Some of the most important players in digestion are hormones released by the cells in your stomach lining. When your stomach senses it's not empty, for instance, your brain signals cells in your stomach lining to release gastrin, a hormone that stimulates the release of gastric juices containing hydrochloric acid to dissolve your food.

Gastrin also contains pepsin, which breaks down proteins and stimulates your liver to produce bile, which allows you to digest fats. Without gastrin, you wouldn't be able to glean much nourishment from a steak or an omelette.