Haemostasis

Question 1

Stage I of haemostasis

Answer 1

• The first stage is vasoconstriction where there is a reflex response to injury that causes local constriction of blood vessels
• happens in response either to direct injury in the vessel wall to vascular smooth muscle cells causing the muscle to contract and restrict
• may also be damage to endothelial cells where molecules are released from injured endothelial cells that form activated platelets which act on smooth muscle to help drive vasoconstriction, although there is no neuronal response there is an influence from pain receptors where there are neuronal signals from local pain receptors
• vasoconstriction causes reduction of blood flow to injured area reduces blood loss and so increases likelihood of contact activation of platelets and coagulation factors as blood is moving more slowly across the injury site so there is more time for the platelets to interact

Question 2

Stage II of haemostasis

Answer 2

• this stage involves platelet activation and platelet plug
• platelets have a conserved structure in which there is an internal membrane network, lots of granules, plasma membrane packed with activation and adhesion receptors (sense damage and respond to signals), and has a highly organised cytoskeleton to maintain their resting shape and important for driving activation
• activated by multiple surface receptors
• the receptors can be divided into 2 classes: ones which are activated by contact and ones which are activated by soluble signals
• activated by contact: respond to activator signals within the vessel wall, e.g collagens which only respond when the vessel is damaged and Von Willebrand factor (blood protein bound to collagen and act via its receptor), also have glycoprotein complexes which recognise activator signals in vessel wall, and also adhesion receptors (such as integrin) which bind to collagen and ECM proteins to active platelets and help them stick to damaged surface
• soluble signals are released from other platelets, from activated cells or generated in the plasma as part of this response, these will mostly consist of G protein coupled receptors which are in the plasma membrane and they will respond to thromboxane signs which are activating signals to thrombin, and these soluble signals also help to drive platelet activation
• shape change: helps to plug damage to vessel and make platelets more likely to stick together, platelet spreading is driven by cytoskeleton rearrangements, in soluble suspension rather than there being highly organised adhesion and spreading, there is a change from a disc shape to a spiky shape that increases the likelihood of the platelets interacting with each other and causing them to aggregate (helps to plug/start the process of stopping blood from leaking out and secondly helps to support aggregation)
• secretion: secrete dense granules and alpha granules, these reinforce platelet activation, activate coagulation cascade, and drive vessel wall/tissue repair
• alpha granules: cell mitogens, haemostatics cofactors (important for linking stage II to the generation of thrombin and production of secondary haemostatic plug), adhesion proteins (help platelets stick down to surface), platelet receptors, platelet specific , and leukocyte recruitment
• dense granules are involved in reinforcing platelet activation through platelet agonists and haemostatic cofactors (interact with coagulation cascade that help to drive clot formation)
• sticky: action of integrin binds fibrinogen which forms bridges between adjacent integrins and cause platelet aggregation

Question 3

Stage III of haemostasis

Answer 3

• includes coagulation and fibrin formation
• fibrinogen is converted to fibrin which is then cross-linked to form a fibrin insoluble mesh that forms the actual blood clot (secondary thrombus)
• driven by thrombin (protease present in the plasma), important for cleaving fibrinogen to fibrin and activating other proteins, such as factor 13, which enables the mesh to be formed (allows for cross-linking role)
• thrombin: generated from its proenzyme pro-thrombin and key points of coagulation cascade include:
  1. A proteolysis cascade converting precursor factors into active factors
  2. Each step in pathway provides an amplification of signal, great increase in amount of activation
  3. End result is the generation of thrombin
  4. The cascade also has negative feedback loop to ensure local and limited thrombin generation

Question 4

Stage IV of haemostasis

Answer 4

• final step that involves fibrinolysis which is the breakdown of the fibrin clot during blood vessel damage, factors are released that bind to fibrin and recruit plasminogen (enzyme) to clot
• plasminogen can be activated to its active form from plasmin via various molecules that are also recruited to the site of the growing clot
• plasmin works to cleave plasmin and give the degradation products
• this process involves both spatial and temporal regulation
• localisation of fibrinolysis to fibrin helps to limit fibrinolysis to the site of injury and prevents excessive thrombus growth