MadSci Network: Anatomy

Re: If H2O enters soaking fingers/toes, why isn't the skin swollen tight?

Date: Sat Jul 17 11:56:27 2004
Posted By: Dave Williams, Science Department Chair, Valencia Community College
Area of science: Anatomy
ID: 1089738260.An


Unfortunately you have been badly misled by the previous answer 
that you have seen on Mad Sci. Young Mr. Carlson is under the 
misapprehension that osmosis moves water into the human body. 
While nothing could be farther from the truth, this is a common 
mistake among new biologists (or old ones who don't think).

One of the main purposes of the skin is as a water barrier. It keeps 
water in and out of the human body.

Water moves into the dead layer of flattened skin cells that make up 
the stratum corneum of the skin (the keratin that Mr Carlson referred 
to) by capillary action, the same way it soaks into a paper towel. No 
doubt it first seeps into the spaces between the flakes of dead skin. 
Then the dead skin cells become rehydrated and the layer swells 
and becomes more expansive, thereby increasing it's surface area.

But...  the stratum corneum is attached to the underlying layers of 
living skin cells in the epidermis which are in turn attached to 
connective tissue in the dermis.  In order to accommodate its new 
surface area the stratum corneum must wrinkle. It is able to do so 
because the tissues to which it is bound are somewhat elastic and 
stretch to accommodate the larger surface. The resulting interaction 
between the swollen dead skin layer and the underlying, slightly 
elastic tissues to which is its connected produces the corrugated 
appearance which we sometimes call "prune fingers".

Some biology instructors may not be prepared to accept this 
explanation. It is a sad fact of modern science education that many 
teachers (even university professors who do very advanced research) 
do not question their initial, simplistic explanations for ordinary 
phenomena outside their area of expertise. The prevailing notion 
among biology teachers that osmosis accounts for ALL water 
movement is as rampant, in my experience, as is the popular but 
unfounded notion that we use only 10% of our brain.

To the best of my limited knowledge, water (on Earth, at least) moves 
in response to four different forces.

1. It responds to gravity, as when water is poured from one container 
into another. This is called "bulk flow" of water. It is responsible for 
the often observed phenomenon that "water seeks its own level".

2. Water will also respond to pressure, as when it is pumped from 
one location to another. Such pressure is called "hydrostatic 
pressure" and is also responsible for some biological water 
movement, as when water is lost from capillary blood vessels near 
the arterial (pressurized) end.

3. Water will move as a result of electrostatic intermolecular forces; 
the repulsion and attraction generated by partial charges on the 
surface of the water molecule. A common type of this movement is 
called "capillary action". This is the movement of water up the sides 
of tubes in apparent defiance of gravity or into small spaces of all 
kinds. This type of water movement is responsible for the fact that 
water from a puddle on a table will soak into our misplaced shirt 
sleeve. Also, it contributes in no small way to the movement of water 
up the stems (trunks) of large trees.

4. Osmosis is, like its parent concept diffusion, powered by the 
thermal energy inherent in the system of which the water is a part 
(reflected in the temperature of the surroundings). Osmosis is the 
diffusion of water molecules through a semipermeable membrane. A 
semipermeable membrane is a thin membrane that will let water 
molecules through but will hold back large solute molecules 
dissolved in the water. Such a membrane must separate two water 
compartments in order to facilitate the diffusion of water.

In each of the four examples the same explanatory motif can be 
invoked: water moves from where it is in greater concentration to 
where it is in lesser concentration. We pour water from a full jug into 
an empty glass. We pump water from the well into the empty sink. 
Water moves from the puddle on the counter top to the many, small 
empty spaces in the paper towel. Water moves from a solution with a 
lower concentration of solutes (where the water is in higher 
concentration) to a solution with a higher concentration of solutes 
(where the water is in lower concentration). The slipshod definition of 
diffusion as "the movement of a substance from higher to lower 
concentrations", when applied to osmosis, leads to much confusion 
stemming from the reverse nature of the relationship between the 
water concentration and that of the solutes.

What is most often missing in the typical presentation of osmosis is 
an explanation of the energy source. Typically, students walk away 
from a lecture on osmosis with the idea that it requires no energy -- 
that it is energy free!

This, of course, is not the case. Osmosis (like diffusion) depends on 
thermal energy which is manifested in the incessant motion of the 
molecules. The point that many instructors are trying to make 
(perhaps without much reflection given to it) is that no energy needs 
to be supplied. It is already present in the system. Unfortunately, this 
is usually carelessly conveyed as "no energy is required for 

The notion that osmosis has any potential (at least on the short term) 
to affect an immersed living human body completely covered by a 
relatively thick layer of dead cells, which does not constitute a 
semipermeable membrane separating two water compartments, is 
suspect. The fact that even extended immersion seems to have no 
effect on the actual water balance of a living human body needs 
some sort of explanation, no doubt.

A thick yet porous membrane may, given extended time of immersion 
in fresh water (perhaps much longer than a typical bath), emulate a 
semipermeable membrane. Of this and its results I am not sure. I 
am sure that the "prune fingers" effect cannot be attributed to 

The $64 question, for those inclined to tackle it, is: "Why does the 
wrinkled effect only appear on the skin of the finger tips (or toe tips) 
and not on the palms (or soles) and other parts of the skin?"

Mr. Carlson's explanation does not hold water (Just a little joke!) for 
the reasons I have given. The answer, I think, lies in the fact that the 
stratum corneum on the palms and soles is so thick that we don't 
notice the (large) wrinkles and that on other parts of the body is so 
thin it can't pull effectively against it's connection with the dermis; it 
can't stretch the elastic, so to speak. Bear in mind, however, that my 
answer is just an educated guess (just like Mr. Carlson's). I may be 

In fact, there is another hypothesis. Quite frankly, I don't buy it but it is 
worth considering. The idea is that water does actually get into your 
body when you soak and the the wrinkling response is from nerve 
activity attempting to keep the water out. Here is a news release 
about this idea:,00500011.htm

I have not found an original source for this study so am reserving my 
judgment. My main source of skepticism about the study is the 

"The test is based on the fact that water seeps in easily through the 
pores in the hand, diluting the body's own liquids and changing the 
electrolyte values."

This statement, as any professor of human anatomy and physiology 
will tell you, does not have the ring of truth. If "water seeps in easily" 
then it would be dangerous to get into the water. What "pores" are 
they referring to? If a slight dilution of the body's own liquids is what 
produces this effect, then why don't our fingers get wrinkled whenever 
we become edematous (have excess water in our systems)?

Don't get me wrong, I am willing to admit and accept that there are 
reasonable answers to these questions but we must have them 
before we can rest with this hypothesis.

I hope that my answer has cleared up this question for you without 
undue confusion. Science is like that. There are few pat answers. 
Most everything is in the 'this-is-the-best-we-can-do-just-now' stage. 
Here are some sources that agree with me:

I sort-of got the last word in on this one!

This one references the previous discussion (which comes up easily 
on Google) but cites someone else who agrees.

This next one is purported to come from the Library of Congress.

Here is a good one.

C. Susan Brown, Ph.D. agrees with me.

Here is another one of Mr. Carlson's answers. He has (quite 
admirably) done some experiments but still has to explain away facts 
(my fingers wrinkle in the ocean). Further, his answer is based on the 
assumption the the human body interacts osmotically with it's 
environment, an assumption which I do no accept.

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