For you information... Haemostasis

This question is taken from the London Feb 2013 exam.

Question:
What are the stages of haemostasis?

Answer:
Haemostasis is a process that causes bleeding to stop. There are 3 steps that occur in rapid sequence.

1. Vasoconstriction (vascular spasm): The damaged blood vessels contract as the first response to injury. This reduces blood flow to the area and limits the amount of blood loss. It is triggered by local pain receptors as well as local chemicals released by the damaged endothelial cells.
2. Platelet plug formation: Platelets adhere to the damaged endothelium to for a platelet plug and seals up the break in the vessel wall. They then release chemical mediators such as adenosine diphosphate (ADP), serotonin and thromboxane A2 that potentiate more platelets to adhere to the platelet plug.
3. Activation of the coagulation pathway: The platelet plug is reinforced by activation of the coagulation pathway and the subsequent products. Fibrinogen is converted to fibrin by thrombin. Clot regulation occurs through a negative feedback loop to prevent excessive clotting.