Page 117 - Biology Form Two

Biology for Secondary Schools

preventing further loss of blood. The
clear fluid called serum which oozes
out of the wound remains when the rest
of the blood clots. The clot stops further
bleeding and prevents entry of bacteria
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into the body through the wound.

Blood groups

Task 5.5

Figure 5.12: Blood platelets Use the library and internet sources to
search for information on ABO blood
Functions of platelets grouping system and Rhesus factor.
Platelets play an important role in Then, write short notes.
clotting blood.

Grouping of human blood is done using
The clotting process: Blood clotting the ABO blood grouping system and the
or coagulation is an important process Rhesus factor. Genetic materials that
that prevents excessive bleeding determine blood groups are inherited
when a blood vessel is injured. Blood from the parents.
platelets and proteins in the plasma
work together to stop bleeding by The ABO blood grouping system
forming a clot over the injury.
The ABO blood grouping system is the
classification of human blood based
Blood platelets at the site of an injury
produce thromboplastin which starts off on two factors. The first factor is the
presence or absence of antigen A or
the clotting process. Thromboplastin, antigen B on the membranes of the
with the help of vitamin K and calcium, red blood cells. The second one is the
neutralises heparin, an anticoagulant in presence or absence of antibody a or
the blood; and converts prothrombin, antibody b in the blood plasma.
which is an inactive plasma protein
to thrombin. This is an active plasma A person cannot have a certain antigen
protein, which catalyses conversion of on the membrane of the red blood cell
soluble fibrinogen to insoluble fibrin. and the corresponding antibody in
Fibrin forms a network of fibers that the plasma. For example, one cannot
trap debris and blood cells. These have both antigen A and antibody a.
result into a clot at the site of the wound This would cause agglutination or
clumping together of red blood cells.

Student’s Book - Form Two 111

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