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Pruny Fingers
Why do the tips of your
fingers shrivel like a raisin when you stay in the water too long?
Tim, Framingham, Massachusetts
Why do our fingers wrinkle
when taking a bath? Cherie, Bellows Falls, Vermont
 A
wrinkly, 'pruny' finger after a long bath. Photo courtesy of Fir0002 and
Wikipedia.
Creating pruny skin is a
two-step process.
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First, we keep our fingers
in water (for example, a swimming pool, bathtub or shower) long enough to soak
oils away from the outer skin. Skin oils greatly reduce evaporation of water
from the skin, and largely block water from entering the skin.
-
Second, once water has
washed off skin oils, the outermost skin surface will absorb water. The outer
skin, called the stratum corneum (Latin for "horny layer") is made up
mostly of dead, dry cells; the layer is thick on the fingers to protect these
appendages as they engage in normal tough activities of everyday life, such as
laying bricks, or, formerly, digging for grubs. The dry, dead skin sucks in
water like a thirsty camel.
As the thick, horny layer
sponges up water, it expands. But deeper layers of skin don’t expand and thus
restrain the swelling. So the outer layer of skin develops folds (where the
skin swelled ) and valleys (where lower layers hold tight).
Although our fingers appear
to shrivel, they actually expand — except where lower-layer entities (like blood
vessels) hold the outermost skin in place.
Further Reading:
Everyday mysteries: pruny fingers, Library of Congress
Physiology forum: pruny fingers
(Answered Oct. 13, 2008)
Comment
Readers' Answers
- The reason our fingers and toes don't swell up like water balloons but
instead shrivel like raisins is due to how skin layers are joined.
When the top layer absorbs water, it swells, becoming larger than the lower
layers to which it's attached. The skin doesn't detach from our fingers, so
the only thing it can do is wrinkle up to accommodate the increase in surface
area.
Johnathan
Lee Yu Hern,
Singapore
- The reason your finger shrivels up when wet is — actually it doesn't! Since there is more salt in your body than in the water,
water crosses into your skin cells to dilute the amount of
salt. This process is called osmosis.
My high school biology teacher explained it as: you have a size 3 finger
and size 3 skin. After you have been in water, you still have a size 3 finger
but now you have size 7 skin.
Rosalind Taylor, Gilroy, California, USA
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Because the skin on our palms and the bottoms of our fingers and feet are
thicker than other parts of our skin, they swell more than the rest. Because
this swelling layer of skin is connected underneath to tissue that does not
swell, the skin buckles. Hence, wrinkled fingers and toes.
Nawal, World
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