THE BRAIN

MONRO-KELLIE DOCTRINE & CEREBROSPINAL FLUID

PART TWO

Sources: 

propofology.com

Dr. David Lyness

80% BRAIN

The Monro-Kellie hypothesis states that the cranial compartment is incompressible,
and the volume inside the cranium is a fixed volume. 

The cranium and its constituents (blood, CSF, and brain tissue) create a state of volume equilibrium, such that any increase in volume of one of the cranial constituents must be compensated by a decrease in volume of another.

There is good maintenance of a constant ICP for displacement of up to 100-125ml

Cerebrospinal fluid (CSF) is a clear, colorless body fluid

Chemical Stability: CSF flows through the ventricular system and is absorbed back into the bloodstream, rinsing the metabolic waste from the central nervous system through the BBB. This allows for homeostatic regulation of the distribution of neuroendocrine factors. e.g.  high glycine concentration disrupts temperature and blood pressure control, and high CSF pH causes dizziness and syncope. 

CSF has a "sink action" by which the various substances formed in the nervous tissue during its metabolic activity diffuse rapidly into the CSF and are thus removed into the bloodstream as CSF is absorbed

The brain produces roughly 500 mL of cerebrospinal fluid per day. This fluid is constantly reabsorbed, so that only 100-160 mL is present at any one time.

The CSF is created from blood plasma and is largely similar to it, except that CSF is nearly protein-free compared with plasma and has some modified electrolyte levels

1400g

Arachnoid granulations (or arachnoid villi) are small protrusions of the arachnoid, through the dura mater. They act as one-way valves for CSF.

Normal ICP is 5 to 15 mmHg. 

Rate of formation of CSF is constant and is not affected by ICP. Absorption of CSF increases linearly as pressure rises >7 cmsH2O. 

It is said that CSF reduces, 'in effect' the net weight of the brain from 1400g to 50g!