Session 2 - Haemostasis and blood groups Flashcards
Function of platelets
help stop blood loss from damaged vessels by forming a platelet plug. Their granules also contain chemicals that promote blood clotting.
Process for making platelets
Thrombopoiesis Thrombopoietin stimulates myeloid stem cells to produce platelets in bone marrow.
1st stage before becoming a platelet
Megakaryocytes Each megakaryocyte splits into 2000 to 3000 fragments, enclosed by a piece of cell membrane. This is a platelet (thrombocyte).
What is Haemostasis
Stoppage of bleeding when blood vessels are damaged
Prevents haemorrhage ( loss of large amounts of blood)
Mechanisms: Vascular spasm, platelet formation, blood clotting
Haemostasis: Vascular spasm
oReflex contraction of smooth muscle of small blood vessels when arteries or arterioles are damaged
o Only for small blood vessel or arteriole.
oCan reduce blood loss for several hours until other mechanisms can take over
Haemostasis: Platlet plug formation
o Platelets store a lot of chemicals in granules needed for platelet plug formation
Platlet plug formation:
Platelet Adhesion
• platelets stick to exposed collagen and damaged endothelial cells in vessel wall
Platelet Release Reaction
- platelets “activated” by adhesion
- extend projections & make contact
- release thromboxane A2, ADP, serotonin
Platelet Aggregation
- activated platelets stick together to form a mass called a ‘platelet plug’
- plug reinforced by fibrin threads formed during clotting process
Blood clotting
Clotting is a cascade of reactions in which each clotting factor activates the next in a fixed sequence resulting in the formation of fibrin threads
oKey Steps
- Formation of prothrombinase
- Prothrombinaseand Ca+2 convert prothrombin into thrombin
- Thrombin converts soluble fibrinogen into insoluble fibrin threads
What are the two pathways of the clotting cascade and their common pathway
Extrinsic (tissue trauma) - seconds
Intrinsic (Blood trauma) - several minutes
Common pathway:
oProthrombinase+ Ca+2
•catalyze the conversion of prothrombin → thrombin
oThrombin + Ca2+
- converts fibrinogen → fibrin threads
- activates fibrin stabilizing factor XIII
Strengthened Fibrin threads (end result) &
Clot retraction = Platelets pull on fibrin threads causing clot retraction (tightening)
Blood vessel repair
oEdges of damaged vessel are pulled together
oFibroblasts and endothelial cells repair the blood vessel
Vitamin K is required for
normal clotting
produced by bacteria in the large intestine
Haemostatic control mechanisms
oFibrinolytic system = Break down of fibrin threads when repair completed or when there are inappropriate clots = fibrinolysisis dissolution of a clot
oInactive plasminogen is incorporated into the clot
•plasminogen becomes plasmin (fibrinolysin) which digests fibrin threads and inactivating fibrinogen, prothrombin, factors V, VIII, XII
oClot formation remains localized
•fibrin threads trap / absorbs thrombin
oEndothelial cells and WBC produce prostacyclins that oppose thromboxane A2 (platelet adhesion and release)
oAnticoagulants - present in blood, supress clotting
Anticoagulant agents
Anticoagulants:suppress or prevent blood clotting
oPatients who are at increased risk of forming blood clots may receive an anticoagulant drug
•Aspirin inhibits vasoconstriction and platelet aggregation through stopping production of TxA2.
•heparin
–administered during haemodialysis and surgery
•warfarin (Coumadin®)
–antagonist to vitamin K so blocks synthesis of clotting factors
–slower than heparin
Thrombolytic agents
are injected to dissolve clots
Universal recipients and universal donors
Universal recepients: People with AB blood have no antibodies in plasma and are able to recieve any type of blood
Universal Donors: People with type O blood cell have no antigens on their cells and are able to donate their blood to anyone - highly wanted for donations
In the ABO system ( blood types) Agglutinogens are
A and B and are found on the surface of RBCs to determine blood types
- display only antigen A -blood type A
- display only antigen B -blood type B
- display both antigens A and B -blood type AB
- display neither antigen -blood type O
(blood types) Agglutinins are
Antibodies present in plasma designated as anti-A and anti-B, that react with agglutinogens (antigen A and antigen B)
- anti-A antibody reacts with antigen A
- anti-B antibody reacts with antigen B
When they react they will stick together and form clots therefore if antigen A (meaning you are Blood Type A) is found on your RBC you require anti-B antibody
