Mechanisms of Blood
Blood coagulation refers
to the process of forming a clot to stop bleeding. Coagulation is a
complicated subject and is greatly simplified here for the student's
To stop bleeding, the
body relies on the interaction of three processes:
involves the first two processes.
1. Vasoconstriction. Vasoconstriction is the body's first response
to injury in the vascular wall. When injury occurs, vessel walls
constrict, causing reduced blood flow to the site of injury.
2. Platelet plug. Platelets
aggregate to the site of the injury. They stick together acting as
a "plug." Platelets also activate the process which causes a fibrin
clot to form, known as secondary hemostasis.
3. Platelets alone are not enough to secure the damage in
the vessel wall. A clot must form at the site of injury. The
formation of a clot depends upon several substances called clotting
factors. These factors are designated by roman numerals I through
XIII. These factors activate each other in what as known as the
clotting cascade. The end result of this cascade is that
fibrinogen, a soluble plasma protein, is cleaved into fibrin, a
nonsoluble plasma protein. The fibrin proteins stick together
forming a clot.
The clotting cascade occurs
through two separate pathways that interact, the intrinsic and the
The extrinsic pathway is activated by external trauma that causes
blood to escape from the vascular system. This pathway is quicker
than the intrinsic pathway. It involves factor VII.
The intrinsic pathway is activated by trauma inside the vascular
system, and is activated by platelets, exposed endothelium,
chemicals, or collagen. This pathway is slower than the extrinsic
pathway, but more important. It involves factors XII, XI, IX, VIII.
Both pathways meet and finish the pathway of clot production in what
is known as the common pathway. The common pathway involves factors
I, II, V, and X.
Instructor's Note: A diagram
may be found in your text illustrating the clotting cascade. The
student does not need to be concerned about
learning the details of these pathways. The student does
need to realize that different factors are involved in each
pathway. If a patient does not clot normally, it is usually due to
a platelet abnormality or deficiency, or by a defect or deficiency
in one of the clotting factors. There are diagnostic tests which
test for deficiencies in the intrinsic pathway, the extrinsic
pathway, and platelet abnormalities. These tests allow the physician
to narrow down and eventually discover the defect which is causing a
patient to bleed excessively.
student may want to study a more in depth chart and explanation of
the coagulation cascade.
See Coagulation Cascade
1. What 3 processes are involved
to stop bleeding?
2. Define the two processes of primary hemostasis.
3. What 2 pathways are involved in secondary hemostasis?
4. What factors can lead to the activation of each pathway?
5. What pathway is factor VIII involved in?
6. What is the end result of the clotting cascade?
7. How can a physician determine what is causing a bleeding