Skin: The Jack of All Trades

The skin is the largest organ of the body and is made of two main layers. The outer layer is the epidermis, a tough sheet with no blood vessels of its own. Beneath it lies the dermis, a thicker layer packed with blood vessels, nerves, glands and hair roots. Because a single organ protects, senses, cools, excretes and even manufactures vitamin D, the skin is rightly nicknamed 'the jack of all trades'.

The epidermis itself has three layers. The outermost is the cornified layer (stratum corneum), made of flat dead cells full of a horny protein called keratin; these are constantly worn away and replaced. The middle granular layer is a thin transition zone. The innermost Malpighian layer (also called the germinative layer) has living, dividing cells that push new cells outward, and it contains the pigment melanin that colours the skin and shields it from harmful sunlight.

The dermis is the living workshop of the skin. It holds blood vessels, sensory nerve endings, hair follicles, sweat glands and sebaceous (oil) glands, all set in tough elastic fibres. Sweat glands release sweat that cools the body and removes a little waste. Sebaceous glands secrete an oily sebum that keeps skin and hair soft and waterproof. The nerve endings make the skin a sense organ for touch, pressure, heat, cold and pain.

Skin keeps body temperature steady. When you are hot, the dermal blood vessels widen (vasodilation) so more warm blood reaches the surface and loses heat, and sweat glands pour out sweat that cools you as it evaporates. When you are cold, the vessels narrow (vasoconstriction) to keep heat in, hairs stand erect to trap a layer of warm air, and shivering generates heat. This automatic control is called thermoregulation.