How your skin is layered and what each layer does

The integumentary system is the skin plus its accessory structures: hair, nails, sweat glands, and oil glands. The skin itself has two main layers, with a third, closely attached layer below.

The epidermis has no blood vessels; the dermis below it is richly supplied with them. The dermis has a thin upper papillary layer that pushes finger-like dermal papillae into the epidermis and a thicker, tougher reticular layer beneath it. The hypodermis is not strictly part of the skin.

The epidermis is a conveyor belt. From deep to superficial, its layers are:

1. Stratum basale: a single layer of stem cells that constantly divide. Every keratinocyte, the cell that makes the tough protein keratin, starts here. Melanocytes and touch-sensing Merkel cells also sit in this layer.
2. Stratum spinosum: 8 to 10 layers of keratinocytes held together by desmosomes. These cells begin making keratin and a water-repelling lipid. Immune Langerhans cells are found here.
3. Stratum granulosum: the cells flatten, fill with keratin and grainy keratohyalin, and die.
4. Stratum lucidum: a clear layer of dead cells found only in thick skin of the palms, soles, and digits.
5. Stratum corneum: 15 to 30 layers of dead, keratin-packed cells that block microbes and water loss and are steadily shed. The whole layer is replaced in about 4 weeks.

Most of the body is thin skin with four layers; only the palms and soles have thick skin with all five.

Melanocytes make the pigment melanin and pass it into nearby keratinocytes in small packets called melanosomes. Melanin shields the DNA of epidermal cells from ultraviolet (UV) damage. Sun exposure triggers more melanin production, which is what a tan is; it takes about 10 days to peak. Carotene and the hemoglobin in blood also affect skin color.

• Eccrine sweat glands are everywhere, especially on the palms, soles, and forehead. Their watery sweat is the main way the body cools itself.
• Apocrine sweat glands are in hairy areas such as the armpits and genital region and usually empty into hair follicles. Their thicker sweat has organic compounds that bacteria break down, causing odor.
• Sebaceous (oil) glands, most of them attached to hair follicles, secrete oily sebum. Sebum keeps skin and hair soft and waterproof and has antibacterial fatty acids.

Hair and nails are made of dead, keratinized cells. A tiny smooth muscle, the arrector pili, pulls each hair upright (goose bumps) to trap a layer of insulating air.

The sympathetic nervous system adjusts the skin to keep core temperature steady.

• Too hot: sweat glands release more sweat, which cools the body as it evaporates. Dermal arterioles dilate, so more warm blood flows near the surface and loses heat. That is why skin flushes during exercise.
• Too cold: dermal arterioles constrict, keeping warm blood in the core. The skin cools and may look pale, but internal organs stay warm.

Skin also makes vitamin D. When UV light hits the epidermis, a cholesterol-derived molecule is converted to vitamin D3. The liver and then the kidneys convert it to its active form, calcitriol. Vitamin D is needed to absorb calcium and phosphorus, so a lack of it weakens bones: rickets in children and osteomalacia (soft bones) in older adults.

Burns are graded by how deep the damage goes, which is why knowing the layers matters.

• First degree: epidermis only. Painful and swollen, but heals on its own in a few days; a mild sunburn is an example.
• Second degree: epidermis plus part of the dermis. Painful blisters; heals in several weeks if kept clean.
• Third degree: the full epidermis and dermis. Nerve endings are destroyed, so it may hurt less than a shallower burn. The body cannot repair it from local tissue.
• Fourth degree: reaches the underlying muscle and bone.

Losing skin removes the barrier, so the biggest threats from large burns are fluid loss and infection. Burn area is estimated with the "rule of nines," which assigns body areas percentages in multiples of nine.