Blood clots form through complex interactions between substances in blood and blood vessel walls. Platelets initiate clot formation by clumping at damaged sites and releasing chemicals. Blood clotting factors then trigger a chemical reaction producing fibrin to strengthen the clot. Anti-clotting proteins regulate clot growth. The body slowly breaks down clots as tissue heals. Blood clots can form abnormally and cause health issues when substances that promote clotting come into contact with blood, such as if a cholesterol plaque or blood vessel ruptures, or if blood flow is impaired.

1. Blood Clots
Blood clots are healthy and lifesaving when they
stop bleeding. However, bloodclots can also form
abnormally, causing a heart attack, stroke, or other
serious medical problems.
How Does Blood Clot?
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. 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 anticlotting 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.

3. 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 Causes Blood Clots?
The process of blood clotting is triggered whenever
flowing blood is exposed to certain substances.
There are many different such substances, which
are called thrombogenic because they promote
formation of thrombus (another name for a clot).
Many thrombogenic substances are located in
the skin or in blood vessel walls. Normally safely
separated from flowing blood, their contact with
blood usually means the blood vessel wall is
ruptured and bleeding. Examples of these
thrombogenic substances are tissue factor,
collagen, and von Willebrand factor.

4. Most heart attacks and strokes result from the
sudden formation of a blood clot on a
waxy cholesterol plaque inside an artery in the
heart or brain. When the plaque ruptures suddenly,
thrombogenic substances inside the plaque are
exposed to blood, triggering the blood clotting
process.
Blood clots may also form when blood fails to flow
properly. Atrial fibrillation is an abnormal heart
rhythm in which blood pools in the heart,
potentially forming blood clots. If a blood clot
dislodges and travels to the brain, it can cause a
stroke. Prolonged immobilization can reduce blood
flow in the legs, increasing the risk for blood clots in
leg veins (deep venous thrombosis, or DVT).
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