Skin Protects From Water Loss

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Biological Strategy

Humans

Mary Hoff

Image: Mélissa Jeanty / Upslash / Free non-commercial use

Prevent Fracture/Rupture

High force impact or stress can cause materials that comprise living systems to separate into two or more pieces (called fracturing) or to break or burst suddenly (called rupturing). For example, a scallop prevents structural failure from fracture because its shell is comprised of two materials of varying stiffness. When a crack moves from the scallop’s stiff material to the less stiff one, the latter reduces the force at the tip of the crack, thereby stopping it from spreading farther.

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Protect From Loss of Liquids

Water is essential to life. Liquids, mostly water, make up 70 to 90% of all living systems, and the loss of even a small percentage can mean the difference between life and death. Living systems must maintain a proper liquid balance, which is especially difficult in dry conditions. To do so, they must control the movement of liquids across their boundaries. Living systems do this using structures or waterproof materials to prevent or slow liquid movement. For example, when humans receive a cut, they must limit blood loss. Scattered throughout the bloodstream are lens-shaped structures that serve to plug the wound.

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Protect From Excess Liquids

While water is essential to life, too much water or other liquids can overwhelm living systems. Excess liquids can, for example, decrease a living system’s access to oxygen, promote excessive bacterial or fungal growth, or strip away soil and nutrients. To prevent the accumulation of excess liquids, living systems must control the movement of liquids across their boundaries or surfaces. They do so using waterproofing materials or structures, slowing flow, and/or facilitating flow to move the liquid away. Plant leaves, for example, commonly have waxy surfaces comprised of water-repelling chemicals to keep water from engorging the leaves or facilitating bacterial and fungal growth.

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Distribute Liquids

Liquids include water, as well as body fluids such as blood, gastric juices, nutrient-laden liquids, and more. To survive, many living systems must move such liquids within themselves or between locations. Because of their properties, liquids tend to disperse unless they are confined in some way. To address this, living systems have strategies to confine fluids for transport, and to overcome barriers such as gravity, friction, and other forces. Some of these same barriers also provide opportunities. Trees and giraffes face the same challenge: how to move fluids (water and blood, respectively) upward against gravity. But their strategies are quite different. The tree moves water using capillary action and evaporation, possibly due to water’s properties of polarity and adhesion. The giraffe’s tight skin provides pressure to assist in blood circulation. and keep blood from pooling in the legs.

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Store Liquids

Many living systems must store liquids, such as water or nectar, so that it is available over long periods of time, including when moisture levels are low. Because of their properties, liquids tend to disperse unless they are confined in some way. Each liquid has its own unique properties. For example, water is polar, exhibiting a strong negative charge on one side of the molecule and a strong positive charge on the other. Living systems have strategies to confine fluids by taking advantage of these properties. A good example of taking advantage of water’s polarity is using materials that repel water. In doing so, a living system can keep water on one side of a barrier, such as a membrane.

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Mammals

Class Mammalia (“breast”): Bats, cats, whales, horses, humans

Mammals make up less than 1% of all animals on earth, but they include some of the most well-known species. We know first-hand some of the characteristics that make mammals unique, like having hair, being able to sweat, and producing milk through mammary glands. Another critical shared feature is a set of highly-specialized teeth. Unlike sharks or alligators, for example, whose teeth are generally all the same size and shape, mammals have differently shaped teeth in different areas of the jaws to target specific foods or foraging strategies.

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The skin of humans regulates water movement with proteins, water-capturing molecules, and fats.

Introduction

Take a moment to think about your skin. It has a challenging task: Keep the good stuff in, and keep the bad stuff out––while still letting some stuff out, and letting other stuff in, all while providing the flexibility required for movement.

One of the most important components of that “stuff” is water. As life moved from liquid to land, one of its biggest challenges became retaining water. The very top layer of skin, known as the “stratum corneum,” plays a big job in keeping its owner from drying out.

Image: Zmarszczki / CC BY SA - Creative Commons Attribution + ShareAlike

Human skin helps limit the movement of water between the inside and outside of the body.

The Strategy

In humans, the stratum corneum consists of 15-20 layers of cells called corneocytes embedded in a flexible fatty matrix. Corneocytes are made up of a skeleton made of keratin (a protein) and natural moisturizing factor (NMF), which in turn is composed of water and different small molecules.

When the corneocytes encounter water they absorb it. In the outermost layer of the stratum corneum, the keratin is packed together tightly, so it doesn’t bind water well, but the NMF is able to hang onto water molecules. In the middle layer, the keratin is unfolded. This provides room for water molecules. Not only that, but as water molecules move in, they elbow the keratin apart, making even more room for more water molecules and giving the skin a sponge like capacity to absorb moisture.

This combination allows the corneocytes to expand but stay mobile, embedded in the fats. As a result, the skin remains soft and pliable while also regulating the ability of water to flow through the stratum corneum into or out of the body. The fats outside the cells also limit the movement of water (oil and water don’t mix) and help keep the NMF from escaping from the cells and reducing the cells’ ability to absorb water.

Image: Mikael Häggström / CC BY SA - Creative Commons Attribution + ShareAlike

The stratum corneum absorbs water, keeping skin pliable and limiting movement of water between a person and the environment.

The Potential

By altering the ability to absorb water with the amount of water available, the skin is able to maintain its own flexibility while also protecting the rest of the body from either drying out or swelling up. This approach could be used to regulate water flow in irrigation systems to keep plants watered but avoid wasting water by overwatering. It could also be applied to textiles to limit water penetration while maintaining pliability.

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References

“The vertebrate integument represents an evolutionary compromise between the needs for mechanical protection and those of sensing the environment and regulating the exchange of materials and energy. Fibrous keratins evolved as a means of strengthening the integument while simultaneously providing a structural support for lipids, which comprise the principal barrier to cutaneous water efflux in terrestrial taxa…How do the structural features of keratin influence its resistance to water movement? Generally, structural features that alter the free volume (equivalent to pores or channels) should alter the permeation of water molecules accordingly. Resistance to diffusion is affected by the molecular mass of side chains and tends to increase with cross-linking beyond certain critical levels (Lieberman et al., 1972)…The stability of cross-linkages is dependent on a large number of intermolecular forces, including covalent, ionic, and hydrogen bonding in addition to van der Waals attractive forces between non-polar amino acid side chains. All of these act to influence the mobility and free volumes of the structure…Studies of human skin have indeed demonstrated that gradients of water exist in the stratum corneum (Warner et al., 1988; Bommannan et al., 1990; Caspers et al., 2001; Bouwstra et al., 2003a). Fourier transform infrared spectroscopy has demonstrated that free water content in stratum corneum is greater in central regions relative to superficial and deeper cell layers at moderate levels of hydration (57%–87%, w/w), whereas at higher levels of hydration (300% w/w) water swells corneocytes in a direction perpendicular to the skin surface except for the deepest cell layers adjacent to the viable epidermis (Bouwstra et al., 2003a). While the mechanism excluding free water from the deeper cell layers of stratum corneum is not understood, it is speculated to play a role in preventing dehydration of the viable epidermis. In relatively dry conditions (18%–26% w/w), only bound water is present in the stratum corneum (Bulgin and Vinson, 1967; Hansen and Yellin, 1972; Bouwstra et al., 2003a).” (Lillywhite 2006:202, 212, 213)

Journal article

Water relations of tetrapod integument

Lillywhite, H. B.

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