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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