Skin
Years wrinkle the skin, but to give up enthusiasm wrinkles the soul. ~ American general Douglas MacArthur
Every living entity, from a cell to the walking colony known as a human, has a physical barrier of delineation. Cell have membranes. Plants and animals have functionally differentiated organs that act as boundaries between the inner and outer world. The human body is largely surrounded by skin.
Skin is one the largest single human organs. Skin covers 5.5 m2 for an average adult male and weighs 3.6 kg: 6–8% of total body weight. A piece of skin the size of a US quarter has 3 million cells, 100 sweat glands, 50 nerve endings, 1 meter of blood vessels, and nearly as many lymph vessels.
The entire skin of a body has 640,000 sensory receptors connected to over 1/2 million nerve fibers. Each fingertip has over 3,000 touch receptors. Sense receptor spacing varies from 7 to 135 tactile points per cm2.
There are distinct task-specialized touch receptors which may work in complementary fashion. Their relative densities vary depending upon an area’s tactile employment.
Meissner’s corpuscles are mechanoreceptors sensitive to light touch. They detect if an object is slipping from your grasp.
Merkel cells detect prolonged pressure. Besides appreciating texture, Merkel cells convey information about an object’s weight and how tightly you are gripping it.
So, Meissner’s corpuscles and Merkel cells work together to let you hold eggs without crushing or dropping them.
Human genitals, which work part-time as pleasure centers, are rich in Meissner’s corpuscles. So too nipples. But few Merkel cells are employed there – which is why you cannot read braille with your penis, clitoris, or breasts.
Melanocyte skin cells sense ultraviolet light using a photosensitive receptor – rhodopsin – also found in the eye. As a protective measure, UV light triggers melanogenesis within hours. Melanogenesis is the production of melanin. Melanin is a pigment, ubiquitous in nature, that acts as a photoprotectant. Melanin absorbs 99.9% of UV radiation, transforming it into harmless heat, which is why sun-exposed skin is excessively warm.
Hair follicle density goes from zero on the lips to 150,000 per cm2 on the scalp.
Skin in thinnest – 1/10th millimeter – on the eyelids, lower abdomen, and external genitalia, and thickest on the palms of the hands and soles of the feet: 3–4 mm.
Skin orchestrates an array of functions:
1) waterproofing and puncture-resistant protection;
2) guarding against foreign invasion as part of the immune system;
3) temperature regulation: skin capillaries constrict to drive blood into the interior, conserving body heat, or enlarge to radiate heat from the interior; sweat also acts to cool via evaporation;
4) a metabolic organ, with metabolism, storage, and catabolism of fat, along with storing and adjusting water and salt levels;
5) a source of moisture and lubrication to other organs;
6) vitamin D synthesis;
7) excretion via the sweat glands;
8) passer of gas, both in and out;
9) blood pressure regulator: in a biological butterfly effect, the opening and closing of capillaries can assist in making large shifts in blood flow;
10) a sensory organ, especially on the primary sources of contact with the external world: the palms and soles.
Skin is both protection and portal: blocking toxins and preventing excessive fluid loss but allowing organic nutrients in as well as expelling waste.
The skin is the body’s most adaptable tissue. From the velvet of a baby bottom to the parchment of a geezer’s forearm, skin tells a health story in thickness, color, shape, and function. Skin responds to the demand made upon it: becoming thin and sensitive or calloused and unfeeling.
Human skin owes its toughness and adaptability to being arranged in layers. The outermost covering, the epidermis, has 4 layers.
The epidermis is continually being shed and replaced. An epidermal lifecycle is ~27 days. The epidermal layers have no blood vessels. They receive their irrigation and nourishment from the underlying dermis.
The dermis is thicker than the other layers, housing all the blood and lymph vessels that service the epidermis; hair follicles, along with their sebaceous glands and erector muscles; sweat glands; and most of the skin’s nerve endings.
The dermis is heavily laced with connective tissue, giving skin both its tensile strength and its pliability and elasticity. The connective tissue is woven into the subcutaneous connective tissue which further adheres to deeper layers. The dermal connective tissues also house the main healing mechanisms for the skin.
The connective dermal web provides the changes in appearance of aging skin. Aging leads to moisture loss, a dynamic where the fullness of skin is literally drained away, leaving it drier, thinner, and wrinkled.
Aged skin is less elastic. Dermal aging shows a person’s biological age as opposed to chronological age. Heredity, diet, disease, trauma, and stress all make statements on the skin.
