Every strand of hair on your head is produced by a tiny organ buried in your skin called a follicle, and the process is far more dynamic than it looks from the outside. Your scalp alone holds between 90,000 and 150,000 of these follicles, each one cycling independently through phases of growth, rest, and shedding throughout your life. Understanding how hair works means looking at what it’s made of, how your body builds it, and why it eventually changes.
What Hair Is Made Of
Hair is mostly protein. The fiber you see and touch is composed of tightly bundled protein strands that account for 70% to 95% of its total mass. The dominant proteins are keratins, the same tough structural material found in your fingernails. These keratin chains are cross-linked by bonds that give hair its strength and flexibility, which is why a single strand can stretch up to 30% of its length before breaking.
Each strand has three concentric layers. The outermost layer, the cuticle, is a shingle-like coating of overlapping cells that protects the strand and resists abrasion. Think of it like the bark of a tree. Beneath that sits the cortex, the thickest layer, which contains the bulk of the keratin and also houses the pigment granules that give hair its color. At the very center of thicker hairs is the medulla, a soft core that plays a minimal structural role and is sometimes absent entirely in fine hair.
How Your Body Builds a Hair Strand
The real action happens below the skin’s surface, at the base of each follicle. Nestled at the bottom is a small cluster of cells called the dermal papilla, which acts as the command center for hair growth. The dermal papilla sends chemical signals to surrounding cells, instructing them to divide rapidly. These rapidly dividing cells are called matrix cells, and they are essentially the factory floor: they multiply, push upward, and harden into the layered structure of the hair shaft as they rise through the follicle.
Blood vessels feed the dermal papilla with oxygen and nutrients, which is why overall health, diet, and circulation can visibly affect hair quality. As the matrix cells move away from their blood supply and toward the skin’s surface, they die and become the tough, compacted protein fiber you recognize as a strand of hair. The visible part of your hair is, technically, dead tissue. All the living biology happens underground.
The Three Phases of Hair Growth
Each follicle on your head cycles through three distinct phases, and they do so independently of one another. That’s why you don’t shed all your hair at once.
The first phase, anagen, is the active growth period. For scalp hair, this lasts about 2 to 8 years. The length of your anagen phase is largely genetic and is the main reason some people can grow hair to their waist while others hit a limit at shoulder length. During anagen, the matrix cells are dividing quickly and the strand is growing at a rate of roughly 0.6 to 3.36 centimeters per month, with about 1 centimeter per month being a common average.
Next comes catagen, a brief transition phase lasting about 2 weeks. The follicle shrinks, growth slows dramatically, and the strand detaches from its blood supply at the base. The hair stays in place but is no longer actively being built.
Finally, the follicle enters telogen, a resting phase that typically lasts 2 to 3 months. The old strand sits loosely in the follicle while, deeper down, a new hair begins forming. When the new strand starts its own anagen phase, it pushes the old one out. This is normal shedding. You can expect to lose between 50 and 150 hairs daily, and each one is simply a strand that reached the end of its cycle.
What Determines Hair Color
Hair color comes from pigment granules embedded in the cortex by specialized cells called melanocytes, which sit near the dermal papilla. These cells produce melanin and inject it into the growing strand as it forms. The cuticle, despite being the outermost layer, contributes nothing to color.
Two types of melanin control the palette. Eumelanin produces dark tones: a large amount yields black hair, a moderate amount gives brown, and very little results in blond. Pheomelanin produces warm, reddish tones. Red hair comes from a high concentration of pheomelanin with only a small amount of eumelanin. The specific shade you’re born with depends on the ratio between these two pigments, which is governed by multiple genes working together.
Gray and white hair appear when melanocytes slow down or stop producing pigment altogether. The strand itself isn’t changing color. It’s simply growing in without any melanin deposited inside it.
Why Hair Is Straight, Wavy, or Curly
Your hair’s curl pattern is determined by the shape of the follicle it grows from. Straight hair emerges from round, vertically oriented follicles. Inside these follicles, cells divide symmetrically, producing an evenly round strand that hangs straight. Curly hair grows from curved, asymmetrical follicles. The cells on one side of the follicle divide and produce proteins differently than the cells on the other side, resulting in a strand with an oval or elliptical cross-section. That uneven shape causes the fiber to curl as it exits the skin.
The degree of curvature in the follicle correlates with how tight the curl is. A slight curve produces waves. A sharply angled follicle produces coils. This geometry is genetic, which is why curl pattern runs in families, though hormonal changes during puberty or pregnancy can sometimes reshape follicles enough to alter texture temporarily or permanently.
What Hair Does for Your Body
Hair serves several biological functions, though their importance to modern humans varies. Each follicle is surrounded by nerve endings, making hair an extension of your sense of touch. You can feel a breeze or an insect on your arm partly because the movement of hair triggers those nerves.
Hair also plays a role in temperature regulation, at least in theory. Tiny muscles called arrector pili attach to each follicle. When you’re cold, these muscles contract and pull the hair upright, creating a layer of trapped air that insulates the skin. This is the mechanism behind goose bumps. In mammals with thick fur, piloerection is highly effective at conserving heat. In humans, who have relatively little body hair, the insulating effect is minimal. When you’re warm, the muscles relax, the hairs lie flat, and air flows more freely across the skin to release heat.
Scalp hair provides more practical protection: it shields the skin underneath from ultraviolet radiation, physical impact, and heat loss from the head, which is one of the body’s most vascularized areas.
How Hair Changes With Age
Hair doesn’t just go gray as you get older. It also tends to get thinner. Researchers at Tokyo Medical and Dental University found the underlying mechanism: over time, hair follicle stem cells accumulate genetic errors that prevent them from regenerating properly. As these damaged stem cells lose function, the follicles they support physically shrink. A shrunken follicle produces a thinner, wispier strand than it once did, or eventually stops producing hair altogether.
This process, called follicle miniaturization, has been confirmed in studies comparing hair follicles from younger and older women. The older the individual, the more shrunken follicles were present, and the degree of shrinkage correlated with the number of mutations in follicle stem cells. In mouse studies, boosting production of a specific structural protein (Col17A1) prevented follicles from shrinking and preserved hair density, pointing to the biological roots of age-related thinning.
Separate research at the University of Colorado identified a protein that keeps follicle stem cells dormant between growth cycles. When this protein is absent, follicles stay permanently active. The balance between dormancy and activation helps explain why some people maintain thicker hair longer than others, and why disruptions to that signaling, whether from aging, hormones, or genetics, can tip the balance toward progressive hair loss.