0:00:00.000,0:00:04.140
We’ve been spending a lot of time lately talking[br]about eating, and digesting, and metabolizing food.

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And those are some of my favorite things in[br]the world! It’s been a really great time.

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But, as with all good parties, or brunch buffets,[br]in the end, we’re left with a mess.

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And I’m not talking about spilled beer and[br]Dorito crumbs, I’m talking about toxic levels

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of garbage that need to be cleaned up before[br]they kill you.

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In your body, a lot of the cleanup that comes[br]after metabolism is handled by the liver, which

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plays a tremendous role in directing dead cells and[br]leftover chemicals to the digestive and urinary systems.

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But your liver can’t actually escort waste[br]out of your person.

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Your lungs can lend a hand, exhaling carbon[br]dioxide, and of course your colon will eventually

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poop out unusable stuff and old cell-parts.[br]But much of your chemical waste still needs

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to be sorted and disposed of, so one system[br]steps in to bat clean-up.

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And that, is your urinary system.

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This system -- and specifically your kidneys[br]-- does all sorts of important homeostatic

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stuff, like regulating your water volume,[br]ion salt concentrations, and pH levels, and

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influencing your red blood cell production[br]and blood pressure.

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But its main purpose -- what we’re going[br]to be focusing on for the next two lessons

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-- is how it filters toxic leftovers from[br]your blood -- like the nitrogenous waste made

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by metabolizing protein -- and ferries it[br]out of the body.

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And — spoiler alert! — this all involves the[br]how, and the why, and the what of your pee.

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Now you probably know that kidneys are filters,[br]and you may imagine them as sieves that strain

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out the bad stuff, leaving it sitting like[br]a hairball at the bottom of the bathtub.

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But that is, in fact, kind of the opposite[br]of what you should be thinking.

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Most of what’s in your blood is totally[br]removed by the kidneys. Then your body pulls

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back what it wants to hold onto, before the[br]rest is sent on a one-way trip to the bladder.

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It’s kinda like this: you don’t clean out your fridge[br]by just taking out the rotten fruit and fuzzy leftovers.

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Instead, you’ve got to take everything out,[br]and put it on the counter, and then sort through

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what goes back in the fridge and what goes[br]in the trash.

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That’s how your urinary system cleans you[br]up. And it is really good at its job.

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So this morning I decided to go the healthy[br]route and instead of eating my normal breakfast

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of nothing, I had a big 32-ounce protein smoothie.

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My digestive system did its thing, and all[br]the protein was hydrolyzed into amino acids,

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which were absorbed by my blood, and sent[br]all over my body to build and repair cells.

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It’s a beautiful thing, but not without[br]consequence.

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Because metabolizing nutrients -- especially[br]protein -- makes a mess.

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You may remember that amino acids are unique,[br]in that they have nitrogen in their amine groups.

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And because we can’t store amino acids,[br]extra ones get processed into storable carbs or fats.

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But the amine group isn’t used in those[br]storage molecules, so it’s converted to

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NH3, or ammonia, which happens to be toxic. So the

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liver converts the ammonia into a less-toxic compound,[br]urea, which our kidneys filter out into our pee.

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Once out of the body, urea can degrade back[br]into ammonia, which is why dirty, pee-soaked

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toilets and cat litter boxes smell like ammonia.

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Now this business of taking out the nitrogenous[br]trash is one of the urinary system’s biggest jobs.

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Its other major duty is to regulate the balance[br]of salt and water in your blood, and both

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of these tasks are processed in the whole[br]system of tubes that is your urinary system.

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So let’s take a look at some basic pee-making[br]anatomy.

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Your kidneys are a pair of dark red, fist-sized,[br]bean-shaped organs that sit on each side of

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your spine against the posterior body wall.

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Kidneys are retroperitoneal, which means they[br]lie between the dorsal wall and the peritoneum

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-- the membrane that surrounds the abdominal[br]cavity -- rather than inside the cavity itself,

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like your intestines and stomach do.

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Each kidney has three distinct layers, beginning[br]with the outermost cortex.

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Beneath that is the medulla,[br]a set of cone-shaped masses

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of tissue that secrete urine into tiny sac-like tubules.

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And finally, the innermost layer is the renal[br]pelvis, a funnel-shaped tube surrounded by

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smooth muscle that uses peristalsis to move urine[br]out of the kidney, into the ureter, and into the bladder.

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Because the kidneys’ main job is to filter[br]blood continuously, they end up seeing a lot of it.

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In fact, at any given moment they hold over[br]20 percent of your total blood volume.

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Oxygenated blood enters the kidneys through[br]the large renal arteries, which deliver nearly

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a quarter of all blood pumped through the[br]heart every minute. That means your kidneys

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filter about 120 to 140 liters of blood EVERY[br]DAY.

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As they enter the kidneys, renal arteries branch many,[br]many times, ending in tons of little capillary groups.

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So a kidney isn’t just one big filter; instead,[br]each one is made up of about a million twisty

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microscopic filtering units called nephrons.

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Structurally and functionally, nephrons are[br]where the real business of blood-processing

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-- which, like, “pee-making” -- begins, in three steps:[br]filtration, reabsorption, and secretion.

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Each nephron consists of a round renal corpuscle[br]that resides up in the cortex, followed by

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a long and winding renal tubule that loops[br]around between the cortex and the medulla.

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The outer part of the corpuscle is a cup-shaped[br]feature called the glomerular capsule, because

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inside it there’s a whole tangle of capillaries[br]called the glomerulus -- that’s from the

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Latin word for “ball of yarn,” which is[br]pretty much what it looks like.

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And the endothelium of these capillaries is[br]very porous. So they allow lots of fluid,

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waste products, ions, glucose, and amino acids[br]to pass from the blood into the capsule -- but

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they block out bigger molecules like blood[br]cells and proteins, so they stay in the blood

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and exit through the peritubular capillaries,[br]also known as the vasa recta.

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Now, all the stuff that get squeezed out of[br]the blood into the glomerulus is called filtrate,

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which is then sent along to the elaborately[br]twisting three-centimeter-long renal tubule.

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Even though it looks like it’s just a tube,[br]it has three major parts, some of which are

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permeable to certain substances, but not others.

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First along is the proximal convoluted tubule,[br]or PCT, which is about as convoluted-looking

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at its name suggests; then the tube drops[br]into a dramatic hairpin turn called the nephron

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loop, or the loop of Henle -- I term I kinda[br]like better, personally -- and finally it

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ends in the distal convoluted tubule or DCT,[br]which empties into a collecting duct.

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All this twisting might make the tubule look,[br]like, super inefficient, but it actually serves

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an important purpose, as you might expect.

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Just like with your small intestines, the[br]long, curly shape of the nephron provides

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more time and space for it to re-absorb whatever[br]useable stuff it can.

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And this meandering path also allows the parts[br]of the tubule that are toward the end, to

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have an affect on processes that take place[br]closer to the beginning, as they pass each other.

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Because a lot of the stuff that winds up in[br]the tube are valuable commodities -- like

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ions and glucose and water -- and we don’t[br]want to just pee all of them out if we can help it.

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So, let’s trace the whole process, starting at the[br]top, with the proximal convoluted tubule or PCT.

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The walls here are made of cuboidal epithelial[br]cells, with big ol’ mitochondria that make

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ATP, to power pumps that pull lots of sodium[br]ions from the filtrate, using active transport.

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These cells also are covered in microvilli[br]that increase their surface area and help

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re-absorb much of the good stuff from the[br]filtrate and back into the blood.

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The remaining filtrate passes from the PCT[br]into the loop of Henle, which starts in the

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cortex, then dips into the medulla before[br]coming back into the cortex.

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And the form of this loop is key to its function,[br]because its primary task is to drive the re-absorption

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of water, by creating a salt concentration[br]gradient in the tissue of the medulla.

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It does this mainly by actively pumping out[br]salts in the ascending limb. This creates

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some very salty interstitial fluid in the[br]medulla, so when new filtrate comes down the

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descending loop in front of it, water passively[br]flows out, and into the super salty interstitial space.

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Since most of this water is picked up by the[br]blood pretty quickly, the saltiness of the

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interstitial space doesn’t get diluted.[br]So it can keep drawing water out of the next

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batch of filtrate in the descending limb.

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Needless to say, this is super important,[br]because if we peed out all the water that

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went into our kidneys, we would die of dehydration[br]really quick.

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But even after all that, we are still only[br]two thirds of the way through the process.

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As we move out of the loop of Henle, into[br]the distal convoluted tubule, and on to the

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collecting duct, the remaining filtrate is[br]now officially urine. But there’s one more

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component that we have to squeeze the most[br]out of before we excrete the stuff. Urea.

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Even though we think of urea as a waste product[br]-- just one more part of that protein shake

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that has to be dumped -- the kidneys actually[br]need it.

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They use it to ramp up the concentration gradient[br]earlier in the process, making the medulla

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even saltier for the filtrate that’s back[br]there going through the ascending limb.

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So in the final steps, after the filtrate[br]leaves the DCT, it enters the collecting duct,

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which runs back into the medulla. And while[br]the salt passively draws even more water out

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of the collecting duct, some urea passively[br]leaves the urine as well.

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Making the medulla even more salty -- and,[br]in turn, more effective at drawing out water

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from the ascending limb a few steps back.

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So there’s essentially a traveling pool[br]of urea that escapes the urine, finds its

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way back into the loop of Henle, and then[br]runs the whole course again back to the collecting

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duct -- an ammonia-scented cycle called urea[br]recycling.

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Now all that’s left is a kind of last call[br]to selectively sneak out any extra waste -- like

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hydrogen, potassium, and certain organic acids[br]and bases -- using active transport.

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This is called tubular secretion, and it transports[br]only select kinds of waste that have already

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made their way into the blood that’s in the[br]peritubular capillaries, ready to leave the kidneys.

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This step is kind of like emptying your pockets[br]of any last wads of tissue or crumpled receipts

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as you’re walking a bag of trash to the[br]curb.

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And that’s how your kidneys clean up the[br]mess left over from the giant party that is

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you metabolizing food. So if you thought that[br]your kidneys were just a kinda fine mesh that

0:08:49.040,0:08:51.660
filtered out bad stuff? Now you know that’s[br]not true.

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If you thought your urinary system was basically[br]a matter of: Water goes in, pee goes out?

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That’s DEFINITELY not true.

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And if you thought we were done talking about[br]your urine, that is also not true, either,

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because next time, we’re going to learn[br]how your body regulates what’s absorbed

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and what’s excreted, and we’ll find out [br]can happen when that regulation goes awry.

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But for now, you learned the anatomy of your[br]urinary system, and how your kidneys filter

0:09:13.700,0:09:17.940
metabolic waste and balance salt and water[br]concentrations in the blood. Specifically

0:09:17.940,0:09:23.000
you learned how nephrons use glomerular filtration,[br]tubular reabsorption, and tubular secretion

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to reabsorb water and nutrients back into[br]the blood, and make urine with the leftovers.

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Thank you to our Headmaster of Learning, Linnea[br]Boyev, and thank you to all of our Patreon

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patrons whose monthly contributions help make[br]Crash Course possible, not only for themselves,

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but for everyone. If you like Crash Course[br]and want to help us keep making videos like

0:09:39.230,0:09:41.580
this one, you can go to patreon.com/crashcourse.

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This episode was filmed in the Doctor Cheryl[br]C. Kinney Crash Course Studio, it was written

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by Kathleen Yale, edited by Blake de Pastino,[br]and our consultant is Dr. Brandon Jackson.

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It was directed and edited by Nicole Sweeney;[br]our sound designer is Michael Aranda, and

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the Graphics team is Thought Cafe.