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.
