Humans are not entirely waterproof, but rather possess a highly effective water-resistant outer layer. While “waterproof” implies complete impermeability, the human body manages its interaction with water, preventing excessive water loss and limiting water entry. This system ensures the body maintains its internal environment despite external conditions.
How Skin Forms a Water Barrier
The skin’s primary defense against water is its outermost layer, the stratum corneum. This layer functions like a protective wall, composed of flattened, dead cells called corneocytes, often likened to bricks. Keratin, a water-insoluble protein within these cells, plays a significant part in sealing the skin barrier.
The “mortar” holding these cellular “bricks” together is a lipid matrix, composed mainly of ceramides, cholesterol, and free fatty acids. These lipids form a continuous pathway that repels external water and prevents excessive water loss from inside the body. This arrangement of cells and lipids creates a formidable barrier crucial for maintaining skin hydration and protecting against environmental elements.
Why We Aren’t Completely Impermeable
Human skin is not entirely impermeable. Prolonged water exposure can lead to some absorption, demonstrating its semi-permeable nature. This selective permeability allows certain small, fat-soluble substances to pass through, which is the basis for transdermal medications. However, it also means some harmful substances can potentially enter the body.
A common observation illustrating skin’s water interaction is the “pruning” or wrinkling of fingers and toes after a long soak. This phenomenon is not due to water absorption causing swelling, as previously thought. Instead, it is an active process controlled by the sympathetic nervous system, involving vasoconstriction, or the narrowing of blood vessels, in the fingertips. This reduction in blood flow creates negative pressure, causing the outermost skin layer to pucker and form grooves, which may enhance grip in wet conditions.
When the Skin Barrier is Compromised
The skin’s protective barrier can be compromised by various factors, making it less effective at resisting water. Physical injuries like cuts, scrapes, and burns directly disrupt the stratum corneum, creating openings for external substances to enter and internal fluids to escape. This damage can lead to increased water loss, where water evaporates from the skin at a higher rate, resulting in dryness and dehydration.
Certain skin conditions also impair the barrier function. Eczema, for instance, is characterized by a weakened or damaged skin barrier, allowing moisture to escape and irritants and pathogens to enter. This can lead to chronic dehydration, increased sensitivity, and a higher risk of infection. Factors like harsh chemicals, environmental pollutants, excessive washing, and hard water can also damage the lipid matrix, reducing the skin’s ability to protect and retain moisture.