Skin is the largest organ in the human body with a surface area of about two square metres and 15% of body weight. It is soft, to allow movement, but still tough enough to resist breaking or tearing. When there is a break or opening in the skin it is called a wound.
Skin varies in texture and thickness from one part of the body to the next and from person to person. For instance, the skin on our eyelids is very thin and delicate (at 0.5mm thick), while skin on the soles of our feet is thicker and harder (at 4mm thick). The skin has more than 18km of blood vessels.
The epidermis is the outermost layer of the skin and the layer you can see. It creates a waterproof barrier to hold moisture in and keep moisture out and is made up of 15% water. The epidermis provides protection to the more fragile tissues below.
It also contains the pigment-producing cells (melanocytes) that give a person’s skin its colour. There is a continuous process of renewal in the epidermis. New cells are formed in the lower layers of the skin rising to the surface of the skin as they mature. These replace the old cells that die and are shed.
The dermis lies just beneath the epidermis and is the inner layer of the skin providing the structure of the skin. It is made up of made up of networks of elastic fibres (elastin) for flexibility and dense fibres (collagen) for strength. The dermis is about 70% water and contains the blood vessels (for transporting nutrients and waste products) and nerve fibres (for sensation). The blood vessels also have a role in temperature regulation. In hot weather or after exercise, these blood vessels expand, bringing body heat to the skin surface. Perspiration floods out of sweat glands and evaporates from the skin, taking the heat along with it. If the temperature is cold, these blood vessels in the dermis contract, which helps to cut down on heat loss.
The subcutaneous is a layer of fat that sits immediately under the dermis. It provides temperature insulation and mechanical protection. It gives smoothness and contour to our body. The fat tissue stored in the subcutaneous is a source of energy as well as providing cushioning. The size of this layer varies throughout the body, and from person to person.
Functions of skin
- Skin is a defence barrier. Skin protects against trauma from water, chemicals, microorganisms, mechanical stress and irradiation.
- Skin regulates temperature. Skin regulates temperature by interacting with a brain structure called the hypothalamus. The hypothalamus has temperature sensors that cause sweating when the body overheats and increase heat production when body temperature drops too low.
- Skin is a sensory organ. It connects the mind and body to the external world. Skin helps us detect temperature, touch and vibration.
- Skin manufactures hormones and neurotransmitters. Skin makes and uses hormones like vitamin D, steroids and thyroid hormones. Skin also makes neurotransmitters and hormones used in the brain.
- Skin assists the immune system. When a foreign material is present, special skin cells (called mast cells) trigger an immune reaction, which signals other components of the immune system to become active.