My biopsychology teacher insists that the CSF makes the brain lighter
due to buoyancy physics.  I insist that this is impossible! 
Everything I've found online and in textbooks does not give me enough
information to throw at my teacher. My class text reads, "The brain
and spinal cord literally float in the cerebrospinal fluid,so the
weight of a 1200- to 1400-gram brain is in effect reduced to less than
100 grams."  What I get from this is that the brain is suspended in
fluid which reduces the pressure it's weight would otherwise put on it
due to gravity, so the net effect of the pressure is as though the
brain were lighter, but the brain itself is not lighter.  Therefore
the neck must still support the entire weight of the brain, plust the
weight of the CSF.

I asked my teacher this and gave the following example which she still
would not accept:
Cup A has a blob of fat in it.  Cup B has a blob of fat floating in
water in it.  Assuming the cups were of equal mass and the blobs of
fat were also of equal mass, Cup B and its contents must be heavier
than Cup A and its contents.

Please explain how CSF really works and cite one or more sources I can
either find online or get from a library so I have evidence to show my
prof.

---

Request for Question Clarification by andrewxmp-ga on 26 Apr 2005 23:24 PDT

The answer to this doesn't lie in physiology, but rather physics. 
Think about those "force diagrams" you drew in intro physics.  In a
hypothetical brain without CSF, you would have the brain resting
predominantly on the spinal column, with all of its weight creating a
force due to gravity (the "normal force" as long as we're speaking
physics).  Now, when you fill up the cranium with CSF, and picture the
brain almost completely submerged, you can add more "forces" to the
diagram.  When a boat is floating in water, why does it not sink?  The
amount of water displaced weighs exactly the same as the mass of the
boat.  These forces equal out, and you could imagine the force due to
gravity of the boat's mass pushing down, with an equal force due to
the water pressure pushing up.  With the brain, the volume of the
brain displaces a certain amount of volume of CSF, and the mass of
this displaced fluid pushes the brain upwards, another "upwards" force
arrow.  This force cancels out much of the downwards force due to
gravity the brain originally exerted.  It doesnt "weigh" less per se,
but the total sum of forces on it or exerted by it (which used to sum
1200 to 1400 grams) is now far less.

I'll note that I have no idea what your teachers secondary example was
supposed to mean.  Seems like a bad one.....but anyway..

does that help at all, or is somethingstill missing?  I would
recommend actually drawig the fore diagram I was describing...I think
it would be helpful.

---

Clarification of Question by kianwilcox-ga on 27 Apr 2005 14:44 PDT

The vector diagram makes perfect sense, and that is what I was trying
to get at with her.  But she insists that bouyancy physics actually
make it weigh less.  I understand that there is less downward force
because of vectors canceling out, but that just means less pressure
between the bottom of the brain and the skull, yes?

This helps me understand, but I need something referencable to show my teacher.

---

Request for Question Clarification by andrewxmp-ga on 27 Apr 2005 16:12 PDT

Yes, this does mean less pressure between the bottom of the brain and
the skull.  IF you want to make this mean something OTHER than "the
brain weighs less" than Isuppose you could, but really it's just a
matter of semantics.  When you're in a pool swimming, your mass hasnt
changed by you do "weigh less" the way everyone talks about it.  Your
call....

---

Clarification of Question by kianwilcox-ga on 28 Apr 2005 22:50 PDT

Thanks, consider the question fully answered. if you'd like the 5
bucks, just put it in the 'answers' and I'll send that to you. thanks!

efn,

The answer to this doesn't lie in physiology, but rather physics. 
Think about those "force diagrams" you drew in intro physics.  In a
hypothetical brain without CSF, you would have the brain resting
predominantly on the spinal column, with all of its weight creating a
force due to gravity (the "normal force" as long as we're speaking
physics).  Now, when you fill up the cranium with CSF, and picture the
brain almost completely submerged, you can add more "forces" to the
diagram.  When a boat is floating in water, why does it not sink?  The
amount of water displaced weighs exactly the same as the mass of the
boat.  These forces equal out, and you could imagine the force due to
gravity of the boat's mass pushing down, with an equal force due to
the water pressure pushing up.  With the brain, the volume of the
brain displaces a certain amount of volume of CSF, and the mass of
this displaced fluid pushes the brain upwards, another "upwards" force
arrow.  This force cancels out much of the downwards force due to
gravity the brain originally exerted.  It doesnt "weigh" less per se,
but the total sum of forces on it or exerted by it (which used to sum
1200 to 1400 grams) is now far less.

I would recommend actually drawig the fore diagram I was describing...I think
it would be helpful.

Yes, this does mean less pressure between the bottom of the brain and
the skull.  IF you want to make this mean something OTHER than "the
brain weighs less" than Isuppose you could, but really it's just a
matter of semantics.  When you're in a pool swimming, your mass hasnt
changed by you do "weigh less" the way everyone talks about it.  Your
call....

Glad this cleared things up.  Good luck!

Regards,
Andrewxmp

As andrewxmp says, it's a matter of semantics, specifically two
distinct meanings of "weigh".  If you put a scale on the bottom of a
swimming pool and stand on it, you will get a lower reading than if
you stand on the scale on land.  Does that mean you weigh less in the
water?  If "weigh" refers to your mass, no, but if "weigh" refers to
how hard you press on the scale, yes.

Your teacher is a moron and your book is misleading.  These sorts of
things happen all the time.  Congratulations to you for remaining
skeptical!

Obviously the brain is not lighter, for the same reason your other
organs are not lighter for 'floating' amongst blood, lymph,
interstitial fluid, and other tissues.  Nor is your whole body
considered 'lighter' for being immersed in the fluidic atmosphere.

And generally speaking, the primary function of CSF is not generally
considered to be for 'floating' the brain (since a big proportion of
CSF is actually in the interior the brain, in the ventricles), but
rather bathing the brain in nutrients & crap like that.

I like your blob-of-fat-in-the-cup argument.