The Skin Illustrates the Properties of Organs

Although we take our skin for granted, it is so important as a barrier against infection and water loss that large-scale destruction of skin, such as by extensive burns, can prove fatal. The epidermis, or outer layer of the skin, is a specialized epithethelial tissue. It is covered by protective later of dead cells produced by the underlying living epidermal cells. These dead cells are packed with the protein keratin, which helps keep the skin both airtight and relatively waterproof.

Immediately beneath the epidermis lies a layer of connective tissue, the dermis. The loosely packed cells of the dermis are permeated by arterioles (small arteries). Arterioles feed blood pumped from the heart into a dense meshwork of capillaries that nourish both the dermal and epidermal tissue and empty into a network of venules (small veins) in the dermis. Loss of heat through the skin precisely regulated by neurons controlling the degree of dilation (expansion) of the arterioles.

When cooling is required, the arterioles dilate and flood the capillary beds with blood, thus releasing excess heat. When heat conversation is required, the arterioles supplying the skin capillaries are constricted; this is what caused frostbite. Lymph vessels collect and carry off extracellular fluid within the dermis. Various sensory nerve endings responsive to temperature, touch, pressure, vibration, and pain are scattered throughout the dermis and epidermis and provide feedback to the nervous system.

The dermis is also packed with glands derived from epithelial tissue. Glands called hair follicles produce hair from protein-containing secretions. Sweat glands produce watery secretion that cool the skin and secrete substances such as salts and urea. Sebaceous glands secrete an oily substance (sebum) that lubricates the epithelium.

In addition to the epithelial, connective, and nerve tissue already mentioned. The skin also contains muscle tissue. Tiny muscles attached to the hair follicles can cause the hairs of the skin to "stand on end" in response to signals from motor neurons, although this reaction is useless for retaining heat in humans most mammals are able to increase the thickness of their insulating fur in cold weather by erecting individual hairs.

Organ systems consist of two or more individual organs (in some cases, located in different regions of the body) that work together, performing a common function. An example is the digestive system, in which the mouth, esophagus, stomach, intestines, and other organs that supply digestive enzymes, such as the liver and pancreases, all function together to convert food into nutrient molecules.

Reference: Skin-Wikipedia, http://en.wikipedia.org/wiki/Skin