Overview: Human Hair
Hair is a protein that is part of the integumentary system. It grows out of hair follicles embedded deep in the dermis. The only part of the hair that is alive is the root. The rest of the hair shaft is made up entirely of dead cells called keratin, which is a type of protein. There are three different types of hair: terminal, vellus and lanugo. Terminal hair is the fully developed, thick hair found on the head, underarms and pubic area of humans. Vellus hair is the short, very fine hair also called "peach fuzz" that covers almost every part of the human body except the palms of the hands, soles of the feet, back of the ears, certain external genital organs and the lips. Lanugo hair is the hair that covers a fetus, and is also sometimes found on anorexics as a way for their bodies to retain heat.
Hair generally grows an average of 1/2 inch per month, with growth generally predetermined by a person's genetic code, meaning that it can only grow so fast. In a year, hair usually grows about 6 inches. As a person ages, hair growth can slow due to cell division also slowing. Hair growth is a result of cell division when new hair cells form in the root and push the hair forward. The new hair is always added at the root as the older hair inches downward slowly. Hair can grow slightly faster in the summer, as the process of cell division accelerates during the warmer months.
Other Factors that Influence Hair Growth
Few outside factors influence hair growth, with the biggest influence being stress. High levels of stress can cause hair loss and premature balding. However, the greatest influence on hair growth is a person's genes. Genetics determine the length, texture, thickness and color of a person's hair. There are no shampoos, conditioners or other hair-care products that can influence the length of one's hair or make hair grow faster. Genetic code predisposes hair to only grow up to a certain length.