Hair does not exist and grow alone, but is part of the pilosebaceous system, which also includes follicles, papillae, and sebaceous glands. A typical human has approximately 100,000 hairs on his or her scalp and loses about 50 to 100 strands a day.

View a second image of a skin sebaceous gland.

Sebaceous glands are scattered over the entire surface of the skin, except for the palms of the hands and the soles and sides of the feet. The simple branched areolar glands exude an oily substance known as sebum. Sebum is usually secreted onto the hair shaft, but in a few regions of the body it is directly emitted onto the surface of the skin. A natural skin softener, sebum helps keep hair from becoming brittle and skin from becoming dry and rough. Sebum also contains a bactericidal agent that inhibits the growth of a variety of bacteria.

Humans produce two primary types of hair, vellus and terminal. Vellus hair is the soft, downy hair that lacks color. The follicles that produce vellus hair are normally shallow and the hair shaft they produce is usually small. Terminal hair, such as that of the scalp or eyebrows, is coarse, pigmented, and deeply rooted. Hair sometimes gradually shifts between types, often because of a change in the hormones of the body.