Hemostasis and blood coagulation

ATHAR KHALIL

4/19/2022

Guyton Chapter-37

Hemostasis and blood coagulation

Hemostasis Events
 Hemostasis means prevention of blood loss.
 Steps of hemostasis
1. Vascular constriction
2. Formation of platelet plug
3. Formation of blood clot due to coagulation
4. Eventual growth of fibrous tissue

1. Vascular Constriction
 After vessel gets ruptured the smooth muscles in vessels constrict and
instantaneously reduces the flow of blood from the ruptured vessel.
 Contraction results from
a. Local myogenic spasm
b. Local autacoid factors (platelets and vascular endothelium)
c. Nervous reflexes
d. Thromboxane A2 from platelets

2. Formation of the platelet plug

 Platelet/thrombocyte characteristics
1. Formed in the bone marrow from megakaryocytes, large cells which
divide into minute fragments in the bone marrow or while they
squeeze through capillaries.
2. Do not have nuclei and do not reproduce
3. Actin, myosin and thrombosthenin that cause the platelets to
contract
4. Residuals of ER and Golgi apparatus which synthesize various
enzymes and store large amounts of calcium ions.
5. Mitochondria and enzyme systems which form ATP and ADP
6. Enzymes that form prostaglandins, vascular reactions
7. Fibrin stabilizing factors,
 8. Growth factor, cause endothelial cells, smooth muscle cells and
fibroblasts to multiply and grow, eventually helps repair damaged
vascular walls.
9. The platelet membrane contains a coat of glycoprotein which repels
the endothelial lining during normal body activity, and gets
attached to the endothelial lining during tissue abrasion.
10. Half life is 8-12 days, removed by macrophages in the spleen.
 Mechanism of Platelet plug formation
1. Damaged vascular surface  irregular platelets forming
pseudopodia secrete contractile vacuoles  Stick to collagen
fibers and vWF  ADP and PAF secreted  form Thromboxane A2
 other nearby platelets activated  Clog  Contract  Fibrin
threads form  Platelet plug.
Importance of platelet mechanism
 Closes minute ruptures in vessels that occur many thousands of times
daily.
Blood Coagulation in vessel
 Clot begins to develop 15-20 seconds if the trauma is severe, and 1-2
minutes if the trauma is minor.
 Further retraction of the clot takes place.
Fibrous organization or dissolution of blood clots
 Either the clot becomes invaded by fibroblasts, which subsequently form
connective tissue all around the clot or it dissolves.
 A clot formed in a small hole is invaded by fibroblasts and converts to fibrous
tissue in about 1 to 2 weeks.
 Whereas, when excess bleeding takes place and unrequired clot is formed
then special substances functioning as enzymes dissolve the clot.
Mechanism of Blood coagulation
 Procoagulats  promote coagulation
 Anticoagulants  Inhibit coagulation
 More than 12 blood coagulation factors are involved and the net result is the
formation of a complex of activated substances called Prothrombin
activator.
 Prothrombin activator causes Prothrombin  thrombin
 Thrombin converts Fibrinogen  fibrin fibers that enmesh platelets (clot).
 Rate limiting factor in blood coagulation is the formation of prothrombin
activator the later processes take place rapidly.
 Platelets take part in prothrombin to thrombin conversion (platelets have
prothrombin receptors).
 Prothrombin is formed continually by the liver and needs vitamin K for
normal activation of prothrombin.
 Conversion of Fibrinogen to Fibrin
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  Fibrinogen is formed in the liver.
 Yet fibrinogen is leaked into the interstitial fluid in very less amount, if the
permeability of blood capillaries increases then more fibrinogen leaks and
may cause clotting as in blood and plasma.
 Action of Thrombin on fibrinogen to convert it to fibrin
 Thrombin catalyzes Fibrinogen to fibrin by removing 4 low-molecular-weight
peptides from each molecule of fibrinogen forming a molecule of fibrin
monomer.
 Initially the fibrin monomers are held together by weak noncovalent
hydrogen bonds.
 Afterwards, fibrin stabilizing factor is released by the platelets entrapped in
the clot, and is activated by thrombin which converts the weak bonding into
covalent bonding.
Blood Clot
 Clot is a meshwork of fibrin fibers entrapping blood cells, platelets, and
plasma.
Clot Retraction and Expression of Serum
 After 20-60 mins of clotting, the clot contracts and expresses most of the
fluid and the expressed fluid is called serum which cannot clot because it
lacks the clotting factors.
 Further fibrin stabilizing factor, actin, myosin, and thrombosthenin are
activated which further contract the clot, contraction causes the damaged
ends of vessels to come close and bind again.
Positive feedback of clot formation
 Thrombin has a direct effect on prothrombin, tending to convert it into still
more thrombin and further acts on clotting factors for prothrombin activator
formation.
 Thus, once a critical amount of thrombin is formed it boosts the coagulation
process.
Formation of Prothrombin activator
 Initiate due to
1. Trauma to the vascular wall or adjacent tissue
2. Trauma to the blood
3. Contact of blood with collagen fibers or elements outside the blood
vessels.
 Prothrombin activator is generally formed in 2 ways Extrinsic pathway
(trauma to vascular wall and surrounding tissue), and Intrinsic pathway
(begins in the blood).
Extrinsic pathway for initiating clotting
 Traumatized tissue releases tissue factor or tissue thromboplastin
(phospholipids+lipoprotein complex) that functions as a proteolytic enzyme.

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  The lipoprotein part of tissue factor further complexes with blood coagulation
factor VII and calcium ions to form activated factor X (Xa).
 The factor X combines with tissue factor phospholipid in the presence of
factor V and forms prothrombin activator.

Intrinsic pathway for initiating clotting

 Activation of factor XII
 Release of platelet phospholipids
 Activation of factor XI (require kininogen and accelerated by prekallikrein)
 Activation of factor IX
 Activation of factor X (role of factor VIII)
 Factor XIII is missing in hemophilia, thus called antihemophilic factor
 Thrombocytopenia lacks platelets (clotting factor).
 Action of factor X to form prothrombin activator
Summary of blood clotting initiation
 Clotting occurs by both pathways simultaneously
 Tissue factor initiates extrinsic pathway whereas contact of factor XII with
collagen initiates intrinsic pathway.
 Extrinsic pathway can be explosive and rapid (15 secs in severe trauma)
whereas intrinsic pathway is relatively slow.
Intravascular anticoagulants
 Factors that stop coagulation
1. Smoothness of the endothelial surface preventing contact and
activation of clotting factors
2. Glycocalyx on the endothelium repelling platelets
3. Thrombomodulin protein binds thrombin and activates a plasma
protein C which inactivates factor V and VIII.
 Endothelial cells also produce prostacyclin and nitric oxide (NO) that
inhibit platelet aggregation and initiation of clotting. PGI2 and NO also
function as vasodilators.
 Antithrombin-heparin cofactor or antithrombin III remove thrombin from the
blood and stop coagulation.
 Formation of fibrin fibers from fibrin.
 Heparin
1. Powerful anticoagulant but low concentration.
2. Widely used as anticoagulant pharmacological agent.
3. Negatively charged conjugated polysaccharide.
4. Not effective alone but when combines with antithrombin III boosts
its effect by thousandfold
5. Produced by precapillary basophilic mast cells in connective tissue
of blood

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 6. The precapillary mast cells are mostly present near lungs and liver
because clots are formed in these veins oftenly due to slow passage
of blood.
Plasmin: lysis of blood clots
 Euglobin plasminogen (profibrinolysin) when activated forms plasmin
(fibrinolysin)
 Resembles trypsin, and digests fibrin fibres, factor XII, factor V, factor VII,
fibrinogen and prothrombin.
 Plasminogen is present within the cells of blood clot and the cells slowly
secrete tissue plasminogen activator which eventually converts plasminogen
to plasmin.
Excessive bleeding conditions
 Vitamin K deficiency (cause decreased prothrombin and clotting factors)
1. Most of the clotting factors are formed in the liver and any
damage in the liver may stop the production leading to over
bleeding
2. Liver carboxylase adds carboxyl group to the glutamic acid and
activates the clotting factors in the presence of Vitamin K,
deficiency of which does not form active clotting factors.
 Hemophilia
1. Exclusively in males
2. Abnormality or deficiency of factor VIII and IX
3. Genetically transmitted from the X chromosome
4. May be caused by novel mutation events
5. Factor VIII has 2 components, larger and smaller. The smaller
component is most important in intrinsic pathway and most of the
hemophilic cases are due to its absence.
6. Von Willebrand disease is caused due to the deficiency of the larger
component.
 Thrombocytopenia (platelet deficiency)
1. Bleeding is from small venules rather than from larger vessels as in
hemophilia
2. Skin shows petechiae, red or purple blotches (thrombocytopenic
purpura)
3. Causes are
i. Decreased platelet production
ii. Peripheral platelet destruction
iii. Pooling of platelets in the spleen
iv. Consumption of platelets in thrombi
v. Dilution of blood.
Thromboembolic conditions
 Abnormal clot in a blood vessel is called thrombus
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  Freely flowing clot is called emboli
 Thrombus formed in the left side or arteries may go to brain and kidneys
whereas one formed in the right side or veins may flow into the lungs and
cause pulmonary arterial embolism.
 May form due to slow passage of blood or roughened endothelial surface
 Can be prevented or cured by injecting tissue plasminogen activator.
 Disseminated intravascular coagulation is caused due to traumatized or
large number of dying tissues which release great amount of tissue factor
into blood and form small clots which combine together to form large clots.
Especially occurs in septicemia, where large amounts of bacteria, or bacterial
toxins (endotoxins) regulate the clotting mechanism.
Anticoagulants for clinical use
 Heparin
 Coumarins (warfarin) inhibit VKORC1 due to which active vitamin K is
diminished and the clotting factor levels falls, it does not completely stop
clotting but decreases the rate of clotting.
Prevention of blood coagulation outside the body
 Blood collected in siliconized containers often does not clot upto 1 hour or
more, because sides of the tube prepared with silicon prevents contact
activation of factor XII and platelets.
 Heparin acts both in vitro and vivo
 Various substances which can decrease the conc. of calcium ions can
prevent clotting e.g. oxalate compound
 Any substance that deionizes calcium can also be used e.g. negatively
charged citrate ion
Blood coagulation tests
 Bleeding time: bleeding lasts for 1-6 mins, if longer than clotting issue
present
 Clotting time: normal clotting time 6-10 mins,
 Prothrombin time: indicates concentration of prothrombin in blood, the blood
removed from patient is immediately oxalated, so that none of the
prothrombin can change into thrombin. Then a large excess of calcium and
tissue factor is added which starts the process of clotting again and the time
taken is called prothrombin time normal is 12 secs.
 Normal INR range is 0.9 to 1.3. high INR results in excessive bleeding,
whereas low INR shows presence of clot.

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