[What causes blood clots?

Blood clots form when there is damage to the lining of a blood vessel, either an artery or a vein. The damage may be obvious, such as a laceration, or may occur on the microscopic level. As well, blood will begin to clot if it stops moving and becomes stagnant.

Venous thrombosis or blood clots in a vein occur when a person becomes immobilized and muscles are not contracting to push blood back to the heart. This stagnant blood begins to form small clots along the lining walls of the vein that gradually grow to partially or completely occlude the vein. An analogy to this process is a slow moving river. Over time, weeds and algae start to accumulate along the banks of the river where the water flows more slowly. Gradually, as the weeds start to grow, they begin to invade the center of the river because they can withstand the pressure of the water.

symptoms-
What are the symptoms of blood clots?

Venous clots do not allow blood to return to the heart and symptoms occur because of this damming effect. Most often occurring in the legs or the arms, symptoms include:

* swelling,

* warmth,

* redness, and

* pain.

Arterial clots do not allow blood get to the affected area.

* Pain is the initial symptom of this ischemia, or oxygen deprivation due to loss of blood supply.

* Other symptoms depend upon the location of the clot, and often the effect will be a loss of function. Heart attack and stroke are self-explanatory.

* In an arm or leg, in addition to pain, the limb may appear white, and weakness, loss of sensation, or paralysis may occur.

* If the blood supply is lost to an area of the bowel, in addition to intense pain, there may be bloody diarrhea.
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http://www.medicinenet.com/blood_clots/article.htm