Integumentary System Health Article

Definition

The integumentary system includes the skin and the related structures that cover and protect the body. The human integumentary system is composed of the skin, and includes glands, hair, and nails. The largest organ in the body, the skin protects the body, prevents water loss, regulates body temperature, and senses the external environment.

Description

The integumentary system serves many protective functions for the body. It acts as a mechanical barrier, simultaneously preventing water from entering the body and excessive water loss. It also limits access of microorganisms that could cause illness, and protects underlying tissues from mechanical damage. Pigments in the skin called melanin, give skin its color, and absorb and reflect the sun's harmful ultraviolet radiation.

Function

In addition to serving as a protective barrier, the skin helps to regulate the body temperature by several mechanisms. If heat builds up in the body, sweat glands in the skin produce more sweat that evaporates and cools the skin. When the body overheats, blood vessels in the skin dilate (expand), bringing more blood to the surface, and allowing body heat to dissipate. When the body is too cold, the blood vessels in the skin constrict, shunting blood away from the body surface, thus conserving heat. Along with temperature regulation, the skin serves as a minor excretory organ, since sweat removes small, clinically insignificant amounts of nitrogenous wastes produced by the body. The skin also functions as a sense organ since 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 thickness ranges from 0.02-0.16 in (0.5-4.0 mm). Skin is composed of an outer layer, or epidermis, and a thicker inner layer, the dermis. A subcutaneous layer of fatty or adipose tissue is immediately 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.

The epidermis

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 disks are touchsensitive 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 considerable friction, the epidermis has five layers. Calluses, abnormal thickenings of the epidermis, 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 that contain keratin.

The dermis

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.

The blood vessels in the dermis contain a volume of blood. When a part of the body, such as a working muscle, needs more blood, blood vessels in the dermis constrict, shifting blood from the skin to supply muscles and other body parts. Sweat glands with ducts that pass through the epidermis and open on the skin surface through pores are embedded in the deep layers of the dermis. Hair follicles and hair roots also originate in the dermis, and the hair shafts extend from the hair root through the skin layers to the surface. The dermis also contains sebaceous glands associated with hair follicles. Sebaceous glands produce an oily substance called sebum. Sebum softens the hair and prevents it from drying, but if sebum blocks a sebaceous gland, a whitehead appears on the skin. A blackhead results if the material oxidizes and dries. Acne pimples are caused by infections of the sebaceous glands.

The skin is an important sense organ and as such includes a number of types of nerves, which are mainly in the dermis, with a few reaching the epidermis. Nerves carry impulses to and from hair muscles, sweat glands, and blood vessels, and receive messages from touch, temperature, and pain receptors. Some nerve endings are specialized such as sensory receptors that detect external stimuli. The nerve endings in the dermal papillae, known as Meissner's corpuscles, detect light touch, 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 skin hairs by stimuli such as touch or wind. Bare nerve endings throughout the skin supply information to the brain about temperature change (both heat and cold), texture, pressure, and trauma.