Essentials of Anatomy and Physiology

3.Chemical clotting—The stimulus for clotting is a
rough surface within a vessel, or a break in the ves-
sel, which also creates a rough surface. The more
damage there is, the faster clotting begins, usually
within 15 to 120 seconds.

The clotting mechanism is a series of reactions
involving chemicals that normally circulate in the
blood and others that are released when a vessel is
damaged.
The chemicals involved in clotting include platelet
factors, chemicals released by damaged tissues, cal-
cium ions, and the plasma proteins prothrombin, fi-
brinogen, Factor 8, and others synthesized by the liver.
(These clotting factors are also designated by Roman
numerals; Factor 8 would be Factor VIII.) Vitamin K
is necessary for the liver to synthesize prothrombin

and several other clotting factors (Factors 7, 9, and 10).

Most of our vitamin K is produced by the bacteria that

live in the colon; the vitamin is absorbed as the colon

absorbs water and may be stored in the liver.

Chemical clotting is usually described in three

stages, which are listed in Table 11–3 and illustrated in

Fig. 11–7. Stage 1 begins when a vessel is cut or dam-

aged internally, and includes all of the factors shown.

As you follow the pathway, notice that the product of

stage 1 is prothrombin activator, which may also be

called prothrombinase. Each name tells us something.

The first name suggests that this chemical activates

prothrombin, and that is true. The second name ends

in “ase,” which indicates that this is an enzyme. The

traditional names for enzymes use the substrate of the

enzyme as the first part of the name, and add “ase.” So

this chemical must be an enzyme whose substrate is

Blood 265

Skin is cut and

blood escapes from a

capillary and an

arteriole.

Capillary

Arteriole

Platelets

Fibrin

In the capillary, platelets

stick to the ruptured wall

and form a platelet plug.

In the arteriole, chemical

clotting forms a fibrin clot.

Clot retraction pulls the

edges of the break together.

Figure 11–6. Hemostasis. Platelet
plug formation in a capillary and
chemical clotting and clot retraction
in an arteriole.
QUESTION:Look at the diameter of
the arteriole (compared to that of
the capillary) and explain why
platelet plugs would not be suffi-
cient to stop the bleeding.