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Did you know..

..that skin is the largest organ of the human body? It stretches over about 1.7 square metres.

Skin covers us from head to toe, keeping the outside out, and our insides in. It also plays an important role in keeping the body at the right temperature.

Human skin is made up of two layers: the epidermis, which is the outer layer, and the dermis, which is the inner layer.

Epidermis: This part of the skin is where we find a chemical layer called melanin, which gives skin its colour. The amount of melanin in each cell increases, following exposure to sunlight. The epidermis varies in thickness, from 0.1mm in the eyelid, to over 1mm on the palms of the hands and soles of the feet. Skin is always replacing itself, layer after layer. New skin forms beneath old skin, and is constantly being pushed out towards the surface as new skin forms underneath. The rate at which we 'shed' skin generally equals the rate at which new cells are formed.

Dermis: Here is where we find the many specialised structures of the skin: nerves; blood vessels; sweat glands and the roots of hairs.

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Nerves: A large number of nerve fibres are present in this layer of the skin. They are one of the most important components of this organ as they not only allow us to feel sensations, such as pain and temperature, but they also regulate the activity of sweat glands and blood

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Blood vessels: The blood vessels provide nourishment for other structures within the dermis as well as the cells of the epidermis. Heat regulation is also dependent on this blood supply. The arterioles (tiny arteries) of the skin contain smooth muscles which contract in response to the cold, and relax in response to heat. This is why certain parts of our bodies may become a slight shade of white or blue in the cold, because the blood supply has been constricted to minimise heat loss. Adequate blood flow to the vital organs, such as the brain and heart, is thus maintained. When it is hot, the walls of the arterioles relax, and being so close to the external environment, the blood that passes through these arterioles is cooled, and thus the body is kept at normal temperatures. This is one mechanism the body uses to cool down. Another is through sweating.

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Sweat glands: These glands consist of coiled tubes in the dermal layer, which extend upwards to open onto the skin surface. When it is hot, or when a person exercises, the body heats up and needs to be cooled down. The watery fluid which is secreted through these sweat glands, which we call sweat or perspiration, is to help the body cool down. Have you ever put water on your face on a hot day, and noticed that it made the breeze slightly cooler? Sweat works in the same way. 

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Hairs: There are two types of hair that are found on the skin: 

  • vellus hairs, which are fine and soft, and cover most of the body, except the palms of hands and soles of feet; and 
  • terminal hairs, which are longer and darker, and are found on the scalp, eyebrows, eyelashes, groin, armpit and the male beard line.

The base of a hair follicle is formed by what is called a papilla, and it is from here that hair grows. In this papilla, there is a layer of melanocytes which gives hair its colour. Once a hair has been shed, the growing phase of that particular follicle starts over and a new one soon emerges.

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Sebaceous glands: These structures

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So, what does the skin do?

Located in our skin are many skin receptors which allow us to feel such things as heat, cold and pain. The receptors in our skin responsible for the sense of touch send impulses to the brain, and thus an object or temperature is felt. If the brain detects that the external environment is too cold or too hot, a homeostatic response is initiated.

When it is cold, the muscles in the arterioles (tiny arteries) of the skin constrict to decrease blood flow to the skin. This is to ensure that blood supply to the vital organs remains constant, and that blood temperature does not become too cold. Erectile muscles situated behind the hair follicles cause hairs to stand up. This is to trap a layer of heat between the hairs and the skin.

When it is hot, the muscles in the arterioles relax. Because these arterioles are closer to the surface of the skin and the external environment, the blood within them is cooled by such things as cold water or a cool breeze. If the body really starts to overheat, sweat glands in the skin go one step further and secrete a watery fluid, which cools the skin down even further.

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