These tubules extend upwards, reaching the epidermis. Tiny holes in the surface of our body – the pores, and they are barely visible to the naked eye. When you work hard or intensely moving, the heat production increases, and then speed up the sweat glands sweat. It is also occurs when the temperature is high. The rate of sweating can exceed the speed of evaporation, especially if humidity is high, as the evaporation rate decreases with increasing humidity. Sweating then it will accumulate in the body in the form of droplets, "Temperature determines how many sweat glands a person has, as well as the amount of the sun determines how much melanin in human skin. People living in hot, humid climates, tend to have more sweat glands, and they are sweating with a lower concentration of salt, than people who live in colder and drier climates. The skin also works as a chemical processing plant for the entire body. When you are outdoors, the skin absorbs ultraviolet light emanating from the sun, and then uses it to convert chemicals into vitamin D. This vitamin is essential for our body, because it helps stimulate calcium absorption. Without calcium, our bones grow thin and brittle, which eventually leads to diseases such as rickets and ostiomelit (a disease of the skeleton, which weakens the bone). In addition, the epidermis contains a special pigment called melanin, which is responsible for the color of our skin. He is also responsible for protection against ultraviolet light.