T2: Blood Physiology (Lecture 6)

blood clotting

-process by which blood is transformed into a solid gel called a clot, or thrombus, and consists of a protein called fibrin.

-prevents excessive bleeding when blood vessels are injured.

essential part of the clot

formation of fibrin

Function of blood clotting

to support the plug and to solidify the blood left around the wound site

secondary hemostasis

-Occurs following a platelet plug formation

-Involves a cascade of enzyme (clotting factors) activation

-Activation of enzymes occur by proteolytic cleavage

-Formation of gel-like fibrin clot, in which red bloods cells become trapped and is called a red thrombus

What cellular element can clotting not occur in the absence of?

platelets, several reaction cascades happen on their surfaces

-clotting can occur in the absence of any other cellular element

Number of factors involved in blood clotting

13

Clotting/coagulation factors

-all present as inactive plasma proteins in blood

-expressed as Roman numerals (order of discovery)

-synthesized in the liver

-many of the activation rxns take place on platelet surfaces.

clotting factors which need vitamin K for synthesis

II, VII, IX, X (2, 7, 9, 10)

Which clotting factor is not a protein factor?

Factor 4 (IV), it is ionized calcium

List the first 4 clotting factors (most important ones)

I (1) - fibrinogen

II (2) - prothrombin

III (3) - tissue factor/tissue thromboplastin

IV (4) - calcium

Which factor is not used for clotting?

VI (factor 6)

Key Step in clot formation pathway

Generation of the active thrombin enzyme

prothrobinase complex

by which inactive enzyme prothrombin is converted to thrombin

thrombin

-active enzyme

-converts the inactive protein fibrinogen to the active form fibrin (long fibrin strands)

fibrin

- insoluble protein

-upon stabilization becomes strong strands and forms a mesh-work, or the fibrin clot, in which cells get trapped

Factor Xa

-common activated factor

-directly involved in formation of active thrombin

2 pathways clotting factors are activated to form factor Xa

intrinsic pathway and extrinsic pathway

-Both meet & activate the common factor in the common pathway leading to the formation of a blood clot

Intrinsic Pathway (Factors)

all factors come from within the blood & the blood vessel itself

extrinsic pathway (Factors)

initiated by some factors outside the blood vessel

3 interconnected pathways

intrinsic, extrinsic, and common

What triggers the intrinsic pathway

triggered by internal trauma or damage that occurs within the blood vessels themselves from:

1. endothelial cell injury

2. exposure to subendothelial collagen

3. contact w foreign or artificial surfaces

endothelial cell injury (1)

-damage to the thin layer of cells (endothelial cells) lining the inside of blood vessels

-can occur due to high blood pressure, inflammation, or vascular disease

exposure of subendothelial collagen (2)

-endothelial layer is disrupted, revealing the collagen underneath

-Subendothelial collagen is a structural protein located beneath the endothelial cells lining the inside of blood vessels & is normally hidden from the BS by the intact endothelial layer

-once the endothelium is compromised, collagen becomes exposed to circulating blood and initiates clotting.

Contact w foreign or artificial surfaces (2)

triggers intrinsic pathway

-exs. medical devices like catheters, heart valves, or synthetic implants that come into contact with blood

Initiation of Intrinsic pathway

when Factor XII (12) encounters negatively charged surfaces and is activated

Activation in vivo (Intrinsic pathway)

-occurs when blood is exposed to collagen fibers underlying the endothelium in the blood vessels

-collagen has a negatively charged surface

Activation in vitro (Intrinsic pathway)

-occurs when blood is exposed to foreign surfaces (ie glass test tube, synthetic plastics)

-uncoated or plain glass in a test tube has a naturally negatively charged surface

Significance of negatively charged surface

can convert factor XII (12) into its active form

--ve charged surfaces resembles a damaged blood vessel

What triggers the extrinsic pathway?

requires exposure to tissue factor (TF/factor III) which is located outside the bloodstream or external to the blood by external trauma

tissue factor/factor III (3)

a transmembrane protein found on cells outside the vascular endothelium

external trauma

-physical injury that damages tissues outside the blood vessels

-causes rupture of the vessel wall, allowing blood to leak into the surrounding tissues (removes barrier that normally keeps TF and blood separate)

Significance of blood able to leak into surrounding tissues

-Factor VII (7) normally circulates in blood in its inactive form

-comes into contact with TF on the damaged extravascular cells, binds to it and becomes activated factor VII

TF-VIIa complex/Factor VIIa (7)

activates Factor X in the presence of phospholipids exposed on activated platelets and calcium

Beggining of Intrinsic pathway

activation of factor XII (12) when comes into contact w negatively charged surfaces to form XIIa

contact activation factor

Factor XII (12)

Silicon coating on glass test tube

-silicon neutralizes the -ve charge, so blood does not clot in vitro and therefore doesn't initiate the intrinsic pathway

Key to the extrinsic pathway

tissue factor (Factor III)

Diffrentiate between activation of the intrinsic pathway & extrinsic pathway

Intrinsic: damage of blood vessels

Extrinsic: tissue damage, not only blood vessel wall damage

key enzyme that starts the common pathway

Factor Xa (Intrinsic & Extrinsic pathways merge at formation of active factor 10 and lead to formation of active thrombin in the common pathway)

prothrombinase complex (Details)

-Factor Xa binds w Factor Va, calcium ions (Ca²⁺), and phospholipids on the surface of activated platelets

-catalyzes the transformation of prothrombin into thrombin

Stable fibrin clot

formed by factor XIIIa (13) cross-linking fibrin strands

Clotting cascade function/Why so many steps?

-allows for strong amplification of initial signal in response to tissue damage and bleeding

-The number of steps decreases the possibility of accidental activation of the clotting process

Under normal physiological conditions, how is clotting initiated?

Clotting is initiated in the extrinsic pathway when Factor VII (7) is activated in the presence of TFs which are exposed when a cut occurs and are found immediately outside the cut site

Factor VIIa (Activated factor 7)

-produces small amount of active factor Xa, which activates a small amount of thrombin and results in the formation of a small amount of fibrin.

Amount of thrombin produced

Small amount of thrombin produced from the extrinsic pathway is too small to produce adequate sustained coagulation

Feedback effect of thrombin

- initial small amount of thrombin provides a powerful feedback effect to many of the factors of the intrinsic pathway, including factors V, VIII and XI and factor XIII.

-initiates the further formation of thrombin, thereby stimulating its own formation.

-activated intrinsic pathway produces a large amount of thrombin, and produces a large fibrin clot.

Intrinsic & extrinsic pathways link

-thrombin is the link between them

-pathways are activated sequentially

deficiency in factor VII (7)

causes serious bleeding

(this factor initiates clotting)

deficiency in factor VIII (8)

Inds lacking factor VIII experience severe bleeding and are known as hemophiliacs

deficiency in factor XI (11)

causes moderate bleeding

deficiency in factor XII (12)

-causes no bleeding problem in vivo

-causes no blood clot in the glass test tube in vitro

-clotting is not initiated by this factor

blood from healthy individual

will clot when placed in a glass test tube with no anticoagulant (silicon)

-When blood touches glass, Factor XII (12) is activated & initiates clot formation in glass test tube

-When glass is coated by silicon, the surface of the glass becomes too smooth to trigger the activation of factor XII. (Hence factor XII does not initiate the intrinsic pathway)

Properties of thrombin in the clotting pathway (4)

-Activation of platelets

-Converts soluble fibrinogen into insoluble fibrin

-Activates several other clotting factors, including factor V, VIII, XI and XIII

-Plays a role in anti-clotting pathways (anti-coagulant activity) by activating protein C

anticoagulants

prevent clot formation where and when it is not required

fibrinolysis

the enzymatic breakdown of the fibrin in blood clots as tissue repair occurs

-role of fibrinolytic system

Anticoagulants in our body

TFPI/tissue factory pathway inhibitor, antithrombin 3, thrombomodulin, protein C & S

TFPI/Tissue factory pathway inhibitor

-inhibits TF (factor III)

-TF initiates the extrinsic pathway of clotting as it is needed to activate factor VII (7), which activates factor X (10)

antithrombin

inhibits the actions of thrombin

thrombomodulin

- protein expressed on surface of undamaged, healthy endothelial cells

- binds to thrombin & activates protein C to prevent formation of a clot

-Activated protein C inhibits Factors VIIIa and Va

Protein C & S

-Bound thrombin activates inactive protein C to form active protein C.

-Active Protein C interacts w protein S & they inhibit factors Va (5a-formation of thrombin) & factor VIIIa (8a-formation of factor Xa).

Calcium chelators

-bind ionized calcium, removing the free ionized calcium & preventing the activation of clotting factors in the clotting pathways

-works in vitro

-ex. sodium citrate

Heparin (Clinical Anticoagulant)

increases the activity of antithrombin 3

-works in vitro & in vivo

Vitamin K antagonists

inhibit the synthesis of certain clotting factors that are dependent on vitamin K in the clotting pathway (factors II, VII, IX and X: 1, 7, 9, 10)

-works in vivo

tissue plasminogen activator (tPA)

-a plasminogen activator

-released from healthy endothelial cells

How do plasminogen activators break down a clot (fibrinolysis)

-tPA (tissue plasminogen activator) converts inactive plasminogen to the active enzyme plasmin

-Plasmin breaks down insoluble fibrin strands into soluble fibrin degradation products to dissolve the clot

clinical clot busters/thrombolytic drugs

-Used to treat patients with heart attacks to dissolve clots

- Thrombolytics work by dissolving a major clot quickly to restart blood flow to the heart and helps prevent damage to the heart muscle

When pro-hemostatic factors fail:

-Severe bleeding disorders which lead to hemorrhage may occur -Failure of hemostatic mechanisms when they ARE required

-Defects in pro-hemostatic pathways may including problems with platelets and clotting factors

problems with platelets

1. Thrombocytopenia: deficiency in numbers of platelets

2. Abnormal platelet function: due to a deficiency of von Willebrand's factor (plasma protein secreted by endothelial cells and platelets that promotes adhesion by binding to platelets and exposed collagen molecules in damaged blood vessels)

Problems with clotting factors

1. Hereditary deficiencies

2. acquired deficiencies: vit K deficiency causes deficiencies of those factors that require it

When anti-hemostatic factors fail:

-Blood clots remain in the circulation for a long time leading to thrombosis (formation of blood clot in the vessel, blocking blood flow)

-Formation of blood clot when they are NOT required

-Includes hereditary disorders & acquired disorders

hereditary disorders

Deficiencies of natural anticoagulants and fibrinolytic factors may cause thrombosis

acquired disorders

may also lead to the formation of blood clots

ex. decreased blood flow