You are supposed to form clots only to protect you from bleeding, but any trauma or inflammation in your body can cause clots to form in blood vessels. If a clot breaks off from a blood vessel wall, it can block arteries to cause a heart attack, stroke or pulmonary embolism.

Clots can form wherever your body responds to injury or infection. When germs get into your body, they release chemicals that cause inflammation characterized by redness, itching, swelling, heat and clotting. Certain cells in your body, such as white blood cells and fat cells, also release chemicals that produce inflammation. Anything that damages tissue can cause inflammation, such as trauma, burns, frostbite, irritants such as corrosive chemicals, ultraviolet or other ionizing radiation, dying tissue from any cause such as lack of oxygen in a heart attack, obesity, periodontal disease, smoking, excessive alcohol, gastroesophageal reflux or allergic reactions.

Other factors that can lead to increased risk for clotting include genetic diseases such as Lp(a), dehydration, or physical blockage of arteries that can be caused by sitting in one position for a long time. Any clot has the potential to break off and cause damage, but you lessen your risk of blockage problems if your arteries are pliable and not already partially blocked with plaques.

You may remember when NBC reporter David Bloom died suddenly while he was covering the war in Iraq, from a pulmonary embolism. A clot that formed in his leg broke lose from the vein, and traveled in his bloodstream to his lungs where it blocked the flow of blood to his lungs. He had a pulmonary embolism, a deadly consequence of "Economy Class Syndrome." When a person sits in a small space for a long time without moving, the knees are bent, which slows the flow of blood through the veins in the back of the legs, so that a clot can form in the deep veins of the calf muscles. David Bloom had been traveling in a vehicle whose interior was designed for safety, not comfort. He had reportedly been complaining of pains in his legs, a classic warning for a blood clot, or thromboembolism. The desert heat also caused dehydration, which thickens the blood and increases risk for clots.

Embolisms of the kind that killed Bloom are responsible for an estimated 60,000 deaths in the United States each year. The underlying cause that makes people susceptible has been a mystery, but many experts believe that there is a genetic susceptibility to forming clots. Italian researcher Dr. Paolo Prandoni reports that clotting is linked to arteriosclerosis or hardening of the arteries that leads to heart attack and stroke. Prandoni and his colleagues performed ultrasound examinations to take pictures of the carotid artery, leading to the brain, in 299 patients with deep venous clots of the legs and 150 healthy people. He found twice as many plaques, the fatty deposits of atherosclerosis in the arteries of people who had suffered clots as people who were healthy.