The gas expelled from the rectum and the gas expelled from the mouth are distinct substances that originate in different parts of the digestive tract. The human body has a one-way system designed to ensure gas produced lower in the intestines exits through the anus, while gas from the stomach exits through the mouth as a belch. However, the contents of the lower digestive tract—including gas—can be forced upward and expelled orally under rare and serious medical circumstances.
Sources of Digestive Gas
The gas expelled from the body’s two main exit points is chemically different because it is generated by two separate processes. The gas released as a belch is primarily composed of swallowed air, known as aerophagia. This gas is rich in nitrogen and oxygen, reflecting the atmosphere’s composition, and collects in the stomach before being vented upward.
The gas released as flatus, commonly called a fart, originates lower in the gut, primarily in the large intestine. This gas is a byproduct of the fermentation process carried out by the trillions of bacteria that make up the gut microbiome. While the majority of flatus is odorless gases like hydrogen, carbon dioxide, and sometimes methane, the characteristic smell comes from trace amounts of sulfur compounds. These odorous gases are created when gut bacteria break down certain foods, particularly those containing sulfur.
The difference in chemical composition—swallowed air versus bacterial byproducts—highlights why gas from the lower tract does not normally exit the mouth. The digestive system is equipped with specific mechanisms that keep these two types of gas separated and direct them toward their respective exits.
The Normal Path of Expulsion
The digestive tract is essentially a one-way tube, and the mechanism that enforces this directionality is a coordinated muscular process called peristalsis. This involves wave-like contractions of the smooth muscle lining the gastrointestinal tract, which push food, fluid, and gas downward from the esophagus to the rectum. The rhythm and force of these contractions ensure a consistent, forward-moving flow of contents toward the anus.
Specialized muscular rings, known as sphincters, also act as one-way valves to regulate the passage of material and prevent backward movement. The lower esophageal sphincter (LES) is a ring of muscle at the junction of the esophagus and the stomach. Its function is to relax only to allow swallowed material to enter the stomach, immediately closing afterward to prevent contents from refluxing back up into the throat.
These valves and the continuous action of peristalsis work in tandem to maintain strict, downward propulsion. This system is designed to keep bacterially produced gas and other contents confined to the lower intestinal tract, where the only exit is the anus.
When Digestive Flow Reverses
The rare scenario where gas from the lower intestines exits the mouth is the result of a serious failure in the digestive tract’s directional system. This reversal occurs when a severe obstruction or dysfunction creates immense pressure below the point of blockage. A mechanical bowel obstruction, where a physical barrier blocks the intestine, prevents the normal downward passage of contents.
As food, fluids, and gas continue to accumulate against the obstruction, the pressure in the intestines begins to build dramatically. This buildup can trigger a pathological reversal of the normal wave-like contractions, known as reverse peristalsis. The sheer force of this process, combined with the intense pressure, eventually causes vomiting.
The substance expelled during this type of vomiting is not just stomach contents, but material forced backward from the small or large intestine. This includes the highly malodorous, bacterially produced gas that would otherwise be expelled as flatus. In these severe medical cases, the gas that reaches the mouth is carried within the reversed stream of intestinal contents.