Overview
Soak in the tub for a while, and watch the skin on your fingers get all wrinkled and puckered, like the surface of a prune. But a prune’s skin gets wrinkly because the fruit underneath it is shrinking as it dries out. As best as scientists can tell, in the tub, your skin gets wrinkly because it’s expanding while the flesh underneath it stays the same size.
Skin
Your skin is made up of several layers. The subcutaneous layer, the lowermost layer, contains the large blood vessels and nerves that serve your skin. The dermis, the middle layer, is where you find your sweat glands, the roots of the hair on your skin, and the tiny capillaries that carry oxygen-rich blood to your cells. The outer level is the epidermis. That’s the protective layer, and it’s made up of dead skin cells that tend to absorb water.
Soaking
Ordinarily, your skin is covered with an ultra-thin layer of oil, called sebum. This is the stuff that leaves smudges and fingerprints. Sebum helps waterproof your skin. But when you spend a long time in the water, the sebum washes away, and the dead skin cells of the epidermis start to soak up water.
Expansion
As the epidermis absorbs water, it grows in volume. But the dermis underneath isn’t expanding along with it. The epidermis is tightly attached to the dermis, which is attached to the tissue beneath it. So even though your outer layer of skin is expanding, it can’t blow up like a balloon. So it forms folds and ripples–that is, it wrinkles.
Extremities
You may notice that the wrinkling seems mostly confined to the hands and feet. According to researchers at the Library of Congress, the extremities–the hands and feet–have the thickest layers of dead cells, because that’s where the skin needs the most protection. Because those areas have a thicker epidermis, the skin there will deform more radically than other areas. You see the effect as wrinkles.
Vasoconstriction
Wrinkly fingers is an area of research that is still evolving. Scientists are also looking into the role of vasoconstriction, or narrowing of the blood vessels, in skin wrinkling. As the Library of Congress explains it, when the skin soaks in water, the nerve endings may send signals that cause blood vessels and other structures in the dermis to contract, or shrink. This makes the dermis level smaller, which makes the wrinkling of the epidermis even more pronounced.
