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Blood · Visual journey

Blood clots: when they protect you and when they block blood flow.

When you cut yourself, your blood forms a plug that breaks down as the cut heals. But a clot in a leg vein can break loose and travel to the lungs.

Illustration of a blood vessel torn open in the middle, with red blood cells and one white blood cell spilling out through the gapTearThis is where blood leaks out

Blood vessel

First, stop the leak.

The vessel narrows, and platelets, which are fragments of cells, stick to the tear and form a plug.

Illustration of the end of a blood vessel sealed by a platelet plug, with yellow strands of fibrin running through it and trapping red blood cellsFibrinStrands that hold the plug in place

Plug

Then, a mesh to hold it in place.

One after another, clotting factors turn fibrinogen into fibrin, a web of strands that anchors the plug.

Under the microscope

Platelets and fibrin in the same clot.

Fibrin shows up red and platelets green. Where they overlap, they look yellow or orange.

Confocal micrograph of a human blood clot: a mesh of red fibrin strands wraps around clusters of green platelets, which look yellow where they overlap with the red

Platelets: they form the first plug.

That mesh depends on vitamin K.

Vitamin K

The vitamin that makes the mesh possible.

Your liver needs vitamin K to make several clotting factors, and an enzyme recycles the vitamin so the liver can use it again. That recycling step is exactly where one medicine and one poison strike.

A medicine

Warfarin slows the recycling on purpose.

As a result, the liver makes fewer active clotting factors. That’s what makes warfarin an anticoagulant, or blood thinner.

It doesn’t dissolve clots. Instead, it keeps existing clots from getting bigger and new ones from forming.

A poison

Some rat poisons attack the same step.

They block the same enzyme, but with no doctor keeping the dose in check. Bleeding can take as long as five days to start.

Illustration of see-through legs with a magnified circle over a deep vein, showing a red clot inside the veinThrombusA clot inside the vein, with no wound to seal

When a clot forms where it shouldn’t

A clot inside a vein in the leg.

This is deep vein thrombosis, or DVT. It usually forms in the calf, thigh or pelvis.

The risk goes up after hours without moving or after surgery, and with cancer, pregnancy, birth control or hormone therapy.

From the leg to the lungs

If a piece breaks off, it can get stuck in the lung.

That piece, called an embolus, passes through the heart and blocks an artery in the lung. This is a pulmonary embolism, and it can be life-threatening.

Illustration of the heart and lungs: a circle marks a branch of the pulmonary artery, and an enlarged inset shows a clot blocking itEmbolusThe clot that traveled here blocks an artery in the lung

What you can control

Keep moving, and catch the warning signs early.

Not every clot can be prevented. But staying active and spotting the signs early are things you can do.

What you can do

On a long trip, get up and walk around every hour or two.

If you can’t get up, move your ankles and knees.

Before surgery or a hospital stay, ask your care team about a plan to prevent clots.

If you take warfarin

Take it exactly as prescribed.

Don’t change the dose on your own, get your INR checked on schedule and keep the amount of vitamin K in your diet steady. Check with your doctor before adding any medicine, supplement or aspirin.

If you take a blood thinner

Tell your doctor right away about any bleeding you can’t explain.

Watch for nosebleeds or bleeding gums that won’t stop, pink or brown urine, red or black stools, bruising, coughing up or vomiting blood, and dizziness or weakness.

At home

Keep rat poison where children and pets can’t get to it.

Could someone have eaten rat poison? Call 911 or Poison Control now, even if they seem fine. If it’s a pet, call your vet.

I’m your friend, Dr. Carlo De la Sancha. Health and science for everyone.

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Where the information comes from

Images

In the illustrations, the original English labels were erased and the white background was made transparent; none of the anatomy was redrawn. The opening microscope image is a version of the cited micrograph with more depth, and the micrograph itself appears cropped further down the page. The arrows were added for this page.

Educational content for the general public. It doesn’t diagnose blood clots or pulmonary embolism, and it doesn’t recommend treatment. Diagnosis and any decisions about blood thinners are up to your health care team.

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