Skin alone can be twelve percent of your body weight
The integumentary system is an animal’s outer shell of organs: skin, hair, scales, feathers, hooves, claws and nails. In people the skin alone covers about 1.5 to 2 square metres and makes up roughly 12 to 15 percent of body weight, which puts it among the largest organs we have.
Beyond acting as a wall between the body and the outside world, this system balances water, shields deeper tissues, gets rid of waste through sweat, helps regulate temperature, and carries receptors for pain, pressure, touch, heat and cold. Skin also releases melanin against sunburn, makes vitamin D under ultraviolet light, and repairs small wounds with new cells from its deepest layer.
Skin has two main tissue layers joined at a basement membrane. The outer epidermis is a stack of flattened cells in four or five strata, with an extra clear layer, the stratum lucidum, in the thick skin of palms and soles; it is renewed from stem cells at its base. Its dominant cell, the keratinocyte, makes the tough protein keratin and secretes the lipids that keep water in, while melanocytes supply colour and Merkel and Langerhans cells complete the cast. Most of our surface is keratinised, but mucous linings such as the inside of the mouth are not.
Underneath, the dermis holds blood vessels, glands and hair follicles in woven collagen and elastin. Its upper papillary layer pushes finger-like ridges into the epidermis, and its deeper reticular layer gives stretch while resisting wrinkles and sagging. Below that lies the hypodermis, mostly fat cells in lobules, which stores energy for hard effort or lean times and insulates against heat loss; collagen bundles tie it to the dermis loosely enough for skin to slide over the tissue beneath.
Keratin also builds nails, which grow from the nail matrix at about a millimetre a week. Only primates have nails; other vertebrates grow claws or hooves instead. Fish and aquatic amphibians coat their skin in constantly renewed mucus, and small invertebrates in wet places breathe straight through their outer layer.
Source: Integumentary system
