The integumentary system, better known to laypeople as “the skin,” is the largest of the body's organ systems and one of the most important. Far from being just a covering to make sure that the body's underlying tissues aren't exposed, the skin serves a number of functions, ranging from helping the body eliminate waste to protecting the body from physical trauma.
In humans, the integumentary system can make up as much as 15% of the total body weight, and it has a large surface area which meets up with a number of mucus membranes such as those which line the mouth, anus, and eye sockets. The skin includes three layers of tissue, along with related structures such as the hair, nails, sebaceous glands, and sweat glands. Together, the components of the integumentary system keep the body protected and insulated from the outside world.
One of the roles of the integumentary system is the elimination of waste. Waste materials can be secreted through the skin to speed elimination, which explains why sometimes people have sweat which smells unusual, as their bodies are expressing waste materials. The skin also provides a layer of waterproofing, and protects the body from insects, bacteria, viruses, and numerous other potential threats from the outside world. Sebaceous glands keep the integumentary system oiled so that it will remain flexible and durable.
The integumentary system also helps to regulate body temperature, and it can both conserve and give off heat as needed. Skin also regulates water loss and retention. Both temperature and water loss regulation are critical to homeostasis, which is why severe burn victims are at risk of death, because their bodies are not able to regulate their temperature and water content. In humans, the skin also synthesizes vitamin D for use by the body.
The epidermis, dermis, and subdermis of the integumentary system also act as a literal cushion to protect the body from impact, and to absorb blows, cuts, and other forms of physical trauma. The layer of fat in the subdermis is an important part of this impact cushion, and it also helps insulate the body to keep the internal temperature stable.
Many humans have noted that the color of someone's skin can vary radically, from very pale to quite dark. This is due to various levels of the pigments carotene and melanin, which are designed to protect the integumentary system and the body in general from UV radiation. Individuals with ancestors who lived in the tropics tend to have darker skin because their ancestors were more at risk from the sun's rays.