Skin
The human skin
The skin (or cutaneous membrane) is the largest organ in the human body. There are about 2 sq m (6 sq ft) of it covering an average adult and it accounts for about an eighth of the total body weight. It's the body's first line of defence against a hostile environment, being waterproof (from both directions) and germ-proof. It also acts as a temperature control mechanism and the main access to one of the senses - touch.
Different layers of skin
There are two distinct parts of the skin, known as Epidermis and the Dermis and a third layer, the Hypodermis (or subcutaneous tissue), not strictly skin, but made up of fatty cells which lie below the skin.
The bit we can see and touch is the epidermis. Most of the epidermis is made up of dead skin cells - the living cells at the bottom of the epidermis rise to the surface of the skin, hardening, flattening, and eventually being shed providing us with the outermost layer of protection. Interestingly, dead skin cells constitute over 90% of house dust.
The lower, living levels of the epidermis also contain melanin - the pigment that gives skin its colour and protects it from the sun - and a chemical compound called ergosterol that produces vitamin D in sunlight.
The bottom layer of skin, the dermis, is richly supplied with blood vessels; this is the part of skin in which hair follicles, sweat and sebaceous glands, and nerve endings are located. The nerve endings are in ridges that push into the epidermis in a slightly different pattern in everybody. This is the reason behind unique fingerprints.
There are two different types of sweat gland. The first, the eccrine glands, produce a salty liquid and these are located all over the skins surface. This is used as a temperature control mechanism, but the liquid is also slightly acidic, which is useful for keeping down bacteria that live naturally on the skin.
The other type - the apocrine glands - are the ones responsible for body odour. These glands are only located in the armpits and genitals and secrete a mixture of protein and fat. The bacteria that live on the skins surface thrive on this and cause body odour. The apocrine glands only start to become active at puberty.
The other type of gland found in the dermis are the sebaceous glands. These secrete an oily substance that keeps the epidermis waterproof and the hair supple.