Describe the formation, circulation and functions of cerebrospinal fluid.
To achieve a pass in this question, candidates needed to state where and how CSF was
formed, where it flows to after formation followed by a list of its functions. Additional
credit was given for knowledge of rates of production and basic CSF composition. The fact
that CSF production is constant whilst its absorption is pressure dependant was often
overlooked. Thus candidates were expected to mention that there is ~ 150 ml of CSF in the
adult, half within the cranium; about 60-70% of the CSF is formed by the choroid plexuses,
the remaining 30-40% by the cerebral vessels lining the ventricular walls; in humans the
CSF turns-over ~ 4 times/day; composition is essentially brain ECF; brain ECF normally
occupies ~ 15% of brain volume; CSF flows out through the foramina of Magendie and
Luschka and is absorbed through the arachnoid villi into the cerebral venous sinuses;
absorption, being largely by bulk flow, is proportional to ventricular pressure [at normal
pressure ~ 7.0-18.0 cmH2O (mean ~ 11), filtration = absorption, when pressure falls below
~ 7 cmH2O absorption ceases] and CSF Functions [buoyancy, constant metabolic
environment, buffers CSF against rapid plasma changes in K+, Ca++, Mg++, transport of
chemical messengers, sink for waste disposal].
A number of candidates embarked on long discussions of how CSF pH affects physiology
to the exclusion of what was asked for in the question. Many answers did not adequately
cover the three components asked for in the question.
Syllabus: CNS 2d
Reference Text: Guyton Chp 61