Platelet Plug Formation

Question 1

what are the three steps in hemostasis?

Answer 1

1. vasoconstriction
2. platelet plug formation
3. coagulation

Question 2

what are the three steps to platelet plug formation?

Answer 2

1. platelet adhesion
2. platelet activation
3. degranulation? check the powerpoint from review
4. platelet aggregation

Question 3

what happens during platelet adhesion?

Answer 3

usually endothelial cells lining the blood vessel express molecules that inhibit platelet adherence

but when a vessel is injured, normal endothelial cells are damaged/removed which exposes the subendothelial collagen fibers to the inside of the vessel

endothelial cells release vWF which binds to the exposed collagen

circulating platelets rolling through the blood vessel begin tethering on vWF-immobilized collagen through a very weak but fast interaction

Question 4

what allows for circulating platelets to start attaching to vWF-immobilized collagen when there’s a blood vessel injury?

Answer 4

1. damaged endothelial cells release vWF which binds to exposed collagen
2. platelets express a protein called GPIb, which binds with vWF

this allows for platelet adhesion

the combination of collagen, vWF, and GPIb allow for platelet adhesion in the first step of plug formation

Question 5

What vessel inhibitors normally prevent platelet adhesion when there isn’t any injury?

Answer 5

1. Nitric oxide
2. endothelial ADPase
3. prostacycline (PGI2)

Question 6

what happens during platelet activation?

Answer 6

after platelets adhere, they have to be activated to form a plug

platelet activation initiates when the subendothelial collagen binds to GPIb via vWF

activation triggers exocytosis of dense granules and alpha granules, leading to release of ADP, vWF, thrombin, and other chemical
modulators (including thromboxane A2, PDGF, VEGF, serotonin, and coagulation factors)

Question 7

what two types of granules do platelets have and what does each secrete?

Answer 7

1. dense granules = ADP

2. alpha granules = vWF, thrombin and growth factors

Question 8

what two things happen during platelet activation?

Answer 8

1. exocytosis of dens granules and alpha granules

2. platelets change shape to flatten and increase surface area

Question 9

what helps reinforce platelet activation?

Answer 9

thrombin

it activates certain receptors on the platelet membrane

Question 10

what receptors do activated platelets express? what do they do?

Answer 10

activated platelets express glycoprotein IIb/IIIa (GPIIb/IIIa) and vWF receptors

GPIIb/IIIa receptor on the platelet binds to fibrinogen

vWF receptor allows the cells to continue to adhere to exposed collagen in the broken endothelium

look at the picture…

Question 11

what is thromboxane A2?

Answer 11

it’s secreted by platelets once they’re activated

it’s an arachidonic acid derivative that activates other platelets and induces vasoconstriction

Question 12

what gets secreted by platelets once they’re activated?

Answer 12

• thromboxane A2 (TxA2)
• serotonin
• platelet dervived growth factor (PDGF)
• vascular endothelial growth factor (VEGF)

Question 13

what is serotonin?

Answer 13

it’s secreted by platelets once they’re activated

short-lived and has a vasoconstrictive effect associated with inflammation during vessel injury

Question 14

what are PDGF and VEGF?

Answer 14

they’re secreted by platelets once they’re activated

they’re growth factors involved in angiogenesis

Question 15

what is angiogenesis?

Answer 15

growth of new blood vessels following injury

Question 16

what is the function of fibrinogen?

Answer 16

it’s a protein that helps form bridges between adjacent platelets, leading to their aggregation

Question 17

what happens during platelet aggregation?

Answer 17

fibrinogen bridges adjacent platelets to cause aggregation

initial wave of aggregation is reversible, but concurrent activation of thrombin stabilizes the platelet plug by causing further activation and aggregation and by promoting irreversible platelet contraction

thrombin also converts fibrinogen into insoluble fibrin which cements platelets into place

Question 18

What is the function of GPIIb/IIIa complex?

Answer 18

It allows adjacent platelets to both bind fibrinogen and form a tethering bridge between platelets

Question 19

what two things allow for platelet adhesion to the damaged endothelial wall?

Answer 19

vWF and GPIa