The question of why fingers and toes wrinkle in the bathtub is a scientific debate that takes into consideration the many layers of the skin and the behavior of the blood vessels, nerves and other elements located just beneath the epidermis. Your skin consists of many living and dead cells, which interact to create the effect of wrinkled fingers and toes when you swim or take a bath.
Sebum
The sebum, or natural oil, on your skin functions to protect and moisten the skin all over your body. It normally repels small amounts of water. When parts of your body remain underwater for a prolonged period of time, however, the sebum on those parts of the body begins to wash away, allowing more water to penetrate the skin. This is the first step that eventually allows the fingers and toes to wrinkle.
Dead Skin
Your fingers and toes wrinkle in water because the outer layer of skin, the epidermis, absorbs the water and swells up. The epidermis is a tough layer of skin that is made up of dead keratin, the protein also found in hair and fingernails. These dead keratin cells soak up water and increase in volume, causing the epidermis as a whole to increase in surface area.
Live Skin
The epidermis is attached to the layer of skin underneath, called the dermis, which is composed of living skin cells. Because the epidermis and dermis are connected, when the dead skin cells in the epidermis swell up while the live skin cells in the dermis stay the same, the outer layer of skin has to wrinkle up to accommodate its extra volume and surface area.
Blood Vessels and Nerves
The blood vessels and nerves in the skin may also play a role in why fingers and toes wrinkle in water. The Library of Congress notes that scientific research has revealed that digital vasoconstriction, or the narrowing of blood vessels, occurs when the hands and feet are placed in water. Along with the blood vessels, the nerves and the skin's temperature regulators also decrease in size, which detracts all the layers of the skin into the body to cause wrinkling. Because the blood vessels and nerves also contribute to wrinkling, people whose nerves do not function properly experience little or no wrinkling when their fingers and toes are placed in water.
Skin Cells
The skin cells on your fingers and toes are different from the skin on the rest of your body, which explains why your hands and feet wrinkle in water while the rest of your body does not. Although the epidermis is composed of dead keratin cells all over your body, the layers of skin cells are thickest on your hands and feet in order to make those body parts tougher and more functional. Because the keratin cells are the part of the skin that absorbs the water and make the epidermis wrinkly, the areas of the body where you have the most keratin cells are the areas most likely to wrinkle.
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