L7: Platelets and hemostasis

Question 1

are platelets cells ?

Answer 1

no , they are cell fragments from megakaryocytes

Question 2

what is the normal platelet count ?

Answer 2

150,000 - 400,000 / mm^3

Question 3

what eliminates platelets from circulation ?

Answer 3

the spleen

Question 4

what is platelet formation stimulated by ?

Answer 4

Thrombopoietin (secreted by the liver )

Question 5

describe the structure of platelets .( cytoplasm )

Answer 5

cytoplasm : outer hyalomere + central granulomere

Question 6

describe the cell membrane of thrombocytes .

Answer 6

contains phospholipids that activate multiple stages in blood clotting process

glycoproteins:
-GPIb: binds to vWF (adhesion)

• GPIIb / IIIa : binds to fibrinogen (aggregation )
• P2Y12: binds ADP ( aggregation)

Question 7

what does the outer hyalomere of thrombocytes contain ?

Answer 7

cytoskeletal fibers (actin and myosin ). these fibers allow the platelet to maintain its shape and contract when necessary

Question 8

what does the central granulomere of thrombocytes contain ?

Answer 8

alpha granules ( contain clotting factors - vWF , Fibrinogen , platelets growth Factor , and platelet factor IV )

delta (dense) granules : contain ADP , ATP , and Ca++ and serotonin

Question 9

what is hemostasis ?

Answer 9

stoppage of bleeding from injured blood vessels

Question 10

what structure is involved in every step of hemostasis ?

Answer 10

platelets

Question 11

what is the first step of hemostasis ?

Answer 11

vasoconstriction of blood vessel to decrease flow of blood to the injured vessel

Question 12

what are some vasoconstricting substances that mediate vasoconstriction ?

Answer 12

thromboxane A2 , serotonin and endothelin

Question 13

what is the second step of hemostasis ?

Answer 13

formation of platelet plug (1-4 MINUTES )

Damage (injury) of  the endothelium blood vessel causes exposure of the underlying collagen. 
Platelets adhere (GP1b) with exposed collagen fibers in the vascular wall

Von Willebrand factor (vWF) forms a bridge between collagen and platelets.

Question 14

describe what happens during platelet activation .

Answer 14

platelet activation occurs when the platelet plug is forming . platelets begin to swell and become irregular in shape , they become more sticky then platelets degranulate . they also secrete ADP and thromboxane A2 which in turn activates nearby plaetlets ?

Question 15

what substances are secreted by platelets to activate nearby platelets ?

Answer 15

thromboxane A2 and ADP

Question 16

what is the third step of hemostasis ?

Answer 16

formation of blood fibrin clot (4-8 minutes)

plasma protein fibrinogen is converted to fibrin tightens around platelet clot and becomes dense , this results in transformation of blood into solid gel ( blood clot).

Question 17

what is thrombocytopathia ?

Answer 17

decreased function of platelets

Question 18

Severe Bleeding disorder occurs when platelet count falls below____

Answer 18

50,000 cells / ul