Blood platelets and homeostasis

Question 1

Where is thrombopoietin secreted ?

Answer 1

Liver

Question 2

What are thrombocytes?

Answer 2

Fragments of megakaryocytes which have broken off.

Question 3

What are the characteristics of thrombocytes?

Answer 3

Platelet production controlled by
thrombopoietin (increases megakaryocytes)
Platelets contain granules with cytokines and
clotting proteins
Platelets become activated when damage
occurs to the blood vessel walls
Small (2-4 μm)
Lifespan (±10 days)
Normal number: 150‐450 x 103/ μ l blood

Question 4

What is thrombocytopenia and what effects does it have ?

Answer 4

A decreased amount of thrombocytes in the blood.
Spontaneous bleeding
Skin (purpura, petechiae), mucous membranes of the
nose, mouth, GI tract, uterine cavity
Prolonged bleeding after injury

Question 5

Outline the heamostatic mechanism of blood and its 3 major steps.

Answer 5

Haemostasis is the process of keeping blood
within the blood vessel by repairing leaks without
compromising the fluidity of blood

1. Vasoconstriction: reduces blood loss and makes it easier for platelets to bind to the injury.
2. Temporary blockage of the break by a platelet
   plug
3. Blood coagulation/ clot forming that seals the
   injury until tissues are repaired

Question 6

Outline platelet plug formation

Answer 6

Exposed collagen binds and activates platelets.
Release of platelet factors
Factors attract more platelets
Platelets aggregate into platelet plug

Question 7

Outline platelet plug formation

Answer 7

von Willebrand factor binds platelets to collagen and platelets bind to each other via fibrinogen

Question 8

what is the role of von Willebrand factor

Answer 8

It binds the vessel wall (collagen) of microfibrils to the glycoprotein 1B of platelet membranes.

Question 9

What is the role of serotonin in platelet function?

Answer 9

It acts as a vasoconstrictor and is involved in platelet aggregation and is released as a result of platelet activation.

Question 10

What is the role of PAF in platelet function ?

Answer 10

Increases capillary permeabiltiy to reduce infection.

Question 11

Role of thromboxane A2 in platelet function

Answer 11

vasoconstrictor and eicosanoid

Question 12

the roles of PDGF in platelet function

Answer 12

promotes wound healing by attracting fibroblasts and smooth muscle cells

Question 13

What are the functions of platelets?

Answer 13

Functions of platelets
1. Form platelet plugs that seal off leaks in vessels
2. Release serotonin and thromboxane A2 →
cause local vasoconstriction
3. Release clotting factors
4. Release growth stimulators: stimulate repair of
vascular lesions and bone lesion

Question 14

What causes blood coagulation ?

Answer 14

Plasma proteins and calcium ions which are released by damaged tissue and activated platelets

Question 15

What is the order that clotting factors appear in the intrinsic pathway ?

Answer 15

12, 11(Ca), 9(Ca), 8

Question 16

Which clotting factors require calcium

Answer 16

1,2,9,10,11

Question 17

why do blood bags contain citrate

Answer 17

prevents coagulation

Question 18

Which clotting factors require Vitamin K

Answer 18

2,9,10

Question 19

What is the role of thrombin in coagulation

Answer 19

Stimulates conversion of fibrinogen to fibrin

Question 20

List the factors involved in coagulation

Answer 20

collagen
Starts intrinsic pathway N/A

von Willebrand
factor (vWF)

Kinonogenand
kallikrein

Tissue factor (tissue
thromboplastin or

Prothrombin and
thrombin (factor II)

Fibrin production N/A
Fibrinogen and
fibrin (factor I)

Fibrin stabilizing
factor (factor XIII)

Ca2+(factor IV) Plasma ions N/A Required for

Vitamin K

Question 21

Outline anticlotting mechanisms

Answer 21

 Intravascular clotting is limited by:
1. Smooth endothelial lining of the cardio‐vascular system
2. Continued circulation of blood
3. Factors counteracting platelet aggregation, i.e.
prostacyclin (released by the intact endothelium)
4. Antithrombin III ‐ a circulating compound (a globulin
produced by the liver) that binds to thrombin and
inactivates it
5. Heparin, a natural occurring anticoagulant (produced by
mast cells

Question 22

What is fibrinolysis

Answer 22

Conversion of fibrinogen into fibrin and subsequent fibrinolysis

Question 23

List the factors involved in fibrinolysis and anti-coagulation

Answer 23

Plasmin
Tissue plasminogen activator
antithrombin 3
prostacyclin

Question 24

Effects of recombinant tPA

Answer 24

acute ischaemic stroke