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The brain is encased in bone.
With the exception of a few foramina, the cranium can almost be thought of as a static container — that is, it doesn't flex to accomodate the volume of its contents.
You can see how this might cause problems.
If we looked inside your skull right now, we'd find different volumes of cerebrospinal fluid (CSF), blood, and brain tissue.
Each of these exert outward force, like air in a balloon. We call this force "intracranial pressure" (ICP).
Which leads us to the Monroe-Kelly doctrine.
According to this doctrine, the volume of brain, blood, and CSF in the skull is roughly stable. If one increases, others must decrease.
This lets us compensate for changes in ICP, but only up to a certain threshold. After all, blood volume is finite, and we need a certain amount to survive.
Speaking of which:
Getting blood to the brain is hard.
Not only are we (usually) pumping against gravity, we also have to overcome ICP.
In practice, this means MAP must exceed ICP for cerebral perfusion pressure (CPP) to be moving in the right direction. Said another way:
CPP = MAP - ICP
Without enough CPP, we cannot survive.
In this context, it's easy to see how haemodynamic changes can have such profound and rapid effects.
If MAP falls too low or ICP climbs too high, the brain doesn't get enough blood. And it's not a huge leap from cerebral hypoperfusion to brain damage.
Then, from brain damage, it's an even smaller leap to get to cerebral oedema.
Unsurprisingly, fluids are fluid.
Under ordinary circumstances, this fluid is kept where it belongs, like inside blood vessels or cells.
But when things go wrong, it can escape.
Abnormal fluid accumulation is known as oedema — in the brain, we call it cerebral oedema.
There are two main types of cerebral oedema:
If enough fluid accumulates, it can exhaust the brain's ability to compensate, leading to brain damage and death.
Of course, cerebral oedema is not the sole cause of raised ICP.
But it works well to illustrate the concepts of oedema (which can occur in many different parts of the body), and raised ICP (which can occur in many different pathologies).
Plus, it ties in nicely with our next topic.