Haemostasis Flashcards
(41 cards)
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
