The New Childrens Encyclopedia

272

Your bodySix billion human beings share planet Earth. Each is unique. We all share certain characteristics, however, notably our basic body systems, from our circulatory to our respiratory system. Body systems are made up of groups of tissues and organs that work together.

The main part of the respiratory system are the lungs.

The respitory tract

draws air

The nervous system into the lungs, extracting oxygen required for life, then expels waste carbon dioxide. The system also enables people to speak.

is

composed of the brain, the spinal cord, nerves, and sense organs. This is what controls everything the body does, much of it automatically.

SKIN, HAIR, AND NAILS

Our skin, hair, and nails form a protective covering and together form a body system called the integumentary system. Your hair and nails grow through your skin, which is the body’s largest organ. Dead skin cells are constantly shed from the surface of the skin.FAST FACTS O

Human hair will grow

between

1 ⁄^4

inch (6 mm)

and

1 ⁄^3

inch (8 mm) every

four weeks.O

On average, a person

sheds about 1 lb (0.5 kg) of dead skin cells each year.O

An adult human’s skin

weighs approximately 11 lb (5 kg).O

Skin is waterproof.

O

Fingernails grow four times

faster than toenails.

The main part of the nervous system is the brain.

Dead hair

Epidermis

Hair follicle

Dermis

Hair bulb

Papilla

THE HUMAN BODY