During a thrombotic stroke a clot forms on pre-existing deposits on the artery walls blocking blood flow in an artery feeding the brain. Many thrombotic strokes occur in large blood vessels. A second type of ischemic stroke, small vessel thrombotic stroke, is called lacunar because a small cavity opens within the brain after this kind of stroke. The second category of stroke is hemorrhagic, a breaking, tearing or bursting in the wall of a blood vessel. Blood not only fails to deliver its nutrients, but it also causes direct physical damage.
Clotted blood takes up space in a closed system, increasing pressure on the brain.
In addition, the blood's weight presses on the brain cells beneath it. Hemorrhagic strokes are called subarachnoid when they occur between the skull and the dura, or the delicate membrane surrounding the brain. When the hemorrhage is below the dura, within the brain, it is intracerebral. Aneurysms are a common source of hemorrhagic strokes. Aneurysms are balloon-like bulges of artery walls.