(in-teg-yoo-MEN-tuh-ree SIS-tem):Includes the skin, hair, nails, sweat glands, sebaceous (oil) glands, and sensory receptors. It serves as the body's first line of defense against environmental damage, regulates body temperature, prevents water loss, synthesizes vitamin D, and houses sensory receptors for pain, touch, and temperature. The skin itself consists of three layers: the epidermis (outer layer), dermis (middle layer), and hypodermis (subcutaneous layer).