Platelets and Haemostasis

Question 1

what are platelets
what is their function

Answer 1

small cell fragments formed by megakaryocytes
clotting

Question 2

how are platelets formed
describe their origin cells

Answer 2

shed off from megakaryocytes
extraordinarily large bone-marrow cells

Question 3

describe clot formation when a vessel is injured

Answer 3

• circulating platelets adhere to and are activated by exposed collagen
• Von Willebrand factor activates platelets causing them to have protrusions
• activated platelets release ADP and thromboxane A2
• these further activate more platelets to join the newly forming plug and release further ADP and thromboxane A2

Question 4

how soon after injury does a blood vessel constrict

Answer 4

vasoconstriction occurs within seconds

Question 5

what prevents platelet aggregation under normal conditions and confines aggregation to plug sites

Answer 5

normal uninjured endothelium releases prostacyclin and nitric oxide to inhibit it

Question 6

what is the effect of aspirin

Answer 6

acts as a blood thinner by inhibiting the COX1 pathway which normally releases prostaglandins that stimulate clotting

Question 7

what is haemostasis
what is the functions

Answer 7

arrest of bleeding from a broken blood vessel; stopping a haemorrhage
prevents blood loss from damaged small vessels

Question 8

features of platelets

Answer 8

no nucleus
some organelles round when inactive, protrusions when active

Question 9

how does blood clot formation get triggered

Answer 9

chain reaction involving plasma clotting factors

Question 10

fibrinogen

Answer 10

plasma protein important for clotting but must be converted to fibrin

Question 11

when is prothrombin converted to thrombin

Answer 11

when blood clotting is desirable

Question 12

what is the clotting cascade
what is the final step

Answer 12

series of sequential reactions that occurs until thrombin catalyses the conversion of fibrinogen to fibrin

Question 13

describe the formation of fibrin mesh

Answer 13

• thrombin cleaves fibrinogen to make a fibrin monomer
• thrombin also cleaves factor 13 -> 13a
• fibrin monomers cross link into a fibrin strand
• cross linked stands form a mesh stabilised by factor 13 cross linking

Question 14

intrinsic clotting pathway

Answer 14

precipitates clotting within damaged vessels, and clotting of blood samples e.g. in test tubes

Question 15

extrinsic pathway

Answer 15

initiates clotting blood that has escaped into the tissues

Question 16

clot retraction

Answer 16

platelets trapped within the clot contract and shrink the fibrin mesh
pulls the edges of the damaged vessel closer together allowing healing

Question 17

at what point do the intrinsic and extrinsic pathway converge

Answer 17

factor 10 activation

Question 18

factor 10 function

Answer 18

together with Ca2+ and factor 5 converts prothrombin to thrombin

Question 19

what activates factor 13

Answer 19

thrombin

Question 20

what are the results of thalassaemia and haemophilia

Answer 20

poor blood clotting and excessive bleeding

Question 21

how is the extrinsic pathway activated when there is damage from outside

Answer 21

• tissue thromboplastin (factor 3) is released from damaged tissue
• acts together with factor 7 and Ca2+ to activate factor 10

Question 22

when damage affects both pathways, which activates factor 10 first

Answer 22

extrinsic

Question 23

what other factors does factor 10 require to act

Answer 23

factor 5 and Ca2+

Question 24

how is the intrinsic pathway activated

Answer 24

• when factor 12 (Hageman factor) is activated by contact with exposed collagen at the surface of damaged vessels
• or by contact with a foreign surface (glass in test tubes)

Question 25

order of factor activation in the intrinsic pathway

Answer 25

12, 11, 9, 10

Question 26

what factor is required between factor 11 and 9 what factor is required between factor 9 and 10

Answer 26

4 8

Question 27

where are clotting factors synthesised what vitamin do many of them require

Answer 27

liver vitamin K

Question 28

what cells form a scar at vessel defects

Answer 28

fibroblasts

Question 29

what dissolves clots how

Answer 29

plasmin becomes trapped in the clot and later dissolves it by slowly breaking down the fibrin meshwork

Question 30

what is the continuous role of plasmin

Answer 30

preventing inappropriate clot formation

Question 31

why would you want to pharmacologically inhibit tissue plasminogen activator

Answer 31

it cleaves plasminogen into active plasmin when you want clotting e.g. haemophilia

Question 32

why would you want to pharmacologically inhibit plasmin what is the physiological inhibitor of plasmin when does it act

Answer 32

when you want clotting alpha2-antiplasmin, when plasmin in blood is 6-8X exceeding therapeutic dose

Question 33

what is a thrombus what is a thromboembolism - what can they cause

Answer 33

abnormal intravascular clot attached to a vessel wall free floating abnormal intravascular clot - stroke

Question 34

causes of strokes

Answer 34

thromboembolism, brain bleed, hypoxia

Question 35

why is liver disease a blood clotting disorder

Answer 35

liver synthesises procoagulants and most clotting factors also produces bile to absorb fat and vitamin K

Question 36

why is vitamin K deficiency a blood clotting disorder

Answer 36

essential for the synthesis of clotting factors

Question 37

why does long term antibiotic use result in bleeding disorders

Answer 37

results in vit K deficiency because half of our Vit K comes from gut bacteria (other half diet)

Question 38

thrombocytopenia definition results cause treatment

Answer 38

number of circulating platelets is too low spontaneous, widespread haemorrhage, visible by small purple spots on the skin caused by damaged bone marrow or cancer treated with whole blood transfusions or platelet transfusion

Question 39

thrombosis definition causes results

Answer 39

inappropriate clotting caused by roughened surface of a vessel, endothelial cell injury (collagen exposed), disrupted flow (stasis - no laminar flow) can cause DVT, ischemia, and tissue death

Question 40

coronary thrombosis cerebral thrombosis

Answer 40

thrombus in a blood vessel of the heart can cause a heart attack thrombus in a blood vessel of the brain can cause a stroke

Question 41

atherosclerosis

Answer 41

build up of oxidised LDL, fatty plaques in vessels

Question 42

what do blood types depend on

Answer 42

surface antigens on erythrocytes

Question 43

when does haemolytic disease of the new-born occur explain why

Answer 43

- occurs when the mother is Rh- during the second or later pregnancies with a Rh+ child - when Rh+ blood from previous pregnancy caused her body to synthesise anti-Rh Abs - mothers Abs cross the placenta and cause agglutination and haemolysis of foetal RBCs

Question 44

how is haemolytic disease of the new-born prevented

Answer 44

injected anti-Rh antibodies eliminate Rh+ cells