Hair is a thick, filamentous structure that grows out of your skin. Its main components are dead, keratinized cells and a protective cuticle. Hair is a very important part of the human body, and it serves several functions. Some of these include protection, sensory input, thermoregulation and communication. It protects the skull and eyes from dust particles, which could contain bacteria and other microbes. It also helps to trap sweat and prevent it from dripping into the eyes. People can communicate emotions with their hair by touching, patting, combing and braiding it, and even compulsively wagging, rubbing and pulling it at times of high anxiety or stress.
Hair grows from a small hole in the skin called a hair follicle. It gets nourishment from a blood vessel that runs underneath the follicle. The follicle then makes hairs by growing new cells from the base of the root. As the cells move upward, they are cut off from their food supply and begin to harden, a process known as keratinization (kair-uh-tih-nuh-ZAY-shun). The keratinized cells then push through the skin’s surface to form the hair shaft, which is visible above the scalp.
The outermost layer of the hair shaft is called the cuticle, which gives it its shine and a waxy coating that repels water. The middle layer is called the cortex, which contains a protein phase that consists of coiled-coil proteins. The cortex is surrounded by the innermost layer, the medulla, which is soft and oily and contains melanocytes (melan-OHS-cuh-tees). The medulla produces the two kinds of pigment that give your hair its color: eumelanin (yoo-MEL-uh-nin) and pheomelanin (fee-hem-OL-uh-nin).
Each strand of hair starts at a small point on the skin called a papilla. The papilla sits just below the dermis, and it is connected to an oil gland called a sebaceous gland. The gland makes a type of oil called lipid-rich sebum, which helps to keep your hair and skin moisturized.
As the strand of hair moves up through the hair follicle, it begins to become a rod of cells that grow long and then elongate into a cord-like structure. The cells that make up the strand are also growing at an accelerated rate during this time, a process called mitosis. As the strand continues to lengthen, it becomes increasingly dense and tough. This is why it can feel very heavy and stiff when you have really long hair. As the strand gets longer and heavier, it may eventually break and fall off. When a hair breaks, it is replaced with a new cell from the same follicle. This process of growth and replacement is what gives you a full head of hair. However, it is a good idea to take care of your hair so that it stays healthy. This is why it is important to shampoo and condition it regularly, and it is recommended that you use a wide-toothed comb or brush when you groom it. Also, be sure to use a gentle shampoo, especially if you have a delicate or sensitive scalp.