Normally, gas accumulates in the GI tract either because of entrance of outside air or production of gases in the body. Accordingly, the gases accumulated in GI tract are classified into two groups:

1. Exogenous gases

2. Endogenous gases.

Exogenous Gases

Exogenous gases form about 90% of accumulated gases. These gases enter the GI tract either by swallowing through mouth or drinking carbonated beverages.

2. Endogenous Gases

Endogenous gases form about 10% of accumulated gases. These gases are produced by digestion of food stuffs and interaction between bacteria and food stuffs in the intestine.


Evacuation of accumulated gases usually occurs by two processes:

1. Belching

2. Flatulence.


Belching is the process by which the gas accumulated in stomach is expelled through mouth. It is also called burping. It occurs because of inflation (distention) of stomach by swallowed air. The distention of the stomach causes abdominal discomfort and the belching expels the air and relieves the discomfort.

Most of the gas accumulated in stomach is expelled through mouth. Only a small amount enters the intestine.

Causes for Accumulation of Gases in Stomach

1. Aerophagia: Swallowing large amounts of air due to gulping the food or drink too rapidly

2. Drinking carbonated beverages

3. During some emotional conditions like anxiety lot of air enters the stomach through mouth.

Act of Belching

Belching is not a simple act and it requires the coordination of several activities such as:

1. Closure of larynx, which prevents entry of liquid or food with the air from stomach into the lungs.

2. Elevation of larynx and relaxation of upper esophageal sphincter. It allows exit of air through

esophagus more easily.

3. Opening of lower esophageal sphincter.

4. Descent of diaphragm, which increases abdominal pressure and decreases intrathoracic pressure.

All these activities are responsible for the expulsion of air from stomach to the exterior via esophagus.


Flatulence is the production of a mixture of intestinal gases. The mixture of gases is known as flatus (in

Latin, flatus = wind). Expulsion of flatus through anus under pressure is called farting or passing gas. Farting is associated with disagreeable odor (due to odorous gases) and sound (due to vibration of anal sphincter).

Quantity of Flatus

Average flatus released by human is about 500 to 1500 mL per day, with 10 to 25 episodes throughout the day.

Source of Gases in Intestine

Flatulence is the mixture of gases present in the intestine. Flatulence by swallowed air is rare.

Common sources of gases in flatulence are:

1. Bacterial action on undigested sugars and polysaccharides (e.g. starch, cellulose)

2. Digestion of some flatulence producing food stuffs such as cheese, yeast in bread, oats, onion, beans,

cabbage, milk, etc.

Constituents of Flatus

Major constituents of flatus:

1. Swallowed non-odorous gases

i. Nitrogen (major constituent)

ii. Oxygen

2. Non-odorous gases produced by microbes

i. Methane

ii. Carbon dioxide

iii. Hydrogen

3. Odorous materials such as

i. Low molecular weight fatty acids like butyric acid

ii. Reduced sulfur compounds (hydrogen sulfide and carbonyl sulfide).


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