The Skin

The skin is an organ because it consists of different tissues that are joined to perform specific activities. It is one of the largest organs of the body in surface area and weight. In adults, the skin covers an area of about 2 square meters, and weighs 4.5 to 5 kg. It ranges in thickness from 0.5 to 4.0 mm, depending on location. The skin is not just a simple, thin coat that keeps the body together and provides protection. It performs several essential functions. Dermatology (der'-ma-TOL-o-je; dermado = skin; logos = study of) is the medical specialty that deals with diagnosing and treating skin disorders.

Anatomy of the Skin

Structurally, the skin consists of two principal parts. The outer, thinner portion, which is composed of epithelium, is called the epidermis. The epidermis is attached to the inner, thicker, connective tissue part called the dermis. Beneath the dermis is a subcutaneous (subQ)
layer. This layer, also called the superficial fascia or hypodermis, consists of areolar and adipose tissues. Fibbers from the dermis extend down into the subcutaneous layer and anchor the skin to it. The subcutaneous layer, in turn, attaches to underlying tissues and organs.

 

Physiology of the Skin

Skin serves several functions, which are introduced here.

Regulation of body temperature.

In response to high environmental temperature or strenuous exercise, the evaporation of sweat from the skin surface helps lower an elevated body temperature to normal. In response to low environmental temperature, production of sweat is decreased, which helps conserve heat. Changes in the flow of blood to the skin also help regulate body temperature.

Protection.

The skin covers the body and provides a physical barrier that protects underlying tissues from physical abrasion, bacterial invasion, dehydration, and ultraviolet (UV) radiation. Hair and nails also have protective functions.

Sensation.

The skin contains abundant nerve endings and receptors that detect stimuli related to temperature, touch, pressure, and pain.

Excretion.

Besides removing heat and some water from the body, sweat also is the vehicle for excretion of a small amount of salts and several organic compounds.

Immunity.

Certain cells of the epidermis are important components of the immune system, which fends off foreign invaders.

Blood reservoir.

The dermis of the skin houses extensive networks of blood vessels that carry 8 to 10% of the total blood flow in a resting adult. In moderate exercise, skin blood flow may increase, which helps dissipate heat from the body. During hard exercise, however, skin blood vessels constrict (narrow) somewhat, and more blood is able to circulate to contracting muscles.

Synthesis of Vitamin D.

Vitamin D is a group of closely related compounds.  Synthesis of vitamin D begins with activation of a precursor molecule in the skin by ultraviolet (UV) rays in sunlight. Enzymes in the liver and kidneys then modify the molecule, finally producing calcitriol, the most active form of vitamin D. Calcitriol contributes to the homeostasis of body fluids by aiding absorption of calcium in foods. According to the synthesis sequence just described, vitamin D is a hormone, since it is produced in one location in the body, transported by the blood, and then exerts its effect in another location. In this respect, the skin may be considered an endocrine organ.

See Diagram: Structure of the skin and underlying subcutaneous tissue. The stratum lucidum shown is not present on hairy skin but is included so that you can see the relationship of all five epidermal strata.