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Blood Clots Overview
Blood clots can occur under many different circumstances and in many different locations. Blood clots that form in response to an injury or a cut are beneficial, stopping potentially dangerous bleeding. However, a number of conditions can cause you to develop blood clots in critical locations, such as your lungs and brain, and they require medical attention.
Blood has a seemingly impossible job: it must flow continuously and smoothly for an entire lifetime, but quickly form a blood clot when bleeding occurs. Blood achieves this through complex interactions between substances in blood and the blood vessel walls.
The major parts of Blood Clot formation are:
1. The platelet plug forms. Platelets are tiny components in blood that initiate blood clots. Platelets become stimulated when they encounter a damaged blood vessel, and flock to the site. The platelets clump together and form a plug, which reduces bleeding. Platelets also release substances that start the chemical reaction of blood clot formation.
2. Chemical reactions grow the blood clot. Blood contains dissolved proteins, also called clotting factors, which promote blood clots. (Most of the proteins have Roman numerals for names, including factors V, VII, VIII, IX, X, and XI). These signal to and amplify each other’s activity in massive numbers at the site of bleeding. This results in a rapid chemical chain reaction whose end product is fibrin, the main protein forming blood clots. A fibrin-formed blood clot is much tougher and more durable than the platelet plug.
3. Anti-clotting processes halt the blood clot’s growth. Once formed, the blood clot must be prevented from growing and spreading through the body, where it could cause damage. Numerous anti-clotting proteins (antithrombin, protein C, protein S, and others) exist in a natural balance with clotting factors. The anti-clotting enzymes neutralize excess clotting factors, preventing them from extending the blood clot farther than it should go.
4. The body slowly breaks down the blood clot. As the damaged tissue heals, the body slowly degrades the blood clot and reabsorbs it. An enzyme called plasmin is responsible for dissolving the tough fibrin strands in a blood clot. Various other substances act together to activate plasmin and help it break down the clot.
What does a blood clot look like?
Blood is a liquid that flows within blood vessels. It is constantly in motion as the heart pumps blood through arteries to the different organs and cells of the body. The blood is returned back to the heart by the veins. Veins are squeezed when muscles in the body contract and push the blood back to the heart.
Blood clotting is an important mechanism to help the body repair injured blood vessels. Blood consists of:
- red blood cells containing hemoglobin that carry oxygen to cells and remove carbon dioxide (the waste product of metabolism),
- white blood cells that fight infection,
- platelets that are part of the clotting process of the body, and
- blood plasma, which contains fluid, chemicals and proteins that are important for bodily functions.
Complex mechanisms exist in the bloodstream to form clots where they are needed. If the lining of the blood vessels becomes damaged, platelets are recruited to the injured area to form an initial plug. These activated platelets release chemicals that start the clotting cascade, using a series of clotting factors produced by the body. Ultimately, fibrin is formed, the protein that crosslinks with itself to form a mesh that makes up the final blood clot.
Causes of Blood Clots
Blood clots may form inside small veins near the surface of your skin (superficial phlebitis), resulting in localized redness, pain and swelling. Superficial phlebitis rarely causes complications and requires minimal treatment. Blood clots that form inside larger, deeper veins (deep vein thrombosis) may cause more-widespread symptoms in the affected area, usually your leg, and can cause more-serious problems.
Blood clots may also break away from their original source and cause damage elsewhere in your body. Blood clots that break away from a deep vein thrombosis and travel to your lungs may cause a potentially life-threatening blood clot in the lungs (pulmonary embolism).
Blood clots that arise in one of the chambers of your heart, usually due to an irregular heart rhythm such as atrial fibrillation, may travel to your brain and cause a stroke. Blood clots that arise in the arteries of the heart itself may block the flow of blood through that artery and cause a heart attack.
Blood clots that arise from within the carotid arteries in your neck may travel to the brain and cause a stroke.
Factors and conditions that can cause blood clots, as well as serious conditions that are associated with blood clots once they form and travel to other parts of your body, include:
- Antiphospholipid syndrome
- Arteriosclerosis / atherosclerosis
- Certain medications, such as oral contraceptives, hormone therapy drugs and some breast cancer medications
- Deep vein thrombosis
- Factor V Leiden
- Family history of blood clots
- Heart arrhythmias
- Heart attack
- Heart failure
- Obesity
- Peripheral artery disease
- Polycythemia vera
- Pregnancy
- Prolonged sitting or bed rest
- Pulmonary embolism
- Smoking
- Surgery
Alterations to the Blood Clotting Process
Medical conditions and medications can alter the process of blood clot formation, making blood clots more or less likely. Some examples include:
- Aspirin, Brilinta, Effient, and clopidogrel (Plavix). Drugs that interfere with platelet function, making blood clots less likely.
- Warfarin (Coumadin), Eliquis (apixaban), Pradaxa (dabigatran), Xarelto (rivaroxaban). Oral drugs that reduce production of clotting factors, reducing blood clotting.
- Heparin. An intravenous or injectable drug that interferes with thrombin, preventing blood clot formation.
- Tissue plasminogen activator (tPA). A clot-dissolving drug that activates plasmin, and is occasionally given as a treatment for heart attack or stroke.
- Hypercoagulable state. An improper balance between clotting factors and clot-reducing substances that results in increased likelihood of abnormal blood clots.
- Hemophilia. A genetic deficiency of certain functioning clotting factors results in poor blood clotting and excessive bleeding.
- von Willebrand factor deficiency. A relatively common condition resulting in slow blood clot formation, which is usually mild.
Risk Factors for Blood Clots?
The risk factors for arterial clots are those that are common to all diseases that cause narrowing of blood vessels, cholesterol plaque formation, and plaque rupture.
- High blood pressure
- High cholesterol levels
- Diabetes
- Smoking
- Family history
- Cancer
Venous clots are formed due to one of two main reasons: 1) immobility, and 2) genetic errors in the clotting mechanism.
Immobility: Most commonly, when the body stops moving, the risk of blood clots increases, since muscle movement is required to pump blood towards the heart. Stagnant blood in a vein is prone to clot.
- This may occur when a person is hospitalized or bedridden after illness or surgery.
- It may also occur with long trips (such as in a car, train, or plane) where hours may pass without a person getting up to walk or stretch.
- Orthopedic injuries and casting also put the person at risk.
- Pregnancy is a risk factor for forming blood clots in the legs and pelvis, since the growing uterus may slow blood flow back to the heart to a sufficient extent that blood clots may form.
Genetic errors in the clotting mechanism: There may be a genetic or inborn error in the clotting mechanism, making a person hypercoagulable (hyper=more + coagulation= clotting) and at greater risk for forming clots.
