How a Stroke Happens

What happens

• A stroke happens when blood flow to part of the brain is cut off by a clot, or a blood vessel in the brain bursts.
• Most strokes are caused by a clot. Brain cells starved of oxygen begin to die within minutes.
• A TIA (mini-stroke) causes signs that go away, but it is a serious warning and an emergency.
• Think FAST: Face drooping, Arms weak, Speech slurred or jumbled, Time to call 9-1-1.
• About 9 in 10 strokes are linked to risk factors we can change. High blood pressure is the biggest.

Know the warning signs

• Face: is it drooping on one side?
• Arms: can the person raise both arms and keep them up?
• Speech: is it slurred or jumbled?
• Time: call 9-1-1 or your local emergency number right away, even if the signs go away.

Note the time the signs started. Don't drive to the hospital. An ambulance can start care on the way and go to a hospital that treats strokes.

Call 9-1-1 or your local emergency number.

What you can do

• Get your blood pressure checked, and if it's high, work with your care team to bring it down.
• Ask about atrial fibrillation (an irregular heartbeat). Treatment lowers the stroke risk it causes.
• Don't smoke, and limit alcohol.
• Stay active, and eat plenty of vegetables, fruit and whole grains, with less salt.
• Keep diabetes under control.

Common questions

What are the signs of a stroke?

Think FAST. Face: is it drooping on one side? Arms: can the person raise both arms and keep them up? Speech: is it slurred or jumbled? Time: call 9-1-1 or your local emergency number right away, even if the signs go away.

What is a mini-stroke (TIA)?

A transient ischemic attack happens when a clot blocks a brain artery only briefly. The signs may last just a few minutes, but a bigger stroke may follow, often within days. Treat it as an emergency and call 9-1-1.

Can young people have a stroke?

Yes. Stroke can happen at any age. Worldwide, about half of all strokes happen in people younger than seventy.

Transcript

Your brain makes up only about two percent of your body weight. Yet it uses about a fifth of all the oxygen your body takes in.

And it keeps almost no oxygen in reserve, so it needs a steady flow of blood, every second. When that flow is cut off, or a blood vessel bursts, that's a stroke. Let's see how it happens, and how to spot one fast.

Blood reaches your brain through four main arteries in your neck: two carotid arteries at the front, and two vertebral arteries at the back.

Inside the skull, they branch into smaller and smaller vessels, reaching every part of the brain.

And each part of the brain has its own jobs. One area moves your arm. Another helps you speak. Another lets you see. So the signs of a stroke depend on which area loses its blood supply.

There are two main kinds of stroke. Most are caused by a clot that blocks an artery. This is called an ischemic stroke.

The clot can form right inside a brain artery that's been narrowed by fatty plaque. Or it can form somewhere else, often in the heart, then travel up with the blood until it gets stuck.

Beyond the clot, brain cells are starved of oxygen. Within minutes, they begin to die.

The other kind is a hemorrhagic stroke. A weakened blood vessel bursts, and blood leaks into or around the brain. Years of high blood pressure are a common cause.

The leaking blood harms brain cells, and builds up pressure inside the skull.

Sometimes, a clot blocks an artery only briefly, and then clears. The signs may last just a few minutes. This is called a transient ischemic attack, or TIA. Some people call it a mini-stroke.

Even if the signs go away, a TIA is a serious warning. A bigger stroke may follow, often within days. So treat it as an emergency, too.

So, how do you spot a stroke? Think FAST. F is for face. Is it drooping on one side?

A is for arms. Can the person raise both arms, and keep them up?

S is for speech. Is it slurred or jumbled?

And T is for time. Time to call 9-1-1 or your local emergency number, right away. Even if the signs go away.

Note the time the signs started, and tell the paramedics. Don't drive to the hospital. An ambulance can start care on the way, and take the person to a hospital that treats strokes.

Every minute counts, because time is brain. In a typical large stroke, almost two million brain cells are lost for every minute without treatment.

In hospital, a brain scan shows whether it's a clot or a bleed. That matters, because a clot-busting drug that helps one could make the other worse.

For a clot, the medicine can dissolve it, but it can only be given within the first few hours. For some large clots, doctors thread a thin tube up to the brain, and pull the clot out.

Now for the good news. A study of strokes in thirty-two countries found that about nine in ten were linked to risk factors we can change.

The biggest one by far is high blood pressure. It usually has no symptoms, so get it checked. If it's high, treatment can bring it down.

Also ask about an irregular heartbeat called atrial fibrillation. It lets clots form in the heart, and makes a stroke about five times more likely. Treatment can lower that risk.

Don't smoke. Stay active, and eat plenty of vegetables, fruit and whole grains, with less salt. Keep diabetes under control, and limit alcohol.

One common myth is that strokes only happen to older people. In fact, about half of all strokes worldwide happen in people younger than seventy.

So, to recap. A clot blocks, or a vessel bursts. Brain cells start to die. Spot it with FAST. And call 9-1-1 right away.

Now you know how a stroke happens, and how to spot one fast. Share FAST with the people you love. It could save a life.

Sources

1. Heart & Stroke Foundation of Canada: Signs of stroke (FAST)
2. Heart & Stroke Foundation of Canada: What is stroke?
3. Heart & Stroke Foundation of Canada: Atrial fibrillation
4. O'Donnell MJ et al. INTERSTROKE, The Lancet 2016
5. Saver JL. Time is brain — quantified, Stroke 2006
6. World Stroke Organization: Global Stroke Fact Sheet 2025