Production, function, and composition of CSF

Modified ultrafiltrate of plasma in the ventricles of brain and the subarachnoid space

Total 150mL (25mL ventricles, 25mL spinal cord, 100mL subarachoid space).

Secretion from coroid plexus - 25ml/hr

Ultrafiltrate of plasma formed in coroidal interstitium

Coroid plexus cells have tight junctions that prevent passive ion flow into CSF

Coroid plexus cells use Na/K ATPase to primary actively transport sodium into CSF

Bicarbonate and chloride follow by secondary active transport with sodium

Water follows transcellularly by osmosis

Secretion is constant, not CPP dependent, but falls when CPP<55

Composition vs plasma

Na similar

K much lower - preserves excitability

Protein almost absent - therefore Cl and HCO3 higher

Glucose \(\frac{2}{3}\) plasma

pH similar to blood to facilitate central chemoreceptor function

Function

Barrier function ("blood-CSF barrier")

Chemical stability and waste removal (flowing CSF prevents accumulation of waste products in CNS)

Buoyancy - reduce net weight of brain to 50g

Mechanical cushion function

Hydraulic pressure buffering - \(\uparrow\)ICP \(\to\) CSF moves into spinal canal

Reabsorption

At arachnoid granulations

Rate \(\propto P_{\text{CSF}} - P_{\text{venous}} \approx P_{\text{CSF}} - 7\)