Integumentary System皮肤 · 毛发 · 指甲

Place · Level 3

身体最大的器官 · 屏障 + 感知 + 温度 + 维 D 合成厂 · 营养在外显 · 多数美容补剂没循证

Three layers · 28-day turnover

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三层结构 · 28 天周转

Skin is the body's largest organ — in adults it covers 1.5-2 m² and weighs 3-5 kg, far more than the liver. It is much more than an outer coat — it is a multifunctional organ:

Barrier — keeps microbes, chemicals, UV, and water loss outSensing — densely packed touch, pressure, temperature, and pain receptorsThermoregulation — sweating plus vasoconstriction / vasodilationVitamin D factory — UVB on 7-dehydrocholesterol → vitamin D3 (covered in detail in the vitamin-d/sun L4)Immune outpost — Langerhans cells and resident T-cell populations
Three layers:

Epidermis (50-100 µm): avascular — it gets nutrients by diffusion. Keratinocytes are born in the deepest basal layer, migrate to the surface over 28 days while flattening, and ultimately form the stratum corneum, a layer of dead cells that flakes off. Melanocytes pick up UV signals and make melanin, which they transfer to neighboring keratinocytes. Langerhans cells are the skin's dendritic cells, acting as antigen sentinels.

Dermis (1-4 mm) is the engineering layer: type I collagen (90%) plus type III provides tensile strength, elastin provides recoil, hyaluronic acid (HA) holds water (1 g of HA can bind 1000 mL); blood vessels, nerves, sweat glands, sebaceous glands, and hair follicles all live here.

Hypodermis is the fat pad — it insulates and cushions.

The epidermis turns over completely in 28 days, while dermal components have half-lives of 5-15 years. This is why dietary changes show up on the skin within 4-8 weeks (mostly epidermal changes), while deep aging is extremely hard to reverse — once dermal collagen is lost, it is very difficult to regrow.

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