The human integumentary system is made up of the skin, hair, nails, and glands, and it serves many protective functions for the body. It prevents excessive water loss, keeps out microorganisms that could cause illness, and protectsthe underlying tissues from mechanical damage. Pigments in the skin called melanin absorb and reflect the sun's harmful ultraviolet radiation. The skin also helps to regulate body temperature. If heat builds up in the body, sweatglands in the skin produce sweat, which evaporates and cools the skin. In addition, when the body overheats, blood vessels in the skin expand and bring more blood to the surface, which allows body heat to be lost. If the body is too cold, on the other hand, the blood vessels in the skin contract, resultingin less blood at the body surface; heat is, thus, conserved. In addition to temperature regulation, the skin serves as a minor excretory organ, because sweat removes small amounts of nitrogenous wastes produced by the body. The skin also functions as a sense organ in that it contains millions of nerve endings that detect touch, heat, cold, pain, and pressure. Finally, the skin produces vitamin D in the presence of sunlight, and renews and repairs damage to itself.
In an adult, the skin covers about 21.5 sq ft (2 sq m) and weighs about 11 lb(5 kg). Depending on location, the skin ranges from 0.02-0.16 in (0.5-4.0 mm) thick. Its two principal parts are the outer layer, or epidermis, and a thicker inner layer, the dermis. A subcutaneous layer of fatty or adipose tissueis found below the dermis. Fibers from the dermis attach the skin to the subcutaneous layer, and the underlying tissues and organs also connect to the subcutaneous layer.
Ninety percent of the epidermis, including the outer layers, contains keratinocytes--cells that produce keratin, a protein that helps waterproof and protect the skin. Melanocytes are pigment cells that produce melanin, a brown-black pigment that adds to skin color and absorbs ultraviolet light, thereby shielding the genetic material in skin cells from damage. Merkel's cells are touch-sensitive cells found in the deepest layer of the epidermis of hairless skin.
In most areas of the body, the epidermis consists of four layers. On the soles of the feet and palms of the hands, where there is a lot of friction, the epidermis has five layers. In addition, calluses, abnormal thickenings of theepidermis, occur on skin subject to constant friction. At the skin surface, the outer layer of the epidermis constantly sheds the dead cells containing keratin. The uppermost layer consists of about 25 rows of flat dead cells thatcontain keratin.
The dermis is made up of connective tissue that contains protein, collagen, and elastic fibers. It also contains blood and lymph vessels, sensory receptors, related nerves, and glands. The outer part of the dermis has fingerlike projections, called dermal papillae, that indent the lower layer of the epidermis. Dermal papillae cause ridges in the epidermis above it, which in the digits give rise to fingerprints. The ridge pattern of fingerprints is inherited,and is unique to each individual. The dermis is thick in the palms and soles, but very thin in other places, such as the eyelids. If a part of the body,such as a working muscle, needs more blood, blood vessels in the dermis constrict, causing blood to leave the skin and enter the circulation that leads tomuscles and other body parts. Sweat glands, the ducts of which pass throughthe epidermis to the outside and open on the skin surface through pores, areembedded in the deep layers of the dermis. Hair follicles and hair roots alsooriginate in the dermis, and the hair shafts extend from the hair root through the skin layers to the surface. Also in the dermis are sebaceous glands associated with hair follicles that produce an oily substance called sebum. Sebum softens the hair and prevents it from drying; if sebum blocks up a sebaceous gland, a whitehead appears on the skin. A blackhead results if the material oxidizes and dries. Acne is caused by infections of the sebaceous glands. When this occurs, the skin breaks out in pimples and can become scarred.
The skin is an important sense organ, and as such it includes a number of nerves, which are mainly in the dermis (a few reach the epidermis). Nerves carryimpulses to and from hair muscles, sweat glands, and blood vessels, and receive messages from touch, temperature, and pain receptors. Some nerve endingsare specialized, such as sensory receptors that detect external stimuli. Thenerve endings in the dermal papillae are known as Meissner's corpuscles, which detect light touch, such as a pat, or the feel of clothing on the skin. Pacinian corpuscles, located in the deeper dermis, are stimulated by stronger pressure on the skin. Receptors near hair roots detect displacement of the skinhairs by stimuli such as touch or wind. Bare nerve endings throughout the skin report information to the brain about temperature change, texture, pressure, and trauma.
Some skin disorders result from overexposure to the ultraviolet (UV) rays insunlight. At first, overexposure to sunlight results in injury known as sunburn. UV rays damage skin cells, blood vessels, and other dermal structures. Continual overexposure leads to leathery skin, wrinkles, and discoloration andcan also lead to skin cancer. Anyone excessively exposed to UV rays runs a risk of skin cancer, regardless of the amount of pigmentation normally in the skin. Seventy-five percent of all skin cancers are basal cell carcinomas thatarise in the epidermis and rarely spread (metastasize) to other parts of thebody. Physicians can surgically remove basal cell cancers. Squamous cell carcinomas also occur in the epidermis, but these tend to metastasize. Malignantmelanomas are life-threatening skin cancers that metastasize rapidly. There can be a 10-20 year delay between exposure to sunlight and the development ofskin cancers.