It provides a protective layer from ultraviolet light by causing tanning. The skin participates in excretory functions, as well. It allows for the excretion of small amounts of water and salts. Of course, if you're hiking to Mt. Rose in August, you are aware that it permits copious amounts of water and salt excretion by the wetness of your t-shirt and the salt deposits on the outside of your boots at the arch the following day. The skin is the site of vitamin D synthesis.
Vitamin D stimulates calcium and phosphate uptake from food for bone growth.
Vitamin D is a hormone, i.e., it is produced in one part of the body and carried via the blood to exert its effect on a different part of the body. Alternatively, the vitamin D will be inactivated and excreted via the kidneys, should we not need it. Figure 1 Illustrates the anatomy of a section of thick skin. A number of structures are identified. This layer is continuously shed and replaced.