Platelets
Platelets are
the smallest of the formed elements of the
blood. Every cubic millimeter of blood contains about 250 million
platelets. There are about a trillion platelets in the blood of
an average adult. Platelets are fragments of the giant
bone-marrow cells called megakaryocytes. When a megakaryocyte
matures, its cytoplasm breaks up, forming several thousand
platelets. Platelets are roughly 
disk-shaped between one-half and
one-third the diameter of a red cell. However, they contain only
about one-thirteenth the volume of a red cell. When released into
the blood, they circulate and die in about ten days.
The primary function of platelets is that of forming blood
clots. Platelets contain contractile protein (actomyosin) and are
prone to 
contract
much as muscles do. This causes the shrinkage
of a fresh blood clot after it stands for a few minutes. When a
wound occurs, numbers of platelets are attracted to the site
where they activate thrombin which starts the clotting process.
Prothrombin is the
precursor of thrombin. Thrombin also makes the
platelets sticky. When exposed to collagen and thrombin, the
platelets aggregate to form a plug. Persons with a low platelet
count have a long bleeding time.
Platelets not only tend to stick to one another, but to the
walls of blood vessels as well. Because they promote clotting,
they play a key role in formation of thrombi.
Platelets are
the smallest of the formed elements of the
blood. Every cubic millimeter of blood contains about 250 million
platelets. There are about a trillion platelets in the blood of
an average adult. Platelets are fragments of the giant
bone-marrow cells called megakaryocytes. When a megakaryocyte
matures, its cytoplasm breaks up, forming several thousand
platelets. Platelets are roughly 
disk-shaped between one-half and
one-third the diameter of a red cell. However, they contain only
about one-thirteenth the volume of a red cell. When released into
the blood, they circulate and die in about ten days.
contract
Prothrombin