The Skin - The Integumentary System
Integumentary (Skin) system
The integumentary system, formed by the skin, hair, nails, and associated glands, enwraps the body. It is the most visible organ system and one of the most complex. Diverse in both form and function—from delicate eyelashes to the thick skin of the soles—the integumentary system protects the body from the outside world and its many harmful substances. It utilizes the Sun's rays while at the same time shielding the body from their damaging effects. In addition, the system helps to regulate body temperature, serves as a minor excretory organ, and makes the inner body aware of its outer environment through sensory receptors.DefinitionThe 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.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.FunctionIn 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.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.
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