The job of the digestive system is to break down the food
we eat into smaller units called nutrients. The nutrients are then absorbed
into the bloodstream and fuel the body’s activities. The MOUTH takes in food and begins the digestive process, which continues in
the STOMACH. The food then passes to the INTESTINES,
which complete the process. Enzymes – substances that speed up chemical
reactions in the body – play a key part in breaking down food.
PARTS OF THE DIGESTIVE SYSTEM
The digestive system consists mainly of a long muscular tube, the
digestive tract. This starts at the mouth, continues via the oesophagus and
stomach to the intestines, and ends at the anus. The system also relies on the
pancreas, liver, and gall bladder to help digest food. Each day the pancreas
releases about 1.5 litres (2 1/2 pints) of enzyme-containing juice into the
tract. The liver produces about 1 litre (1 3/4 pints) of bile, which is stored
temporarily in the gall bladder.
The mouth is a cavity formed between the tongue at its
base, cheeks at the side, hard and soft palate in its roof, and teeth at the
front. The teeth tear and grind food, which is then churned through movements
of the jaws and tongue. Breaking the food into smaller pieces creates a larger
surface area for the action of enzymes in saliva; these begin to digest the
food.
Each tooth has a crown, which can be seen above the gumline, and
one or more roots, which are embedded in the jawbone. In the centre of the
tooth lies the pulp cavity, which contains blood vessels and nerves. This is
surrounded by dentine, a strong material that forms most of the tooth. The
outer layer of the crown is made of enamel, the hardest substance in the body.
These glands produce a mucus-rich fluid, saliva, that moistens
food and so helps form it into a ball, called a bolus, that is easily
swallowed. Saliva also contains enzymes that begin the digestive process. When
food enters the mouth, the brain sends messages to the glands, triggering a
rush of saliva. Food must mix with saliva before it can stimulate the taste
buds, which check that the food is safe to eat.
Teeth come in four main types: incisors, canines, premolars, and
molars. Each is shaped for a specific function. People have two sets of teeth
during their lives: the 20 milk teeth emerge between the ages of about six
months and three years. The 32 permanent teeth (like those seen here) emerge
from the age of about six years into the early twenties.
The stomach is a muscular bag that begins the digestive
process. On entering the stomach, food is mixed with gastric juices and churned
by contractions of muscles in the stomach wall. Food usually stays in the
stomach for about four hours. By that time, it has become a semi-liquid (called
chyme) that can be released in spurts into the duodenum, the first part of the
small intestine.
When food enters the stomach, glands in its lining release an
enzyme, pepsin. This begins to break down protein in food. The lining also
produces hydrochloric acid, which maintains the acid environment needed for
pepsin to work and kills any bacteria present.
The intestines form a continuous tube that is about 8 m
(26 ft) long in adults. Food is pushed along this tube by contractions of
muscles in the intestinal walls. In the small intestine, muscle contractions
mix chyme with enzymes produced by the pancreas and intestinal lining. It is
here that nutrients are absorbed into the bloodstream. In the large intestine,
water is absorbed from the digestive waste, and the remaining waste is formed
into stools.
The lining of the small intestine has a huge surface area due to
the folds in its lining as well as villi, tiny fingerlike structures that
project from the lining. Once digested, nutrients are absorbed into tiny blood
vessels within the villi. They are then carried in the blood to the liver for
processing. Food can remain in the small intestine for up to five hours.
The intestines
are home to more than 500 species of bacteria, which they need to function
effectively. Some of these bacteria are thought to protect the intestines from
disease. Certain bacteria in the large intestine make vitamin K, which the body
needs for blood clotting.
