Hair is a protein filament that grows from follicles found in the dermis, or skin. The human body, apart from areas of glabrous skin, is covered in follicles which produce thick terminal and fine vellus hair. Terminal hairs are long, coarse and pigmented, while vellus hairs are short, fine, and unpigmented. Both types of hair are found on most parts of the human body, with the exception of the lips, palms of hands, and soles of feet.
Hair serves a number of important functions, including protection of the skin, regulation of body temperature, and communication. The hair shafts, made of a hard protein called keratin, provide insulation and protection to the scalp and skin. Hair also helps regulate body temperature by providing insulation, and by trapping a layer of air near the skin. The hair on the head, for example, helps to keep the scalp warm, while the hairs on the arms and legs help to regulate temperature by spreading sweat and heat from the skin’s surface.
The color and texture of hair are determined by genetics, as well as by a number of environmental factors such as age, health, and hormonal status. The natural color of hair is determined by the presence of melanin, which is a pigment that is produced by specialized cells in the hair follicles. The amount of melanin produced determines the color of the hair, with more melanin resulting in darker hair and less melanin resulting in lighter hair.
Hair texture is determined by the shape of the hair shaft, which is influenced by the size and shape of the hair follicle. The size and shape of the follicle determines the thickness and curl pattern of the hair, with larger follicles producing thicker, straighter hair and smaller follicles producing finer, curlier hair. Hair texture can also be influenced by environmental factors such as chemical treatments, heat styling, and exposure to environmental pollutants.
The growth cycle of hair consists of three phases: anagen, catagen, and telogen. During the anagen phase, which can last for several years, the hair actively grows from the follicle. The hair reaches its full length during this phase, and the length of this phase determines the maximum length to which the hair will grow. During the catagen phase, which lasts for a few weeks, the hair stops growing and the follicle shrinks. During the telogen phase, which lasts for several months, the hair is at rest and eventually falls out, to be replaced by a new anagen hair.
Hair loss can occur for a variety of reasons, including hormonal changes, genetics, medical conditions, and aging. Androgenetic alopecia, also known as male or female pattern baldness, is a common type of hair loss that is caused by a combination of genetics and hormones. In this condition, hair follicles shrink over time, producing finer and shorter hair until they eventually stop producing hair altogether. Alopecia areata is an autoimmune condition that causes hair loss on the scalp and other parts of the body, while trichotillomania is a psychological disorder that causes individuals to pull their hair out.
Hair can be styled in a variety of ways, including braiding, cornrowing, twisting, and braiding. Hair styling can also involve the use of chemicals, such as relaxers, perm solutions, and hair coloring products, which can alter the texture, color, and overall appearance of the hair. Heat styling, such as blow-drying, flat ironing, and curling, can also be used to style hair, but frequent use of heat styling tools can cause damage to the hair shaft, leading to dryness and brittleness.