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BLOOD
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Everybody is familiar with the sight of blood - the red
fluid that oozes out of your body when you've sustained
a cut or a deep injury, which is slightly denser and
approximately 3-4 times more viscous than water.
 Blood volume
Blood volume is variable, but tends to be about 8% of
body weight. Factors such as body size, amount of
adipose tissue, and electrolyte concentrations all affect
volume. The average adult has about 5 liters of blood.
Blood composition
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 Blood consists of cells which are suspended in a
liquid. The components of blood can be separated by
filtration, however, the most common method of
separating blood is to centrifuge (spin) it.
 Three layers are visible in centrifuged blood. The
straw-colored liquid portion, called plasma, forms at
the top (~55%). A thin cream-colored layer, called
the buffy coat, forms below the plasma. The buffy
coat consists of white blood cells and platelets. The
red blood cells form the heavy bottom portion of the
separated mixture (~45%).
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1-The plasma
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 The Plasma is a straw colored liquid, most of which is
water. It makes up 55% of the blood and serves as a
transport medium for blood cells and platelets.
 By far the greatest constituent of plasma is water, which
accounts for >90% of the total volume. Other
constituents fall into the following main categories:
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Ions (Na+, Cl-, etc) ,
Plasma proteins (albumins, globulins, fibrinogen, etc) ,
Dissolved gases (O2, CO2) ,
Circulating Nutrients (glucose, amino acids, etc) ,
Circulating Tissue products (urea, creatinine, lactate, etc)
Circulating Hormones (insulin, adrenaline, etc)
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 The water in the plasma is an important solvent for
ions such as Na+, Cl-, K+, etc. It also dissolves
nutrients like simple sugar, lipids, amino acids and
vitamins. Sometimes, it would help transport
enzymes and hormones, but that depends on the
activities of the body.
Serum: Plasma differs slightly from what is termed
serum, which is generally taken to mean the fluid
part of the blood which remains after the blood has
coagulated (clotted).
2-The red blood cells
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 The red blood cells form a major part of the blood.
These are the familiar discotic shaped cells which
make up 99% of the cells in the blood. They are the
principal carriers of the red colored hemoglobin
molecules. Hemoglobin is an iron containing protein
and binds about 97% of all oxygen in the body;
hence, the most principled function of red blood cells
is to carry oxygen gas around the body. Although
they do carry a little carbon dioxide sometimes, most
of this unwanted gas is transported by the plasma
3- The white blood cells
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 The white blood (leukocytes) cells are a vital source
of defense against external organisms. White blood
cells also serve as 'sanitary engineers' cleaning up
dead cells and tissue debris that would otherwise
accumulate to and lead to problems. There are five
classes of leukocytes: neutrophil, eosinophil,
basophil, monocyte and lymphocytes.
4-The platelets
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 These are not true cells but fragments of cytoplasm
from certain bone marrow cells. They play a part in
the clotting of blood.
Plasma and platelets in
clotting:
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 If damage occurs to a blood vessel, circulating platelets
immediately get trapped at the injury site. On accumulating the
platelets 'plug' the leak in the vessel providing a first step in
damage control. This mechanism is supplemented by 'blood
coagulation', or clotting, which is the most important means of
defense against bleeding. As mentioned plasma contains
several dissolved proteins. Fibrinogen is a rod shaped soluble
protein which in the presence of a catalyst thrombin gets
converted to an insoluble protein fibrin.
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 The combined action of the platelets and 'fibrin web'
is sufficient to prevent a dangerous loss of blood. In
cases where the formation of fibrin and hence
formation of a clot is impaired due to some reason
(e.g. a genetic disorder as in hemophilia) a person is
at great risk of bleeding to death.
BLOOD CELLS
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