Blood clots are clumps that occur when blood hardens from a liquid to a solid.

• A blood clot that forms inside one of your veins or arteries is called a thrombus. A thrombus may also form in your heart.
• A thrombus that breaks loose and travels from one location in the body to another is called an embolus.

A thrombus or embolus can partly or completely block the flow of blood in a blood vessel.

• A blockage in an artery may prevent oxygen from reaching the tissues in that area. This is called ischemia. If ischemia is not treated promptly, it can lead to tissue damage or death.
• A blockage in the vein will usually cause fluid buildup and swelling.

Clot; Emboli; Thrombi; Hypercoagulable state

Situations in which a blood clot is more likely to form include:

• Being on long-term bed rest
• Crossing your legs for long periods of time when sitting, or sitting for long periods of time, such as in a plane or car
• During and after pregnancy
• Not having enough water in your body (dehydration)
• Taking birth control pills or estrogen hormones (especially in women who smoke)
• Using an intravenous catheter long-term

Blood clots are also more likely in people with cancer, recent surgery or injury, obesity, and liver or kidney disease.

A buildup of cholesterol that narrows an artery may change or slow the flow of blood, making it easier for a blood clot or thrombus to form.

Conditions that are passed down through families (inherited) may make you more likely to form abnormal blood clots. Inherited conditions that affect clotting are:

• Factor V Leiden thrombophilia
• Prothrombin G20210A mutation
• Other rare conditions such as protein C, protein S, and antithrombin III deficiencies

A blood clot may block an artery or vein in the heart, affecting the:

• Heart (angina or a heart attack)
• Intestines (mesenteric ischemia) or mesenteric venous thrombosis
• Kidneys (renal vein thrombosis)
• Leg or arm arteries
• Legs (deep vein thrombosis)
• Lungs (pulmonary embolism)
• Neck or brain (stroke)

Schafer AI. Thrombotic disorders: hypercoagulable states. In: Goldman L, Schafer AI, eds. Cecil Medicine. 24th ed. Philadelphia, Pa: Saunders Elsevier; 2011:chap 179.