The integumentary system is the physical system that forms the barrier between the external environment and the internal systems of the body. In humans, this system consists of skin, hair, nails, and related glands. Altogether, the integumentary system forms the largest organ in the body. The main function of this system is to protect bones, organs, and other internal structures from harm. In addition, the integumentary system performs important immune functions, cell fluid maintenance, synthesis of Vitamin D, body temperature regulation, and detection of stimuli.