Biomedical Engineering Reference
In-Depth Information
3.2.1 The protagonists of hemostasis
Platelets (thrombocytes)
Platelets are cells with no nucleus, produced in the bone marrow and specialized
to perform a number of functions in the clotting process. They have a diameter
of 2-4
m, a life span of 5-9 days, and a discoid shape. However, during the
clotting process they undergo deep morphological changes, emitting protuberances
(
pseudopods
) which favor mutual aggregation, as well as adhesion to other elements
constituting the clot. This is one of the many phenomenon following
platelets acti-
vation
. Their normal concentration in blood is 1.5-4
μ
mm
3
.
6
Their membrane participates actively in the coagulation process. It is provided
with receptors (we will explain the functions of the receptors GPIb and GPIIb-IIIa),
intervening in the binding process with other elements. They possess corpuscles,
called
10
5
×
/
) granules
. The first class is able to secrete some
among the substances taking part in the coagulation process (namely
Fibrinogen
and vWF, see below), while the dense granules contain ADP (AdenosineDiPhos-
phate),
serotonin
(5-hydroxytryptamine) and
Calcium
(contributing to platelets ac-
tivation when released), and synthesize
Thromboxane A2
and other platelet activa-
tors, which are inhibited by aspirin (explaining the well known anticoagulant action
of that drug). Already from this preliminary illustration one can infer that platelets
intervene in the clotting process boosting a rapid positive feedback. The list of sub-
stances stimulating platelets is much longer (see [19]).
α
granules
and
dense (or
δ
The numbered Factor pairs
Many proteins (generally called
Factors
) which intervene in various ways in the
clotting process come in two forms: nonactivated and activated. They are numbered
(usually with Roman numbers) in the chronological order of their discovery. So for
instance we have the pair FVII, FVIIa, denoting the two respective forms of Factor
seven. The nonactivated form is referred to as
zymogen
and once activated it becomes
an
enzyme
(protease), namely a molecule able to modify other complex molecules by
cleaving some strategic chemical bond. In normal conditions only the nonactivated
factors are present in the blood, with the notable exception of FVIIa, which circulates
along with FVII, though in minimal concentration. However, it must be said that
its activity manifests itself only in the presence of the
Tissue Factor
(see below),
which becomes available as a consequence of an injury. The role of each factor in
6
The data referring to blood and its components are reported here just as an order of magnitude.
They vary according to sex, size and health state of the individual and in the same individual
even during the day. Some more information about other cells in the blood can be useful. There
are approximately 5 liters of blood in the human body. 45 % of the volume is made of cells, the
rest (plasma) is 92 % water: Red Blood Cells (RBCs) or erythrocytes: diam. 7
μ
m, concentration
mm
3
, lifespan 120 days (approximate data), no nucleus; White Blood Cells (WBCs) or
leukocytes have a nucleus and come in several different types (among which the monocytes have
some importance in the coagulation process), concentration 6
×
10
3
10
6
5
×
/
/
mm
3
, their size is much larger
than the one of RBCs [12].
