Anyone who loves the water knows
that if you stay in the pool or the bathtub long enough, your fingers and toes
will seemingly shrink to become wrinkly and prune-like. The question is
"Why?"
How does it make sense that by
spending significant time in the water, fingers and toes look as if they've
lost water? Think about where the term "pruney" comes from--having a quality that is consistent with a
dehydrated plum.
The truth is that your fingers and
toes actually gain water as you swim your 100 laps in the pool or sit in a hot
tub for longer than the posted 15 min time limit. You know who you are.
The wrinkly effect is a matter of which layers in the skin absorb the
water.
The skin is made up of 3 major
layers, the hypodermis, a fatty
layer at the base of the skin, the dermis,
a middle layer, packed with blood vessels, nerve endings, sweat glands, etc.,
and the epidermis, a self-regenerating protective
layer that forms the surface of the skin.
It turns out that the epidermis
itself has 5 layers, including (from the surface down)...
•
Stratum corneum
•
Granular layer
•
Squamous cell layer
•
Basal cell layer
 |
| The stratum corneum (surface layer shown here in pale yellow) absorbs water from the outside. |
The stratum corneum (surface layer) is composed of dead skin cells
filled with keratin. Due to
the rules of osmosis, water flows
into these cells, because the concentration of solutes on the outside of these
cells is lower than the concentration of solutes on the inside.
As these cells absorb water, the
stratum corneum becomes larger, giving it greater surface area than the layers
below. As a result, the fingers and toes wrinkle to accommodate the
difference in surface area between the stratum corneum and the layers
underneath.
GREAT! Case closed… Well
actually, there might be more to this story. Recent research has found
that people who have lost nerve function in their hands or toes exhibit either
no wrinkling, or only slight wrinkling of their hands and toes when immersed in
the water.
This discovery has led researchers
to believe that vasoconstriction
(the narrowing of blood vessels) must play a role in the wrinkly digit mystery.
Here's why:
 |
| The narrowing of blood vessels near the surface of the skin keep blood from flowing close to the surface, also reducing the volume of the skin in near the surface. |
When you're cold, thermoreceptors (sensory neurons that
gauge temperature) in your skin direct your body to narrow blood vessels near
the surface of the skin to prevent warm blood from coming into close proximity
to cold air. In this way, you are able to conserve the heat in your blood
for your body's core. Water immersion seems to trigger this same
response. As a result, blood is kept from the surface of your skin,
especially in your fingers and toes. If the blood is kept away from these
places, the fingers and toes can lose volume, and thus appear wrinkly.
People with a loss of nerve function in their feet or hands wouldn't send
these nerve signals, and thus have less wrinkling.
Believe it or not, researchers are still working to refine their
scientific explanations for this phenomenon.
In a new approach, mathematicians are using 3D geometric modeling to
explore how keratin (the all-important protein in the stratus corneum) is able
to absorb relatively large amounts of water and yet maintain its strength.
You can read about that research here:
Sources for this post—
“Everyday Mysteries.” Library of
Congress Science Reference Services. Library of Congress, n.d. Web. 19
November 2012.
Bull, C and Henry, J.A. Finger wrinkling as a test of autonomic
function. British medical journal,
Feb. 26, 1977: 551- 552.
Hseih, Ching-Hua et al. Paradoxical response to water immersion in
replanted fingers. Clinical
autonomic research, v. 16, June 2006: 223-227.
Lappe. Marc. The body's edge: our
cultural obsession with skin. New York, Henry Holt and Company, 1996. 242
p.
Robbins, C.R. Skin in the
Encyclopedia of human biology. Edited by Renato Dulbecco. v. 8. San Diego,
Academic Press, c. 1997. p. 39-47.
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