Before we dive into talking about brain injury, we want to introduce something scientists call the Monroe-Kellie Doctrine, which is important for understanding how traumatic brain injury (TBI) happens. Think about your skull like a suitcase. If you are packing for a trip, you can only fit in a certain number of items. So, if you pack too many clothes, you would not be able to pack other things, like your favorite book. The Monroe-Kellie Doctrine states that there is a limited amount of space in the skull to hold all of the cells that make up the brain tissue, the blood that supplies oxygen to the tissue, and a special type of fluid that cushions the brain, called cerebrospinal fluid. So, because the space is limited, if there is an increase in any one of these three things, one or both of the other two will have to decrease. That means that if you hit your head and the brain swells, there will be less room for blood, which could mean the brain will become even more injured.

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