1 00:00:00,000 --> 00:00:03,660 Picture this: You’re getting ready to give a big presentation in front of, like, a lot 2 00:00:03,670 --> 00:00:07,100 of important people. You’re practicing in front of your mirror, and then just for a 3 00:00:07,100 --> 00:00:09,550 second you forget how to speak. 4 00:00:09,550 --> 00:00:14,580 Suddenly, you feel that familiar sting of anxiety, like an icy hand on the back of your neck. 5 00:00:14,580 --> 00:00:18,340 You look at yourself in that mirror and you start imagining some of the worst, worst-case 6 00:00:18,340 --> 00:00:21,640 scenarios. Like, what if you totally lose your train of thought up there? What if you 7 00:00:21,640 --> 00:00:25,439 barf? What if everybody gets up and leaves? Now you’re really nervous. I’m getting 8 00:00:25,439 --> 00:00:27,099 freaked out just talking about it. 9 00:00:27,099 --> 00:00:31,339 So ou start taking quick, shallow breaths, and you’re feeling light-headed, and seeing 10 00:00:31,340 --> 00:00:34,280 stars, and now you, my friend, are hyperventilating. 11 00:00:34,280 --> 00:00:38,520 When we talk about respiration, we tend to focus on oxygen -- and who could blame us? 12 00:00:38,520 --> 00:00:42,600 It’s easy to forget the equally important role that carbon dioxide plays in maintaining 13 00:00:42,610 --> 00:00:46,920 homeostasis. Your internal balance between oxygen and carbon dioxide factors heavily 14 00:00:46,920 --> 00:00:50,880 into all sorts of stuff -- especially in your blood, where it can affect your blood’s 15 00:00:50,880 --> 00:00:53,240 pressure, its pH level, even its temperature. 16 00:00:53,240 --> 00:00:58,000 And now -- at, like, T-minus 5 minutes to your presentation -- all of those things are 17 00:00:58,000 --> 00:01:01,600 out of whack, because you’re exhaling more CO2 than you should. 18 00:01:01,600 --> 00:01:05,840 You’re just about to faint, when a friend suddenly hands you a paper bag to breathe 19 00:01:05,840 --> 00:01:10,340 into. And you’ve never been so grateful for a lunch bag in your life, because, somehow, 20 00:01:10,340 --> 00:01:11,480 it does the trick. 21 00:01:11,490 --> 00:01:13,340 Within seconds, you’re back to normal. 22 00:01:13,340 --> 00:01:17,799 The drop in CO2 that occurs in your blood when you hyperventilate is called hypocapnia, 23 00:01:17,799 --> 00:01:21,549 and it signals a breakdown in one of the most complex and important functions that your 24 00:01:21,549 --> 00:01:23,420 respiratory system performs. 25 00:01:23,420 --> 00:01:28,399 That is: the exchange of gases inside your blood cells, where the stuff your body doesn’t 26 00:01:28,399 --> 00:01:31,149 want is swapped out for what it desperately needs. 27 00:01:31,149 --> 00:01:35,299 This exchange -- between carbon dioxide and oxygen -- is regulated by a whole series of 28 00:01:35,299 --> 00:01:39,289 biological signals that your blood cells use to communicate with your tissues, about what 29 00:01:39,289 --> 00:01:41,979 they have, what they want, and what they need to get rid of. 30 00:01:41,979 --> 00:01:45,859 It’s almost like a code, one that’s written into your blood’s chemistry, in the folding 31 00:01:45,859 --> 00:01:48,619 of its proteins -- even in its temperature and acidity. 32 00:01:48,620 --> 00:01:51,680 It’s what allows you to perform strenuous physical tasks, like climbing a mountain. 33 00:01:51,680 --> 00:01:57,680 It’s also what lets you reboot your whole respiratory system, with nothing more than a paper bag. 34 00:02:08,940 --> 00:02:12,000 I’ll admit it: when we’ve talked about the chemistry of your blood so far, we’ve 35 00:02:12,000 --> 00:02:13,760 tended to keep things pretty simple. 36 00:02:13,770 --> 00:02:17,630 Like, hemoglobin contains four protein chains, each of which contains an iron atom; since 37 00:02:17,630 --> 00:02:21,930 iron binds readily with oxygen, that’s how hemoglobin transports oxygen around your body. 38 00:02:21,930 --> 00:02:22,760 Ba-da-bing. 39 00:02:22,760 --> 00:02:26,940 But the fact is, hemoglobin’s affinity for oxygen isn’t always the same. 40 00:02:26,940 --> 00:02:30,680 In some places, we want our hemoglobin to have a high affinity for oxygen, so it can 41 00:02:30,680 --> 00:02:33,160 easily grab it out of the air. And in others, we want it to have 42 00:02:33,160 --> 00:02:37,100 a low affinity for oxygen oxygen, so it can dump those molecules to feed our cells. 43 00:02:37,100 --> 00:02:41,860 So how does your hemoglobin know when to collect its precious cargo and when to let it go? 44 00:02:41,870 --> 00:02:45,780 Well, a lot of it has to do with a principle of chemistry known as partial pressure. 45 00:02:45,780 --> 00:02:50,890 One of the things that fluids always do is move from areas of high pressure to low pressure. 46 00:02:50,890 --> 00:02:55,160 And molecules also diffuse from areas of high concentration to areas of low concentration. 47 00:02:55,160 --> 00:03:00,060 But when we talk about gases in a mixture, we need to combine the ideas of pressure and concentration. 48 00:03:00,070 --> 00:03:04,100 See, air is a mixture of molecules. And when you’re studying the respiratory system, 49 00:03:04,100 --> 00:03:08,140 you often need to focus on the oxygen, which makes up about 21% of it. 50 00:03:08,140 --> 00:03:12,100 But that doesn’t tell us how many oxygen molecules there are. For that, we need to 51 00:03:12,100 --> 00:03:16,860 know the overall air pressure, since more molecules in a certain volume means more pressure. 52 00:03:16,860 --> 00:03:20,580 So, partial pressure gives us a way of understanding how much oxygen there is, 53 00:03:20,580 --> 00:03:22,660 based on the pressure that it’s creating. 54 00:03:22,660 --> 00:03:27,880 Example: The pressure of air at sea level is about 760 millimeters of mercury. But since 55 00:03:27,880 --> 00:03:33,100 only about 21 percent of that air is oxygen, oxygen’s part of that pressure -- or partial 56 00:03:33,100 --> 00:03:38,760 pressure of oxygen -- is 21% of 760, or about 160 millimeters of mercury. 57 00:03:38,760 --> 00:03:41,520 Now, that’s just outside, at sea level. 58 00:03:41,520 --> 00:03:45,640 When that air mixes with the air deep in your lungs -- including a lot of air that you haven’t 59 00:03:45,640 --> 00:03:50,700 exhaled yet -- the partial pressure of oxygen drops to about 104 millimeters of mercury. 60 00:03:50,710 --> 00:03:54,410 And in the blood that’s entering your lungs -- after most of its oxygen has been stripped 61 00:03:54,410 --> 00:03:59,730 away by your hungry muscles and neurons -- the oxygen partial pressure is only about 40 millimeters. 62 00:03:59,730 --> 00:04:05,520 This big differences in pressure make it easy for oxygen molecules to travel from the outside air into 63 00:04:05,520 --> 00:04:11,620 your blood plasma, because, as a rule dissolved gases always diffuse down their partial pressure gradients. 64 00:04:11,620 --> 00:04:15,180 This is why it’s so much harder to breathe at higher altitudes. When you climb a mountain, 65 00:04:15,180 --> 00:04:20,680 the concentration of oxygen stays at about 21%. But the pressure gets lower, which means the 66 00:04:20,680 --> 00:04:26,180 partial pressure of oxygen also decreases to about 45 millimeters of mercury at the top of Mt. Everest. 67 00:04:26,180 --> 00:04:29,960 So the partial pressure of oxygen at the top of the highest peak in the world, is almost 68 00:04:29,970 --> 00:04:33,410 the same as the de-oxygenated blood that’s entering your lungs. 69 00:04:33,410 --> 00:04:37,350 So basically there is no partial pressure gradient, which makes it really hard to get 70 00:04:37,350 --> 00:04:40,440 oxygen into your blood. But, let’s get back to the red blood cells. 71 00:04:40,440 --> 00:04:44,860 Remember that the globin in your hemoglobin is a protein -- and when proteins bind to 72 00:04:44,870 --> 00:04:49,270 stuff, they tend to change shape. And that shape-change can make the protein more or 73 00:04:49,270 --> 00:04:51,640 less likely to bind to other stuff. 74 00:04:51,640 --> 00:04:55,560 When an empty hemoglobin runs into an oxygen molecule, things are a little awkward. 75 00:04:55,570 --> 00:04:58,190 It’s like a first date -- bonding isn’t so easy. 76 00:04:58,190 --> 00:05:02,870 But once they finally bind, hemoglobin suddenly changes shape, which makes it easier for other 77 00:05:02,870 --> 00:05:06,590 oxygen molecules to attach, like friends gathering around the lunch table. 78 00:05:06,590 --> 00:05:11,650 That affinity for joining in -- or cooperativity, as it’s known -- continues until all four 79 00:05:11,650 --> 00:05:14,750 binding sites are taken, and the molecule is fully saturated. 80 00:05:14,750 --> 00:05:21,150 Now your hemoglobin is known as oxyhemoglobin, or HbO2. It is not...not why the cable network 81 00:05:21,150 --> 00:05:24,160 is called that. That’s the “Home Box Office.” Anyway. 82 00:05:24,160 --> 00:05:28,420 By the time the blood leaves the lungs, each hemoglobin is fully saturated, the oxygen 83 00:05:28,430 --> 00:05:32,790 partial pressure in your plasma is about 100 millimeters, and now it is ready to be delivered 84 00:05:32,790 --> 00:05:34,490 to where it is needed most. 85 00:05:34,490 --> 00:05:38,660 Active tissues, like the brain, heart, and muscles, are always hungry for oxygen. They 86 00:05:38,669 --> 00:05:43,419 burn through it quickly, lowering the oxygen partial pressure around them to about 40 millimeters. 87 00:05:43,419 --> 00:05:47,150 So when the blood arrives on the scene, oxygen moves down the gradient from the plasma to 88 00:05:47,150 --> 00:05:49,210 the tissues, to feed those hungry cells. 89 00:05:49,210 --> 00:05:53,160 That makes the oxygen partial pressure in your plasma drop, so your hemoglobin starts 90 00:05:53,160 --> 00:05:55,560 to give up more of its oxygen to the plasma. 91 00:05:55,560 --> 00:06:00,540 BUT! Partial pressures are only part of what’s prodding your hemoglobin to give up the goods. 92 00:06:00,540 --> 00:06:04,700 All of that metabolic activity going on in your tissues is also producing other triggers, 93 00:06:04,700 --> 00:06:08,160 in the form of waste products -- specifically heat and CO2. 94 00:06:08,160 --> 00:06:13,900 Both of those things activate the release of more oxygen, by lowering hemoglobin’s affinity for it. 95 00:06:13,900 --> 00:06:17,360 Say you’re climbing that mountain again, and your thighs are feeling the burn. Red 96 00:06:17,360 --> 00:06:21,660 blood cells saturated with oxygen are going to the muscle tissue in your quads, where 97 00:06:21,660 --> 00:06:26,660 the hemoglobin can dump a bunch of O2, because of the lower partial pressures of oxygen in your muscles. 98 00:06:26,669 --> 00:06:31,060 But a hard-working quad will also heat up the surrounding tissues, and that rise in 99 00:06:31,060 --> 00:06:37,000 temperature changes the shape of hemoglobin -- and it does it in such a way that lowers its affinity for O2. 100 00:06:37,000 --> 00:06:41,260 So when those red blood cells hit that warm active tissue, they release even more oxygen 101 00:06:41,260 --> 00:06:44,980 -- like 20 percent more -- beyond what partial pressures would trigger. 102 00:06:44,980 --> 00:06:46,840 But wait! There’s more! 103 00:06:46,840 --> 00:06:51,540 Carbon dioxide triggers the release of oxygen, too, because it also binds to the hemoglobin, 104 00:06:51,540 --> 00:06:56,320 changing its shape again, lowering its affinity for oxygen still more. And as oxygen jumps 105 00:06:56,320 --> 00:06:58,880 ship, the hemoglobin can pick up more CO2. 106 00:06:58,880 --> 00:07:02,240 Finally, JUST IN CASE the hemoglobin isn’t getting the message at this point, there’s 107 00:07:02,240 --> 00:07:06,680 one more trigger that your respiratory system has up its sleeve. The spike in CO2 that’s 108 00:07:06,680 --> 00:07:10,000 released by your active muscle tissues actually makes your blood more acidic. 109 00:07:10,000 --> 00:07:14,550 Since your blood is mostly water, when CO2 dissolves in it, it forms carbonic acid, which 110 00:07:14,550 --> 00:07:19,790 breaks down into bicarbonate and hydrogen ions. Those ions bind to the hemoglobin, changing 111 00:07:19,790 --> 00:07:23,669 its shape yet again, further lowering its affinity for oxygen. 112 00:07:23,669 --> 00:07:27,979 So now, at last, your tissues have the oxygen they need, and your red blood cells are stuck 113 00:07:27,979 --> 00:07:30,410 with all this CO2 that they need to get rid of. 114 00:07:30,410 --> 00:07:32,780 Your red blood cells ride the vein-train back to the lungs, 115 00:07:32,780 --> 00:07:35,800 where they encounter a new wave of freshly inhaled oxygen. 116 00:07:35,800 --> 00:07:40,620 And when that O2 binds to the hemoglobin -- which, again, is hard at first -- it eventually changes 117 00:07:40,620 --> 00:07:45,520 its shape back to the way it was when we started, which decreases its affinity for CO2. 118 00:07:45,520 --> 00:07:50,060 So the hemoglobin drops its carbon dioxide, which moves down its partial pressure gradient 119 00:07:50,060 --> 00:07:54,980 into the air of your lungs, so you can exhale it, and the whole thing can start all over again. 120 00:07:54,980 --> 00:07:58,100 Now if that isn’t enough to make you hyperventilate, I’m not sure what is. 121 00:07:58,100 --> 00:08:02,360 But this brings us back to that unfortunate episode you had before your big presentation. 122 00:08:02,360 --> 00:08:06,540 This whole complex code of chemical signals that I just described? Well, it assumes that 123 00:08:06,540 --> 00:08:09,360 what your cells and tissues are telling each other is actually true. 124 00:08:09,360 --> 00:08:14,600 But as we all know, sometimes our bodies don’t mean what they say. Thanks, body. 125 00:08:14,600 --> 00:08:17,400 Like, when you’re freaking out about your presentation, your sympathetic nervous system 126 00:08:17,400 --> 00:08:21,500 makes your heart race and your breathing increase, to prepare you to fight or flee. 127 00:08:21,500 --> 00:08:25,820 The problem is: there’s nothing to actually fight or flee from, so your muscles aren’t 128 00:08:25,820 --> 00:08:29,850 actually doing anything, so they’re not using all the extra oxygen you’re breathing in. 129 00:08:29,850 --> 00:08:34,770 And they also aren't producing the extra CO2 that you're suddenly exhaling all over the place. 130 00:08:34,770 --> 00:08:38,740 So when you start to exhale CO2 faster than your cells release it, its concentration in 131 00:08:38,750 --> 00:08:43,390 your blood drops. And with less carbonic acid around, your blood’s pH starts to rise. 132 00:08:43,390 --> 00:08:47,270 And you know what else? While low blood pH does things like change the shape of your 133 00:08:47,270 --> 00:08:51,480 hemoglobin to deliver oxygen, high pH causes vasoconstriction. 134 00:08:51,480 --> 00:08:54,820 Normally, this is supposed to divert blood from the parts you’re not using during times 135 00:08:54,820 --> 00:08:57,960 of stress, like your digestive organs, to the parts that you are using. 136 00:08:57,970 --> 00:09:01,899 But when you hyperventilate, this constriction happens everywhere, which means less blood 137 00:09:01,899 --> 00:09:04,570 is delivered to your brain, which makes you light-headed. 138 00:09:04,570 --> 00:09:07,820 Luckily, that trick with the breathing into the paper bag -- it really does work. 139 00:09:07,820 --> 00:09:12,180 It works because it lets you breathe back in all of the CO2 you just breathed out. So 140 00:09:12,180 --> 00:09:16,500 the partial pressure of carbon dioxide in the bag is higher, which forces that CO2 into 141 00:09:16,500 --> 00:09:19,850 your blood, which lowers its pH, and you get back to homeostasis. 142 00:09:19,850 --> 00:09:25,780 And of course, homeostasis is the key to life...and you know, also to a successful presentation. 143 00:09:25,780 --> 00:09:30,340 If you were able to remain calm today, you learned how your blood cells exchange oxygen 144 00:09:30,350 --> 00:09:35,010 and CO2 to maintain homeostasis. We talked about partial pressure gradients, and how 145 00:09:35,010 --> 00:09:39,620 they, along with changes in blood temperature, acidity, and CO2 concentrations, change how 146 00:09:39,620 --> 00:09:44,140 hemoglobin binds to gases in your blood. And you learned how the thing with the bag works. 147 00:09:44,140 --> 00:09:48,580 Of course, we must say thank you to our patrons on Patreon who help make Crash Course possible through 148 00:09:48,580 --> 00:09:53,460 their monthly contributions, not just for themselves, but for everyone. If you like Crash Course and want to 149 00:09:53,460 --> 00:09:57,360 help us keep making videos like this one, you can go to patreon.com/crashcourse. 150 00:09:57,360 --> 00:10:01,180 This episode was filmed in the Doctor Cheryl C. Kinney Crash Course Studio, it was written 151 00:10:01,180 --> 00:10:05,660 by Kathleen Yale, the script was edited by Blake de Pastino, and our consultant is Dr. 152 00:10:05,660 --> 00:10:10,270 Brandon Jackson. It was directed and edited by Nicole Sweeney; our sound designer is Michael 153 00:10:10,270 --> 00:10:12,790 Aranda, and the graphics team is Thought Cafe.