The pressure of blood inside vessels keeps them distended. If there was no blood in them, they would collapse, arteries or veins.
Intracranial vessels face two pressures:
1.The distending pressure of the blood within -> determined by the MAP
2.The collapsing pressure from outside -> this is the ICP
If the ICP increases to greater than the MAP, it will force intracranial vessels to collapse. This will cause ischemia. This could happen, eg, if the ICP is 200mmHg and the MAP is 150mmHg.
Before outright collapse, though, you can have varying degrees of compression. Eg a CPP of 100mmHg might allow intracranial vessels to remain at an optimum cross sectional area, but a CPP of 40mmHg could cause some compression and reduce that area. Area reduction means increased resistance, which means decreased flow.
In strokes etc, bleeding and/or intracranial hemorrhage increases ICP. The body reflexively increases the MAP to maintain CPP.
