Question submitted by Patrick Hawkings.
Blood clotting is the body's way of protecting itself against blood loss when tissue is damaged.
When tissues such as skin or veins are damaged, they release chemicals into the blood that cause a number of clotting factors to react and form a blood clot at the site of damage. If blood didn't clot when the body were injured, even a small cut could be fatal. Normally, clotting occurs only when there is some kind of damage done to a tissue, and then only at the site of damage. However, when blood circulation is poor, such as in a person that is bed-ridden, doesn't exercise, or has a high cholesterol level, blood clotting factors tend to accumulate and some times form a clot in the middle of a vein or artery even when there is no damage there. A person also can be more susceptible to this type of clot after a major injury or surgery, when high levels of clotting factors are present in the blood stream.