The skin, or integument, is considered an organ because it consists of all four tissue types. The skin also consists of accessory organs, such as glands, hair, and nails, thus making up the integumentary system. A section of skin with various accessory organs is shown in Figure 1.
Figure 1 .A section of skin with various accessory organs.
The skin consists of two layers, the epidermis and the underlying dermis. Although technically not part of the skin, the hypodermis (subcutaneous layer, or superficial fascia) lies beneath the dermis.
The skin performs a variety of functions:
- Protection is provided against biological invasion, physical damage, and ultraviolet radiation.
- Sensation is provided by nerve endings for touch, pain, and heat.
- Thermoregulation is supported through the sweating and regulation of blood flow through the skin.
- Synthesis of vitamin D occurs in the skin.
- Blood within the skin can be shunted to other parts of the body when needed.
- Excretion of salts and small amounts of wastes (ammonia and urea) occurs with the production of sweat.