the cranial cavity or everything within the skull contains three main constituents blood cerebral spinal fluid and brain tissue in total this makes up a volume of 1700 mils where blood both arterial and venous is 150 ml or 10 of the total volume csf 150 mils or 10 of the total volume and brain tissue 1400 mils or 80 of the total volume each of these three volumes contributes to the intercranial pressure which needs to be maintained between 7 and 15 millimeters of mercury now because the skull is rigid any increase in one of these three volumes will increase the intercranial volume and therefore the intercranial pressure this relationship is known as the monroe kelly doctrine or the monroe kelly hypothesis where the interchangeable volume equals the brain volume plus the csf volume plus the blood volume now if there is a small rise in one of these three volumes let's say blood volume increases through vasodilation the other two volumes will decrease to maintain the intercranial volume and therefore the intercranial pressure to protect the brain but if there's a significant rise let's say there's a brain tumor in the brain tissue and the brain volume increases there's only so much that the other two can decrease until the intercranial pressure starts to rise and therefore the brain will be negatively impacted my next video will talk about the increases in intercranial pressure