Page 150 - The Cell in 40 Topics

The Cell in 40 Topics

Blood platelets—or thrombocytes, the smallest of the bone-marrow
cells—have one vitally important characteristic. These cells are the main el-
ement in blood clotting. A protein known as the Von Willebrand factor en-
sures that the thrombocytes traveling through the bloodstream do not
bypass the site of any injury. The thrombocytes caught here release a sub-
stance that brings other thrombocytes to the same site. Together, these cells
then close up the open wound.
Once they have discharged their duties, the thrombocytes die. Their
self-sacrifice is just one part of the blood-clotting system (Figure 115).
Another protein responsible for blood clotting is thrombin, a sub-
stance produced only where there is an open wound. This production must
be neither to great nor too small. In addition, it must take place and cease at
exactly the right time. To date, more than 20 bodily chemicals that play a
role in thrombin production have been described, and all are enzymes.
These enzymes can halt or initiate their own production. The process is so
tightly controlled that thrombin forms only in the event of tissue injury. As
soon as all the enzymes required for clotting reach the requisite levels in the
body, structural substances—long fibers which are actually proteins,
known as fibrinogen —are formed. In a short time, a whole network of fib-
rinogen fibers is established where the blood is flowing to the outside. The
thrombocytes in the blood become caught in this network and accumulate
in it.

Figure 115.
A protein known as
the von Willebrand
factor prevents the
thrombocytes travel-
ing through the
bloodstream from
bypassing the site
of the injury.

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