Your skin does more than just cover your body; it has a number of structures like hair and sweat glands. Learn more about the hair that protects your body in this lesson.

Skin Accessory Structures

Did you know that all mammals have hair on their skin, even if you can’t see it, or that underneath all that white fur, the skin of a polar bear is actually black? You see, your skin, in addition to providing protection for your muscles and organs, contains all sorts of other cells to produce color, relay touch, excrete waste, and grow hair!

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You have hair all over your body; it’s on almost all your body surfaces even if you can’t see it! You also have sweat glands that help you cool off, sebaceous glands (they are the ones that produce all that oil that tends to give you pimples on your face), and then of course you have your nails. Did you know they are made up of the same protein that your hair is made of? All of these structures are called accessory structures of the integumentary system. Your integumentary system is just the scientific word used to refer to the largest organ of your body – your skin and all of its accessory structures.

Hair Functions

Ever wonder why hair keeps growing back after you shave it? Or, why it takes longer to grow back after waxing than shaving? And, did you know that it is found everywhere except for the palms of your hands and the soles of your feet?

The human body has somewhere around 2.5 million hairs on it! Well, give or take a few for those who have gone bald as they aged. Of these hairs, 75% are not even on your head! They are on the rest of your body.

Hair is actually a non-living structure produced in an organ called a hair follicle. Now, the hair follicle is living even though the hair it produces is not. That’s why it hurts when someone pulls your hair out, but not when they cut it. Imagine if your hair was alive and hurt every time you got it cut. We’d all be walking around with hair down to our ankles!

Okay, so if hair isn’t a living structure, then why do we have it? What does it do? Well, for anybody who has lost the hair on their head, they probably know the first function.

Hair protects your head from the UV radiation from the sun. It also helps cushion and insulate your head. People without hair tend to lose more heat through their scalp than those with hair. Other hairs, like those in your nose, prevent entry of foreign particles into the body, and others, like those all over the rest of your body, act as sensory receptors, helping you notice when something like a mosquito lands on your arm. And hopefully, with enough warning, you can get the mosquito before it gets you!

Structure of the Hair

Each hair on your body, whether it’s on your head, in your nose, or above your eyes, has a similar structure.

All the way down at the bottom is the root of the hair called the root hair plexus. Now, don’t let the word ‘plexus’ confuse you; it just means a network. In this case, it is a network of sensory nerves that surrounds the base of the hair follicle. It is because of the plexus of nerves that you can feel the movement of even a single hair, like when that pesky mosquito lands on your arm.

Surrounding each hair follicle is a sheath of tissue, kind of like a pocket of tissue that the hair sits in. Attached to this sheath is a really super tiny muscle called the arrector pili. Did you even know you had muscles that small? Well, the arrector pili is what is responsible for goosebumps! When you get cold, or maybe even scared, your arrector pili muscles contract, making your hair stand erect. Hmmm… ‘erect’ kind of sounds like ‘arrector,’ doesn’t it? That should help make it easier to remember. While it’s harder to see this in humans, you can see it a lot easier in hairy animals when they get frightened or angry.

Now, if you take a moment to move inside that sheath of tissue – you know, the one the arrector pili is attached to – we will see the hair root. This is the part that anchors the hair to the skin. It extends from right below the skin surface all the way down to the bottom, also called the hair bulb. This is the bottom of the hair root that kind of looks rounded like a light bulb.

The hair bulb is where hair production takes place. There are cells at the base of the bulb that divide to produce new hair cells. As more cells are produced, your hair grows upward, from the hair bulb towards the surface. Did you know that the average person’s hair – well, the hair on their head – grows about a half an inch per month? That’s why you have to get your hair cut every few months to keep your same hair style!

Okay, well, that covers the parts beneath the skin, but what about the part above the skin? You know, the part that isn’t alive? Well, that is called your hair shaft, and it refers to the part of the hair above the surface of the skin. As cells approach the surface of the skin, they stop dividing and have become keratinized. Keratinization is the process of producing the protein keratin, which makes hair cells thick and water-resistant.

Hair Loss

So, if I were to ask you what hair structure is damaged when people lose their hair, what would you say? The shaft? No, that part’s dead, right? How about the hair follicle? Well, yes, but maybe let’s be a little more specific. I know – what about the hair bulb? Isn’t that where hair production takes place?

You see, while baldness can be caused by many things – like too much DHT (one the forms of the hormone testosterone), genetic disposition (guys, did you know male baldness is actually inherited from your mother?), and even stress – the physical problem lies in the hair follicle. Down there, all the way at the hair bulb – that’s where hair production starts and stops. Losing hair is a normal process as new hairs replace old hairs. But typically, baldness occurs when the cells that produce hair stop dividing, and the follicle begins to shrink. Then there are no more new hairs to replace the lost hairs.

Lesson Summary

And, that’s it! Now you know how much your hair grows each month, how baldness occurs, and the structure and function of hair. Let’s take a quick moment to review.

Hair is found all over your body. It protects your head from the UV radiation from the sun, cushions and insulates your head, prevents entry of foreign particles into the body, and acts as sensory receptors, helping you notice those pesky mosquitoes that land on your arm.

The hair you see, the part above your skin, is the hair shaft, which is actually made up of dead skin cells. Below the skin, the hair lives in an organ called the hair follicle. Parts of the hair follicle include the hair root, which extends from beneath the skin surface down to the hair bulb. The hair bulb is where hair production takes place as hair cells grow and divide. Attached to the sheath of tissue surrounding the hair follicle are the arrector pili muscles. These are the ones that cause your hair to stand erect, like when you have goosebumps!

Learning Outcomes

After this lesson, you’ll be able to:

  • Define the integumentary system and name its accessory structures
  • Describe the structure of hair and identify its components, both above and below the skin
  • Summarize the functions of hair
  • Recall what keratinization is
  • Explain what causes hair loss