Childrens Illustrated Encyclopedia

HUMAN BODY

263

Digestive system

the mouth, esophagus, stomach, and intestines are

part of the digestive system. these organs work

together to break down food into particles that are

small enough to pass through the lining of the

intestine and into the blood. the mouth and teeth

chop and chew food, and the stomach churns it with

powerful digestive chemicals. the liver is the main

organ for converting absorbed nutrients into forms

more suitable for use by the various organs. the

large intestine deals with wastes and leftover food.

There are several stages of development in everyone’s

lifetime—from birth through childhood, adolescence,

and adulthood, to old age.

Growth and development

As the human body grows, it develops

many skills. Babies learn to smile, sit up,

crawl, walk, and talk. Learning continues

at school. On average, the peak of physical

abilities is reached between 18 and 25 years

of age. Later, more changes occur with

age. the skin becomes wrinkled and less

elastic, the joints are less flexible, bones

become more brittle, muscles are less

powerful, and there is some loss of

height and graying of hair.

In many older people, decrease in

physical strength is offset by the wisdom

and knowledge gained from a lifetime

of experience.

muscuLAr system

there are about 650

muscles in the body. some,

such as the arm muscles, can

be controlled at will, to pull

on the bones of the skeleton

and move the body. Others,

such as the muscles of the

heart and intestine, work

automatically.

Find out more

Brain and nerves

ears

eyes

Heart and blood

Lungs and breathing

reproduction

skeletons

teeth

urinAry system

the kidneys filter waste

substances from the blood

to form a fluid, urine, which

is stored in the bladder.

Lung

Heart

Neck

muscles

tilt and

twist head.

Upper arm

muscles

bend and

straighten elbow

and move

upper arm.

Chest muscles

help in breathing.

Thigh muscles,

together with the

buttock muscles,

are the body’s most

powerful muscles.

Tendons join

muscles to bone.

Stomach

Small intestine

respirAtOry AnD

circuLAtOry systems

the lungs, airways, and

throat and nasal passages

make up the respiratory

system. the lungs absorb

vital oxygen from the air.

the blood transports this

oxygen around the body,

pumped through the

blood vessels by the heart.

Esophagus

(gullet)

Mouth

Teeth

Large

intestine

Abdominal

muscles bend

and rotate

the trunk.

Rectum Anus

Trachea

(windpipe)

Veins return blood

to the heart.

Bladder

Kidney

Arteries

carry

blood

from

the

heart.

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