Week 6: Hemostasis

What is Hemostasis?

formation of blood clot at the site of vessel injury to stop the blood flow

What are the 2 stages of hemostasis called?

1. primary hemostasis

2. secondary hemostasis

What is the difference between 1° vs 2° Hemostasis?

1° - rapid formation of a temporary platelet plug at the site of injury

2° - reinforcement of the platelet plug through coagulation cascade to form fibrin network

they happen vert quickly and almost at the same time

What is another name for platelet?

thrombocyte

Define thrombocytopenia

low platelet count

What are the 2 reasons someone could develop thrombocytopenia?

1. something is consuming or damaging the platelets

2. problem with bone marrow where platelets are produced

What is the average life span of a platelet?

about 7-10 days

What are 3 interesting facts about the structure and function of platelets? (what can they NOT do)

1. no nucleus

2. can’t make new proteins

3. can’t recover from irreversible inhibition of COX1

Describe the 6 steps of Primary Hemostasis?

1. injury

2. adhesion of platelets

3. interaction of vwf to platelets

4. activation of platelet

5. aggregation

What does injury of the blood vessel lead to? (STEP 1)

injury to endothelial cells of the blood vessel leads to exposure of subendothelial components to the blood

What is the subendothelial space/tissue?

the space outside of the blood vessel that contains molecules that are sequestered from the blood

What are the 2 factors located in subendothelial space that are exposed to the blood through injury?

1. TF (tissue factor)

2. collagen

What occurs during adhesion of platelets to the site of injury? (STEP 2)

platelets gather around the site of injury

vwf binds collagen

vwf (+collagen) binds its receptor on the platelet

What is vwf?

von willibrand factor → large protein made by endothelial cells that helps platelets stick together and form a plug at site of injury

Describe what happens when vwf interacts with platelets during adhesion? (STEP 3)

the binding of vwf + platelets leads to Ca⁺⁺ and ADP release from the platelet

What are Ca⁺⁺ and ADP important for?

Ca⁺⁺ = important for 2° hemostatis coagulation cascade

ADP = signaling molecule that binds receptors on platelets to activate them

Which receptor does ADP act on?

acts on P2Y₁₂ receptor located on the platelet

What happens during platelet activation? (STEP 4)

after ADP binds P2Y₁₂ and activates platelet → the platelet changes conformation and leads to increased receptor expression and release of mediators from the platelet and the start of PLATELET AGGREGATION

What are the names of the 3 mediators released by platelets?

1. ADP

2. TXA-2 (thromboxane A2)

3. 5HT (serotonin)

Which receptor does TXA-2 bind to?

TP receptor

How is TXA-2 made and what does it do?

TXA-2 made by COX1 inside the platelet

binds TP receptor on the platelet

Describe the function of COX1

cycolooxygenase enzyme isoform that is found in platelets that is required to make TXA2

Which receptor does 5HT use?

serotonin receptor

Where is 5HT made and what does it do?

it is NOT made by the platelet

it binds 5HT receptor on platelet and is taken up and stored via SERT inside platelet

What is the overall function of these 3 mediators and what could happen if we had issues with them?

overall function: promote platelet plug formation

if had issues with them → we would have decreased platelet formation

What happens during platelet aggregation? (STEP 5)

aggregation = platelets clump together

when mediators are released, they bind their receptors on other platelets, leading to:

1. vasoconstriction

2. more activation of platelets

3. aggregation/clumping of platelets for plug formation

How does vasoconstriction help platelet formation?

vasoconstriction → decreased blood flow → decreased blood loss

What do we call the proteins that are involved in the coagulation cascade/2° hemostasis pathway?

clotting factors - denoted by roman numerals

What are the 3 parts to the coagulation cascasde?

1. intrinsic pathway

2. extrinsic pathway

3. common pathway

What are the names of the 3 cofactors used in these 3 pathways?

1. vwf: VIII_a (factor 8)

2. TF

3. V_a (factor 5)

What are the 4 factors that are involved in the INTRINSIC PATHWAY?

XII_a (12)

XI_a (11)

IX_a (9)

VIII_a (8)

What does the subscript ‘a’ denote?

tells us that we are talking about the ACTIVE form of the factor

(factors need to be converted to their active forms in the pathway)

What are the 1 factor involved in the EXTRINSIC pathway?

VII_a (7)

What are the 4 main factors involved in the common pathway and what is a pneumonic we could use to remember them?

“SMALL BILLS” = 10/5/2/1

X_a (10)

V_a (5)

II_a (2)

I (1)

Draw out the common pathway and include the factors and cofactors

factor X_a acts with V_a to promote prothrombin (II) conversion to thrombin (II_a)

II_a converts fibrinogen (I) to fibrin monomers

XIII_a (13) helps fibrin monomers to from fibrin crosslinks

What is the importance of fibrin crosslinks?

they are the “glue” that glues everything together to form an effective platelet plug for blood loss

Draw out the intrinsic pathway; include cofactors

XII_a → XI_a → IX_a

vwf:VIII_a promotes activation of X_a

common pathway starts: X_a + V_a → prothrombin (II) to thrombin (II_a ) → II_a catalyzes fibrinogen(I) to fibrin monomers → XIII_a catalyzes fibrin crosslink formation

Draw out the extrinsic pathway; include cofactors

VII_a binds TF → activation of X_a

X_a + V_a to convert II (prothrombin) to II_a (thrombin)

II_a converts I (fibrinogen) to fibrin

XIII_a catalyzes fibrin monomers to form fibrin crosslinks

Which factor can VII_a (7) activate in addition to X_a ?

factor IX_a in the INTRINSIC pathway

this tells us that XII_a and XI_a may not be super important for intrinsic pathway activation

Where are clotting factors made?

in the LIVER

Why is Ca⁺⁺ so important in 2° hemostasis?

Ca⁺⁺ binds the (-) charged phospholipids located on the platelets and on clotting factors

it serves as a bridge → connecting the clotting factors to the platelet surface

What is the name of the process that clotting factors undergo in order to make them become fully functional?

vitamin K dependent carboxylation at glutamic residues on clotting factors inside of the liver (REQUIRED to become FULLY functional)

What are the names of the 4 vit K dependent carboxylation clotting factors?

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

What are the names of the 2 vit K dependent carboxylation anti-coagulation factors?

C and S

What are coagulation tests?

they are TIMED tests that measure how LONG it takes to FORM a CLOT based on what is added (mimics NORMAL conditions)

If a coagulation test is longer than normal, what does this result indicate?

indicates there is a PROBLEM with CLOTTING (taking longer to clot)

What are the names of the 2 coagulation tests we typically do?

1. PT/ INR (prothrombin test/international normalized ratio)

2. _aPTT or PTT (partial thromboplastin time)

What does the PT coagulation test measure?

measures the time it takes to clot when you add substances that activate the EXTRINSIC pathway

Which 3 substances do we add in a PT test?

1. TF

2. phospholipids

3. Ca⁺⁺

Which 2 pathways are assessed by the PT test?

EXTRINSIC + COMMON

How should PT/INR change in response to a PROBLEM with the extrinsic or common pathway?

PT/INR will INCREASE and be LONGER than normal

takes longer time to form a clot

What does the PTT coagulation test measure?

measures the TIME it takes to form a clot when substance that activate the INTRINSIC pathway are added

Which 3 substances are added in a PTT test?

1. intrinsic pathway activation substances

2. Ca⁺⁺

3. phospholipids

Which 2 pathways are assessed by the PTT test?

INTRINSIC + COMMON

How should PTT change in response to a PROBLEM with the intrinsic or common pathway?

PTT INCREASES and is LONGER than normal (takes longer to form a clot)

What is the name of the other test that is done but is not actually termed a coagulation test?

Anti-X_a → measures the presence of a direct/indirect inhibitor of factor X_a

What are the names of the 3 major endogenous anticoagulants?

1. antithrombin III (ATIII)

2. thrombomodulin

3. tissue plasminogen activator (t-PA)

Where is ATIII synthesized?

the liver

Describe the mechanism of ATIII and the effect it has on clotting

ATIII inhbits X_a (10) and II_a in the COMMON pathway

this PREVENTS coagulation

What is thrombomodulin?

an endothelial cell receptor that binds thrombin II_a

What is the importance of thrombomodulin?

this receptor is important for preventing coagulation in stable areas when it is NOT needed

What does activation of thrombomodulin receptor by II_a lead to?

activation of PROTEIN C to ACTIVATED PROTEIN C (APC)

What is the role of APC and how does it carry out its role?

APC = anti-coagulant

binds PROTEIN S to form APC:S complex → this complex INHIBITS V_a (5) and VII_a (8) (cofactors in common and intrinsic pathway)

PREVENTS coagulation

What is the process C and S must undergo to be functional?

vit K dependent carboxylation in the liver

What is tPA?

tissue plasminogen activator

What is the role of tPA in clotting?

endothelial cell activaates tPA → tPA catalyzes the conversion of plasminogen (precursor) to plasmin

plasmin → DISSOLVES fibrin clot

What are the names of the 2 antiplatelet molecules made by endothelial cells?

1. nitric oxide (NO)

2. prostacyclin (PGI2) (made by COX2)

What are the effects of NO and PG12 on platelet function?

both VASODILATORS that prevent unnecessary formation of blood clots (prevents activation of platelets)

What is a pathological thrombus?

inappropriate formation of clots

What is the main cause of pathological thrombus?

statis of blood (stagnant blood)

What are 4 examples of blood stasis?

1. immobilization

2. prolonged sitting

3. atrial fibrillation (stasis in atria)

4. surgery

What are the 5 lifestyle related causes of pathological thrombus?

1. obesity

2. DM

3. smoking

4. trauma

5. oral contraceptives

How do oral contraceptives contribute to blood clot formation?

they contain estradiol → this increases hepatic clotting factor

Which pathological problem can trigger coagulation and is associated with antibodies?

antiphospholipid antibodies → neutralize phospholipids so clotting factors can’t bind platelets

What is the name of the mutation that can cause thrombus?

Factor V (5) Leiden

What is Factor V Leiden?

a mutation in factor V that renders it RESISTANT to APC:S inactivation → basically reduces the ability to turn off clotting