Biomedical Engineering Reference
In-Depth Information
3.2 The modern view of blood coagulation
The aim of this section is to explain the most recent model of the blood coagulation
process in a form accessible to people not having a specific knowledge in the field.
Therefore we shall avoid the intricacy of the chemistry of the many proteins parti-
cipating in the formation of a blood clot (as well as the description of their highly
complicated structure), and we will concentrate on illustrating their action, present-
ing a general sketch of the phenomenon and trying to use a plain language. The
circulatory system is equipped with an incredibly complex chemical and mechani-
cal machinery ready to repair lesions which may occur to blood vessels by sealing
them with a
clot
(or
thrombus
). A clot is a gel like structure consisting of a polymer
(
Fibrin
) network entrapping various blood components. The ingredients necessary
to lead to the clot formation (and that will be described in detail very soon) are either
present in the blood, or reside in the endothelium of the blood vessels, namely in the
external membrane (
adventitia
), in the main body of the vessel (
tunica media
), and
in the innermost, thin membrane (
intima
). Actually, the ones contained in the blood
vessel walls become available immediately after a lesion of the intima, so even in
the presence of a tiny internal lesion. The important thing to notice is that in normal
conditions they do not come in contact with blood.
Clearly, it is absolutely crucial that the clotting mechanism is set in motion only
when it is really necessary, remaining silent in normal conditions, and at the same
time that the coagulation process is terminated before it occludes the vessel, allow-
ing blood to flow normally. Indeed, after the process of
hemostasis
5
is completed,
the thrombus will be gradually removed by means of another process known as
fibri-
nolysis
. To be more precise, it is not correct to look at these processes in a sequential
way. The hemostasis machine and the fibrinolysis machine are both active at the
same time and whether a thrombus is growing or retreating is the result of an un-
balance between the two processes. Besides the physiological process of hemostasis
following an injury, it is well known that unwanted coagulation can take place due
to temporary reduced blood flow rate or stasis (or sometimes in stagnation points of
the flow generally accompanying abnormal vortices), leading to
deep vein throm-
bosis
(DVT), a disease also induced by bone traumatization e.g. during prosthetic
implants. We will briefly deal with that phenomenon too in the next section. In order
to explain at least the basic steps of hemostasis it is advisable to describe the vari-
ous elements contributing to the final formation of a clot. It is important to get some
knowledge of the functions performed by these substances, and the reader can use
the following list as a guideline through the core of the model.
5
In all words with the prefix
hemo-
the spelling
haemo-
can also be used, though not frequent in
the medical literature.
