All hair on the human scalp grows from a hair follicle. These are placed at a slant in the dermis, or inner skin, and are the thickest at the base where the follicle forms a bulb which contains the dermal papilla attached to the follicle by the basal stalk.

Each strand of hair contains three distinct layers. The cuticle of the hair is made up of a single layer of scales which interlock with the cells of the hair's inner root sheath to firmly anchor it in the follicle. The cortex is composed of keratinized cells which are tightly bound around each other. These fibril bands provide the hair with strength. The medulla consists of large, loosely connected cells with intracellular air spaces. By reflecting light these air spaces determine the sheen and color tones of the hair.

Hair density on the human scalp varies with the color and texture of the hair. These differ as follows: blonde hair has an average of 140,000 hairs; brunettes or dark-haired people will average 110,000 hairs; and red-haired individuals will have the least density with an average of 90,000 hairs.