The Integumentary System

The Largest Organ

Skin is an organ, and it is the largest one the body has, covering roughly two square meters in an adult and accounting for about 16 percent of body weight.

It does five jobs: it keeps pathogens out, it keeps fluid in, it regulates body temperature, it senses touch, pressure, temperature and pain, and it manufactures vitamin D on exposure to sunlight.

Skin is also the organ you assess first and most often, because it reports on circulation faster than almost anything else you can measure.

The Three Layers

Epidermis. The outer layer, made of stacked sheets of epithelial cells with no blood vessels of its own. New cells form at the bottom, push upward, flatten, fill with keratin and die, and the surface you touch is dead cells being shed continuously. The whole layer replaces itself roughly every month.

Pigment cells called melanocytes sit at the base and produce melanin, which gives skin its color and absorbs ultraviolet light.

Because there are no vessels in the epidermis, a wound confined to it does not bleed.

Dermis. The layer beneath, and where nearly everything functional lives: capillaries, nerve endings, sweat glands, sebaceous (oil) glands, hair follicles and lymphatic vessels, all held in a framework of collagen and elastin.

A wound reaching the dermis bleeds and hurts, because that is where the blood supply and the nerve endings are.

Subcutaneous layer. Below the dermis, made of fat and connective tissue. It insulates, cushions against impact, stores energy and anchors skin to the muscle and bone underneath. Thickness varies by body region and by person, and it thins with age, which is part of why older adults chill quickly and bruise easily.

Temperature Regulation

The body holds core temperature near 37 degrees Celsius, and the skin is the main tool for doing it. Heat is lost or conserved through five mechanisms.

• Radiation. Heat leaving the body as infrared energy to cooler surroundings. The largest route at rest.
• Convection. Heat carried away by moving air or water. Wind multiplies it.
• Conduction. Heat transferred by direct contact, as with a patient lying on cold ground. Water conducts heat far faster than air does.
• Evaporation. Heat consumed turning sweat into vapor. Its effectiveness falls as humidity rises.
• Respiration. Heat lost in exhaled air.

The hypothalamus is the thermostat. Too warm, and it dilates the dermal vessels to bring blood to the surface and activates sweat glands. Too cold, and it constricts those vessels to keep blood deep and triggers shivering, which generates heat through muscle activity.

Those same responses explain skin findings.

A patient in shock has pale, cool, sweaty skin because the sympathetic response has clamped the dermal vessels shut while simultaneously activating the sweat glands. Nothing about that is temperature regulation. It is the circulatory compensation showing through the skin.

Heat exhaustion leaves the patient sweating heavily, with skin that is pale, cool and clammy, because the cooling system is still working and is simply losing the race.

Heat stroke is the failure of that system. The patient stops sweating, and the skin becomes hot and dry along with an altered mental status. Sweating means exhaustion. Hot and dry means stroke, and it is a true emergency.

Reading the Skin

Skin gives you three findings quickly, and together they are one of the most useful assessments you have.

Color. Pale suggests constriction, blood loss or shock. Flushed suggests fever, heat exposure or a distributive process. Cyanosis, a blue tint, means poor oxygenation and appears first in the lips, the nail beds and around the mouth. Jaundice, a yellow tint, points at the liver. Mottling, a blotchy irregular pattern, is a sign of poor perfusion, particularly in children.

Assess color where pigmentation interferes least: the nail beds, the inside of the lips, the mucous membranes of the mouth and the conjunctiva of the eye.

Temperature and moisture. Assess with the back of your hand. Cool and clammy points at shock. Hot and dry points at heat stroke or a high fever. Cold and dry points at cold exposure.

Capillary refill. Press a nail bed or the sternum until it blanches, release, and count how long color takes to return. Under two seconds is normal. It is most reliable in children and least reliable in cold environments and in older adults.

Wounds

Closed wounds leave the skin surface intact. A contusion is bruising from bleeding into the tissue. A hematoma is a larger collection of blood forming a raised mass. A crush injury damages structures underneath with little to see on the surface.

Open wounds break the surface.

Bleeding is described by its source. Arterial bleeding is bright red and spurts with the pulse. Venous bleeding is darker red and flows steadily. Capillary bleeding oozes and usually stops on its own.

Burns

Burn depth describes how far down through the layers the injury reached.

Superficial (first degree). Epidermis only. Red, dry, painful, no blisters. Sunburn is the common example. Heals without scarring.

Partial thickness (second degree). Through the epidermis into the dermis. Blisters, moist appearance, intense pain. This is the most painful category, because the nerve endings are exposed and still alive.

Full thickness (third degree). Through the entire dermis, possibly into the subcutaneous tissue. The surface can be white, brown, leathery or charred, and the area itself is often painless because the nerve endings have been destroyed. Painless is not reassuring here, it is a finding, and the surrounding partial thickness areas still hurt considerably.

Burn extent is estimated with the rule of nines, dividing the adult body surface into sections of nine percent. In children the proportions differ, most importantly the head, which accounts for a far larger share of body surface. The patient's own palm, including fingers, represents roughly one percent of their body surface, which is useful for scattered burns.

Two physiologic consequences drive burn care. Skin that is gone can no longer hold fluid in, so large burns lose fluid volume and can produce shock over hours. Skin that is gone can no longer keep organisms out, so infection becomes the leading later threat.

Circumferential burns, meaning burns encircling a limb or the chest, are a specific concern. Burned tissue tightens as it swells, and a circle of it can cut off circulation to the limb or restrict the chest from expanding.

Skin and Aging

Older skin is thinner, less elastic and has less subcutaneous fat. Capillaries are more fragile, so bruising happens with minor contact and can look far worse than the mechanism suggests. Sweat production falls and the vessel responses are slower, which is why older adults tolerate heat and cold poorly. Wounds heal more slowly and become infected more readily.

Skin turgor, tested by pinching the skin and watching how quickly it returns, is a dehydration check that becomes unreliable with age, because aging skin returns slowly regardless of hydration.

What to Take Away

Three layers: epidermis with no vessels, dermis with everything functional, subcutaneous fat below. The skin regulates temperature by moving blood toward or away from the surface and by sweating. Pale, cool and sweaty means the sympathetic response, which usually means shock. Sweating means heat exhaustion, hot and dry means heat stroke. Burn depth describes layers reached, and the full thickness center is painless while the edges hurt.