thrombosis, embolism, ischaemia and infarction

Question 1

what 2 factors mean that clots are rare?

Answer 1

Laminar flow – cells travel in the centre of arterial vessels and don’t touch the sides

Endothelial cells which line vessels are not ‘sticky’ when healthy

Question 2

what is thrombosis?

Answer 2

the formation of a solid mass from blood constituents in an intact vessel in a living person .

Question 3

outline how a thrombus is formed

Answer 3

1. endothelial wall damaged
2. platelets stick to the exposed collagen and release chemicals which cause platelet aggregation.
3. trapped RBCs + clotting factors + platelets trigger the clotting cascade and a large protein molecule fibrin forms and deposits and forms the clot

Question 4

what type of feedback loop is the formation of a thrombus?

Answer 4

Positive feedback loop - once started, difficult to stop

Question 5

what is Virchow’s triad?

Answer 5

summarised the 3 causes of thrombosis:

• change in vessel wall eg. endothelial injury
• change in Blood flow (stasis of blood)
• change in blood constituents eg. hyper coagulation

Question 6

what can be prescribed to reduce the risk of thrombosis?

Answer 6

Low dose aspirin

because it inhibits platelet aggregation

Question 7

what is an embolism?

Answer 7

the process of a solid mass in the blood being carried through the circulation to a place where it gets stuck and blocks the vessel

Question 8

define an embolus

Answer 8

a mass of material in the vascular system able to become lodged within a vessel and block it

Question 9

give 5 examples of causes of an embolism?

Answer 9

• part of a thrombus that has broken off
• Air (pressurised systems of intravenous fluids/ bloods)
  -a Tumour
  -Amniotic fluid (rare in pregnant women)
  -Fat (severe trauma with fractures)

Question 10

what happens if an embolus enters the venous system?

Answer 10

An embolus in the venous system will go onto the vena cava and then through the pulmonary arteries and become lodged in the lungs causing a pulmonary embolism. This means there is decreased perfusion to the lungs.

Question 11

what happens if an embolus enters the arterial system

Answer 11

it can travel anywhere downstream of its entry point.
The consequences could be stroke, MI, gangrene etc.

Question 12

what is the most common cause of:
a) an arterial thrombosis?

b) a venous thrombosis?

Answer 12

a) an atheroma (plaque of fatty material built up inside arteries)

b) blood stasis (abnormal blood flow, blood not reaching areas of the body it should)

Question 13

how do arterial/venous thrombosis differ in terms of :

a) pressure
a) main constituents of the thrombus
c) treatment

Answer 13

a) arterial has high pressure, venous has low pressure

b) arterial thrombus is made of platelets (white thrombus) whilst venous is made of coagulation factors (red thrombus)

c) arterial treated with antiplatelets drugs eg. asprin, venous treated with anticoagulants eg. warfarin

Question 14

define ischemia

Answer 14

reduction of blood flow to a tissue without any other implications

Question 15

define infarction

Answer 15

a reduction of blood flow to a tissue that is so reduced that it cannot even support mere maintenance of the cells in that tissue so they die

Question 16

define end artery supply

Answer 16

an organ that only receives blood supply from one artery.

Question 17

why are end artery supplies problematic?

Answer 17

if a thrombus forms In it, the whole blood supply to that organ is cut off, leading to infarction.

Question 18

name 3 organs that have multiple arterial supplies

Answer 18

1. Pulmonary arteries and bronchial arteries supplying the lungs
2. Portal vein and hepatic artery supplying the liver
3. Some parts of the brain – circle of Willis

Question 19

what is deep vein thrombosis?

Answer 19

a deep venous thrombosis of the leg veins, breaks off and causes embolism through the large veins (inferior vena cava) and right side of the heart to the lungs

Question 20

what are the 5 stages in clot formation

Answer 20

1. vascular spasm
2. platelet plug - aggregation
3. coagulation
4. clotting
5. fibrinolysis

Question 21

Damage to endothelial cells in the vessel causes some of the cells to lift away from the vessel wall, and expose what?

Answer 21

collagen

Question 22

how do platelets adhere to the collagen?

Answer 22

Platelets adhere to collagen via von Willebrand factor (vWF) plasma protein secreted by endothelial cells (glycoprotein-1b)

Question 23

what chemicals do platelets secrete once they are adhered to the collagen, which causes platelet aggregation and the formation of the platelet plug ?

Answer 23

ADP and thromboxane-A2

Question 24

describe the intrinsic coagulation pathway in forming a thrombus

Answer 24

1. RBCs and clotting factors get trapped within the aggregating platelets
2. cascade: 12→11→9→9a+8→10→prothrombin activator
3. prothrombin activator converts prothrombin 2 → thrombin 2a
4. thrombin creates positive feedback loop, and activates soluble fibrinogen to polymerise into insoluble fibrin. clotting factor 13 is activated and forms crosslinks between the fibrin strands, creating the fibrin mesh ( ie. the thrombus)

Question 25

describe the extrinsic coagulation pathway in forming a thrombus

Answer 25

Tissue factor 3 migrates from tissue into blood vessel
3→7→9 and then same as intrinsic cascade to form a fibrin polymer

Question 26

what is a thrombus composed of mainly?

Answer 26

fibrin

Question 27

What can happen if ischaemia is rectified?

Answer 27

Re-perfusion injury can occur due to the release of waste products.

Question 28

Through which blood system would an embolus have travelled if it resulted in a pulmonary embolism?

Answer 28

venous system