why you might suddenly feel the urge to pass gas after a meal (eating triggers these larger contractions through a reflex arc).
How Your Body Knows It’s Gas
One of the more remarkable things happening inside you is that your body can actually tell the difference between gas, liquid, and solid material without you having to think about it. The lining of your intestines contains specialized sensory cells that work like a sense of touch for your gut. These cells detect the physical properties of whatever is pressing against the intestinal wall, similar to how your fingertips can distinguish between textures.
Solid material creates a different pattern of pressure than gas does. Gas produces lighter, more diffuse stretching across a wider area, while solids create concentrated, heavier pressure points. Liquids generate a shearing sensation as they flow along the mucosal surface. Your deeper tissue layers are better at sensing large, solid objects, while the surface lining picks up lighter stimuli like gas.
When gas finally reaches your rectum, it triggers a reflex that briefly relaxes your internal anal sphincter. This “sampling reflex” lets the nerve-rich tissue of the anal canal confirm what’s there. Your brain gets the signal: it’s gas, not something else. That’s how you can (usually) pass gas with confidence, a feat of biological engineering that most people never think twice about.
How Much Gas You’re Carrying
A healthy person passes gas up to 25 times a day, and the total volume produced over 24 hours is typically between 500 and 1,500 milliliters. But at any single moment, the amount actually present in your intestines is modest, that 30 to 200 milliliter range. The system is constantly producing, absorbing, and expelling gas in a rough equilibrium.
Some of the gas never makes it out as a fart at all. A portion gets absorbed through the intestinal wall into your bloodstream, travels to your lungs, and leaves your body when you exhale. Hydrogen and methane in particular can take this route, which is why doctors use breath tests to measure bacterial fermentation in the gut. So those dark pockets visible on an X-ray represent just a snapshot of a continuously cycling system, gas forming, moving, being partially reabsorbed, and eventually either breathed out or passed through.