A stroke happens when blood flow to part of the brain is cut off or when a blood vessel in the brain bursts. In both cases, brain cells begin dying within minutes. The brain loses roughly 1.9 million neurons, 14 billion synapses, and 7.5 miles of nerve fibers for every minute blood flow is disrupted. That speed is what makes stroke a medical emergency and why the phrase “time is brain” exists in emergency medicine. Globally, about 94 million people are living with the effects of stroke, and it kills more than 7 million people each year.
The Two Main Types of Stroke
Strokes fall into two broad categories based on what goes wrong with blood flow. Roughly 87% of strokes are ischemic, meaning a blockage stops blood from reaching part of the brain. The remaining cases are hemorrhagic, meaning a blood vessel ruptures and bleeds into or around the brain.
A third, related event is a transient ischemic attack, sometimes called a “mini-stroke.” A TIA produces the same symptoms as a full stroke but usually resolves within minutes and almost always within an hour. It doesn’t cause permanent damage on its own, but it’s a serious warning: about 1 in 3 people who have a TIA will go on to have a full stroke, and roughly half of those strokes happen within the following year.
How Ischemic Strokes Block Blood Flow
Two mechanisms cause the blockage in an ischemic stroke. The first is thrombosis, where a clot forms directly inside a brain artery. This usually happens at a spot already narrowed by fatty plaque buildup on the vessel wall. The clot grows until it chokes off blood flow at that location.
The second is embolism. Here, a clot or piece of debris forms somewhere else in the body, often in the heart or a large artery in the neck, breaks loose, and travels through the bloodstream until it lodges in a narrower vessel in the brain. Either way, the tissue downstream of the blockage is suddenly starved of oxygen and glucose.
How Hemorrhagic Strokes Cause Damage
When a blood vessel in the brain ruptures, the damage comes from two directions at once. First, the pooling blood forms a mass that physically compresses surrounding brain tissue. This pressure, called mass effect, can push structures in the brain out of position. When the bleed is large (greater than about 60 cubic centimeters), the compression can force brain tissue downward into the brain stem, a situation called herniation that is fatal in over 90% of cases within 30 days.
Second, blood itself is toxic to brain tissue. As red blood cells break down over the following days and weeks, they release hemoglobin and iron into the surrounding brain. Iron triggers a cascade of chemical reactions that produce highly destructive molecules called free radicals. These free radicals attack the proteins, fats, and DNA inside nearby cells. Iron concentrations in the tissue around a brain bleed continue rising for up to 30 days, which is why damage from a hemorrhagic stroke unfolds over a much longer timeline than the initial event might suggest. Swelling around the bleed, driven by both iron toxicity and the body’s clotting response, adds further pressure on healthy tissue.
What Happens Inside Dying Brain Cells
The chain of events that kills brain cells during an ischemic stroke is called the ischemic cascade, and it starts almost immediately. When oxygen and glucose stop arriving, neurons can no longer maintain the electrical balance across their membranes. They depolarize, essentially firing uncontrollably, and release large amounts of a chemical messenger called glutamate into the spaces between cells.
Glutamate is normally a routine signaling molecule, but in these concentrations it becomes toxic. It forces open channels on neighboring neurons that flood them with calcium. This calcium overload is the turning point. It activates enzymes that begin dismantling the cell from the inside, damages the cell’s energy-producing structures (mitochondria), and triggers the production of free radicals that tear apart cell membranes and DNA. The mitochondria, now overwhelmed, start leaking more free radicals, creating a feedback loop of destruction.
On top of this, the dying cells activate inflammatory pathways. Receptors on the cell surface trigger a programmed self-destruct sequence. The combined effect of calcium overload, free radical damage, and inflammation means that cells die in waves, not all at once. The core of the affected area dies within minutes, but surrounding tissue remains at risk for hours, which is why restoring blood flow quickly can save a significant amount of brain.
Recognizing a Stroke: BE-FAST
The symptoms of a stroke depend on which brain region loses blood supply, but a screening tool called BE-FAST captures the most common warning signs:
- Balance: sudden difficulty walking or loss of coordination
- Eyes: sudden vision loss, double vision, or blurring
- Face: one side of the face droops or feels numb
- Arm: sudden weakness or numbness in one arm
- Speech: slurred speech or difficulty finding words
- Time: call emergency services immediately
The “B” and “E” were added to the older FAST acronym because balance problems and vision changes account for a meaningful number of strokes that people fail to recognize in time.
Why High Blood Pressure Is the Biggest Risk Factor
Blood pressure is the single most important modifiable risk factor for stroke. The risk begins climbing at any reading above 115/75 and rises steadily from there. A 10-point increase in systolic blood pressure (the top number) is associated with a 38% higher stroke risk. The relationship works in reverse too: a 10-point reduction through treatment lowers stroke risk by about 31%, regardless of which type of blood pressure medication is used. Other major risk factors include atrial fibrillation, diabetes, smoking, and high cholesterol, but none of them carry the same dose-dependent relationship that blood pressure does.
How Treatment Works Against the Clock
For ischemic strokes, the goal is to reopen the blocked artery as fast as possible. A clot-dissolving medication can be given intravenously within the first few hours. For strokes caused by a large clot in a major brain artery, a procedure called mechanical thrombectomy can physically retrieve the clot using a catheter threaded up from an artery in the groin. Current guidelines support thrombectomy within 6 hours of symptom onset, and in selected patients with brain imaging showing salvageable tissue, the window extends to 16 or even 24 hours.
Hemorrhagic strokes are treated differently. The priorities are controlling blood pressure to slow the bleeding, managing brain swelling, and in some cases surgically draining the accumulated blood. There is no equivalent of a clot-busting drug for hemorrhagic stroke, and in fact, giving one would make the bleeding worse, which is why brain imaging to distinguish the two types is always done before treatment begins.
How the Brain Recovers After a Stroke
The brain’s ability to reorganize itself after injury, known as neuroplasticity, is what makes recovery possible. In the weeks and months following a stroke, surviving neurons near the damaged area undergo physical changes. They sprout new branches, remodel their connections, and form new synapses. These structural changes are driven by specific genetic programs that activate in the tissue surrounding the injury.
At the network level, something more remarkable happens. The brain selectively recruits the most excitable and responsive surviving neurons into circuits that previously ran through the now-dead tissue. Researchers describe this as “functional allocation,” where neurons are essentially reassigned to take over lost duties based on their activity levels and molecular readiness. Over time, brain activity patterns begin to normalize, and movement-related signals that were disrupted by the stroke gradually return to something closer to their pre-stroke patterns.
This is why rehabilitation works. Physical therapy, speech therapy, and occupational therapy are not just exercises. They provide the repetitive input the brain needs to select which neurons get allocated to which circuits. The balance between excitatory and inhibitory signaling in the brain shifts after a stroke, and targeted rehabilitation helps restore that balance, which is critical for regaining function. Most recovery happens in the first three to six months, but meaningful improvement can continue for a year or longer depending on the severity of the stroke and the intensity of rehabilitation.