The hair of humans and fur of animals are one of the main characteristics of what is deemed a mammal.
Hair is made mostly of a protein called keratin.
Hair fibers or strands, grow from an organ in the area under the skin called a follicle, which is found in the dermis skin layer.
The only "living" part of a hair is found in the follicle as it grows. The hair strand above the skin has no biochemical activity and so is considered "dead".
The cross-section of a hair strand is made up of 3 key layers. The outer layer is called the cuticle, within that is the cortex (which contains the keratin), while the center layer is called the medulla.
There are two main types of hair that the body produces, vellus hair and terminal (or androgenic) hair.
Vellus hair develops from childhood covering most of the human body, it is a short, fine, light-colored hair that is often barely noticeable.
Terminal hair is a thick, long and dark hair that is less common than vellus hair but much more noticeable, often replacing vellus hair on certain parts of the body during puberty. The hair on our heads is terminal hair, along with facial and chest hair in men and pubic and armpit hair in both genders.
On humans, hair can grow everywhere except for a few places such as on the palms of the hands, the soles of the feet and on the lips.
The hair on our head serves as a heat insulator and coolant, it also helps to protect us from the sun's UV rays. The function of hair in other locations is debated as we still require clothing to keep us warm unlike other mammals.
Goose bumps which form on skin when the body is cold are created when muscles attached to hair follicles stand up, which causes hair in these follicles to also stand, creating a heat-trapping layer in the skin.
Straight hair has round hair fibers while wavy or curly hair will usually have irregular and oval shaped hair fibers.
Two types of hair pigment are responsible for all natural hair colors. Eumelanin pigment is dominant in dark-blond, brown, and black hair, while pheomelanin is dominant in red hair. Little pigmentation in the hair strand results in blond hair.
Eyebrows protect the eyes from dirt, sweat and rain, and are a key part of non-verbal communication, displaying emotions like sadness, anger, and excitement.
The eyelash protects the eye from dirt, dust and other potentially harmful objects.
Human facial hair grows faster than any other hair on the body.
On average, we lose 50 to 100 strands of hair a day from the scalp.
On average, the lifespan of a human hair is 2 to 7 years. The hair on our scalp goes through 3 phases, the anagen phase, catagen phase, and telogen phase.
Only a few mammals are regarded as being pretty much hairless, these include elephants, rhinoceroses, hippopotamuses, walruses, pigs, whales and naked mole rats.