Covering an average of 20 square feet, the skin is the body's largest and heaviest organ. Its most obvious job is to protect our insides from the outside, but there is much more to the skin than that.

Skin layers The skin has three basic levels — the epidermis, the dermis, and the hypodermis: Epidermis Main roles: makes new skin cells, gives skin its color, protects the body. Layers of skin Basic diagram of skin's structure. The epidermis is the outermost layer; it is a waterproof barrier that gives skin its tone. Dead cells are shed continuously from the epidermis as new ones take their place. We shed around 500 million skin cells each day. In fact, the outermost parts of the epidermis consist of 25–30 layers of dead cells. New cells are made in the lower layers of the epidermis. Over the course of around 4 weeks, they make their way to the surface, become hard, and replace the dead cells as they are shed. Keratinocytes are the most common cell type within the epidermis; their job is to act as a barrier against bacteria, parasites, fungi, viruses, heat, ultraviolet (UV) rays from the sun, and water loss. The epidermis contains no blood vessels. The color of our skin is produced by a pigment called melanin, which is produced by melanocytes; these are found in the epidermis and protect the skin from UV rays. The epidermis is subdivided into five layers: stratum corneum stratum lucidum stratum granulosum stratum spinosum stratum germinativum Between the epidermis and the dermis is a thin sheet of fibers called the basement membrane. Dermis Main roles: makes sweat and oil, provides sensations and blood to the skin, grows hair. The dermis is mostly connective tissue, and it protects the body from stress and strain; it gives the skin strength and elasticity. If the dermis is stretched a lot, for instance, during pregnancy, the dermis can be torn, and this shows up as so-called stretch marks. The dermis houses hair follicles, blood vessels, and lymphatic vessels. It is also home to a number of glands, including sweat glands and sebaceous glands, which produce sebum, an oil that lubricates and waterproofs hair. The dermis is further split into two layers: Papillary region: made of loose connective tissue, it has finger-like projections that push into the epidermis. These projections give the dermis a bumpy surface and are responsible for the patterns we have on our fingertips. Reticular region: made of dense, irregularly organized connective tissue. Protein fibers in the reticular region give skin its strength and elasticity. Subcutaneous tissue Main roles: attaches dermis to the body, controls body temperature, stores fat. The deepest layer is called subcutaneous tissue, the hypodermis, or subcutis. It is not technically part of the skin but helps attach the skin to underlying bone and muscle. Subcutaneous tissue also provides skin with nerves and blood supply. The hypodermis is mostly made of fat, connective tissue, and elastin (an elastic protein that helps tissues return to their normal shape after stretching). The high levels of fat help insulate the body and prevent us from losing too much heat. The fat layer also acts as protection, padding our bones and muscles. Some hormones are made by fat cells in the hypodermis, vitamin D, for instance. Functions of the skin Skin goosebumps Goosebumps help us to retain heat when we are in a cold environment. Some of the many roles of skin: Protection: against pathogens. Langerhans cells in the skin are part of the immune system. Storage: stores lipids (fats) and water. Sensation: nerve endings detect temperature, pressure, vibration, touch, and injury. Control water loss: the skin prevents water from escaping by evaporation. Water resistance: it prevents nutrients from being washed from the skin Thermoregulation: by producing sweat and dilating blood vessels, the skin helps keep the body cool. "Goosebumps" and blood vessel constriction, help us retain heat. Skin color Human skin color can vary from almost black through to almost white. Most of this variation is due to a pigment called melanin. It is worth noting that the coloration of light skin is mostly determined by the whitish-blue color of connective tissue below the dermis and hemoglobin in the veins of the dermis. Melanin's primary role is to protect the skin from damaging UV light from the sun, which can cause skin cancer. When skin is exposed to UV light, melanocytes start producing melanin, creating a suntan. Populations that live in parts of the world that receive higher levels of UV light, for instance, nearer the equator, tend to have higher levels of melanin and, therefore, darker skin. Conversely, populations that receive less sunlight (toward the poles) tend to have lighter skin with less melanin. In general, females have lighter skin than males. This may be because women need more calcium during pregnancy and while breast-feeding; vitamin D, which is produced when skin is exposed to the sun, is important for absorbing calcium.