I. Introduction

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.


II. Compisition

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

rest.


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

clot.


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