Damage to blood vessels is inevitable in life. This injury may not only take the form of gross mechanical trauma but can also occur during very minor movements that can damage vessels at the microscopic level. A system that allows breaches in blood vessels to be sealed is essential, as is a process that can halt coagulation once it has achieved its goal, then organise and removed the clot when the wound has been properly repaired.

Coagulation of the blood can be triggered by three separate factors (Virchow's triad). These factors typically operate in combination.

Haemostasis is not limited just to the mechanisms that permit a blood clot to form. A system needs to exist by which a clot that has served its purpose can be removed and the blood vessel healed. There are also naturally occuring anticoagulants that hold the clotting system in check and prevent it from acting in an overzealous fashion.

The following sections cover the procoagulant and anticoagulant aspects of haemostasis, as well as introducing some of the diseases that can arise if haemostasis malfunctions. The sequence given below is a suggested reading order.