Skin forms the largest organ of the body, accounting for about 16 per cent of a person's weight. It performs many vital roles as both a barrier and a regulating influence between the outside world and the controlled environment within our bodies, acting as a barrier to bacteria and filtering harmful rays from the sun. Internal body temperature (core temperature) is controlled through several processes, including the combined actions of sweat production and the rate of blood flowing through the network of blood vessels within the skin.
In very cold conditions skin blood flow drops very low, retaining heat in the centre of the body and preserving the flow of warm blood to the vital organs within the chest and abdomen and to the brain. Skin and the fat layer beneath it also act as good insulators. When in a hot environment or when generating heat from muscle activity skin vessels open up, potentially increasing skin blood flow to as much as a third of the total output of blood from the heart. Thus the skin turns into a heat radiator. The evaporation of sweat from the skin greatly increases the rate at which heat can be lost from the body.
The physical toughness of the skin prevents the ingress of harmful chemicals and invading organisms such as bacteria and viruses. It also provides resistance to shocks for the more sensitive tissues underneath. At the same time however skin needs to be supple and be able to stretch to accommodate movement.
Skin exposed to sunlight is the main site of manufacture of vitamin D, which is essential for the growth and maintenance of our bones. The extensive network of nerves within the skin feeds information constantly to the brain concerning our surroundings. On the one hand we are thus warned of harmful extremes of temperature or of other dangers while on the other touch can be one of the most powerfully soothing and pleasurable of sensations.
Much importance is attached to the appearance of skin, especially in our modern society. Medical conditions affecting the skin can have marked effects not only on our state of well being but also on the ways we interact with other people, on our suitability for certain occupations and on the sorts of pastimes we can enjoy. Some of the consequences of skin disease such as rashes and itching may be obvious and others, such as the psychological impact, can be subtler although just as important.
There are three main layers of skin: epidermis, dermis and the subcutaneous layer.
Epidermis
This is the outermost layer. In most parts of the body the epidermis is about 0.1 mm thick but on the soles of the feet and the palms of the hands it can be 1mm thick or more. The main skin cell that makes up the epidermis is called the keratinocyte, thus named because it produces a tough protein called keratin. Keratin is also the protein from which nails and hair are formed. It gives skin much of its resistance to physical wear and tear and makes skin waterproof.
Keratinocytes arise in the deepest level of the epidermis and new cells are constantly being produced. As this happens the older cells migrate up to the surface of the skin and eventually are worn off. On average it takes about 60 days for a new keratinocyte to migrate to the surface and to be shed. Skin scales are the result and it is perfectly normal for all people to lose about a gram of skin each day in this way. A proportion of the dust that is picked up by your vacuum cleaner is composed of these dead skin cells.
Keratinocytes change in the their size and shape from square cells at the base of the epidermis, gradually becoming flatter towards the surface, by which time they have also lost their internal structure. In healthy skin these surface cells lie closely together in overlapping fashion, which adds to their protective function.
Dermis
The dermis lies immediately underneath the epidermis and is about four times thicker. It contains numerous specialised supporting tissues as well as blood vessels, nerves, hair roots and sweat glands. For any cosmeceutical product to be effective it should penetrate to this layer. The dermis is thickest on the scalp and back and thinnest on the eyelids. It comprises of 95% collagen and 5% elastin.
Throughout the dermis other types of protein, notably collagen and elastin, give it strength and flexibility. A reduction in these proteins with age is normal and contributes to the more fragile skin of elderly people. Medications, in particular steroid drugs, also weaken the collagen fibres, causing thinning of the skin in the long term and an increased tendency to bruising.
Hair arises from root structures called follicles that run the whole depth of the dermis. The hair shaft travels through the epidermis to appear on the skin surface. In the deep dermis there are different types of sweat gland that connect to the surface of the skin through narrow, spiral-shaped sweat ducts - what we normally call the ‘pores' of the skin.
The subcutaneous layer
The subcutaneous layer is the fatty layer underneath the skin (‘subcutaneous'). It can vary considerably in thickness from person to person depending mostly on whether they are overweight. You can think of the subcutaneous layer as the foundations of your skin, as we age, it is responsible for sagging skin due to aging, stretch marks and cellulite.
