Hair

Hair consists of fine filaments growing from the skin of the scalp. Hair is composed primarily of keratin, a tough protein that also makes up the skin and horns of some animals. Hair emerges from a small pit in the scalp called a hair follicle.

Hair is not living tissue once it emerges from the scalp; the visible part is dead tissue filled with keratin. Growth occurs in the follicle below, where new cells stack and push the hair shaft upward.

Structure

The hair shaft is divided into two parts: the root, embedded in the scalp, and the shaft, which is visible above the skin. The shaft has three layers: the cuticle (the outer layer of overlapping scales), the cortex (the middle layer that contains pigment and gives hair its strength), and the medulla, which is sometimes absent.

Color

Hair color depends on melanin, a pigment produced by cells in the scalp and hair follicle. Two types of melanin contribute to hair color: one that is brown-black and one that is red-yellow. When melanin production decreases—typically with age—hair turns gray.

Growth cycle

Hair goes through a cycle with distinct phases: a phase of rapid growth, a transition phase, and a resting phase during which hair sheds. Each follicle cycles independently, which means hair loss is staggered rather than simultaneous.

Function

Hair helps regulate body temperature by insulating the scalp and allowing air circulation. It also protects against direct sunlight, dust, and debris, functioning like eyebrows and nasal hair. In some animals, hair serves as an extended sense of touch.