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9

VetBooks.ir Coagulation

ElizabEth J. thomovsky* and aimEE C. brooks

Purdue University College of Veterinary Medicine, West Lafayette, Indiana, USA

9.1 Basic Physiology and Anatomy Willebrand’s factor (vWF), serotonin, adenosine
diphosphate (ADP) and calcium. The latter three
Coagulation is a complex process that involves the items are in the dense granules and the other listed
interaction of the body (typically the vascular factors are in the alpha granules. Some of these
endothelium), the platelets, and a host of coagula- items are involved in clot formation (factors V and
tion factors with the goal to prevent the patient XIII, vWF, fibrinogen, Ca), others are involved in
from bleeding excessively after injury. There are two mediating vascular tone to vasodilate locally to
parts to the coagulation system – primary (platelet- allow for ease of clot formation (serotonin), and
based) and secondary (coagulation factor-based) still others aid in platelet activation (ADP).
hemostasis. The historic belief was that primary Platelet plug formation is divided into three phases:
coagulation occurs first with platelets coming to an initiation, activation, and adhesion (also known as
area, creating a platelet plug, and then activating the aggregation) (Fig. 9.2). Initiation occurs when a plate-
secondary coagulation system (clotting factors). let that is rolling through the blood vessel detects
This standard ‘cascade’ for secondary coagulation disruption or damage to the endothelium. Damage to
was described as a linear flow of coagulation fac- the endothelium leads to exposure of collagen and vWF.
tors (Fig. 9.1) that occurs after the platelet becomes Exposed vWF interacts with GPIb/V/IX (a platelet
activated. Since this does not adequately describe glycoprotein receptor) causing adhesion of platelets
the actual interactions of the various cells involved to the damaged site on the endothelium. In addition,
in coagulation, an ‘advanced’ model of coagulation the glycoprotein receptor GPVI on the platelet surface
was introduced in 1992 (see below).
binds to exposed collagen, and both collagen and the
remaining exposed subendothelial materials bind to
GPIIb/IIIa (another platelet surface glycoprotein
Primary hemostasis
receptor; also known as integrin α /β ) and other
3
IIb
Platelets are typically thought of as the first platelet surface receptors. This initial binding to dam-
responders for coagulation. The platelet itself is an aged endothelial surfaces constitutes the initiation
anuclear cell created in the bone marrow by mega- phase of platelet plug formation.
karyocytes. Its main function is to provide a surface The activation phase occurs as these platelets that
for coagulation and to store various factors that are bound to vWF and/or collagen are stimulated to
are important in the events related to coagulation change shape from a disk to a sphere. As they
and inflammation. It is covered with a variety of change shape, the platelets both create pseudopods
glycoproteins that mediate adhesion to the endo- that can ‘grab’ more platelets, and are stimulated to
thelium and to other platelets, as well as mediating release the contents of their alpha and dense gran-
signaling between the platelets and other cells. This ules. These granule contents work to further enhance
‘crosstalk’ is critical in activating platelets and call- platelet adhesion and activation (see Table 9.1).
ing other cells to the area. The two most important Additionally, the activated platelets have various
storage organelles in the platelet are the alpha and enzymes including flippases, floppases, and scram-
dense granules (see Table 9.1). Each contain vari- blases, which work to change the charge of the cell
ous cytokines and other proteins such as platelet surface as well as change the population of lipid-
factor 4, factors V and XIII, fibrinogen, von based cell surface markers to make the platelet

* Corresponding author: ethomovs@purdue.edu

© CAB International, 2020. Basic Monitoring in Canine and Feline Emergency Patients 177
(eds E.J. Thomovsky, P.A. Johnson and A.C. Brooks)