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Blood is the river of life that flows through the body. We
cannot live without it. The heart pumps blood to all our
body cells, supplying them with oxygen and food. At the
same time blood carries carbon dioxide and other waste
products from the cells. Blood also fights infection and
keeps our temperature steady, and carries chemicaks that
regulate many body functions. Finally blood even has
substances that plug broken blood vessels to prevent us
from bleeding to death.
The amount of blood in your body depends on the size of
your body and the altitude of where you live. An adult who
weighs 160 pounds has about 5 quarts of blood. An 80
pound child has about half that. People who live in high
altitudes, where the air contains less oxygen , may have up
to two more quarts than people that in lower regions. The
extra blood delivers extra oxygen to the cells.
Blood consists of cells that move about in watery liquid
called plasma. The cells are known as formed elements
because they have definite shapes. Three types of cells
make up the formed elements: (1) red blood cells, (2) white
blood cells, and (3) platelets. A microliter (1/30,000 of an
ounce) of blood normally contains 4 million to 6 million
red blood cells, 5,000 to 10,000 white blood cells, and
150,000 to 500,000 platelets. Plasma is the liquid straw-
colored part of blood. It makes up about 50 to 60 percent of
the total blood volume. The formed elements count for the
Plasma consists of about 90 percent of water. Hundreds of
other substances make up the balance. They include
proteins that enable blood to clot and fight infection;
dissolve nutrients; and waste products. Plasma also carries
chemicals called hormones, which control growth and
certain other body functions.
Red blood cells consists mainly of hemoglogin, an
oxygen- carrying protein that gives them their red color.
The cells alco consist of chemicals, mainly enzymes.
Enzymes enable the cells to carry out necessary chemical
processes more effectively.
White blood cellsalso called leukocytes, fight infections
and harmful substances that invade the body. Most of the
cells are round and colorless. They have several sizes, and
their nuclei vary in shape. Some kind of white blood cells
kill bacteria by surrounding and digesting them. Other
kinds produce antibodies, proteins that destroy bacteria,
viruses, and other invaders or make them harmless.
Platelets, also known as thrombocytes, dislike structures
that stop bleeding. They are the smallest formed elements.
If ablood vessel is cut , platelets stick to the edges of the
cut and the other, forming a plug. They can release
chemicalc that can react with the fibrinogen and certain
other plasma proteins, leading to the formation of a blood
III. What Blood Does in the Body
The major of jobs are to transport oxygen and nutrients to
body tissues and remove wastes. To accomplish those
tasks, blood must flow to all parts of the body. It does so
by means of our circulatory system, which consists of the
heart, a vast network of blood vessels, and the blood itself.
The heart pumps blood to all the body tissues. Blood
leaves the heart through arteries and returns through the
veins. within the tissues, the arteries become smaller and
smaller. The smallest blood vessels are the capillaries. They
connect the tiniest arteries and the tiniest veins. Oxygen,
food, and other substances pass from the blood through the
thin capillary walls into the tissues. Carbon dioxide and
other wastes from the tissue also pass through the capillary
walls and enter the bloodstream. Blood returns to the heart
through even larger veins.
Your cells use oxygen to produce energy. The process
creates carbon dioxide which passes from the cell through
the capillary walls. Most carbon dioxide enters the plasma,
but some ataches to the hemoglobin. When the blood
reaches the capillaries in your lungs, the carbon dioxide
enters the alveoli and is exhaled.
IV. Protecting Against Disease
White blood cells play an important role in your immune
system, which helps your body resist disease-causing
substances. The invasion of harmful substances activates
the white blood cells. Then they work to destroy it. Some
proteins in the pasma also help fight disease. There are five
main groups of
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