Each hair sits in a hair follicle. Humans have approx. 100,000 – 150,000 hair follicles on the scalp. All hair follicles are formed in the womb. Hair grows on all skin except the palms of the hands and soles of the feet.
The hair follicles extend from the skin surface and deeper into the skin. At the bottom of the hair follicle is the papilla, which is the area where hair growth is regulated. The hair itself is essentially dead material and mostly consists of keratin. At the bottom of the hair follicle, the hair is soft, and it becomes stiffer as the hair grows up from the hair follicle.
A hair follicle is connected to blood vessels (which provide nourishment), a sebaceous gland (which lubricates) and a small muscle. It is this muscle that makes the hair stand up and gives the skin the appearance of goosebumps.
There are two main types of hair; vellus hair and terminal hair. Vellus hairs are thin, short and without pigment. Terminal hairs are thick, long and pigmented.
During puberty, hormone-sensitive vellus hair is stimulated to become terminal hair (pubic hair). Increased transformation from vellus to terminal hair can also be seen in various hair conditions.
Each hair follicle has a dynamic growth cycle that is divided into three phases; growth phase (anagen), transitional phase (catagen) and resting phase (telogen). The durations of the various phases mean that approx. 90% of the hair follicles are in the growth phase, 1 percent in the transition phase and 10 percent in the resting phase.
In humans, these phases are asynchronous. That is, they do not occur at the same time. This results in diffuse, even hair loss, rather than periodic massive hair loss. Humans thus differ from many animals, where periodic hair loss is common ("shedding periods"). A person loses between 30 and 150 hairs every day.
The hair's function is to protect the skin and help with heat regulation. In humans, scalp hair has no function beyond the social and cosmetic, as well as protecting the scalp from sun damage to a certain extent.
Hair appearance, such as color and texture (straight, wavy, curly), is largely determined by genes. More specifically, different hair colors are due to differences in the amount and type of pigment. Red hair is seen when there is an increased amount of pheomelanin, which is the red color pigment. Eumelanin gives brown/black hair. Gray hair is due to reduced pigment production by the pigment cells at the bottom of the hair follicles.
Hair loss can have several causes and is experienced as psychologically burdensome. Examples of causes are fragile hair, medicines and various hair loss conditions. Brittle hair can be a consequence of deficiency disease (eg iron), genetic disease and external damage (heat, chemicals, brushing). Hair loss conditions are divided into two main groups depending on whether or not it results in scarring. Common hair loss conditions that do not cause scarring are androgenetic alopecia ("male pattern baldness"), telogen effluvium and alopecia areata. Examples of hair loss conditions with scarring include discoid lupus, lichen planopilaris, frontal fibrosing alopecia, folliculitis decalvans and acne keloidalis nuchae. Fungal infections of the hair and scalp (tinea capitis) can also cause hair loss - both with and without scarring. In case of scarring hair loss, biopsies (tissue samples) from the scalp are appropriate for correct diagnosis and treatment.
Increased hair growth is also a problem for many, especially women. This is often genetically conditioned, and where socio-cultural factors come into play. Hirsutism is an expression of "male hair growth" among women, for example the formation of moustaches, beards and chest hair. Such cases may have hormonal causes. Hypertrichosis is a term for abnormally increased hair growth.
