Digestion kicks in once the food reaches the stomach. The stomach contains powerful muscles that churn food continuously, essentially beating the contents to a pulp. The stomach secretes juices that convert food into a liquid-like substance known as chyme. Most people know that the stomach contains acid, but fewer people know that this type of acid is hydrochloric acid. And fewer still know that the primary purpose of stomach acid is not to break food down but to provide an acidic environment that allows powerful digestive enzymes to do their work.
It is these enzymes, not the acid itself, that perform most of the chemical breakdown of food. Even if a piece of meat is placed in concentrated hydrochloric acid, it will not break down, but if you add enzymes, it breaks down quickly. Collectively, the combination of acid and enzymes is known as gastric juice, even though hydrochloric acid only comprises about 0.5% of the total volume of the gastric juice. Your body produces 2–3 liters of gastric juice daily.
The stomach, when completely full, can be stretched to hold about 2 liters of food. The average time for gastric emptying is about 4 hours; this time varies according to the quantity and type of food consumed. A light meal can be emptied from the stomach in about 2 hours, while a large heavy meal may require up to 6 hours. Forensic pathologists will often examine the stomach contents of a deceased person in an attempt to pinpoint the time of death.
To prevent the gastric juice from digesting the stomach itself, a thick layer of mucus coats the inside of the stomach. If the stomach produces too much acid, either due to eating too much or from a medical condition, heartburn can result. Heartburn has nothing to do with the heart, but rather is caused by gastric juice moving upward into the esophagus, creating a burning sensation.
This can be alleviated by taking antacids, which contain substances that neutralize acids. A common antacid is Tums, which contains calcium carbonate (CaCO3). Calcium carbonate neutralizes excess acid according to the following equation: