Blood - Platelets and Hemostasis

Question 1

How long do platelets circulate in the blood?

Answer 1

9-12 days

Question 2

What is thrombocytopenia?

Answer 2

Low platelet count

Question 3

What is thrombocytosis?

Answer 3

Excessive amount of platelets

Question 4

Where are platelets produced?

Answer 4

Red bone marrow

Question 5

What 3 factors influence platelet production?

Answer 5

Thrombopoietin, interleukin-6, and multi-CSF

Question 6

What are the three phases of hemostasis?

Answer 6

Vascular phase, platelet phase, and coagulation phase

Question 7

How long does the vascular phase last?

Answer 7

Around 30 minutes

Question 8

What happens during the vascular phase of hemostasis?

Answer 8

• Vascular spasm occurs, which decreases the diameter of the vessel at the site of injury
• Endothelial cells contract and expose the basement membrane to the bloodstream
• Endothelial cells begin releasing chemicals which promote smooth muscle contraction and promote vascular spasms

Question 9

When does the platelet phase start in terms of time after injury and triggers?

Answer 9

15 seconds after injury and when platelets start sticking to endothelial surfaces, to the plasma membrane, and to exposed collagen fibers

Question 10

What is platelet aggregation?

Answer 10

As the number of platelets at the site of injury increases, they start sticking to each other

Question 11

What is platelet adhesion?

Answer 11

The attachment of platelets to exposed surfaces

Question 12

When do platelets activate?

Answer 12

Platelets become activated when they arrive near the injury site

Question 13

What is a sign of platelet activation?

Answer 13

First sign of activation is that they get more spherical and they develop cytoplasmic processes that extend toward nearby platelets

Question 14

What compounds to platelets release during activation and what do they do?

Answer 14

• Adenosine diphosphate: stimulates platelet aggregation and secretion
• Thromboxane A2 and serotonin: stimulate vascular spasms
• Calcium ions: Help platelet aggregation and several steps in the clotting process
• Prostacyclin: Inhibits platelet aggregation and is released by endothelial cells

Question 15

When does coagulation phase start?

Answer 15

After 30+ seconds

Question 16

What are the three coagulation pathways?

Answer 16

Extrinsic, intrinsic, and common

Question 17

How does the extrinsic pathway start?

Answer 17

Starts when damaged endothelial cells release factor III

Question 18

What does the extrinsic pathway do?

Answer 18

It then combines with Ca2+ and Factor VII to form an enzyme that activates factor X, the first step in the common pathway

Question 19

How does the intrinsic pathway start?

Answer 19

Begins with the activation of proenzymes (Usually factor XII) exposed to collagen fibers at the injury site

Question 20

What does the intrinsic pathway do?

Answer 20

Platelets release many other things that accelerate the reactions of the intrinsic pathway, ultimately making an enzyme complex capable of activating Factor X

Question 21

What is the common pathway?

Answer 21

When factor X combined with 5a and calcium activates thrombin, causing further activation of platelets and the enhanced formation of fibrin, which holds the platelets together in strands

Question 22

What is the speed of both pathways in relativity to the other and which one is more important?

Answer 22

Intrinsic is slower than the extrinsic but the intrinsic is more important

Question 23

What proteins and chemicals stop clotting?

Answer 23

• Plasma contains anticoagulants which inhibit clotting
• Heparin: accelerates the activation of antithrombin-III
• Thrombomodulin: a protein released by endothelial cells that binds to thrombin and converts it to an
  enzyme that activates protein C

Question 24

What is fibrinolysis and how does it happen?

Answer 24

• Clot gradually dissolving
• Plasminogen is activated by thrombin and tissue plasminogen activator
• Activated plasminogen produces plasmin, which digests the fibrin strands in the clot