You have soaked in a tub or pool for an extended period of time and notice the skin on your hands looks less smooth and more prune-like than when you first got in. The biology behind how the skin works gives us the answer as to why hands and toes wrinkle after prolonged exposure to water.
The skin is made up of three layers; the deepest of which is called subcutaneous tissue. The middle layer is the dermis, which is home to blood vessels, sweat glands and more. The outermost layer is the epidermis, which keeps water from evaporating off the body, locking in moisture and protecting the inner layers from damage. The epidermis is the layer most related to skin wrinkling.
Dissecting the Epidermis
The epidermis is further divided into four protection-providing layers: the stratum corneum, the granular layer, the basal cell layer and the squamous cell layer. The epidermis also makes sebum, which is an oily substance that resists water. This sebum is why sweat stays on top of your skin instead of being reabsorbed for moisture.
The stratum corneum is made up of dead keratin cells, which protect the skin's outer layer. When you soak in water for an extended period of time, it washes the sebum layer off the skin and the dead keratin cells begin to absorb the water. Your skin wrinkles because the water puffs it up. The skin on the fingers and toes contains the most keratin, according to the Library of Congress. This explains why these are the areas most likely to be wrinkled.
Another theory as to why the skin wrinkles is the principle known as vasoconstriction. This term refers to the narrowing of the blood vessels. Scientists propose that when the skin is exposed to water for a long period of time, the nerve fibers in the hands shrink, which in turn wrinkles the skin.
Returning to Normal
As you dry off, the water absorbed by your skin begins to evaporate and the nerve fibers return to original size. The body then produces more sebum and balance is restored to the skin.