Updated May 28, 2014.

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

Why Does Blood Clot?

Your blood is designed to react to injury in a way that prevents blood loss by clotting at the site of an injury -- a normal process. If blood was unable to clot, a simple injury such as a cut on your finger could cause you to bleed to death.

What is a Blood Clot?

All injuries cause damage to blood vessels. A bruise is an injury in which a blood vessel was damaged and blood then leaked out, becoming visible under the skin.

When you have an injury that damages one of the tiny blood vessels in the body, platelets in your blood go to the site of the injury and begin to bond together to plug the leak. Once platelets start to work, a protein called fibrin forms to create a web of fibers that strengthens the clot.

Think of the blood vessel as a dam: the dam suddenly develops a tiny crack, and water begins to leak through. The platelets are the first responders on the scene, helping patch the hole temporarily. Fibrin is the dam-repair crew that shows up later, strengthening the dam until it's strong again.

Your body is forming several tiny clots at any given time; if you bumped into something and have a tiny bruise, for example, there's a clot at the site inside the blood vessel. Without this process, we would die from bleeding after minor injuries.

When Are Blood Clots Bad?

A blood clot, also known as a thrombus, can become an issue in two ways: first, a thrombus causes problems when it becomes an embolus, dislodging from the site where it was formed and traveling through the blood vessels, eventually lodging in a place where it shouldn't be.

A thrombus also becomes a problem when it grows so large that it obstructs the blood vessel where it was formed -- a scenario that causes most myocardial infarctions, many ischemic strokes, and venous thrombosis.

Arteries get smaller and smaller as they move away from the heart, so a clot that starts near the heart will eventually lodge in a smaller vessel. This prevents oxygenated blood from reaching any areas fed by that artery. Embolic strokes, for example, are the most common type of stroke, and are often caused by blot clots traveling to the brain and starving a segment of brain tissue of blood and oxygen.

Veins, on the other hand, get larger as they return blood to the heart, so blood clots that form in veins can travel all the way to the heart and then get pumped to lungs, where they can create a life-threatening condition called a pulmonary embolism. They can also lodge in blood vessels, most commonly in the legs.

Signs and Symptoms of a Blood Clot

The signs and symptoms of a blood clot vary with the location of the clot—whether it's in a vein or artery—and size. The severity of symptoms also varies widely, ranging from a stroke to leg pain.

Some general signs and symptoms of an arterial blood clot include:

• Moderate to severe pain that begins quickly
• Pale skin in the area of the clot and beyond
• Tingling
• Weakness
• Swelling above the area of the clot

If the clot is in an artery leading to the brain, confusion or paralysis may be present, but a clot in the upper arm could lead to pain, a pale hand, tingling in the arm below the clot, an inability to lift or grasp objects, or tingling. A clot in the arteries that feed the heart is a myocardial infarction, or heart attack.

Signs and symptoms of a venous clot include:

• Warmth in the area of the clot
• Swelling
• Tenderness
• Mild to moderate pain that increases over hours or days

For example, a blood clot in the leg, known as a deep vein thrombosis, would make the area of the leg where the clot formed painful and warm. It may be larger than the other leg due to swelling, and the area may be tender to the touch.

<sub>Sources:

Blood Clots. Medline Plus. Accessed October 2012. http://www.nlm.nih.gov/medlineplus/ency/article/001124.htm</sub>