What Causes Blood Clots?

What causes blood clots?

Blood clots form when there is damage to the lining of a blood vessel, either an artery or a vein. The damage may be obvious, such as a cut or laceration, or may not be visible to the naked eye. Blood also will begin to clot if it stops moving and becomes stagnant, or in diseases that cause the blood to clot abnormally.

Blood clots in a vein (venous thrombosis) occur when a person becomes immobilized and muscles are not contracting to push blood back to the heart. This stagnant blood begins to form small clots along the walls of the vein. This initial clot can gradually grow to partially or completely block (occlude) the vein and prevent blood from returning to the heart.

Blood clots in an artery (arterial thrombi) occur by a different mechanism. For those with atherosclerotic disease, plaque deposits form along the lining of the artery and grow, which causes the vessel to narrow. This disease process may cause:

• Heart attack
• Stroke
• Peripheral artery disease (PAD)
• If a plaque ruptures, a blood clot can form at the site of that rupture and can completely or partially block the blood flow at that point.

What are other causes blood clots?

Blood clots in the heart: In atrial fibrillation, the upper chamber (atrium) of the heart does not beat in an organized manner. Instead, it jiggles, and blood tends to become stagnant along the walls of the atrium. Over time this may cause small blood clots to form. Clots also can form in the ventricle after a heart attack when part of the heart muscle is injured and unable to contract normally. Since the damaged area doesn't contract with the rest of the heart, blood can start to pool or stagnate, leading to clot formation.

Blood leaking out of a blood vessel: Blood clots can form when blood leaks out of a blood vessel, and this process can be very beneficial because the clot helps stop further bleeding at the site of injury. 

A few examples of how bleeding is controlled by the body's clotting mechanism are

• Cuts or scrapes
• Broken bones
• Sprains and strains
• Nosebleed

Blood clots causing other medical problems: Sometimes, normal blood clotting can cause medical problems because of its location. For example, blood in the urine may occur from any of a variety of reasons (such as infection, trauma, or tumor/cancer), and clots may form over the urethra, the tube that empties the bladder, preventing the bladder from emptying, causing urinary retention.

Clot formation in the uterus may cause pain when the clots are passed through the cervix and can lead to vaginal bleeding, either as part of menstruation or as abnormal vaginal bleeding (menorrhagia, dysmenorrhea).

Signs and symptoms of blood clots in the veins

Blood clots in the veins do not allow blood to return to the heart, and symptoms occur because of this "damming effect." 

These clots often occur in the legs or the arms, symptoms include:

• Swelling
• Warmth
• Redness
• Pain

Most often, only one leg or arm is affected and the swelling occurs over the course of many hours. Because the leg or arm becomes red, warm, and swollen, it is sometimes difficult to decide whether the cause is a DVT (deep vein thrombosis) or an infection.

On occasion, the whole leg may become very swollen, painful and turn bluish in color due to a blood clot located in the femoral vein in the upper leg or the iliac vein in the pelvis. This is called phlegmasia cerulia dolens. A similar situation may occur in the arm if the blood clot affects the subclavian vein located in the chest.

Signs and symptoms of blood clots in the arteries

Blood clots in the arteries do not allow blood to pump to an affected area. Body tissue that is deprived of blood and oxygen begins to die and becomes ischemic. Symptoms of blood clots in the arteries depend upon the location of the clot.

Heart attack: Blood clots in the coronary arteries of the heart cause a heart attack. 

Symptoms of a heart attack may include:

• Chest pain or pressure
• Shortness of breath
• Nausea
• Sweating
• Indigestion
• The pain also may radiate to the arm, jaw, or back.

Stroke or transient ischemic attack (TIA or "mini-stroke"): Blood clots to arteries in the brain may cause a stroke or transient ischemic attack (TIA). Symptoms may include:

• Loss of speech
• Loss of vision
• Profound dizziness
• Weakness and/or loss of sensastion on one side of the body

FAST is the memory tool to remember regarding the symptoms for stroke:

• Facial drooping
• Arm weakness
• Speech difficulties

Time (time is of the essence to try to reverse the stroke process. Call 911 and go to a hospital.)

Blood clots that involve the mesenteric arteries that supply the intestine can cause significant abdominal pain, vomiting,blooating and blood in the stool. This is called mesenteric ischemia.

What are the risk factors for forming 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, including:

• High blood pressure
• High cholesterol levels
• Diabetes
• Smoking
• Family history of early heart attack or stroke
• Cancer

Blood clots in the veins are formed due to one of two main reasons: 1) immobility, and 2) genetic errors in the clotting mechanism. There are other associated risk factors including smoking and the use of birth control pills.

Immobility: Commonly, when the body stops moving, the risk of blood clots increases since muscle movement is required to pump blood toward the heart. Stagnant blood in a vein is prone to clot. 

Examples of how blood clots may occur from immobility include:

• Being hospitalized or bedridden after illness or surgery
• Taking long trips (such as in a car, train, or plane), when hours may pass without standing to move, walk, or stretch, and blood pools in the leg veins and may potentially clot
• Orthopedic injuries and/or has casts placed over broken bones or limbs
• Undergoing knee or hip replacement
• Pregnancy is a risk factor for forming blood clots in the legs and pelvis, due to insufficient blood flow back to the heart.
• Immobility due to paralysis from a stroke or spinal cord injury.

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.

source: medicinenet