"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
Creating pruny skin is a two-step process:
- 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)
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."
- style="list-style: none"> 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."
- style="list-style: none"> Rosalind Taylor, Gilroy, California, USA
- "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.
- style="list-style: none"> Nawal, World
April Holladay lives in Albuquerque, New Mexico. Her column, WonderQuest, appears every second Monday of the month on globetechnology.com. To read April's past columns, please visit her website . If you have a question for April, visit this information page . You can also comment on this article at Google Groups