Blood acts as a vehicle to the body. It carries
food and oxygen. It takes away waste materials, and distributes heat evenly.
Blood also fights disease and infection.
A drop of blood contains millions of corpuscles-cell-like
bodies suspended in a watery fluid called plasma. There are three main kinds of
corpuscles, and platelets.
Red corpuscles are the bloods oxygen carriers. They
contain hemoglobin – a red
substance which gives blood its color. The
disc-shaped red cells are so tiny that a drop of blood the size of a pin head
contains 41/2 to 5 million of them while corpuscles are somewhat larger
than red ones and have irregular shapes. There are only about 5000 to 1000 of
them in a drop of blood the size of a pin-head. Their job is to prevent
infection and fight disease by surrounding and destroying any germs that enter
the body.
Platelets help the blood to clot (stick together),
and prevent too much blood being lost from an open wound.
The blood of any one person differs slightly from
that of any other, owing to the presence or absence of certain substances in
the red blood cells. Two such substances, simply called A and B are the basis
of the ABO blood grouping. People in group A have only substance A in their
blood, group b people have only B, group AB have both A and B, and group O have
neither. In blood transfusion, patients are generally given blood of their own
group. But group o can be safely given to any group, and is called the
universal donor. Group AB is called the universal recipient. Because it can
receive any group..
Man is not the only creature to have blood. All but
the very simplest animals also have it. Crabs, crayfish, and some snails have
blood colored blue by a pigment called hemocyanin.
Only in mammals and birds is the blood temperature
kept constant. These are the warm-blooded animals. In cold-blooded creatures,
the blood temperature varies according to the temperature of the surroundings.
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