Hydration, in the context of human physiology, refers to maintaining appropriate water levels within the body’s tissues and systems, specifically the volume of water circulating in the bloodstream and stored in cells. While it is tempting to think that direct contact with water can replenish internal stores, the body possesses sophisticated barriers and mechanisms that strictly regulate how water is absorbed. The pathway water must take to truly affect systemic hydration is internal, not external.
The Skin’s Protective Barrier
The body’s largest organ, the skin, is designed primarily to keep the outside world out and the inside world in, acting as an extremely effective watertight barrier. This protective function is largely due to the stratum corneum, the outermost layer of the epidermis, which is often likened to a brick-and-mortar wall. The “bricks” are tough, flattened skin cells called corneocytes, and the “mortar” is a lipid matrix composed of ceramides, cholesterol, and fatty acids that create a seal. This specialized structure prevents the passive movement of external water into the deeper, living tissues and the bloodstream, thereby blocking systemic hydration through soaking.
The skin’s barrier primarily functions to prevent trans-epidermal water loss (TEWL), which is the natural evaporation of water from the body’s interior into the atmosphere. A healthy skin barrier maintains a low rate of TEWL, indicating its robust ability to retain moisture. Because the skin is so effective at preventing water from escaping, it is equally effective at preventing large volumes of water from entering the body’s internal systems.
Systemic Hydration: The Role of Digestion
For water to contribute to systemic hydration, it must be absorbed into the body’s circulation, a process strictly managed by the digestive system, not the skin. When water is consumed orally, it travels through the esophagus to the stomach and then passes into the small and large intestines. The intestines are the primary sites where water is absorbed into the bloodstream. Once in the bloodstream, water is distributed to all tissues and organs to support crucial bodily functions. This absorbed water lubricates the digestive tract, aids in the production of saliva and digestive enzymes, and transports essential nutrients to cells throughout the body.
Surface Effects: What Soaking Does to the Skin
Although soaking in water does not hydrate the body internally, it causes a noticeable and temporary effect on the skin’s surface. When the skin is submerged for an extended period, the outermost layer absorbs water, which is a form of skin hydration rather than systemic hydration. This absorption is attributed to water moving into the dead cells of the stratum corneum.
This superficial absorption leads to the temporary swelling of the skin’s outer layer, visible particularly on the hands and feet. The well-known “pruney fingers” effect is not just swelling from osmosis; it is a controlled response by the nervous system. The autonomic nervous system signals the blood vessels beneath the skin’s surface to constrict (vasoconstriction), which reduces the volume of the underlying tissue. This reduction causes the overlying, water-swollen skin to pucker and wrinkle, which some scientists suggest is an adaptation to improve grip on wet objects.