Blood Clots
Medical Author: Benjamin C. Wedro, MD, FAAEM
Medical Editor: Melissa Conrad Stöppler, MD
What are blood clots?
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 propelled back to the heart in the veins. When muscles
contract, they squeeze the veins and allow the blood to be pushed back to the
heart.
Blood clotting is an important mechanism to help the body repair injured
blood vessels. Blood consists of:
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, activating a series of
clotting factors. Ultimately, fibrin is formed, the protein that crosslinks
with itself to form a mesh that makes up the final blood clot.
The medical term for a blood clot is a thrombus (plural=
thrombi). When a
thrombus is formed as part of a normal repair process of the body, there is
little consequence. Unfortunately, there are times when a thrombus (blood clot) will form
when it is not needed, and this can have potentially significant consequences.
Picture of a blood clot is formed
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 laceration, or may occur
on the microscopic level. As well, blood will begin to clot if it stops moving
and becomes stagnant.
Venous thrombosis or blood clots in a vein 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 lining walls of the
vein that gradually grow to partially or completely occlude the vein. An analogy
to this process is a slow moving river. Over time, weeds and algae start to
accumulate along the banks of the river where the water flows more slowly.
Gradually, as the weeds start to grow, they begin to invade the center of the
river because they can withstand the pressure of the water.
Arterial thrombi (blood clots in an artery) occur by a
different mechanism. For those with atherosclerotic disease, plaque deposits
form along the lining of the artery and grow to cause narrowing of the vessel.
This is the disease process that may cause
heart attack, stroke, and
peripheral
artery disease. If a
plaque ruptures, a blood clot can form at the site of the ruptured plaque that
can completely or partially occlude the blood flow at that point.
Blood clots can also form in the heart. In
atrial
fibrillation, the atrium or
upper chamber 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 can also 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.
Next: What are the risk factors for blood clots? »
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