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Journal of Pediatric and Neonatal Individualized Medicine  2020;9(1):e090106
doi: 10.7363/090106 
Received: 2019 Apr 14; revised: 2019 Jul 13; accepted: 2019 Jul 15; published online: 2019 Nov 22

Review

The hemostatic system. 1st Part

Doris Barcellona, Francesco Marongiu

Department of Medical Science and Public Health, University of Cagliari, Cagliari, Italy

Abstract

The hemostatic system is a complex ancestral pathway physiologically

dedicated to protect the individual from bleeding. It starts immediately after an
endothelial injury. Platelets and blood coagulation act synergically to provide
a strength clot able to stop bleeding. In healthy subjects, the hemostatic system
is able to work to avoid an excess of fibrin formation and deposition within
the blood vessels on the one hand but is ready to stop bleeding on the other.
To reach this crucial objective, a fine regulation of its activity is required. In
other words, all actions of the hemostatic system are under control to assure
a perfect balance to maintain people distant from both Scylla (bleeding) and
Charybdis (thrombosis). Fibrinolysis is a complementary defensive system
essential to regulate fibrin deposition via its dissolution. It is, in turn, well
controlled to avoid bleeding and thrombosis by a fine control of its inducers
and inhibitors. The aim of this review is to provide a picture of global
haemostasis for helping in understanding this complex topic.

Keywords

Primary hemostasis, platelets, blood coagulation, Vitamin K, fibrin formation,

fibrinolysis.

Corresponding author

Doris Barcellona, Department of Medical Science and Public Health, University of Cagliari, Cagliari,
Italy; tel.: +39 07051096083; e-mail: doris.barcellona@unica.it.

How to cite

Barcellona D, Marongiu F. The hemostatic system. 1st Part. J Pediatr Neonat Individual Med.
2020;9(1):e090106. doi: 10.7363/090106.

Introduction

Hemostasis is a complex phenomenon. Blood coagulation and

platelets belong to a defensive system developed against bleeding. Blood

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coagulation starts immediately after a lesion of the to receptors P2Y1 and P2Y2, and Thromboxane
endothelium, coordinated with platelets, which A2 (TXA2) whose synthesis happens inside the
form a hemostatic plug providing a first block platelets when activated. TXA2 then binds to
of the hemorrhage [1]. Clot is the final product its receptor on the platelet membrane, further
of blood coagulation activation triggered by enhancing platelet aggregation [10]. Another
extravascular compartment rich in Tissue Factor agonist is serotonin, which is released from
(TF), a molecule able to trigger the coagulative activated platelets contributing to platelet
cascade [2]. Fibrinolysis is a further step of the aggregation [11]. Moreover, a further mechanism
system because its role consists in the digestion that enhances platelet aggregation is that of
of the clot [3]. All these actions are under the thrombin, the final protease of blood coagulation.
control of natural anticoagulants and fibrinolysis Thrombin cleaves two Protease-Activated
inhibitors. This system is always working, such Receptors (PARs) on platelets, thus starting a cell
as a car engine idles, since fibrin deposition and signaling process that results in platelet granule
dissolution in the endothelium is a continuous secretion, integrin activation and a cytoskeleton
phenomenon. It is ready to greatly accelerate remodelling, i.e. the Phosphatidylserine (PS)
its function when either bleeding or a vascular exposure. PS translocation to the outer leaflet, in
occlusion occurs. In this review, we will describe turn, contributes both to the absorption of blood
the hemostatic function of platelets and blood coagulation factors and to the release of the stored
coagulation, followed by that of fibrinolysis. ones, thus facilitating further thrombin formation
[12]. PS-exposing platelets also shed extracellular
Primary hemostasis vesicles (microparticles) [13], which have the
opportunity to further amplify the hemostatic and
Primary hemostasis is due to platelets, coagulative properties of platelets (Fig. 1).
which are small fragments that come from
megakaryocytes, large cells located in the bone A.
marrow [4]. Normal platelet count ranges from
150 to 450 x 109/L, and they live in the circulation
for about 10 days [5]. Normally, platelets do not
adhere to surfaces or aggregate within the blood
vessels. However, if an endothelial damage
occurs, they are ready to start a contact with the
sub-endothelial matrix leading to adhesion and
aggregation [6]. Platelets receptor GPIb-IX-V
binds to A1 domain of von Willebrand Factor
(vWF), a large multimeric protein released from
endothelial cells and megakaryocytes that is
present both in plasma and in the sub-endothelial B.
matrix [7]. Another receptor dedicated to platelet
adhesion is GPVI, which binds to the collagen of
the sub-endothelial matrix at the side of injury [8].
Adhesion of platelets to the endothelial matrix
leads to their activation, which consists in a
conformational change of αIIbβ3. It is an integrin,
that exposes, after clustering on the membrane,
binding sites for fibrinogen, vWF, collagen and
fibronectin, thus inducing platelet aggregation Figure 1. Primary haemostasis (A) and clot formation
(B). After endothelium injury, adhesion and aggregation
[9]. Other integrins are involved in this pathway of platelets mediated by von Willebrand Factor and
but have a less important role. However, a fibrinogen, respectively, occur. A small amount of
further activation of platelets for an optimal thrombin is concomitantly produced by Tissue Factor
aggregation and formation of the hemostatic exposed by fibroblast cells of the sub-endothelial space
and Factor VII. Thrombin is able to enhance its production
plug is required. This activation is induced by
by activating Factors XI, V, VIII and platelets. The final
agonists released by platelets themselves, such outcome is a stable clot that reinforces the first hemostatic
as Adenosine Diphosphate (ADP), which binds platelet plug. Red Blood Cells are involved within the clot.

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Blood coagulation Thrombin, which is able to induce also the

activation of FVIII and FV, further enhances
Blood coagulation in vivo is triggered by TF, the coagulation burden and it is now ready to
a 47-kDa type 1 transmembrane glycoprotein, activate fibrinogen to fibrin, an insoluble clot
abundant in the sub-endothelial space, which for preventing blood loss and inducing wound
forms a complex with Factor VII (FVII) once the healing [18].
endothelial barrier is interrupted [14]. A small A negative feedback is provided by a series
amount of thrombin is then formed, which in of natural anticoagulants, which control blood
turn amplifies the coagulation cascade activating coagulation, avoiding an excess of activity.
Factor XI (FXI) [15] on the one hand and platelets Antithrombin and the system of Protein C are the
on the other. Thrombin is the product of the main actors that limit the activation of thrombin,
coagulation cascade activated by the complex TF- thus obtaining a perfect balance between blood
FVII that activates Factor X (FX). However, FX is coagulation activation and its inhibition. First,
also activated by Factor IX (FIX), which belongs thrombin becomes an anticoagulant factor
to the waterfall intrinsic activation of blood after binding to Thrombomodulin (TM), an
coagulation involving Factors XII (FXII) and FXI. endothelium-bound glycoprotein. Once this
This enzymatic reaction is greatly accelerated by complex is formed, thrombin loses its capacity to
Factor VIII (FVIII). In other words, a loop named transform fibrinogen into fibrin on the one hand
“the Josso’s triangle” is therefore formed [16]. but activates Protein C, a Vitamin K dependent
Many years after, a positive feedback has been protein, on the other. Activated Protein C binds to
discovered: the TF-FVIIa and TF-FVIIa-FXa Protein S, another Vitamin K dependent protein,
complexes are able also to activate FVIII. This forming a new complex, which inactivates FV
mechanism can, therefore, enhance the active and FVIII proteolytically, so downregulating
FVIIIa-FIXa (intrinsic Xase) confirming the thrombin formation [19]. Secondly, antithrombin,
existence of the Josso’ triangle [17] (Fig. 2). a member of the serine protease inhibitors,
FXa is able to activate prothrombin to thrombin inactivates multiple coagulation factors, mainly
with the acceleration of Factor V (FV) (the thrombin and FXa, and to a lesser extent FIXa,
prothrombinase complex). Platelet activation is FXIa, and FXIIa [20] (Fig. 3). Third, the TF
an essential step since it provides a phospholipid pathway inhibitor limits the activity of the
surface (phosphatidylserine) on which the TF-FVII complex, the main trigger of blood
adsorption of clotting factors is optimal. coagulation in vivo [21].

Figure 3. The main negative feedbacks of blood coagu­

lation. Antithrombin inactivates mainly thrombin and Factor
Xa, and to a lesser extent Factor IXa, Factor XIa, and
Factor XIIa. Thrombin becomes an anticoagulant factor
Figure 2. The waterfall enzymatic activation of blood after binding to Thrombomodulin (TM); once this complex is
coagulation. A pivotal role is that of the Factor Xa formed, thrombin loses its capacity to transform fibrinogen
activation by both the complexes Tissue Factor-Factor into fibrin on the one hand but activates Protein C, on
VII and Factor IX-Factor VIII (the Josso’s triangle). Fibrin the other. Activated Protein C binds to Protein S, forming
is the final product. a new complex that inactivates Factor V and Factor VIII
TF: Tissue Factor, II: Prothrombin, IIa: Thrombin, PL: proteolytically so downregulating thrombin formation.
Phospholipids. AT: Antithrombin; IIa: Thrombin; TM: Thrombomodulin.

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The role of Vitamin K

Vitamin K has an important role in blood

coagulation as an anti-haemorrhagic factor.
Vitamin K (Koagulation) is a fat-soluble vitamin
identified by the Danish biochemist Henrik Dam
in 1934 [22] after he demonstrated that chicks fed
with a low-fat diet showed a hemorrhagic disease.
Dam noticed that this vitamin was present in
abundance in pig liver fat but also in plants.
Vitamin K identifies a group of lipophilic and
hydrophobic compounds belonging to the class
of 2-methyl-1, 4 naphthoquinone derivatives.
In humans, the main source of Vitamin K is Figure 4. The role of Vitamin K in blood coagulation.
represented by phylloquinone (Vitamin K1) Vitamin K induces the carboxylation of glutamate
contained in vegetables such as spinach, cabbage, residues of proteins (Gla residues) to form gamma-
and broccoli and in fruits such as kiwi and bananas. carboxyglutamate. The carboxylated Gla residues in
this way can bind Ca ions, which are essential for the
Cooking food does not significantly reduce Vitamin coagulability of four factors of the coagulation cascade (II,
K content. Vitamin K2 (menaquinone) is produced VII, IX, and X) on the phospholipid membranes.
by intestinal bacteria, but it is uncertain whether
it contributes to the requirement of this vitamin in
humans. The adequate intake (AI) of Vitamin K 1.5-4.5 mg/mL. It consists of two D and one E
ranges from 2.0 and 2.5 µg/day in infants of 0-6 domains, which contain a pair of three disulfide-
months and 7-12 months, respectively, while the linked chains (Aα, Bβ, and γ). The passage from
amount of this vitamin raises to 30-55 µg/day in fibrinogen to fibrin is a crucial step in blood
infants between 1 and 8 years. In adults, the AI coagulation because it is essential for stopping
ranges from 90 and 120 µg in women and men, the bleeding but also in the pathophysiology of
respectively [23]. vascular obstruction and thrombosis. Thrombin,
But why is Vitamin K so important in preventing coming from the activation of its zymogen,
bleeding? prothrombin, cleaves fibrinopeptides A and B
Vitamin K induces the carboxylation of from the N-terminal portions of the Aα and
glutamate residues of proteins (Gla residues) to Bβ chains of fibrinogen, thus inducing the
form gamma-carboxyglutamate. The carboxylated formation of fibrin monomers, which in turn go
Gla residues in this way can bind Ca ions, which toward polymerization [27]. At the next non-
are essential for the coagulability of four factors of enzymatic step, monomeric fibrin self-assembles
the coagulation cascade (II, VII, IX, and X) on the spontaneously to form fibrin oligomers that
phospholipid membranes [24] (Fig. 4). This step lengthen to make two-stranded protofibrils. They
is also crucial for the function of other proteins put themselves both laterally and longitudinally to
involved in the control of blood coagulation form fibers that branch to yield a three-dimensional
activity, such as Protein C, Protein S, and Protein network [28]. The final outcome is the fibrin I
Z, natural anticoagulants. However, Vitamin K is formation characterized by the interaction of one
also important for the function of proteins involved E domain of one molecule with the D domain of
in bone metabolism, such as osteocalcin, periostin, another, generating a fibrillary pattern so that a
and the matrix Gla protein [25]. The discovery clot starts to develop. Furthermore, thrombin
of Vitamin K was crucial for the development activates Factor XIII, a transglutaminase, which
of Vitamin K antagonists, i.e. coumarins is able to cross-link fibrin at lysine residues of
anticoagulants, whose use as antithrombotic drugs adjacent fibrin monomers, as well as the a-chains
is still widespread in the world [26]. of opposing monomers to form D-dimer and
a-polymers, respectively [29] (Fig. 5). This
Fibrin formation phenomenon, in the end, gives stability and
strength to the clot (fibrin II). Also, plate­ lets
Fibrin formation starts from fibrinogen, a 340- aggregate and Red Blood Cells participate since
kDa protein that circulates at concentrations of they become incorporated into its structure [30].

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of the fibrin network into soluble fragments, such as

(DD)E: a complex formed by of D-dimer coming
from cross-linked adjacent D domains (DD) non-
covalently bound to fragment E. Plasmin causes
proteolysis of fragment E from the (DD)E complex
[39]. D-dimer then circulates in plasma with a half-
life of about 8 hours, cleared by the kidneys and
the reticuloendothelial system [40]. Since D-dimer
comes from cross-linked fibrin, it can be considered
a marker of activation of the coagulative and
Figure 5. Fibrin formation and D-dimer production. Factor fibrinolytic systems [38].
XIII cross-links D domain to each other. Plasmin cannot
lyse the cross-linked D domain. D-dimer are then released
Conclusions
into the circulation.

The hemostatic system is complex, but in the

Fibrinolysis last years more knowledge has been reached on
this topic, especially in the field of bleeding and
Fibrinolysis deals with the dissolution of the thrombosis. To know how this system works is
clot being a defensive system devoted to prevent of paramount importance for understanding the
unnecessary accumulation of intravascular fibrin mechanisms underlying several pathological
[31]. Plasmin is the pivotal protease of fibrinolysis conditions such as inflammation, cancer, bleeding
and is activated from plasminogen by either and thrombosis.
tissue Plasminogen Activator (tPA) or urokinase Another important point is that referred to drugs
Plasminogen Activator (uPA). tPA is synthesized and employed either as hemostatic or antithrombotic
released by endothelial cells, while uPA is produced agents.
by monocytes and urinary epithelium [32]. They
have a short half-lives (4-8 minutes) because of the Declaration of interest
presence of potent inhibitors such as Plasminogen
Activator Inhibitors (PAI-1 and PAI-2) [33]. The The Authors declare that there is no conflict of interest.
role of these inhibitors is devoted to avoid an
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