The human digestive tract, while living, measures approximately 20 feet long. Your mouth is the entrance, and your anus is the main exit. When you swallow foods or beverages, you also swallow air, which contains oxygen and nitrogen. Carbonated sodas contain carbon dioxide, which might cause belching, flatulence or indigestion. Sodas also contain water and caffeine, which is extracted through digestion.
Your mouth houses salivary glands that produce saliva when you eat or drink. Saliva promotes swallowing and contains an enzyme called amylase, which initiates the process of digestion by converting starches in a food or beverage into simple sugars. It helps wash contents down the throat. You might voluntarily initiate a gulp of soda but once it begins, it becomes involuntary and is controlled by the glossopharyngeal, vagus and hypoglossal nerves.
Because a carbonated soda is a liquid, it easily passes down the esophagus, which links the throat and stomach. The National Digestive Diseases Information Clearinghouse reports that a "ring-like muscle" called the lower esophageal sphincter sits at the junction of the esophagus and stomach. When you swallow a gulp of soda, this muscle relaxes and opens to allow its entrance into the stomach.
Upon receiving the carbonated beverage for further digestion, the stomach executes three main functions. First, the upper portion of the stomach relaxes to store the swallowed soda. The digestive glands in the stomach lining produce stomach acid, or hydrochloric acid, containing pepsin, the enzyme responsible for protein digestion. Next, the stomach mixes the digestive juices with the soda. Finally, the stomach releases the contents into the small intestine for absorption.
The Small Intestine
Upon entering the small intestine, soda mixes with two gastric juices produced by the pancreas and liver: The juice from the pancreas digests carbohydrates, fats and proteins. The liver releases bile, which is stored in the gallbladder between meals and binds with fat. After eating or drinking, the gallbladder releases the bile to initiate gastrointestinal lipolysis, or the breakdown of fat in the small intestine. Soda has no nutritional value, so only caffeine, water and sodium pass into the bloodstream through the lining of the small intestine. The unused contents of the small intestine, when they're solids, then move into the colon, or large intestine, and are eliminated through the anus as feces. Liquids, such as soda, are eliminated through the kidneys, bladder and urethra.
Belching and Flatulence
The small bubbles in carbonated sodas contain carbon dioxide, which is introduced into the water during production. As you continue to drink the carbonated beverage, the carbon dioxide accumulates in the stomach. When enough carbon dioxide has collected, your body will release the gas through belching. When your stomach has exceeded its capacity, it can make room for its contents by releasing gas via the anus. As your body digests the soda, you might notice prolonged periods of belching, flatulence or mild-to-intense stomach discomfort.
An 8 ounce glass of carbonated soda contains 35 milligrams of caffeine. According to the National Drug and Alcohol Research Center, Americans consume approximately 200 milligrams of caffeine daily. Caffeine might cause intestinal upset because of its laxative effect on the digestive tract. Caffeine triggers the release of additional gastric juices and can cause the premature transfer of food into the small intestine before being fully digested. It also causes the relaxation of the esophageal sphincter muscle. When this occurs, gastric juices might enter the esophagus and cause acid reflux, or heartburn.