The human skin is the largest organ, accounting for around 12% to 15% of a person’s total weight. Along with nails, hair, nerves, and glands, the skin is part of the integumentary system. The skin has several layers that help protect the muscles, ligaments, internal organs, and bones from external threats. Skin pigmentation and lubrication changes depending on a number of factors.
The visible layer of skin is the epidermis. Not only is the epidermis waterproof, but it also prevents infections. Unlike the underlying tissue of the body, the epidermis has no blood vessels. Instead, this layer nourishes itself by taking in oxygen from the surrounding air and blood capillaries from the lower layers. The sublayers that make up the epidermis are the corneum, lucidum, granulosum, spinosum, and the germinativum. The lucidum only exists within the palms and the bottoms of the feet. The epidermis becomes replenished with fresh, healthy cells as part of keratinization.
Human hair, the epidermis, and the horns and claws of animals consist of several structural materials including keratin, which protects the skin by giving it its strength and preventing the skin from drying out. During keratinization, cells move up the layers of the skin through cell division. The layers’ compositions then change and fill with keratin as the cells pass through. When the cells reach the corneum, the body desquamates them -- in other words, the dead skin flakes off. The process usually takes a few weeks to complete.
The middle layer of the skin, the dermis, sits just under the epidermis. This is the thickest layer of the skin because it contains many structures and tissues. Within the dermis are nerves, blood vessels, sweat glands, oil glands, and hair follicles, though it primarily consists of collagen, a protein that gives the skin strength and contributes to its flexibility. The various structures of the dermis also enable its many functions, such as sensing touch and pain, producing sweat, growing hair, fighting infection, and supplying the epidermis. Like the epidermis, the dermis has sublayers -- the papillary and reticular regions.
The first layer of the dermis, the papillary region, is loose areolar tissue, a type of connective tissue with spaces wide enough for fluids to travel through it. It is also quite strong, despite being soft and flexible. This region receives its name from its projections, or papillae, that reach toward the epidermis. When these papillae reach the epidermis in areas such as the fingers, palms, toes, and soles, they create distinct patterns. On the fingers, these are the fingerprints.
Deep within the papillary region sits the significantly thicker reticular region. Its name comes from a concentration of strong fibers that weave through the sublayer, creating a soft, supporting mesh. This is the layer of the dermis that contains the hair roots, blood vessels, oil glands, sweat glands, and sensory receptors. The ink from tattoos and stretch marks also lies in the dermis.
Though this isn’t technically a layer of the skin, experts often treat it as such because it is part of the integumentary system. Additionally, experts often refer to the subcutaneous tissue as the hypodermis to relate it to the epidermis and the dermis. This layer is primarily loose connective tissue and stores of fat lobules. Many of the structures and tissues of the dermis are also present in the hypodermis. These structures originate below the skin and must travel through the hypodermis to reach the dermis.
Human skin performs many responsibilities and functions. The first and most obvious is protection. Without the skin and its keratin, pathogens could easily reach important parts of the body. Sweat contains lysozyme, an antimicrobial enzyme that attacks bacteria. The sensory receptors in the dermis provide the ability to feel touch, pain, and temperature. The dermis also contains more blood than the skin actually uses, which allows the skin to regulate the body’s temperature. The skin’s keratin helps create a barrier that prevents fluids from washing essential nutrients out of the body.
Melanin is a pigment the epidermis produces to protect the skin from the sun’s rays. The more melanin a person has, the darker their skin tends to be. This is why ethnicities that developed in tropical regions typically have darker skin tones. Without melanin, various colors are more visible through the skin. Underneath the dermis is bluish-white connective tissue, which is why some individuals appear to have "white" skin. This is also why exercise or certain emotions may make some people red and others not: arterioles dilate and allow more blood flow during these events. Some individuals may appear to have a more yellow, purple, or red coloring to their skin. Carotene is a yellow or orange pigment in the dermis. Hemoglobin is a purple color in the blood, and oxyhemoglobin is a red color in the blood.
Within the dermis are the sebaceous glands or oil glands. The palms of the hands and the soles of the feet are the only areas of the body that do not contain sebaceous glands. These glands produce sebum, an oily fluid that lubricates the skin. Sebum also breaks sweat into many fine droplets to prevent dehydration. If the glands malfunction or are overactive, they can produce too much sebum. Oily skin is not necessarily a negative, as oily skin is less prone to signs of aging, such as wrinkles. However, the oil can lead to pores becoming clogged and dead skin cells accumulating on the skin’s surface.
Keratinization constantly providing new cells for the skin does not prevent the skin from aging. Typically, skin is fairly soft and smooth. Once a person reaches the age of 20, their body produces one percent less collagen every year. This is why the skin becomes thinner. The body also produces less elastin, the protein responsible for the skin's elasticity. Experts refer to this as intrinsic aging and it would typically cause only minor wrinkles. Smoking, being in the sun, gravity, pollution, and even making facial expressions are examples are extrinsic aging. Intrinsic and extrinsic aging pair together to create the defined wrinkles and other features we associate with aged skin.
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