When people think of systems of vital organs, they think of the internal systems, such as the circulatory, respiratory or digestive systems. The immune system may also come to mind, but there is another component to the body: the integumentary system. This bodily system consists of the parts that cover the outside of the body and acts as the first line of defense against foreign microorganisms. The components of the integumentary system are the skin, the hair and the nails. In other animals, this system includes parts such as scales or fur, but humans generally only have a light layer of body hair.
The skin is the largest organ of the body and acts as the outermost line of defense against bacteria and other foreign organisms. It also protects the body's core from sudden temperature changes. The skin contains nerve endings that alert the brain of sensations like heat, cold, pain and pressure. These nerve endings are more or less concentrated in different areas, which causes some parts of the body to be more sensitive to touch than others. Due to its additional functions for temperature maintenance, skin is one of the most important components of the integumentary system.
The majority of hair on humans grows from the scalp, though depending on the sex of the person, several other areas tend to have hair, such as the pubic region or the underarms, chest, arms and legs. The main purpose of scalp hair is to provide insulation for the head against heat and cold. Eyebrows catch sweat and divert it away from the eye; this was vital to survival in humanity's hunter-gatherer days. Smaller hairs, such as those in the nostrils and ears, catch dust and other particles to prevent infection. Limb hair serves mainly to keep the limbs warm even if there is a small amount of hair; the so-called "goosebumps" occur when the limb gets cold.
Nails are the smallest components of the integumentary system and serve less of a purpose than hair and skin. They grow on the tips of fingers and toes; their main function is to provide support and a method for grasping small objects that the fingers cannot handle. In addition, the fingernails contain sensory nerve endings that supplement the feeling of the fingertips; toenails serve the same function for the tips of toes. As the majority of people now wear shoes most of the time, however, this function isn't commonly thought to be as important.
Other components of the integumentary system include sweat glands and sebaceous glands. These create lubrication for the skin and hair. Sweat glands in particular remove heat from the interior of the body via evaporation; water has a high specific heat and carries the excess body heat with it.