Food enters the body through the mouth. Inside, mechanical breakdown begins immediately as it is chewed and mixed with saliva. This breakdown continues as the food travels down the esophagus to the stomach. Before the food reaches the stomach, it passes the esophageal sphincter, a circular one-way valve that closes and opens as needed.
The upper part of the stomach, the fundus, acts as the initial contact for food when it meets the stomach lining. There, pepsin — a protein-digesting enzyme — is activated and begins working to digest the food along with hydrochloric acid, a powerful digestive acid that kills bacteria and creates a pH level that allows pepsin and other enzymes to work.
While the food is being broken down, a mucus layer excreted by the stomach’s lining protects the stomach itself.
The lower, narrower portion of the stomach, the pylorus, uses muscular action (termed peristalsis) to mix enzymes, food, and liquid before it empties these into the duodenum, or the first part of the small intestine. The pylorus facilitates the rate at which the food empties into the small intestine.
The body of the stomach is composed of three layers of muscle.
The innermost layer of the stomach muscle, the inner oblique layer, aids in digestion by grinding the food together with digestive juices. The product is a substance known as chyme, a mixture of partially digested food, acids, enzymes, and liquid. It takes anywhere from 40 minutes to a few hours for the stomach to turn food into chyme. Proteins, such as meat, and fats take the longest to digest. Carbohydrates digest the quickest.
The inner oblique layer also has small wave-like ridges called rugae, or gastric folds. These ridges allow the stomach to expand as it fills. They also allow the stomach to grip the food as it churns, further breaking it down.
The exterior muscle, called the outer longitudinal layer, runs longitudinally as its name suggests. The middle layer, called the middle circular layer, is thick with circular muscle.
Digestive Hormones also play a vital role in digestion. The main hormones that affect the stomach are:
- Ghrelin: Produced in the stomach, this stimulates the appetite.
- Peptide YY: This hormone inhibits appetite in response to food.
- Gastrin: This signals the production of acids in the stomach.
- Secretin: This hormone signals the pancreas to produce digestive juices that neutralize stomach acid as it enters the small intestine. It also signals the stomach to produce pepsin.