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144 Human Body


The process of breaking down food into molecules the


body can absorb is called digestion. The digestive


system begins with the mouth and involves many


organs in the abdomen. The digestive organs produce


. ENZYMES which break down food chemically.


WHY IS THE SMALL INTESTINE SO LONG?
The small intestine measures 6.5 m (21 ft) and is the
main organ of digestion and absorption. As food
moves slowly along the great length of the intestine, it
has plenty of time to break down properly. The size of
the small intestine maximizes the area of the inner
lining that food passes and the absorbtion of nutrients.

HOW IS FOOD ABSORBED?
The lining of the small intestine is covered with
microscopic projections called villi, which dramatically
increase its surface area. Molecules of nutrients
dissolve in fluids in the intestine and pass into the
villi. From there, nutrients pass through the thin walls
of the blood vessels in the villi, and into the body.

HOW DOES THE STOMACH WORK?
The stomach is a stretchy, muscular bag that stores
food and churns it around until it is a thick liquid.
Glands in the lining of the stomach secrete gastric
juice, which contains pepsin and hydrochloric acid.
Pepsin is an enzyme that digests protein molecules;
hydrochloric acid kills germs and helps pepsin work.

WHERE DOES FOOD GO AFTER THE STOMACH?
Food is squirted from the stomach into the small
intestine. Here it mixes with digestive juices from the
liver and pancreas. Bile from the liver breaks up fat.
Pancreatic juice neutralizes stomach acid and contains
enzymes that digest carbohydrates, proteins, and fats.

DIGESTIVE SYSTEM 3
The job of the digestive system
is to turn the food we eat into
material that the body can use
for energy, growth, and to repair
itself. It is a long system of
different organs, each section
working in a slightly different
way to extract nutrients
and water from food until only
waste remains.

Digestion

1 VILLI IN SMALL INTESTINE
Finger-like villi line the inside of the small intestine, giving it a velvety
texture and increasing the area for absorbing food.

Salivary glands
produce saliva to
moisten food so
that it travels
comfortably
down the throat

The pancreas
produces enzymes that
digest fat, protein, and
carbohydrate in the
small intestine

The stomach
churns food into a thick liquid
and produces acid and an
enzyme that digests protein

The large intestine
is about 1.5 m (5 ft) long
and absorbs water from the
undigested remains of food

The liver
produces bile, which
breaks down fats

The small intestine
is where most
digestion and nutrient
absorption take place

The tongue
tastes food and
moves it around in
the mouth and
down the throat

The oesophagus
takes swallowed food
to the stomach

The gall bladder
stores bile and releases it
into the small intestine

The appendix
is a small dead-end
tube with no function

The rectum
stores undigested
waste (faeces) until
it can be expelled

Teeth
cut and chew
food into
small pieces
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