The brain depends on a nourishing blood supply to work right. High blood pressure may affect the brain in the following ways:

Transient ischemic attack (TIA). Sometimes this is called a ministroke. A TIA happens when the blood supply to part of the brain is blocked for a short time. Hardened arteries or blood clots caused by high blood pressure can cause TIAs. A TIA is often a warning sign of a full-blown stroke.
Stroke. A stroke happens when part of the brain doesn't get enough oxygen and nutrients. Or it can happen when there is bleeding inside or around the brain. These problems cause brain cells to die. Blood vessels damaged by high blood pressure can narrow, break or leak. High blood pressure also can cause blood clots to form in the arteries leading to the brain. The clots can block blood flow, raising the risk of a stroke.
Dementia. Narrowed or blocked arteries can limit blood flow to the brain. This could lead to a certain type of dementia, called vascular dementia. A single stroke or multiple tiny strokes that interrupt blood flow to the brain also can cause vascular dementia.
Mild cognitive impairment. This condition involves having slightly more troubles with memory, language or thinking than other adults your age have. But the changes aren't major enough to impact your daily life, as with dementia. High blood pressure may lead to mild cognitive impairment.