Oh well, another great example of the crap i was taught at school and
never considered properly ;-)
So, just to get this straight, The process actually goes the other
way, bathwater is drawn into the skin by osmosis, making it larger and
wrinkly? If you bathe in very salty water the osmotic pressure is the
other way and water is sucked out of your skin?
ST
On Jan 14, 2008 8:50 AM, Len Warner <novost@...> wrote:
> At 6:41 am ((PST)) Sun Jan 13, 2008, in Digest 2374 Stefan Trethan wrote:
> >[snip]
> >I also see the wrinkling when put in water, but i believe this is
> >different to the thermal expansion/contraction. Paper tends to get a
> >lot larger when taking up water.
>
> This is because it is made of fibres of cellulose, a long-chain
> carbohydrate molecule, which absorb the water and swell up
> in all directions - but only the X & Y directions are particularly
> noticeable. Water also loosens the bonds between fibres
> leading to permanent effects, such as the wrinkling of a
> previously smoothly calendered sheet due to the disturbance
> of the fibres being retained on drying.
>
> >[snip] It is different to your fingers getting all
> >wrinkly by the way,
>
> Not very different, except that skin has a high content of
> structural protein rather than carbohydrate. Proteins
> generally have quite a high affinity for water.
>
> >this is because the salty water inside you is
> >drawn out into the less salty water on the outside,
>
> Not true, body membranes are quite retentive of electrolytes:
> the presence of salt in sweat is only because the glands
> actively pump out the ions (and the water follows) not
> because the salt leaks from your body.
>
> Otherwise bathing would be a very risky activity rather than
> a pleasure, a pastime and a sport.
>
> As a further example of this, consider death by drowning.
> (That's not a recommendation of a course of action, btw ;-).
>
> If a child drowns in fresh water and is resuscitated there is
> still a possibility of death through electrolyte imbalance
> from the amount of water which has passed through the
> thin lung membrane, diluting the child's small blood.volume.
>
> If a person drowns in sea water and is resuscitated there
> is still a risk of secondary drowning caused by salty
> residues accumulating water out of the bloodstream.
>
> In both these cases the water chases the electrolytes,
> the "salt" doesn't move: look up "osmosis".
>
> >not because of your skin expanding
> >(although it may do that, too).
>
> Nope, that's exactly what it is.
>
> The skin is fairly waterproof, so that we don't leak away,
> and relatively dry on the outside since most of us live in a
> low RH atmosphere. (There are comfort and health problems
> with living in a high humidity environment.)
>
> So the outer layer, high in the protein keratin, has a lower
> hydration level than if it were in equilibrium contact with water.
>
> Given prolonged contact with water, there isn't enough real
> estate on your body to accommodate the skin's expansion in
> area but since your skin is anchored in place it has to wrinkle
> rather than sliding off one end or the other.
>
> >I'm told if you bathe in very salty water (e.g. dead sea)
> >this effect does not occur.
> >I am not certain if this is a myth or true.
>
> The effect can be true and your explanation a myth:
> it is the high osmotic pressure of the salt water that
> counteracts the hydrophilic nature of the skin protein
> restricting any water absorbtion to a level below which
> wrinkling is evident.
>
>
> Regards, LenW
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