The skin

Skin forms the body’s largest organ. An adult’s skin has a surface area of approximately 19 square feet (1.75 square meters) and weighs about 6 pounds (2.7 kilograms).
Skin provides the body with a tough, flexible barrier that protects against disease, injury, and loss of water from the moist internal tissues. It also helps control the body’s temperature, excretes some wastes, and serves as a major sensory organ, registering pressure, pain, and temperature. An area of skin about the size of an adult thumbnail may contain about three million cells, 3 feet (90 centimeters) of blood vessels, 12 feet (3.7 meters) of nerves, and 100 sweat glands.
Skin has three layers of tissue: epidermis, dermis, and subcutaneous tissue.

Freckles are small patches of melanin pigment in the Malpighian layer of the epidermis. The same pigmentation causes the skin to darken in response to sunlight and is responsible for the dark skin of some people.

Epidermis

A slice cut through the epidermis would reveal multiple layers of cells, the lowest alive and multiplying, the topmost cells dead and flaking off.

Cells of the stratum basale, or Malpighian layer, resemble close-packed posts or columns. As they multiply they give rise to the “prickle” cells of the stratum spinosum, the layer just above, some five to ten cells deep. Next comes the stratum granulosum. There are no blood vessels here to bring nutrients or carry away wastes, so this layer’s cells die off and accumulate granules of protein waste. Above these is a clear layer called the stratum lucidum, where granules have changed into the tough fibrous protein keratin—the substance nails and hair are made. The stratum corneum, or cornified layer, the topmost layer of the epidermis, consists of flat, dead, keratinized cells that are continuously flaking off.

The epidermis lacks blood vessels and has few nerves, but contains granules of the pigment melanin. This dark brown substance helps to determine the color of the skin. The skins of dark-skinned people contain much melanin. The freckles and suntans of paleskinned people are also caused by melanin. Strong sunlight stimulates production of this pigment, which helps protect the skin from damage caused by overexposure to the sun’s ultraviolet radiation. Ultraviolet light also has a beneficial effect, however, by acting on the skin to help the body synthesize vitamin D an important factor in the healthy growth of bones.

The skin has three principal layers—the epidermis, dermis, and a layer of subcutaneous tissue, composed of fat, a fibrous layer, and a layer of subcutaneous blood vessels and muscle. The epidermis itself has three main layers—the Malpighian generative layer, a layer of living cells, and an outer layer of dead “cornified” cells. The dermis contains numerous tiny blood capillaries, nerve endings, sweat and sebaceous glands, and lymph vessels. Hair follicles run through this layer. Associated with hair follicles are a blood supply, a sensory nerve supply, a muscle which can cause the hair to rise, and a sebaceous gland.

Dermis

This is a closely woven network of connective tissue, thinnest in the eyelids, thickest in the back, and everywhere far thicker than the epidermis. Tough protein fibers in the dermis give skin its tensile strength and bulk. The dermis also contains scattered blood vessels, lymph vessels, nerve endings, hair follicles, and glands connected with the epidermis.

Thousands of tiny projections called papillae jut up from the dermis and fit into tiny pits in the bottom of the epidermis. Papillae grouped in rows form the ridges on fingers, giving each individual a distinctive set of fingerprints. Each of these papillae has a rich supply of tiny capillaries blood vessels bringing nourishment to growing skin, and regulating heat loss from the body. Little heat escapes from the skin when its capillaries constrict to block the flow of blood, but heat loss is substantial when capillaries expand and let blood pass through freely. Papillae also contain nerve endings sensitive to touch.

Nail is a specialized horny tissue that grows at the ends of the fingers and toes. The nail root is buried in a fold of skin. The body of the nail grows continually from this until it extends beyond the end of the finger or toe.

Subcutaneous tissue

This consists mainly of blood vessels, connective tissue, and cells that store fat. This tissue helps protect the body from blows and other injuries and also helps retain body heat.

Sensors in the skin

Hundreds of thousands of sensitive nerve endings are embedded in the skin, especially in such regions as the lips and finger pads. Between them, different kinds of sensor detect touch, pressure, heat, cold, and pain. Touch receptors are shaped like bulbs. Other nerve endings form a mesh embracing the roots of hairs: these sensors are activated when a hair bends. Free nerve endings, resembling branching twigs, may register pain as well as touch and pressure. But pain is felt by several kinds of nerve if these are subjected to intense pressure. All these sensors are mechanoreceptors: receptors that fire off signals to the central nervous system when deformed by touch or pressure. Besides these, the skin has two types of thermoreceptor, in the form of nerve endings sensitive to temperature change. One type senses cold, the other heat.

A cross section through skin shows two hairs in their follicles in the layer of subcutaneous fat just below the dermis.

Glands in the skin

There are two important kinds of gland in skin: sebaceous glands and sweat glands.
Sebaceous glands open into hair follicles in the dermal layer of the skin. These glands secrete sebum, a fatty substance that lubricates the hairs and their surrounding skin. Sebum accumulating in a blocked gland may produce a soft, sebaceous cyst, which may appear alarming, but is benign.
Some 2.4 million sweat glands activated by the autonomic nervous system excrete water, salt, and the body wastes, lactic acid and urea. These escape through narrow ducts with openings forming the tiny holes called sweat pores. Certain sweat glands (called apocrine sweat glands), located in the temples, armpits, and the genital area, produce a thick secretion under emotional stress. Others (called eccrine sweat glands) are widespread in the skin and produce a dilute salt solution when the body temperature becomes uncomfortably high, either in response to external heat or to physical activity. This solution helps to cool the body as it evaporates.

A man marching through a hot desert may sweat 2.5 gallons (10 liters) of water in a day-half from his trunk, a quarter from the legs and thighs, and a quarter from the arms and head. Copious sweating causes salt loss and this may produce cramping in people who replace the water by drinking without also taking salt to restore its concentration in the blood.