Haemostasis Flashcards
Give the dimensions of a platelet.
4 um in diameter
1 um thick
What is platelet production called?
Thrombocytopoiesis
Where does thrombocytopoeisis take place?
Takes place in the bone marrow
What cells are usually found in normal marrow?
Megakaryocytes
What are megakaryocytes?
Large cells with large nuclei
What do megakaryocytes do?
They synthesise structural proteins, enzymes and membranes
How do megakaryocytes form platelets?
These cells shed cytoplasm in membrane enclosed packets to form platelets
What stimulates the formation of platelets?
3
Thrombopoietin (TPO), produced in the kidney
Interleukin - 6
Multi CSF
What are the three main functions of platelets?
Form a platelet plug in the wall of damaged blood vessels
Transport and release chemicals important for clotting
Active contraction after clot has formed
What is thrombocytopenia?
2
Abnormally low counts of platelets
Results in bleeding along GIT, skin and CNS
What is thrombocytosis?
2
Too many platelets
Accelerated production due to infection, inflammation or cancer
What are the four stages of clotting?
Vascular spasm
Platelet plug
Coagulation
Fibrinolysis
What are the main benefits of haemostasis?
Prevents loss of blood through damaged vessels
Establishes a framework for repair
When does the vascular phase of haemostasis begin?
Immediately
When does the platelet phase of haemostasis begin?
After about 15 seconds
When does the coagulation phase of haemostasis begin?
After a few minutes
When does the fibrinolysis phase of haemostasis begin?
Following the repair of the blood vessel wall
What happens when a blood vessel wall is cut?
6
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
What happens during the platelet phase?
2
Platelets stick to the ‘sticky’ endothelial cells and exposed collagen
The platelets become activated and release clotting factors that stimulate haemostasis
What happens during the coagulation phase?
4
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
What is the extrinsic clotting pathway?
It begins outside the bloodstream
What is the intrinsic clotting pathway?
Begins inside the bloodstream
What is the common clotting pathway?
1 and 2 converge
What happens during the extrinsic pathway?
2
Factor VIII is released by the endothelial cells or peripheral tissues
Factor VIII, calcium and Factor VII combine to form factor X
What is factor X?
An enzyme complex made from the combining of Factor VIII, calcium and factor VII
`What happens during the intrinsic pathway?
2
Factor XII is activated
Factor VII, IX and Ca++ combine to form an enzyme factor capable of activating Factor X
What happens during the common pathway?
4
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
Which pathway is activated when a vessel is damaged?
2
Both the intrinsic and the extrinsic pathways are activated
The extrinsic pathway is activated first
What two nutrients are required for blood clotting?
Calcium
Vitamin K
Why is calcium needed for clotting?
Calcium is involved in all 3 clotting pathways
Why is vitamin K needed for clotting?
For the formation of 4 clotting factors in the liver
What happens as platelets get trapped in the fibrin strands?
They contract and the clot retracts
Why does the clot retract?
This reduced the size of injury and makes repair easier
What is fibrinolysis?
The dissolution of the clot
What carries out fibrinolysis?
The enzyme plasminogen
How does plasminogen work?
2
Plasminogen is activated by tissue plasminogen activator (tPa)
Plasminogen produces plasmin an enzyme that digests fibrin strands
Give the three abnormal types of haemostasis that can occur?
Thrombus
Embolus
Infarct
What is a thrombus?
A clot in a blood vessel
What may cause a thrombus?
2
An injury
Plaque/atherosclerosis
What is an embolus?
A drifting blood clot
What is an infarct?
Tissue damage/malfunctioning tissue as a result of circulatory blockage
