Britannica Student Encyclopedia - 2010

can affect one or both eyes. Some blind

people have no sight at all. Others can

see light and shadows or blurry shapes.

Color blindness is a different kind of

condition. Color-blind people have

trouble telling colors apart. Some can

see no color at all. Color blindness is

inherited, or passed on from one generation

to another.

Causes

Sometimes blindness is inherited. An

injury to the eyes or the brain may also

cause blindness. However, a number of

diseases and disorders are the main

causes of blindness. These include cataracts,

glaucoma, macular degeneration,

and diabetes mellitus.

A cataract is the clouding of the lens of

an eye. Glaucoma is a disease that can

damage the optic nerve, which connects

the eye to the brain. Macular degeneration

is a disease that damages the retina,

or the lining of the eye. Diabetes mellitus

is a disease of the body that can also

damage the retina. All these problems

cause blindness mainly in older people.

Prevention and Treatment

Proper medical care can prevent or cure

some common types of blindness. For

example, doctors can treat glaucoma

with special eye drops or surgery. Doctors

can use surgery to treat cataracts,

too. They remove the clouded lenses and

replace them with artificial (man-made)

lenses.

A blind mountain climber

crosses a temporary bridge on

the way to the top of Mount

Everest.

A disease

called river

blindness is a

serious problem

in parts of

Africa. It is

caused by a

small worm

that is spread

to humans by

the bite of the

black fly.

90 Blimp BRITANNICA STUDENT ENCYCLOPEDIA

 

Sometimes blindness cannot be prevented

or treated. Nevertheless, many

blind people are able to live on their

own. Some have specially trained guide

dogs that help them to get around.

Many can read books and signs that are

printed in Braille. This is a code of

raised dots that people read with their

fingers.

#More to explore

Braille • Eye

Bloemfontein

Population

(2005 estimate)

379,000

The country of South Africa has three

capitals—Bloemfontein, Pretoria (Tshwane),

and Cape Town. Bloemfontein is

the judicial capital, or where the country’s

highest court meets. The city’s

name means “fountain of flowers.” It is

known for its many parks and gardens.

Bloemfontein is a center of South Africa’s

transportation systems. Many

people in the city work in government

offices or tourism.

The British founded Bloemfontein in

1846 as a fort and settlement. The British

and the Boers were struggling for

control of the area. The Boers were

descendants of earlier Dutch settlers.

Bloemfontein became the area’s capital

under the British and later the Boers.

South Africa became an independent

country in 1910. Bloemfontein was

made its judicial capital. In 2000

Bloemfontein was combined with some

nearby towns to create a larger area

called Mangaung.

#More to explore

Cape Town • Pretoria • South Africa

Blood

In humans and many other animals,

blood is a liquid that carries nutrients

and removes waste. Blood moves

through the body’s cardiovascular system.

This includes the heart and a vast

network of blood vessels. The heart

pumps blood through the blood vessels

to all parts of the body.

Structure

Blood cells make up about half the volume

(amount) of blood. These cells

form inside bone marrow, which is a soft

Gardens surround a court building in

Bloemfontein, South Africa.

BRITANNICA STUDENT ENCYCLOPEDIA Blood 91

 

tissue inside bones. There are three main

types of blood cell: red cells, white cells,

and platelets.

Red blood cells are the most numerous

kind of blood cell. Their main job is to

transport oxygen. In each cell, an ironrich

substance called hemoglobin carries

the oxygen. Hemoglobin and oxygen

together give blood its red color.

White blood cells, or leukocytes, help to

keep the body healthy. Some swallow up

tiny living things called bacteria or other

foreign substances. Others release proteins

that attack invading substances.

Still others help to break down and

remove dead cells.

Platelets are the smallest of the blood

cells. They are able to stick to one

another and form blood clots. Clots

plug holes that may develop in the walls

of blood vessels. This helps to stop

bleeding.

The watery part of the blood is the

plasma. Most of the plasma is water.

Plasma also contains nutrients, chemicals,

hormones, and wastes.

Functions

As blood passes through the lungs it

picks up oxygen. The oxygen-rich blood

then travels throughout the body. The

body’s cells take in this oxygen. In

exchange, they send a gas called carbon

dioxide into the blood. The blood carries

the carbon dioxide back to the

lungs, which push the gas out of the

body.

Blood is made up of red blood cells, white

blood cells, platelets, and plasma.

Blood is an important part of the body, but

people can lose a small amount of their

blood and feel fine. Doctors can examine a

patient’s blood to tell if they have a particular

disease. People can also give some of

their blood to others who may need extra

blood.

92 Blood BRITANNICA STUDENT ENCYCLOPEDIA

 

Blood also carries nutrients (proteins,

fats, sugars, salts, vitamins, and minerals)

to the body’s cells. It takes away

various wastes made by the cells. The