The structure and physiology of the skin are obviously much more complex than we have been able to describe in the present work. Nevertheless, by knowing them even in part it is easier to identify the functions that our skin fulfills:
The function of the skin's barrier is to protect the skin and, therefore, the body from the entry of chemical substances and also preventing the loss of bodily substances. This function is valid from the outside to the inside and vice versa. Only lipid-soluble substances can penetrate it, provided that their molecular weight is not excessive. This selective permeable barrier is mainly due to the basal membrane, horny layer, and intercellular lipids. Thanks to this functional barrier, water, the essential source of life, does not escape from our bodies.
The skin performs a protective function against biological (bacteria, viruses, and mycetes), physical, and chemical agents. An alkaline substance placed on the skin is neutralized by the hydrolipid film and the horny layer before it can damage the organs below. In the same way the sun's radiation is neutralized, at least in part, by melanin or by the horny layer. Finally, the skin plays the essential role of mechanical protection that we all appreciate every day when large or small mechanical traumas are cushioned by our skin.
The first site of entry for foreign substances and bacteria is the skin. With the Langerhan cells the skin is able to identify these and to prepare a defence. Sometimes, as in the case of contact dermatitis, the defences themselves do us harm, resulting in inflammation that is normally the essential response marking the invasion of a foreign agent.
The skin's secretory functions are carried out both by the cutaneous glands and the epidermis itself. Sebum, sweat, and epidermal lipids are products that perform functions for the skin (protecting it) and for the whole body. In fact sweating, like keratinization, is one of the means by which drugs and harmful substances are removed from the body. The apocrine sweat glands participate in this function, as becomes clear, for example, when these expel dietary herbs and spices such as garlic.
The mechanisms used by our skin in thermoregulation are insensible perspiration, eccrine sweating, and changes in cutaneous vascularization. By these mechanisms the skin is able to adapt our body temperature as a function of the ambient temperature. When the individual is hot it means that they have not been able to exchange the heat produced by the body with the environment. If this exchange does not occur, then life may be in danger because an increase in body temperature, as with fever, can block cellular reactions and, therefore, cause their death. This is why, when one is hot, the sweat glands become active and the blood vessels dilate to allow more blood to flow just below the skin, producing heat loss both in physical ways (irradiation, convection, and conduction) and by sweating. Perspiration, on the other hand, remains quite stable and changes occur only under conditions of intense heat. The opposite occurs when one is cold: there is a reduction in sweating and a restriction of the blood vessels with consequent lower blood flow and less heat under the skin. In other words, the ability to increase or decrease the quantity of blood flowing under the skin provides the means of dissipating more or less heat into the environment.
The chance of survival in an environment is linked to the capacity of the individual to be in contact with it. Together with the senses of sight, hearing, and smell, the skin's sensitivity provides the individual not only with the sense of touch, but also allows us to recognize our position and its variation in space. In addition the skin is able to sense pain due its nerve receptors and through other receptors present in the encephalon to read the blood, so facilitating the identification of hot and cold. The skin also detects itching, which, together with pain, heat, and cold, is vital for the survival of the individual, as these sensations warn us of danger or injury. Without the skin and its sensitivity we could burn ourselves without being aware of it or we could freeze without knowing.
Strictly linked to the skin's barrier function, absorption allows substances applied to the skin to be conveyed into the blood system. This important function is being ever more exploited by medicine to avoid the damage that can be caused by gastrolesive drugs when administered orally, or to favour a continuous slow drug absorption.
This function obviously varies according to the area and thickness of the epidermis. Eyelids and other analogous regions characterized by a thin layer of skin will absorb more of the substances applied to them than do other areas. Other regions, like the scalp and the armpits, which are rich in sweat glands and hair follicles, will absorb through these structures more than in other regions. This function can also have negative aspects in that it can also cause absorption of dangerous and even lethal substances. Logically, any change in the horny layer will favour absorption by the skin. Absorption is larger for lipid-soluble substances and is divided into three stages:
1. Penetration or passage of a substance from the hydrolipid film into the epidermis
2. Permeation, which is the diffusion through the cells of the epidermis and dermis
3. Reabsorption, the penetration of local blood vessels by a compound that has reached the dermis
Though with limitations, especially for large molecules such as collagen and other proteins, the epidermis is easily permeable to many substances, particularly if they are appropriately carried. Liposomes, resembling the constitution and structure of membranes and intercellular lipids, penetrate the skin better than other substances and can carry many active principles. For these characteristics, liposomes, like other special structures (some even smaller than liposomes), are particularly well used in cosmetology and medicine.
Two other functions are also worth remembering-the skin communicates our image to others and has a fundamental role in the production of vitamin D, the compound necessary for normal bone development. The skin receives ultraviolet rays and utilizes them in the production of vitamin D. The discovery of this role is quite recent. In fact, at the beginning of the industrial age, when the main fuel was coal, the skies over the cities of Northern Europe were darkened by pollution, which, in combination with climate, resulted in those populations receiving little sunlight. Children, therefore, grew up affected by rickets. Exposure to sunlight was found to improve the condition, leading to the discovery that the skin is the organ that synthesizes the active anti-ricket compound, vitamin D.
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