Haemostasis

Question 1

Give the dimensions of a platelet.

Answer 1

4 um in diameter

1 um thick

Question 2

What is platelet production called?

Answer 2

Thrombocytopoiesis

Question 3

Where does thrombocytopoeisis take place?

Answer 3

Takes place in the bone marrow

Question 4

What cells are usually found in normal marrow?

Answer 4

Megakaryocytes

Question 5

What are megakaryocytes?

Answer 5

Large cells with large nuclei

Question 6

What do megakaryocytes do?

Answer 6

They synthesise structural proteins, enzymes and membranes

Question 7

How do megakaryocytes form platelets?

Answer 7

These cells shed cytoplasm in membrane enclosed packets to form platelets

Question 8

What stimulates the formation of platelets?

3

Answer 8

Thrombopoietin (TPO), produced in the kidney

Interleukin - 6

Multi CSF

Question 9

What are the three main functions of platelets?

Answer 9

Form a platelet plug in the wall of damaged blood vessels

Transport and release chemicals important for clotting

Active contraction after clot has formed

Question 10

What is thrombocytopenia?

2

Answer 10

Abnormally low counts of platelets

Results in bleeding along GIT, skin and CNS

Question 11

What is thrombocytosis?

2

Answer 11

Too many platelets

Accelerated production due to infection, inflammation or cancer

Question 12

What are the four stages of clotting?

Answer 12

Vascular spasm

Platelet plug

Coagulation

Fibrinolysis

Question 13

What are the main benefits of haemostasis?

Answer 13

Prevents loss of blood through damaged vessels

Establishes a framework for repair

Question 14

When does the vascular phase of haemostasis begin?

Answer 14

Immediately

Question 15

When does the platelet phase of haemostasis begin?

Answer 15

After about 15 seconds

Question 16

When does the coagulation phase of haemostasis begin?

Answer 16

After a few minutes

Question 17

When does the fibrinolysis phase of haemostasis begin?

Answer 17

Following the repair of the blood vessel wall

Question 18

What happens when a blood vessel wall is cut?

6

Answer 18

Vascular phase begins

Smooth muscle contracts for 30 minutes

This either slows or stops the blood loss (sufficient in small vessels)

Damaged endothelial cells contract to expose the basement membrane to the bloodstream

Endothelial cells release chemical factors and hormones which stimulate the spasm and accelerate repair (division of cells)

Endothelial cells become sticky

Question 19

What happens during the platelet phase?

2

Answer 19

Platelets stick to the ‘sticky’ endothelial cells and exposed collagen

The platelets become activated and release clotting factors that stimulate haemostasis

Question 20

What happens during the coagulation phase?

4

Answer 20

Fibrinogen (soluble plasma protein) becomes fibrin (insoluble fibres)

The network of insoluble fibres trap blood cells and platelets and seal the platelet plug

Clotting factors (calcium and protein factors 1 to XII

Three clotting pathways: extrinsic, intrinsic, common

Question 21

What is the extrinsic clotting pathway?

Answer 21

It begins outside the bloodstream

Question 22

What is the intrinsic clotting pathway?

Answer 22

Begins inside the bloodstream

Question 23

What is the common clotting pathway?

Answer 23

1 and 2 converge

Question 24

What happens during the extrinsic pathway?

2

Answer 24

Factor VIII is released by the endothelial cells or peripheral tissues

Factor VIII, calcium and Factor VII combine to form factor X

Question 25

What is factor X?

Answer 25

An enzyme complex made from the combining of Factor VIII, calcium and factor VII

Question 26

`What happens during the intrinsic pathway?

2

Answer 26

Factor XII is activated

Factor VII, IX and Ca++ combine to form an enzyme factor capable of activating Factor X

Question 27

What happens during the common pathway?

4

Answer 27

Activated Factor X forms prothrombinase

This converts prothrombin into thrombin

Thrombin converts fibrinogen into fibrin

Fibrin then stabilises the platelet plug by trapping RBCs and sealing the damaged blood vessel

Question 28

Which pathway is activated when a vessel is damaged?

2

Answer 28

Both the intrinsic and the extrinsic pathways are activated

The extrinsic pathway is activated first

Question 29

What two nutrients are required for blood clotting?

Answer 29

Calcium

Vitamin K

Question 30

Why is calcium needed for clotting?

Answer 30

Calcium is involved in all 3 clotting pathways

Question 31

Why is vitamin K needed for clotting?

Answer 31

For the formation of 4 clotting factors in the liver

Question 32

What happens as platelets get trapped in the fibrin strands?

Answer 32

They contract and the clot retracts

Question 33

Why does the clot retract?

Answer 33

This reduced the size of injury and makes repair easier

Question 34

What is fibrinolysis?

Answer 34

The dissolution of the clot

Question 35

What carries out fibrinolysis?

Answer 35

The enzyme plasminogen

Question 36

How does plasminogen work?

2

Answer 36

Plasminogen is activated by tissue plasminogen activator (tPa)

Plasminogen produces plasmin an enzyme that digests fibrin strands

Question 37

Give the three abnormal types of haemostasis that can occur?

Answer 37

Thrombus

Embolus

Infarct

Question 38

What is a thrombus?

Answer 38

A clot in a blood vessel

Question 39

What may cause a thrombus?

2

Answer 39

An injury

Plaque/atherosclerosis

Question 40

What is an embolus?

Answer 40

A drifting blood clot

Question 41

What is an infarct?

Answer 41

Tissue damage/malfunctioning tissue as a result of circulatory blockage