Page 329 - Essential Haematology

Chapter 24 Platelets, blood coagulation and haemostasis / 315

The normal haemostatic response to vascular Components of the h aemostatic
damage depends on a closely linked interaction r esponse
between the blood vessel wall, circulating platelets
and blood coagulation factors (Fig. 24.1 ). Platelets
An efficient and rapid mechanism for stopping

bleeding from sites of blood vessel injury is clearly Platelet p roduction
essential for survival. Nevertheless, such a response
needs to be tightly controlled to prevent extensive Platelets are produced in the bone marrow by frag-
clots developing and to break down such clots once mentation of the cytoplasm of megakaryocytes, one
damage is repaired. The haemostatic system thus of the largest cells in the body. The precursor of the

represents a delicate balance between procoagulant megakaryocyte – the megakaryoblast – arises by a
and anticoagulant mechanisms allied to a process process of differentiation from the haemopoietic

for fi brinolysis. Th e five major components involved stem cell (see Fig. 1.2 ). The megakaryocyte matures

are platelets, coagulation factors, coagulation inhib- by endomitotic synchronous replication (i.e. DNA

itors, fibrinolysis and blood vessels. Th ese are replication in the absence of nuclear or cytoplasmic
described later in the haemostatic response section division) enlarging the cytoplasmic volume as the
on p. 324 . number of nuclear lobes increase in multiples of

Vessel injury

Collagen exposure
Platelet adhesion
Tissue
factor
Platelet activation
Shape change, granule secretion
and activation of GPIIb/IIIa
Serotonin Platelet
phospholipid
Vasoconstriction Blood
coagulation
Thromboxane A 2 , ADP
cascade
Platelet aggregation
Thrombin
Reduced Primary
blood flow haemostatic plug
Fibrin

Stable
haemostatic plug

Figure 24.1 The involvement of blood vessels, platelets and blood coagulation in haemostasis.

ADP, adenosine diphosphate.