and North Khartoum, lie just across the
rivers. Omdurman is the largest city in
Sudan. It is also the place where the
country’s lawmakers meet.
Khartoum is Sudan’s center of
transportation and trade.Many people
in the city work for the government.
Factories there make cotton cloth and
other products.
Khartoum began in 1821 as an Egyptian
army camp. It soon grew into a town.
Egypt and Great Britain shared control
of the area.
In the 1880s local Muslim forces
rebelled against foreign rule. They captured
Khartoum in 1885. British and
Egyptian troops took back the city in
1898. Khartoum was the center of British
and Egyptian rule of Sudan until
1956. In that year Sudan became an
independent country with Khartoum as
its capital.
..More to explore
Nile River • Sudan
Khwarizmi, Al-
Al-Khwarizmi was an Arab mathematician
and astronomer of the AD 800s. He
is known as the Father of Algebra.
Muhammad ibn Musa al-Khwarizmi
was born in about 780. Historians
believe that either he or his ancestors
came from Khwarezm, a region in central
Asia. The region is now part of the
countries called Turkmenistan and
Uzbekistan.
As an adult al-Khwarizmi lived in Baghdad,
in what is now Iraq. There he
worked at the House ofWisdom, a center
for scientific research. Al-Khwarizmi
studied the works of Arab, Greek, and
Indian scholars.
A man walks past a mosque in
Khartoum, Sudan.
“Khartoum”
means
“elephant’s
trunk” in the
Arabic
language.
BRITANNICA STUDENT ENCYCLOPEDIA Khwarizmi, Al- 61
Al-Khwarizmi invented new ways to
solve mathematical problems. One of
the books he wrote explained a problemsolving
system that is now known as
algebra. The word algebra comes from
the Arabic word al-jabr, which appears
in the title of al-Khwarizmi’s book.
European universities used the book to
teach mathematics from the 1100s to
the 1500s.
Another one of al-Khwarizmi’s books
introduced Arabic numerals to Europe.
Arabic numerals are the 10 digits (0, 1,
2, 3, 4, 5, 6, 7, 8, 9) that most people
use today.
Al-Khwarizmi was interested in more
than mathematics. He also wrote important
books on astronomy and geography.
He died in about 850.
#More to explore
Numbers and Number Systems
Kidney
All living things must remove wastes
from their bodies. Reptiles, birds, mammals,
and some other animals remove
some of these wastes through two body
parts called kidneys. Kidneys are part of
the urinary system. They take out extra
water and wastes from the blood. These
wastes leave the body in the form of
urine.
Structure
Human kidneys are reddish brown and
shaped like beans. They sit in the lower
back, one on each side of the backbone.
Each kidney is about 4 to 5 inches (10
to 12.5 centimeters) long.
The inner curve of the kidney has a dent
in the middle called the hilus. The hilus
is the point where blood vessels enter
and leave the kidney. These blood vessels
include a renal artery and a renal vein.
The word renal means “kidney” in the
Latin language. Blood enters the kidney
A postage stamp honors the
mathematician known as
Al-Khwarizmi.
62 Kidney BRITANNICA STUDENT ENCYCLOPEDIA
through the renal artery and leaves
through the renal vein.
Also at the hilus is a funnel-shaped
structure called the renal pelvis. The
renal pelvis is a hollow area that collects
urine. A tube called the ureter leads
from the renal pelvis out of the kidney.
The ureter runs down to the bladder.
Blood flows through the kidneys in tiny
blood vessels called capillaries. The capillaries
lead to tiny tubes called nephrons,
which make urine. Each kidney
has at least a million nephrons.
How KidneysWork
Every few minutes all the blood in the
body runs through the kidneys. The
nephrons remove water and some substances
from the blood. Some of these
substances are waste. Others are nutrients,
or things that are useful to the
body. The nephrons pass the nutrients
and most of the water back into the
blood. The wastes and the extra water
stay in the nephrons. This liquid is the
urine. Drop by drop, the urine passes
from the nephrons into the renal pelvis.
From there it goes into the bladder,
which stores the urine until it leaves the
body.
Sometimes wastes, especially salts,
remain in the kidneys. These salts can
form hard lumps called kidney stones.
Medicines can help to dissolve these
stones.
..More to explore
Blood • Urinary System
Kiev
Population
(2006 estimate)
2,718,000
Kiev is the capital of the country of
Ukraine. It is one of the largest cities in
eastern Europe. Kiev has a port on the
Dnieper River. The city is Ukraine’s
center of culture and business.
The economy of Kiev depends on trade,
industry, and businesses that provide
services. Many people in Kiev work in
government or business offices. Other
people work in publishing, engineering,
or manufacturing. Factories in Kiev
make tools, electronics, chemicals,
foods, and other goods.
Kiev was founded several hundred years
ago. In the late 800s Vikings from
The Church of Saint Andrew, in Kiev,
Ukraine, was built in the 1700s. It is known
for its five green and gold domes.
BRITANNICA STUDENT ENCYCLOPEDIA Kiev 63
northern Europe took over the area.
They made Kiev the capital of their
