Haemostasis

Immediately after a blood vessel has been severed or
ruptured, bleeding is stemmed by local vasoconstriction
resulting from nervous refl exes, vascular spasm,
and humoral factors released from the traumatized
tissues and blood platelets. The damaged vessel is
then sealed temporarily with a plug of aggregated
platelets. Localized blood coagulation leads to sub-
Background biochemistry 77
sequent formation of a blood clot composed of a
three-dimensional meshwork of fi brin fi bres entrapping
blood cells, platelets and plasma. A large amount
of plasminogen is also trapped in the clot along with
other plasma proteins. The clot begins to develop in
15–20 s if the trauma to the vascular wall has been
severe and in 1–2 min if the trauma has been minor.
Within 3–6 min after rupture of a vessel (if the opening
is not too large), the entire opening or broken end
of the vessel is fi lled with clot. After 20 min to 1 hour,
the clot retracts, closing the vessel still further. Clot
retraction is effected by entrapped platelets which
activate themselves to contract, thereby pulling attached
fi brin fi bres towards them and compressing
the fi brin meshwork into a smaller mass. Within a few
hours after clot formation, the clot is invaded by fi broblasts
and the deposition of fi brous connective tissue
throughout the clot seals the vessel permanently. A
day or so after the clot has stopped the bleeding, the
injured tissues and vascular endothelium very slowly
release tissue plasminogen activator. This activator
converts the plasminogen within the clot to plasmin,
which dissolves the clot. After 1 or 2 weeks, the clot has
been replaced by connective tissue.
4.4.2 Formation of the platelet plug
Platelets are colourless, round or oval discs of diameter
1–4 μm (cf. erythrocytes with a mean diameter of
7.8 μm). They arise by fragmentation of megakaryocytes
within the bone marrow or soon after the megakaryocytes
enter the blood. Although platelets have
no nuclei and cannot reproduce, they have many
functional characteristics of whole cells. The presence
of contractile proteins in their cytoplasm enables
them to contract; they have the capacity to synthesize
various enzymes, generate ADP and ATP, and store
large quantities of calcium ions; and they can synthesize
prostaglandins, fi brin-stabilizing factor and
a growth factor that stimulates the proliferation of
vascular endothelial and smooth muscle cells. On the
platelet surface is a coat of glycoproteins (the glycocalyx)
that repulses adherence to normal endothelium
and yet causes adherence to injured areas of the vessel
wall. Adherence is mediated by von Willebrand factor,
a multimeric plasma adhesive protein that binds
to a specifi c receptor, glycoprotein 1b, on the platelet
membrane.
When platelets come in contact with a damaged
vascular surface, such as the collagen fi bres in the vessel
wall, the platelets themselves undergo an immediate
and drastic change, an event known as activation.
They swell to a spherical form and extend long pseudopodia;
their contractile proteins contract forcefully
and cause the release of granules that contain multiple
active factors; they become sticky so that they adhere
to exposed collagen in damaged vessels and tissues;
and they secrete large quantities of ADP. The ADP in
turn acts on nearby platelets to activate them as well,
and the stickiness of these additional platelets causes
them to adhere to the originally activated platelets.
Activated platelets also release thromboxane A2,
which is both a vasoconstrictor and an aggregating
agent (Marcus, 1978). The recruitment of platelets to
the damaged site in this manner leads to the formation
of a platelet plug.
An increase in the cytosolic Ca2+ concentration is
a major signal underlying platelet activation. Physiological
agonists such as collagen, thrombin, ADP,
platelet-activating factor and thromboxane A2 mobilize
Ca2+ by stimulating its release from intracellular
stores in the endoplasmic reticulum and also
its entry into the cell across the plasma membrane.
After stimulation, the cytosolic Ca2+ concentration is
restored to the resting level by sequestration of the ion
into the endoplasmic reticulum and extrusion out of
the cell. These movements of Ca2+ are carried out by
Ca2+-ATPases (calcium pumps).