Hair provides insulation but also physical protection for the skin including protection from ultraviolet radiation. 'Guard' hair predominates while finer 'down' hair offers insulation, unless you’re a Persian cat where down is prolific. Selective breeding has eliminated the protective, insulting downy coat from some breeds of cats and dogs. Yorkshire terriers for example have only about 100 hairs per square centimetre compared to the 500 hairs per square centimetre of Nordic dog breeds. There is a similar although not as dramatic difference between the Siamese and Persian coats.
Hair grows from its follicles in cycles consisting of a rapid growth phase (anagen phase), followed by a long resting stage (catagen phase). Inevitably for most dogs and cats, shedding (telogen phase) follows. Hair detaches from its anchor in the hair follicle. Licking, rubbing, rolling or grooming remove the loosened hair. The hair's growth cycle is partly affected by the surrounding temperature a pet lives in, (eg always indoors or indoors and outdoors), but more so by increasing or decreasing daylight. Body hormones, nutrition, stress and genetic factors also influence hair growth and shedding. As we see each day, pets drop their hair when they are stressed, which is why they leave so much of it on our examining tables. The state of your pet's coat is an excellent reflection of its general health.