After food passes down the esophagus it enters the stomach where it is stored and digested. At the entrance to the stomach is the cardiac sphincter, which opens to allow food to enter the stomach. After the food is in the stomach it contracts, so that even when you stand on your head the food does not go back into the esophagus.
The size, shape and position of the stomach can change depending on the food in the stomach and the strength of the stomach muscles. An average stomach has a capacity of one litre (one quart) and contains enzymes, strong acids including hydrochloric acid and other gastric juices. It takes just over half a litre of gastric juice to digest an average meal. The acids found in the stomach are so strong that it must have a mucus lining to stop the acid from eating through the stomach wall. The hydrochloric acid causes the contents of the stomach to become acidic, it has a pH of between 1-3 which kills all bacteria that had been living on the food. An important enzyme formed in the stomach is pepsin, which breaks down protein.
There are three main sections in the stomach. The fundus is located at the entrance and contains a gas bubble after a meal. Some of the gas is air swallowed when food is eaten, the rest is gas produced by the digestion of food. The body of the stomach is the largest section and acts as a storage space for food and liquids. The lower part of the stomach is funnel shaped and tends to curve to the right. It is the narrowest section of the stomach, at 2 cm (0.8 in) thick.
The walls of the stomach produce muscular contractions which move the food and help digestion. They are similar to the contractions in the esophagus and the process is also called peristalsis. It starts as a ring of contraction at the top of the stomach and moves down towards the lower end. When the direction of the waves is reversed the food rises back up and vomiting occurs. They occur at the rate of about three a minute and each wave takes a minute to pass from the top to the bottom of the stomach. It is therefore possible to see three contractions occurring together. Their purpose is to continue to digest the food, turning it into a thick liquid called chyme and move it towards the intestine. These hunger contractions continue even when there is no food in the stomach. As they get stronger they give a sensation of emptiness and a need for food. When the stomach has no food being digested it shrinks like an empty balloon. The walls contract and form folds and wrinkles as they touch together. When food enters the stomach it expands and the folds disappear.
The walls of the stomach are covered with cells that secrete 1.2-1.5 litres (12.5-3.1 pints) of gastric juice every day. It is acidic due to the high levels of hydrochloric acid and is rich in enzymes. The juice starts digestion and converts the food into the semiliquid chyme. It does this by making the food particles soluble and adding liquid in the form of hydrochloric acid. A normal adult human will secrete 2-4 millilitres of acid every hour, people with ulcers will create two to three times this amount.
Gastric juice is secreted continuously in a small amount by the stomach.The amount increases when we smell, see or taste food, and more is produced if the sensation is pleasurable. So a person who is not hungry may see or smell food and this will cause the stomach to start to produce gastric juices, which may continue for up to an hour. When food actually reaches the stomach more secretions are produced. The food causes a hormone called gastrin to be released from the wall of the stomach into the bloodstream. It reaches the glands that produce the secretions and causes them to release gastric juice. This then acts on the food and begins to break it down, where it can be passed into the small intestine. Only a small amount of absorption occurs in the stomach, the majority occurs in the upper small intestine.
At the bottom of the stomach the pyloric sphincter opens to release the chyme into the top section of the small intestine. It is activated by the waves of peristalsis, allowing the chyme to move out of the stomach.
