A stroke happens when blood flow to part of the brain is cut off or when a blood vessel in the brain bursts. Brain cells begin dying within minutes from lack of oxygen and nutrients. About 87% of strokes are caused by blockages (ischemic strokes), while the rest are caused by bleeding (hemorrhagic strokes). Stroke accounts for roughly 6% of all deaths across developed countries.
Ischemic Stroke: Blocked Blood Flow
The vast majority of strokes occur when a blood clot or piece of debris blocks an artery feeding the brain. This can happen in two ways. In a thrombotic stroke, a clot forms directly inside one of the brain’s own blood vessels, usually at a spot already narrowed by fatty buildup. In an embolic stroke, the clot forms somewhere else in the body, breaks free, and travels through the bloodstream until it lodges in a smaller brain artery.
The distinction matters because it points to different underlying problems. A thrombotic stroke typically signals disease in the brain’s arteries themselves. An embolic stroke means the source of trouble is upstream, often in the heart or the large arteries of the neck.
How Plaque in the Arteries Triggers a Stroke
Atherosclerosis, the gradual buildup of fatty deposits inside artery walls, is one of the most common drivers of stroke. The carotid arteries on each side of the neck are a frequent trouble spot. Over years, cholesterol and inflammatory cells accumulate beneath the artery’s inner lining, forming a plaque capped by a layer of fibrous tissue. As long as that cap stays intact, blood flows past without issue.
Problems start when the cap thins. Inflammatory cells called foam cells infiltrate the cap, weakening its structure. Bleeding within the plaque itself can also thin and destabilize it. If the cap ruptures, the fatty core underneath is exposed directly to flowing blood, which triggers rapid clot formation at the site. That clot can either block the artery on the spot or break apart and send fragments into the brain. Research comparing symptomatic and asymptomatic patients found plaque rupture in 74% of those who had experienced stroke symptoms, compared to just 32% of those without symptoms.
The Role of High Blood Pressure
Chronic high blood pressure damages the brain’s smallest arteries in ways that set the stage for both types of stroke. The walls of tiny penetrating arteries thicken with scar-like collagen deposits, a process that gradually narrows the vessel and restricts blood flow. In more severe or poorly controlled hypertension, segments of the vessel wall can undergo a destructive process where the normal structure breaks down and is replaced by a glassy, weakened deposit. This weakening can cause tiny bulges to form in the vessel wall, which may eventually rupture and bleed into surrounding brain tissue.
High blood pressure also accelerates atherosclerosis in larger arteries, compounding the risk. It is the single most important modifiable risk factor for stroke of any type.
Heart Conditions That Send Clots to the Brain
The heart itself is a major source of the clots responsible for embolic strokes. Atrial fibrillation, an irregular heart rhythm affecting millions of people, is the most significant cardiac cause. During atrial fibrillation, the upper chambers of the heart quiver instead of contracting in a coordinated way. Blood pools and stagnates in those chambers, and stagnant blood clots. If a clot forms and then ejects into the bloodstream, it can travel directly to the brain.
Over time, atrial fibrillation also remodels the heart’s structure, further worsening the stagnation and raising clot risk even in periods when the rhythm seems to have returned to normal.
A Hole in the Heart
About one in four adults has a small opening between the upper chambers of the heart called a patent foramen ovale (PFO), left over from fetal development. Normally, the lungs act as a filter, trapping small clots from the venous system before they can reach the brain. A PFO creates a shortcut: a clot from a leg vein or the pelvis can cross directly into the arterial side and travel to the brain. The average PFO is about 10 millimeters wide, large enough for a clot to pass through and block the main artery supplying the brain or its branches. Clots can also form within the PFO tunnel itself.
Hemorrhagic Stroke: Bleeding in the Brain
Hemorrhagic strokes occur when a blood vessel in the brain ruptures. The escaped blood compresses surrounding tissue and deprives downstream areas of their blood supply. High blood pressure is the leading cause, weakening small arteries over time until they give way.
Brain aneurysms, balloon-like bulges in artery walls, are another cause. These can exist for years without symptoms and then rupture suddenly, often with a devastating headache. Arteriovenous malformations (AVMs) are a rarer cause, accounting for about 2% of all hemorrhagic strokes each year. An AVM is a tangle of blood vessels where arteries connect directly to veins without the normal network of tiny capillaries in between. This puts extreme pressure on vessel walls that were never built to handle it, making them thin and fragile. Over time, the walls may bulge or rupture.
Artery Tears in Younger Adults
Cervical artery dissection is a common cause of stroke in people between the ages of 40 and 60. It occurs when the inner lining of a carotid or vertebral artery in the neck tears. Blood seeps into the vessel wall at the tear site, and a clot forms. If that clot breaks loose, it can travel to the brain and block blood flow, causing an ischemic stroke. These dissections can happen after trauma, vigorous exercise, or chiropractic manipulation of the neck, but they also occur spontaneously in people with no obvious trigger.
Lifestyle Factors That Raise Your Risk
Several everyday health conditions dramatically increase the likelihood of stroke. Smoking damages blood vessel walls, accelerates plaque buildup, and makes blood more prone to clotting. Combined with prediabetes (blood sugar that’s elevated but not yet in the diabetic range), long-term tobacco use triples the risk of stroke even in otherwise healthy young adults aged 18 to 44. That finding held up after researchers accounted for other variables like age, race, income, and alcohol use.
Type 2 diabetes damages small blood vessels throughout the body, including in the brain. Obesity promotes inflammation and is closely linked to high blood pressure, high cholesterol, and diabetes, all of which independently raise stroke risk. Physical inactivity, heavy alcohol use, and diets high in sodium and processed food further contribute. Most strokes are not random events. They are the end result of years of accumulating vascular damage from treatable conditions.
Transient Ischemic Attack: The Warning Stroke
A transient ischemic attack (TIA) is caused by the same mechanisms as a full stroke, a temporary blockage of blood flow to the brain, but symptoms resolve within minutes to hours and don’t cause permanent damage. It is not a minor event. About 1 in 3 people who experience a TIA will eventually have a full stroke, and roughly half of those strokes occur within the first year. A TIA is the clearest warning the body gives that a major stroke is on its way, and it represents a narrow window to identify and treat the underlying cause before permanent brain damage occurs.