system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do brain bleeds (like stroke or aneurysm) kill us? Isn't there plenty of other veins in our head that can transport blood around while the broken one heals?
gpt
Strokes (like heart attacks) occur when a vessel that supplies blood  becomes blocked, cutting off blood supply distal from the area of blockage and causing cell death. 

You are correct that brain tissue is very vascularized. The brain can be subdivided into main vascular 'territories' that are each supplied by a major cerebral artery. Although a given part of the brain has a 'main' blood supply, it also has other 'collateral' blood vessels that it can draw blood from if the main artery becomes blocked. However, the collateral vessels tend to be much smaller and cannot supply as much blood compared to the major arteries, and thus cannot fully compensate for the loss of a major cerebral artery. So collateral vessels can limit the extent of damage to the brain, but they cannot fully eliminate it.

Regarding brain aneurysms, they are not deadly because of blood loss - most brain aneurysms are quite small, and the amount of blood loss from an aneyrysm rupture is relatively minimal (not enough to kill you). The reason why brain aneurysms are deadly is the blood itself. Blood contains a lot of substances that cause inflammation. Blood has a toxic effect when in direct contact with brain cells. Blood normally does not come into direct contact with brain cells, but when a brain aneurysm ruptures, blood leaks into the subarachnoid space (the fluid pocket in which the brain bathes) and damages brain cells. The inflammatory reaction also causes vasospasm (sudden constriction of blood vessels) which cuts off blood supply to the brain and further damages brain tissue similarly to a stroke.