You may think that digestion happens in your stomach, but this is not the case. This process actually begins in your mouth. Your teeth and saliva kick start the food breakdown process. Upon swallowing, the food travels down a long muscular tract called your esophagus. Then, and only then, does the food enter your stomach, where a big part of digestion occurs. Acids and enzymes in your stomach are responsible for breaking down the ingested food so your body can use the nutrients. Otherwise, there where be no point in eating.
Hydrochloric acid, just call it HCl, is a very acidic juice released in your stomach. It acts to lower your stomach's pH from four to five to a pH of one to two. This is necessary to break down the food and to deactivate some of the tiny organisms in food, such as bacteria. Your body doesn't want you to get sick, after all. The high acidity is also important for activating a zymogen called pepsinogen.
A zymogen is basically a precursor to an enzyme, which is a type of protein that speeds up chemical reactions in your body. When the HCl is released in the stomach, the zymogen pepsinogen gets activated and becomes the enzyme pepsin. All you need to know about pepsinogen is it works to break down the protein from food into small amino acid molecules, the building blocks of protein.
Chyme is basically the partially digested food in your stomach. It is the leftovers from HCl and pepsinogen digestion. When this chyme leaves your stomach, it enters the first part of the small intestine, called the duodenum, where a digestive juice known as bile is released from your gallbladder. The bile enters the duodenum and it breaks down fats in the chyme. If you suffer from bile reflux, some of this bile also enters into the stomach and that's why you start to feel heartburn and/or stomach pain.
Mucus and Bicarbonate
In order to avoid digesting itself, the stomach's cells release mucus to form a protective layer around the inside of the organ. These cells also release an alkaline compound -- the opposite of acidic -- called bicarbonate, which mixes into the stomach juices and helps to lower the high acidity that resulted from the release of HCl. This is important for returning your stomach's pH to its normal levels of four to five after the chyme exits into your small intestine.