Haemostasis and Control of Coagulation

Question 1

Hemostasis

Answer 1

For our purposes here: Small venules, small arterioles- small vessels where you might get damage
Events:
cellular & biochemical (clotting cascade)
Preventing blood loss or preventing haematoma formation internally

Question 2

Hemostasis balance

Answer 2

Balance between coagulation and fibrinolysis- need both things to happen. Keeping a clot under control.

Question 3

Fibrin

Answer 3

Stabilizes clot at the site of injury. A net that covers the platelets when they are activated

Question 4

Coagulation Pathway

Answer 4

Produces fibrin.

Question 5

Blood flow past injury

Answer 5

Brings in the factors we need and take other things away

Question 6

Stopping the bleeding in short

Answer 6

1. Injury damages vascular endothelium and exposes sub-endothelial collagen
2. vWF cements platelets to collagen to form a cellular plug
3. Fibrin deposits on platelet plug and crosslinks
4. Plasmin digest fibrin clot and endothelium regenerates

Question 7

Von Willebrand Factor

Answer 7

Reinforcing the blood. Blood glycoprotein. On the surface of the platelet.
Sub endothelial collagen binds vW factor
Have a high avidity- strength of binding- very strong bond- multivalent

Question 8

Tissue injury

Answer 8

1. Vasoconstriction (slow blood flow)
   - neural
   - platelet reinforced
2. Platelet activation
   - adhesion
   - aggregation
3. Coagulation
   - blood clot: thrombin generation and fibrin polymerization (activation of thrombin produces fibrin)
4. Fibrinolysis- blood clot dissolution
5. Vascular patency restored

Question 9

Why do we need to maintain blood flow?

Answer 9

Regulation of pH, transport (O2, nutrients, CO2, waste)

Question 10

Vascular constriction

Answer 10

Limits blood flow. Results from:

• inherent vascular response to injury
• release of activators by platelets
• Sympathetically induced

Question 11

Platelets contain?

Answer 11

Store secretory products in granules
High concentrations of actin and myosin (contraction of the cell- contract and solidify blood clot or platelet plug)
** Calcium- you need for contraction of actin and myosin.
(important for activation, adhesion, aggregation)

Question 12

What are platelets?

Answer 12

Tissue injury, exposure of collagen, binding of platelets by von Willebrand Factor (on cell membrane of platelets), Platelets release a number of factors (activators)

Question 13

Plates release?

Answer 13

Serotonin (vasoconstriction), Growth factors to stimulate the endothelial cells to replace themselves, factor V (20%), Factor XIII (alpha subunit), Platelet factor IV (heparin binding activity and chemotactic agent for neutrophils and monocytes), Lipoproteins, Thromboxane A2, ADP

Question 14

Pathways of platelet aggregation

Answer 14

Activated in response to collagen, vWF, and tissue factor when there is injury to endothelial lining.

• a host of other stimuli such as ADP, 5 HT, epinephrine, thromboxane A2, can initiate aggregation via specific receptors.
• the end point is the rise in cytosolic Ca2+ which induces platelet activation/ aggregation
• Externalization of glycoprotein Gp IIb/IIa receptors cause fibrin bands to bind to different platelets
• Aspirin interferes with the conversion of AA to TXA2 and inhibits rise of cellular Ca2+ and subsequent platelet aggregation

Question 15

Altered endothelial surface =

Answer 15

collagen exposed. Platelets activated and aggregation- discharge of mediators and synthesis of thromboxane A2… eventually vasoconstriction

Question 16

What limits platelet aggregation?

Answer 16

Need inhibitors so the clot doesn’t form on normal endothelial cells.
ADP is in the granules of the platelets- production of ADP–> acts on normal endothelium on either side of the injry. Platelets themselves release ADP- induces them to produce prostacyclin and nitric acid. INHIBIT PLATELET AGGREG.

Question 17

Platelet plug physically seals endothelial injury but also??

Answer 17

Increase cytosolic Ca2+ therefore actin and myosin contracting which compacts and strengthens the plug
Release of a number of chem. e.g. serotonin, epinephrine, thromboxane A2 (vasoconstrictors)

Question 18

Coagulation

Answer 18

Transformation of liquid to solid gel, clot formation provides strength and support to platelet plug.
Ultimately results in the conversion of fibrin from fibrinogen by thrombin. Formation of meshlink that traps blood components.

Question 19

What converts fibrin from fibrinogen?

Answer 19

Thrombin localized to platelet surface that you get reinforcement of the clot

Question 20

Intrinsic and extrinsic coag pathways generally?

Answer 20

Happen together, in parellel, in vivo
Ultimately result in the cleavage of fibrinogen (soft clot)
Cross linked by enzyme transglutaminase

Question 21

Ca importance?

Answer 21

Actin and mysoin inside the platelets. AND important on the outer surface of the platelets to capture and allow thrombin complex to bound to the outside of the platelets.

Question 22

Activation of platelets causes?

Answer 22

Expression of negatively charged phospholipids and attachment of Ca2+ ions to the surface

• some of the clotting factors attach to calcium ions (VII, IX, X, prothrombin, protein C)
• amino terminal domains contain gamma carboxyglutamate resides (gla)– DEPENDENT ON Vitamin K in the liver. No vit K= no coagulation.

Question 23

What is dependent on vit K?

Answer 23

Gamma carboxy glutamate.

Question 24

Zymogen activation

Answer 24

Each protein is produced in a pro form. Not activated in an enzyme sense until it is cleaved by another protease. e.g. instrinsic pathway: Factor 11 is cleaved by Factor 12a for example. subscript “a” means activated.

Question 25

Intrinsic pathway

Answer 25

Activated by injury
Expresses collagen- activates extrinsic- final common pathway
** IMP for us: Cleavage of thrombin to end up with fibrin (from fibrinogen)

Question 26

Intrinsic pathway

Answer 26

• Within damaged blood vessels
• 7 steps set off by factor XII (hageman factor)
• activated by negatively charged surfaces e.g. collagen
• occurs simultaneous with platelet plug formation
• platelets secrete PF3- essential for clotting cascade; enhanced platelet aggregation (Ca ion binding to neg. charged phosphate ions on surface of platelets)

Question 27

Phospholipid (PF3)

Answer 27

Expressed by activated platelets, surface phospholipid, acceleration of coagulation cascade- promotes platelet recruitement, aggregation, fibrin formation

Question 28

Extrinsic pathway

Answer 28

4 steps, requires contact with tissue factors, injured tissue releases a protein complex- tissue thromboplastin
Directly activates factor X

Question 29

Thrombin

Answer 29

• Generated from pro-thrombinase complex (produced in pro form and need to activate this complex by cleaving bits off the end)
• consists of Xa, II (prothrombin), Va bind via Ca2+ to anionic phospholipids on platelet membranes
• Ca2+ binding mediated by Gla domains (gamma carboxyglutamate)- the domain where you get change in glutamate residues where you get change and it binds calcium– because of the extra carboxyl group

Question 30

Gamma carboxyglutamate residue

Answer 30

Needs vitamin K. (Warfarin outcompetes vit. K, therefore you don’t get blood clotting- because it is structurally very close to vitamin K- all about not producing gamma carboxyglutamate- so you can’t bind calcium- it will still be there but cannot bind to the site of injury)… platelets will be there but they are not strong enough

Question 31

Thrombin- pro coagulant activities?

Answer 31

Stim. conversionfibrinogen to fibrin to form stabilized meshwork
Activates factor 13 itself
Activate itself (thrombin activate prothrombin)
Enhances platelet aggregation
Induces secretion of platelet factor 13 (phospholipid)
** cyclic amplification of the system
(monomers of fibrinogen- thrombin actually acts to cleave off bits and therefore they are able to polymerize and form their meshwork)

Question 32

Thrombin anti-coagulant activities?

Answer 32

Activates protein C, which inactivates clotting factors VIIIa and Va (8 and 5)

Question 33

Cross linking of Fibrin

Answer 33

Initial soft plug is characterized by hydrogen bonding, cross linking of fibrin after it has been cleaved, you get much stronger cross links happening mediated by transglutaminase (factor XIIIa).
The covalent bonding– A lot stronger than hydrogen bonding

Question 34

Less thrombin and more plasmin?

Answer 34

Haemorrhage

Question 35

More thrombin and less plasmin?

Answer 35

Thrombosis

Question 36

What limits clot formation to the site of injury?

Answer 36

Clots must rapidly form but be confined to the area of injury
Labile clotting factors- diluted by blood flow, removed by liver, anticoagulant mechanisms (tissue factor pathway inhibitor (TFPI), thrombodulin, anti-thrombin III)

Question 37

Anti coagulant mechanism Antithrombin III

Answer 37

Inactivates thrombin by forming a complex with it (irreversible). Enhanced/stabilized by heparin. Also blocks other factors XIa, IXa, Xa. (thrombin no longer able to participate in coag. cascade)

Question 38

Thrombomodulin

Answer 38

Forms complex with thrombin, thrombomodulin and Ca2+ act as co-factors for thrombin’s activation of protein C
Protein C and Protein S inactivate factors Va and VIIIa through proteolytic cleavage

Question 39

Thrombomodulin

Answer 39

On endothlial cell surface. Bind thrombin. Through Protein C activated which inhibits other clotting factors. Happening on HEALTHY endothelium. Therefore you won’t get too much clot formation.

Question 40

Plasmin

Answer 40

Dissolves clot (this would occur when intact endothelial cells are present underneath) (endothelial cells release plasminogen, which is activated to plasmin)

Question 41

Fibrinolysis

Answer 41

Plasminogen is inactive zymogen of plasmin.
Plasmin degrades fibrin into peptides, dissolving the blood clot.
Plasminogen is activated by tissue plasminogen activator (tPA) only when bound to fibrin.
Thus fibrin promotes its own breakdown.
** activator produced by endothelial cells AND we need plasminogen to bind to fibrin (this is only activated when bound to fibrin and only activated when bound to a clot)