Flatus, or gas in the digestive tract, is a natural bodily function resulting from swallowed air and the metabolic activities of gut bacteria. While most swallowed air is belched out, the bulk of gas expelled through the rectum is a byproduct of bacteria in the colon fermenting undigested carbohydrates. This fermentation produces gases like hydrogen, carbon dioxide, and methane. These gases typically exit the body through the anus, but what happens when this natural exit is temporarily blocked?
The Immediate Effects of Gas Retention
The decision to hold in gas involves the voluntary contraction of the anal sphincter muscle, preventing the expulsion of flatus accumulated in the rectum. This retention causes a rapid increase in pressure within the lower colon.
This buildup of pressure forces the gas to remain trapped inside the intestinal lumen. The result is often discomfort, abdominal bloating, and sometimes cramping pain. The trapped gas may also temporarily migrate back up the digestive tract, causing gurgling sounds as it moves around the intestines.
How Trapped Gas Re-Enters the Body
When gas is held under pressure in the colon, a portion of the gaseous components undergoes reabsorption. The intestinal lining acts as a semi-permeable membrane, allowing gases like hydrogen, methane, and carbon dioxide to diffuse across it. These gases pass from the colon into the surrounding capillaries and enter the body’s circulation.
Once these gaseous molecules pass through the intestinal wall, they enter the portal vein system. The body treats these reabsorbed gases like any other absorbed substance from the gut. The absorbed gases are then transported through the circulatory system, beginning their journey toward final elimination.
The Final Route of Elimination
The gases reabsorbed into the portal blood travel through the liver and circulate throughout the body. This blood eventually reaches the lungs, the body’s primary organs for gas exchange. Volatile compounds, such as hydrogen and methane, are transferred from the blood into the air sacs of the lungs, known as alveoli.
From the alveoli, these gases are expelled from the body through exhalation. A portion of the gas initially prevented from leaving via the rectum ultimately exits through the mouth and nose. Although holding in gas can cause temporary pain and bloating, it is not medically dangerous because the body eliminates the gaseous components through the breath.