Skin

The skin functions in protection, sensation, heat regulation, secretion and excretion, and respiration.

All layers of the skin participate in providing protection for the body. The tough, elastic nature of the entire skin protects against mechanical injury. The cornified outer layers of the- epidermis, imbibed with lipids, protect somewhat against penetration by water and also against loss of fluid from within. The keratinizing layers of the skin (stratum granulosum and stratum lucidum) form an electrically polarized membrane that functions as an important water barrier. The keratinized and keratinizing layers are poor conductors of heat and electricity and are practically inert chemically. Together with the melanin pigment deeper in the epidermis, these layers tend to screen out injurious ultraviolet radiation. The relatively impervious nature of the outer layers of the epidermis and their acid reaction (pH about 5.0) also protect the skim and the body against invasion by microorganisms and parasites.

Sensory functions are performed by the various nerve elements and end organs. The four major sensory modalities of touch, cold, warmth, and pain are not perceived diffusely over the skin surface, but rather at discrete "points" corresponding with projections of the specific sensory end organs to the surface. Pain points are set much more closely than the other receptor points, there being about 170 per square centimeter. Accuracy of sensory localization depends on the density of receptor points over a given area.

Touch and pressure sensations evoked by painless deformation of the skin are mediated by relatively thick myelinated fibers more than 10 microns in diameter and having a conductivity rate of 30 to 60 meters (100-200 feet) per second. They enter the spinal cord through the posterior columns and ascend to the brain. Pain and temperature sensations are mediated by nerve fibers about one half as thick as touch fibers. The pain fibers do not end in special sensory end organs, but merely terminate as delicate, free nerve endings in the epidermis. Any stimulus of sufficient intensity can evoke pain. Non-myelinated nerve fibers coursing in the antero-lateral tract of the spinal cord are believed to carry pain sensations at the slow rate of about 1 meter (3 feet) per second. Itching is mediated by this same type of nerve fiber. In skin denuded of its epidermis only pain sensations can be perceived. Other sensory modalities, such as tickle, wetness, softness, hardness, appreciation of surface texture, form, force, and weight, are all perceived and mediated as combinations of the four major sensory modalities.

Heat is regulated by the skin through its physical properties, through vascular responses, and by sweat secretion, all of which control the rate of heat loss. On heating, para-sympathetic vasomotor nerve impulses cause blood vessels to dilate, bringing a greater amount of blood close to the skin surface. This increases the skin temperature and thus the rate of heat loss by radiation, conduction, and convection. Sweating, which increases heat loss by evaporation, is activated by sympathetic nerve fibers. Moisture also increases the skin's conductivity of heat so as to favor heat loss. On chilling, sweating is reduced, leading to dry skin, which serves as a good insulator and tends to conserve body warmth, and the constriction of blood vessels caused by sympathetic vasomotor nerve impulses tends to conserve heat.

The secretory and excretory functions of the skin are carried out by the sebaceous glands and sweat glands. The oily secretion of the sebaceous glands keeps the hair and surface layers of the epidermis soft, prevents maceration by moisture, and contributes to their protective barrier function. No direct secretory innervation to sebaceous glands has been found, but hormones, such as androgens and progesterone, increase sebum output by causing sebaceous-gland hyperplasia. Increases in skin temperature also increase the rate of sebum formation by lowering the viscosity of sebum and allowing it to flow from the ducts of the glands. In addition, increased sweat secretion tends to emulsify sebum and to carry it away, helping to prevent occlusion of the sebaceous-gland duct.

Sweat secretion, besides its role in heat regulation, also serves an excretory function, helping the body to regulate fluid and electrolyte balance. In aiding the excretion of toxic products such as occur in uremia, it has a very slight effect not comparable to that of the kidneys. The sweat glands work continuously, even at low temperatures, secreting immediately evaporating, invisible sweat droplets. At a critical environmental temperature of about 30°-31°C, there is a sudden outbreak of visible fluid sweat all over the body surface. This sudden outbreak of sweat is under reflex control from the brain-stem. Under extreme conditions as much as six to eight liters of sweat may be produced. The loss of water and salt that this represents under such conditions must be replaced if life is to continue. Insensible perspiration refers to water loss through the skin without the formation of water droplets on the surface. It is composed both of sweat and of water lost directly through the epidermis and amounts to about 500 grams (almost 18 ounces) daily. A portion of this water loss originates as a residue of the keratinization process, as is seen by the great increase in insensible perspiration when keratinization is accelerated.

Absorption of most substances through the intact skin occurs only to a slight extent and probably takes place largely through the appendages. Substances readily soluble in both lipid and water are generally better absorbed than those soluble in only one or the other.

The skin also performs a respiratory function in the body, but it is relatively insignificant.