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Today, Hank answers the question, "Why do we use lotion?" We'll take a look at your integumentary system and all the hard work it does to protect you from and help you interact with the world around you.

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Introduction: Why Do We Use Lotion? 00:00
Protective & Sensory Functions of the Integumentary System 1:23
More Functions: removing waste, storing blood, and regulating body temperature 2:16
Skin Discoloration: Cyanosis, Jaundice, and Erythema 3:46
Melanin, Vitamin D, and Skin Tone 4:39
How Does Hair Conditioner Work? 5:40
How Do Hair, Skin, and Nails Grow? 6:17
Types of Sweat Glands: Eccrine, Apocrine, Mammary, and Ceruminous 6:55
Sebaceous (Oil) Glands 8:17
Review 9:05
Credits 9:33

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  Introduction (0:00)

Look at all this stuff!

Soaps, lotions, conditioners, polishes - all from that huge section of your local store that’s dedicated solely to the grooming of your skin, hair, and nails.

Some might see these things as the trappings of vanity. But me? I see them as the tools for the care, maintenance, and - sure, sometimes decoration - of the integumentary system.

Now, true, in spite of what the cosmetics industry may lead you to believe, your integumentary system is more than just a place to put eye shadow and hair product in the hopes of attracting a mate. This collection of resilient tissues, ranging from the sharp and hard to the soft and fluffy, serve a whole panoply of functions, the majority of which you never even notice.

But, when you do notice what your integumentary system is doing, the results are often uncomfortable or ugly or both, and that is what this stuff is mostly for.

Your sweat glands can make you smell. Your oil glands can give you zits.  Your skin can become either scaly or greasy, and - in rare cases - it can even change color. And hair, well, let’s just say it takes a lot of science to tame this mane.

But each of these tissues - frustrating as they may be at times - has a purpose, and without them you’d be cold and vulnerable and dead.

I’m not gonna lecture you on personal hygiene today. But hopefully by the end of this you’ll understand the important functions of your integumentary system, and maybe why it’s worth a little bit of time and effort to keep it healthy.

And hey, it might even score you a date.

[Opening music]

 The Skin's Functions (1:21)

If you recall our recent run-ins with rogue nails and tattoo needles, you’ll probably remember that the first and most vital purpose of your integumentary system is to act as a protective barrier. Your skin, hair, nails and sweat and oil glands all work together to shield you from all the things out there that are out to get you: excessive sunlight, infections, abrasions, and just, you know, getting poked by sharp sticks and stuff.

But beyond that, this system is also vital to how you sense the world around you. Your skin is loaded with structures that are actually part of your nervous system - called cutaneous sensory receptors - they’re what receive stimuli from the outside environment and send them to your brain.

These receptors, or corpuscles, as they’re sometimes called, register all of the different sensations that you associate with touch. Your tactile corpuscles, for instance, are what make you constantly aware of the tag that’s scratching at the back of your neck, while your lamellar corpuscles register the sense of pressure, like when someone puts their hand on your shoulder. Your hair follicles have receptors, too, which is why you can feel a slight breeze on your skin or through your hair.

Now, on the less-sexy front, your integumentary system also plays a role in the excretion of waste, though not as big as a role as we’re often led to believe. Most nitrogen-containing wastes like urea, uric acid, and ammonia are disposed of through your urine, but small amounts are eliminated through your skin in sweat.

But despite what you may be told at the beginning of your hot-yoga class, there isn’t much evidence that suggests that heavy sweating actually rids your body of any extra toxins - if anything, you’re just losing more water.

When you do exercise, though, you call on another of your skin’s lesser-known functions, as a handy blood storage unit.

About 5 percent of your entire blood volume is retained in your skin at any given time. And when you suddenly need more blood supplied to your organs, like when you’re working out, your nervous system constricts your dermal blood vessels to squeeze that extra blood into circulation.

Now, during these times of exertion, both your blood and your sweat glands work together to perform a key function: regulating your body temperature.

Even without exercise, your body oozes out about half a liter of sweat per day, in an effort to keep you at a comfortable temperature. That’s just your normal, barely noticeable sweat called insensible perspiration. But on a hot day, or if you’re on the dance floor exerting yourself, that sweat becomes much more noticeable. Such sensible perspiration could produce as much as 12 liters of sweat per day!

Now, if the temperature gets chilly, the surface of your skin can lose a lot of heat, because it has so much warm blood behind it. To regulate that heat loss, your dermal blood vessels constrict, causing your blood to head deeper into your tissues and help keep your vital organs warm. Once things warm up, those blood vessels in the skin gradually relax, and allow that blood to return to the surface.

You’ve probably noticed that if you’re cold for too long, your skin may lose some of its color - or even turn pale blue if you’re light-skinned - as that blood retreats from the surface.

And in fact, like a litmus test for your body, changes in the color of your skin can indicate a number of homeostatic imbalances.

Blue skin, or cyanosis, in Caucasian people may indicate heart failure, poor circulation, or severe respiratory issues. That’s because blood that’s been depleted of oxygen turns darker in color, and when seen through the tissue of lips or skin, it can look bluish.

A yellowing of the skin, called jaundice, usually signifies liver disorder, as yellow bile starts accumulating in the blood stream. Reddened skin, or erythema, could indicate a fever, inflammation, or allergy -- all of these conditions cause blood vessels to expand and more blood to flow to the skin’s surface.

Of course, human skin color spans a pretty wide spectrum, so some of these conditions are easier to diagnose by looking for discolorations of other tissues, like mucous membranes and the beds of finger and toenails.

However light or dark your skin color is, though, you can thank your melanin for it. You’ll remember that melanin is a pigment produced by the melanocyte cells in your epidermis. Melanin has two forms, producing pigments that range in color from reddish yellow to brownish black.

Because its main job is to protect us from the sun’s ultraviolet rays, it makes sense that, in the distant past, the distribution of these different skin tones was not at all random.

Historically, where solar radiation is more intense, higher concentrations of deep-colored melanin became an advantage for the protection it provided. But closer to the poles, where those solar rays are weaker and more diffuse, lower concentrations of melanin allowed people to collect what sunlight was available, to manufacture vitamin D.

‘Cause the fact is, we all need some level of sunlight to hit our skin to survive.

Your bones require vitamin D to keep producing new bone cells, and it’s the only vitamin that your body can actually produce on its own. Your skin cells contain a molecule that converts to vitamin D when it comes in contact with UV light.

From there the vitamin heads through your bloodstream to your liver and kidneys where it truly becomes activated D, also called calcitriol, which is circulated to all the bones of your body.

 Your Hair (5:41)

But let’s not just be skin-deep here - your integumentary system also involves your so-called skin appendages - your hair, nails, sweat and sebaceous, or oil, glands, which can each be fascinating as well as frustrating in its own way.

If you’re like some people I know and you spend a fortune on hair conditioner? That’s because your cuticles are out of control. All of your hairs, or pili, are basically just flexible strands of dead keratin protein cells, like your fingernails.

And the outermost layer of these dead cells, called the cuticle, looks like it’s made of overlapping roof shingles. So what you’re paying the conditioner to do is even out the rough surface between those cells of the cuticle to make it look smooth.

Now if you pluck out a strand of your hair you will be in pain, but you will also have the opportunity to notice that it has two main regions - the shaft - where the keratinization is complete - and the root - the part inside the follicle where keratinization is still happening.

Each follicle is just a tube of epidermal cells, and just like in your epidermis, the cells at the bottom of each follicle are young and fresh, continually dividing and pushing older cells up through the skin and into the open air.

And your finger and toenails pretty much grow the same way, starting at the back of the nail bed where new cells divide at the root and get pushed forward, creating the scaly-hard keratin that you paint with polish and keep trimmed during flip-flop season.

  Sweat and Oil Glands (6:56)

But there’s probably no other part of the integumentary system that you spend more money on trying to control than your sweat and oil glands.

You’ve got up to three million tiny sudoriferous, or sweat glands distributed throughout your body. These guys secrete your salty, watery sweat, and they come in two types: eccrine and apocrine.

Your eccrine sweat glands are more abundant - they’re in your palms, forehead, and in the soles of your feet. They’re just simple coiled tubes that start in the dermis, extend through a duct, and open into a pore on the surface of your skin.

Your apocrine sweat glands are a slightly different story. You only have about 2000 of these, and they start cookin’ around puberty, emptying into the hair follicles around your armpits and groin.

These glands secrete a kind of deluxe sweat, with fats and proteins in it. It’s more viscous and sometimes yellowish in color. When bacteria on the skin get a hold of this sweat, it gets odorific, creating what we generically call body odor.

Deodorants don’t affect how much you sweat, but they do reduce those smells by attacking the stink-making bacteria, while antiperspirants do the opposite, using ingredients like aluminum to block your sweat glands and actually keep you from perspiring.

Some researchers believe these glands may be the human equivalent of other animals’ musky sex scent glands. So while you might not want to stink up a whole room, a little bit of body odor might actually get you a mate.

Mammary glands, which secrete milk in lactating people, and ceruminous glands, the ones that make your cerumen, or earwax, are two other types of modified apocrine sweat glands.

Finally, your sebaceous, or oil glands are found everywhere but the thick skin in your palms and foot soles. Their ducts are smaller on your limbs, but they’re pretty big on your face, and neck, and upper chest.

Most of your sebaceous glands secrete their sebum, an oily substance, into hair follicles where it can travel to the surface of your skin. And while yes, they cause wicked pimples, their primary goal is to soften and lubricate your skin and hair, and help slow water loss from the skin in dry environments.

Try to remember that the next time you have a break-out before a big date or job interview. Maybe it’ll make you feel better. It probably won’t.

The irony here is that about half of the things I showed at the beginning of this episode are used to wash away our natural protective moisturizing oils, while the other half are there to add them back through lotions and conditioners!

Deodorant, though. I think we’re all glad that exists.

  Review and Credits (9:05)

Today you learned how your integumentary system protects your body, senses the outside world, helps excrete waste, stores blood, regulates temperature, makes vitamin D, indicates signs of poor health, and gives you zits. We also talked about how your hair and nails grow, the difference between your eccrine and apocrine sweat glands, and your sebaceous oil glands.

Thank you for watching, especially to all of our Subbable subscribers, who make Crash Course possible for themselves and also to everyone else. To find out how you can become a supporter, just go to

This episode was written by Kathleen Yale, edited by Blake de Pastino, and our consultant, is Dr. Brandon Jackson. Our director and editor is Nicholas Jenkins, the script supervisor and sound designer is Michael Aranda, and the graphics team is Thought Café.