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Human Body^145

WHICH ORGANS MAKE DIGESTIVE ENZYMES?

The salivary glands, stomach, pancreas, and small

intestine all produce digestive enzymes. These

enzymes break down proteins, carbohydrates,

and fats into the units they are made up of –

amino acids, sugars, and fatty acids. These

tiny units are so small they can pass into the

villi lining the intestine.

HOW DOES FOOD MOVE ALONG

THE DIGESTIVE SYSTEM?

The oesophagus, stomach, small intestine, and

large intestine all have muscular walls. When these

muscle walls contract, the hollow organ narrows,

pushing the food forward. The contraction happens

in waves that move along the organs. This action

is called peristalsis.

WHY DON’T THE DIGESTIVE ORGANS

DIGEST THEMSELVES?

The lining of the stomach and intestines secretes a

thick, slippery liquid called mucus, which helps food

slide along and protects the digestive organs from

acid and enzymes. Even so, the intestinal lining does

gradually wear away, but it continually renews itself

by producing new cells, just as skin does.

WHAT HAPPENS IN THE LARGE INTESTINE?

Undigested leftovers pass into the large intestine,

which absorbs water and minerals. Harmless bacteria

flourish in the waste and produce some vitamins,

which are also absorbed. The semi-solid wastes

(faeces) collect in the rectum before being expelled.

HOW DO ENZYMES WORK?

Enzymes are catalysts, which means that

they speed up the rate of a chemical reaction.

The molecules of each enzyme have a very

particular shape which enables them to bind to a

specific molecule in the body and make it react.

BACTERIA IN THE LARGE INTESTINE 3

This image shows rod-shaped bacteria

(in pink) on the surface of the large

intestine. Billions of useful and

harmless bacteria live in the

large intestine. This is one

of the most common

species, which helps

release vitamins

from food.

Most chemical reactions in living

organisms are controlled by

enzymes. There are many types,

each suited to a particular task.

FIND OUT MORE. Liver 146 • Micro-organisms 85 • Mouth 142 • Teeth 143

An average meal takes

between 18 and 30 hours to

pass through the digestive

system, depending on the

amount and type of food.

Meals rich in carbohydrates

(such as sugar and starch) are

digested quickest, and fatty

foods take longest.

4 ENZYMES IN THE PANCREAS

This magnified view shows granules of

enzymes (in yellow and orange) made by

the pancreas. These will be released into the

small intestine to help the digestion of food.

1 PERISTALSIS

The muscular walls of most

sections of the digestive system

contract in a wave-like pattern,

known as peristalsis, to push

food through.

ENZYMES

3. The large
   intestine is where
   most of the water is
   removed and
   reabsorbed.

THE TIMETABLE OF THE DIGESTIVE SYSTEM

Large intestine

Muscles

contract

to make the

intestine

narrower

Semi-solid

faeces are

formed after

12–36 hours in

the large

intestine

Semi-digested

food — many of its

nutrients have been

absorbed

2. The small
   intestine takes up
   to 6 hours to
   process food.
   4. The rectum holds
   faeces until there is a
   convenient moment
   to get rid of them.


3. The stomach
   churns food for
   2–4 hours.

Waste – the

watery, indigestible

remains of food –

leaves the small

intestine

Swallowed food

takes about 10

seconds to reach

the stomach

Food is squirted

into the small

intestine

Muscles relax,

allowing the

intestine to

widen

Food

Rectum

digestion