Skin is selectively permeable, not simply permeable or impermeable. This means it allows certain substances to pass through while blocking others. While it acts as a significant barrier against many external threats, it's not completely impenetrable.
Understanding Skin's Permeability
The skin's outer layer, the stratum corneum, forms the primary permeability barrier. This barrier is crucial for maintaining our internal environment and preventing water loss (ScienceDirect Topics: Skin Permeability). However, its selectivity allows for some substances to penetrate.
- What passes through: Fat-soluble substances can more easily pass through the epidermis than water-soluble ones (Biology LibreTexts: Skin). This is why some topical medications are formulated to be lipid-soluble for better absorption.
- What is blocked: The skin effectively blocks most bacteria, viruses, and other harmful substances from entering the body. This barrier function is vital for protection (The epidermal permeability barrier).
- Factors influencing permeability: Several factors can affect skin permeability, including temperature (The effect of heat on skin permeability), the size and charge of molecules (Effect of Size and Surface Charge of Gold Nanoparticles), and the condition of the skin itself.
Historically, skin was considered impermeable, but this view has evolved. Modern understanding recognizes its ability to act as a portal of entry for some substances, making it a target for transdermal drug delivery (Skin permeability: dermatologic aspects of transdermal drug delivery). Even electrolytes can permeate to some extent (The permeability of human skin to electrolytes). However, the degree of permeability varies greatly depending on the substance in question. For example, sea snake skin is permeable to water but not sodium (Sea snake skin: Permeable to water but not to sodium). The skin's permeability is also influenced by the structure and concentration of lipids within its membranes (Omega-O-Acylceramides in Skin Lipid Membranes).
