2. Hydrochloric acid, the digestive acid in our stomach, is as acidic as battery acid. The body has a system of defenses to keep the acid from eating through our skin, starting with the epithelial (wall) cells of the stomach (pictured below). They make a basic mucus coating to cover the inside of the stomach. Even then, these stomach cells have one of the fastest life cycles of any in the human body, dying after days.