Blood clots are not a normal condition, but occur when blood coagulates, or hardens. These form during injury to the body. A thrombus, or blood clot, is made when blood cells lump together with fibrin, a stretchy, thread-like protein. Research has shown fibrin to be a strong material that can absorb blood from cuts to help heal them, but is also responsible for making blood clots hard to break down. Medications are now available to help destroy blood clots by working to break up the fibrin.

Thrombi, or blood clots, develop and remain in the heart or in a blood vessel. Emboli are blood clots that are located somewhere in the body besides the heart or a blood vessel and this condition is called an embolism. An embolism called a thromboembolism occurs when a part of the thrombus, or blood clot, causes blockage to the heart, brain or lungs and stops or restricts blood flow. When this happens, a heart attack could result.

Blood clots can also cause destruction of the body's tissue. This is because, as the blood flow is restricted by the emboli and thrombi, oxygen as well as blood flow is kept out of the tissues in that part of the body. Ischemia is the name of the condition that occurs when no blood flow or oxygen reaches the tissue. If ischemia is left unchecked, body tissues in the affected region can become damaged or dead.

Blood clots can cause death to persons with conditions like deep venous thrombosis (DVT). DVT occurs when a thrombus, or blood clot, enters a vein in the lower part of the body such as the pelvis, thighs or lower part of the legs. Blood clots in deep venous thrombosis cases restrict the blood flow.

Sometimes, swelling and redness occur in DVT, but some people do not experience any systems. Other people experience a lot of pain due to the formation of blood clots. Genetic factors are thought to be partly responsible for the causes of deep venous thrombosis. Poor circulation caused by sitting in long airplane flights is thought to be another causative factor in blood clots in some cases of DVT.