Hair is much more complicated than it appears. It helps transmit sensory information. It acts as a barrier to foreign particles. It's an important part of the appearance and creates gender identity. It's also the only bodily structure that can completely renew itself without scarring. There is hair on almost every surface of the human body. Here's a complete overview of its biology, structure, and function.

How Hair Forms

A developing fetus has all of its hair follicles formed by week 22. At this time, there are 5 million follicles on the body. One million of those are on the head and 100,000 are on the scalp. This is the largest number of hair follicles a human will ever have because follicles do not continue to grow during life. As we grow older, the density of hair follicles decreases.

Anatomy

A diagram of hair anatomy may look straightforward, but it's actually one of the most complicated structures in the body. Hair is made up of two separate structures: the hair follicle, which exists below the skin, and the hair shaft, which is the hair that we see.

Hair Follicle

The hair follicle is the living part of the hair. It's a stocking-like structure that contains cells and connective tissue. The papilla exists at the base of the hair follicle. It contains tiny blood vessels (capillaries) that nourish the cells. The follicle also contains the germinal matrix, which is where cells produce new hairs.

The bulb is the stocking-like structure that surrounds the papilla and germinal matrix. It's fed by capillaries. The bulb is home to several types of stem cells that divide every 23 to 72 hours, faster than any other cells in the body. It also contains hormones that affect hair growth and structure during different stages of life, like puberty.

The follicle is surrounded by an inner and outer sheath that protects and molds the growing hair shaft. The inner sheath follows the hair shaft and ends just before the opening of the sebaceous gland. The outer sheath continues all the way up to the sebaceous gland. The arrector pili muscle, a tiny bundle of muscle fiber, is attached to the outer sheath. When the muscle contracts, it causes the hair to stand up, otherwise known as goosebumps. The sebaceous gland produces sebum, or oil, which is the body's natural conditioner. More sebum is produced during puberty, which is why acne is common during the teen years. Sebum production decreases with age, causing the skin to become dry.

Hair Shaft

The hair shaft—the hair that we can see—is actually dead. It's made up of three layers of keratin, a hardening protein. Here are more details about those three layers.

The Innermost Layer: This is called the medulla. Depending on the type of hair, the medulla isn't always present.

This is called the medulla. Depending on the type of hair, the medulla isn't always present. The Middle Layer: This is called the cortex, which makes up the majority of the hair shaft. Both the medulla and the cortex contain pigmenting cells that are responsible for giving hair color.

This is called the cortex, which makes up the majority of the hair shaft. Both the medulla and the cortex contain pigmenting cells that are responsible for giving hair color. The Outermost Layer: This is called the cuticle, which is formed by tightly packed scales in an overlapping structure that resemble roof shingles. Many hair conditioning products are formulated to even out the cuticle by smoothing out its structure.

The Growth Cycle

The hair on the scalp grows about a half a millimeter a day—about 6 inches per year. Unlike in other mammals, hair growth and loss is random and not seasonal or cyclical. Hairs are always in various stages of growth and shed at any given time. There are three stages of hair growth: anagen, catagen, and telogen.

Stage 1: The anagen phase is the active or growth phase of the hair. Most hair is constantly growing and spends three to four years in this stage. A new hair forms and pushes the club hair up and out of the follicle. During this phase, hair grows approximately 1 centimeter every 28 days. Some people have difficulty growing their hair beyond a certain length because they have a short anagen phase. Conversely, people who have very long hair and have no trouble growing hair have a long anagen phase. The anagen phase for eyelashes, eyebrows, and leg and arm hair is also very short—about 30 to 45 days—which explains why these hairs are so much shorter than scalp hair.

The anagen phase is the active or growth phase of the hair. Most hair is constantly growing and spends three to four years in this stage. A new hair forms and pushes the club hair up and out of the follicle. During this phase, hair grows approximately 1 centimeter every 28 days. Some people have difficulty growing their hair beyond a certain length because they have a short anagen phase. Conversely, people who have very long hair and have no trouble growing hair have a long anagen phase. The anagen phase for eyelashes, eyebrows, and leg and arm hair is also very short—about 30 to 45 days—which explains why these hairs are so much shorter than scalp hair. Stage 2: The catagen phase is a transitional stage, and 3% of all hairs are in this phase at any given time. It lasts for two to three weeks. During this time, growth slows down and the outer root sheath shrinks and attaches to the root of the hair, forming what is known as a club hair.

The catagen phase is a transitional stage, and 3% of all hairs are in this phase at any given time. It lasts for two to three weeks. During this time, growth slows down and the outer root sheath shrinks and attaches to the root of the hair, forming what is known as a club hair. Stage 3: The telogen phase is the resting phase, which lasts for about three months and accounts for 10% to 15% of all hair. During this phase, the hair follicle is at rest and the club hair is completely formed. Pulling out a hair in this phase will reveal a solid, dry, white material at the root. The body sheds approximately 50 to 100 scalp hairs a day.

How It Gets Its Shape

Some people have corkscrew curly hair, while others have thick, straight, shiny hair. The natural appearance of hair is attributed to the shape of the hair. The amount of natural curl that a hair has is determined by its cross-sectional shape. Straight hair has a mostly circular circumference. Strands of curly or kinky hair are flat. The more circular the hair shaft, the straighter the hair. The flatter the shaft, the curlier the hair.

The cross-sectional shape if a hair also determines the amount of shine that the hair has. Straighter hair is shinier because sebum from the sebaceous gland can travel down the hair more easily. The kinkier the hair, the more difficulty the sebum has traveling down the hair, and the more dry and dull the hair looks.

And your hair can change color, texture, and thickness (and its location can change—too much in some areas and too little in others) over time as you get older.