WEBVTT 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 00:00:03.670 --> 00:00:07.100 of important people. You’re practicing in front of your mirror, and then just for a 00:00:07.100 --> 00:00:09.550 second you forget how to speak. 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. 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 00:00:18.340 --> 00:00:21.640 scenarios. Like, what if you totally lose your train of thought up there? What if you 00:00:21.640 --> 00:00:25.439 barf? What if everybody gets up and leaves? Now you’re really nervous. I’m getting 00:00:25.439 --> 00:00:27.099 freaked out just talking about it. 00:00:27.099 --> 00:00:31.339 So ou start taking quick, shallow breaths, and you’re feeling light-headed, and seeing 00:00:31.340 --> 00:00:34.280 stars, and now you, my friend, are hyperventilating. 00:00:34.280 --> 00:00:38.520 When we talk about respiration, we tend to focus on oxygen -- and who could blame us? 00:00:38.520 --> 00:00:42.600 It’s easy to forget the equally important role that carbon dioxide plays in maintaining 00:00:42.610 --> 00:00:46.920 homeostasis. Your internal balance between oxygen and carbon dioxide factors heavily 00:00:46.920 --> 00:00:50.880 into all sorts of stuff -- especially in your blood, where it can affect your blood’s 00:00:50.880 --> 00:00:53.240 pressure, its pH level, even its temperature. 00:00:53.240 --> 00:00:58.000 And now -- at, like, T-minus 5 minutes to your presentation -- all of those things are 00:00:58.000 --> 00:01:01.600 out of whack, because you’re exhaling more CO2 than you should. 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 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, 00:01:10.340 --> 00:01:11.480 it does the trick. 00:01:11.490 --> 00:01:13.340 Within seconds, you’re back to normal. 00:01:13.340 --> 00:01:17.799 The drop in CO2 that occurs in your blood when you hyperventilate is called hypocapnia, 00:01:17.799 --> 00:01:21.549 and it signals a breakdown in one of the most complex and important functions that your 00:01:21.549 --> 00:01:23.420 respiratory system performs. 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 00:01:28.399 --> 00:01:31.149 want is swapped out for what it desperately needs. 00:01:31.149 --> 00:01:35.299 This exchange -- between carbon dioxide and oxygen -- is regulated by a whole series of 00:01:35.299 --> 00:01:39.289 biological signals that your blood cells use to communicate with your tissues, about what 00:01:39.289 --> 00:01:41.979 they have, what they want, and what they need to get rid of. 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 00:01:45.859 --> 00:01:48.619 of its proteins -- even in its temperature and acidity. 00:01:48.620 --> 00:01:51.680 It’s what allows you to perform strenuous physical tasks, like climbing a mountain. 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. 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 00:02:12.000 --> 00:02:13.760 tended to keep things pretty simple. 00:02:13.770 --> 00:02:17.630 Like, hemoglobin contains four protein chains, each of which contains an iron atom; since 00:02:17.630 --> 00:02:21.930 iron binds readily with oxygen, that’s how hemoglobin transports oxygen around your body. 00:02:21.930 --> 00:02:22.760 Ba-da-bing. 00:02:22.760 --> 00:02:26.940 But the fact is, hemoglobin’s affinity for oxygen isn’t always the same. 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 00:02:30.680 --> 00:02:33.160 easily grab it out of the air. And in others, we want it to have 00:02:33.160 --> 00:02:37.100 a low affinity for oxygen oxygen, so it can dump those molecules to feed our cells. 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? 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. 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. 00:02:50.890 --> 00:02:55.160 And molecules also diffuse from areas of high concentration to areas of low concentration. 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. 00:03:00.070 --> 00:03:04.100 See, air is a mixture of molecules. And when you’re studying the respiratory system, 00:03:04.100 --> 00:03:08.140 you often need to focus on the oxygen, which makes up about 21% of it. 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 00:03:12.100 --> 00:03:16.860 know the overall air pressure, since more molecules in a certain volume means more pressure. 00:03:16.860 --> 00:03:20.580 So, partial pressure gives us a way of understanding how much oxygen there is, 00:03:20.580 --> 00:03:22.660 based on the pressure that it’s creating. 00:03:22.660 --> 00:03:27.880 Example: The pressure of air at sea level is about 760 millimeters of mercury. But since 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 00:03:33.100 --> 00:03:38.760 pressure of oxygen -- is 21% of 760, or about 160 millimeters of mercury. 00:03:38.760 --> 00:03:41.520 Now, that’s just outside, at sea level. 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 00:03:45.640 --> 00:03:50.700 exhaled yet -- the partial pressure of oxygen drops to about 104 millimeters of mercury. 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 00:03:54.410 --> 00:03:59.730 away by your hungry muscles and neurons -- the oxygen partial pressure is only about 40 millimeters. 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 00:04:05.520 --> 00:04:11.620 your blood plasma, because, as a rule dissolved gases always diffuse down their partial pressure gradients. 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, 00:04:15.180 --> 00:04:20.680 the concentration of oxygen stays at about 21%. But the pressure gets lower, which means the 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. 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 00:04:29.970 --> 00:04:33.410 the same as the de-oxygenated blood that’s entering your lungs. 00:04:33.410 --> 00:04:37.350 So basically there is no partial pressure gradient, which makes it really hard to get 00:04:37.350 --> 00:04:40.440 oxygen into your blood. But, let’s get back to the red blood cells. 00:04:40.440 --> 00:04:44.860 Remember that the globin in your hemoglobin is a protein -- and when proteins bind to 00:04:44.870 --> 00:04:49.270 stuff, they tend to change shape. And that shape-change can make the protein more or 00:04:49.270 --> 00:04:51.640 less likely to bind to other stuff. 00:04:51.640 --> 00:04:55.560 When an empty hemoglobin runs into an oxygen molecule, things are a little awkward. 00:04:55.570 --> 00:04:58.190 It’s like a first date -- bonding isn’t so easy. 00:04:58.190 --> 00:05:02.870 But once they finally bind, hemoglobin suddenly changes shape, which makes it easier for other 00:05:02.870 --> 00:05:06.590 oxygen molecules to attach, like friends gathering around the lunch table. 00:05:06.590 --> 00:05:11.650 That affinity for joining in -- or cooperativity, as it’s known -- continues until all four 00:05:11.650 --> 00:05:14.750 binding sites are taken, and the molecule is fully saturated. 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 00:05:21.150 --> 00:05:24.160 is called that. That’s the “Home Box Office.” Anyway. 00:05:24.160 --> 00:05:28.420 By the time the blood leaves the lungs, each hemoglobin is fully saturated, the oxygen 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 00:05:32.790 --> 00:05:34.490 to where it is needed most. 00:05:34.490 --> 00:05:38.660 Active tissues, like the brain, heart, and muscles, are always hungry for oxygen. They 00:05:38.669 --> 00:05:43.419 burn through it quickly, lowering the oxygen partial pressure around them to about 40 millimeters. 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 00:05:47.150 --> 00:05:49.210 the tissues, to feed those hungry cells. 00:05:49.210 --> 00:05:53.160 That makes the oxygen partial pressure in your plasma drop, so your hemoglobin starts 00:05:53.160 --> 00:05:55.560 to give up more of its oxygen to the plasma. 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. 00:06:00.540 --> 00:06:04.700 All of that metabolic activity going on in your tissues is also producing other triggers, 00:06:04.700 --> 00:06:08.160 in the form of waste products -- specifically heat and CO2. 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. 00:06:13.900 --> 00:06:17.360 Say you’re climbing that mountain again, and your thighs are feeling the burn. Red 00:06:17.360 --> 00:06:21.660 blood cells saturated with oxygen are going to the muscle tissue in your quads, where 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. 00:06:26.669 --> 00:06:31.060 But a hard-working quad will also heat up the surrounding tissues, and that rise in 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. 00:06:37.000 --> 00:06:41.260 So when those red blood cells hit that warm active tissue, they release even more oxygen 00:06:41.260 --> 00:06:44.980 -- like 20 percent more -- beyond what partial pressures would trigger. 00:06:44.980 --> 00:06:46.840 But wait! There’s more! 00:06:46.840 --> 00:06:51.540 Carbon dioxide triggers the release of oxygen, too, because it also binds to the hemoglobin, 00:06:51.540 --> 00:06:56.320 changing its shape again, lowering its affinity for oxygen still more. And as oxygen jumps 00:06:56.320 --> 00:06:58.880 ship, the hemoglobin can pick up more CO2. 00:06:58.880 --> 00:07:02.240 Finally, JUST IN CASE the hemoglobin isn’t getting the message at this point, there’s 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 00:07:06.680 --> 00:07:10.000 released by your active muscle tissues actually makes your blood more acidic. 00:07:10.000 --> 00:07:14.550 Since your blood is mostly water, when CO2 dissolves in it, it forms carbonic acid, which 00:07:14.550 --> 00:07:19.790 breaks down into bicarbonate and hydrogen ions. Those ions bind to the hemoglobin, changing 00:07:19.790 --> 00:07:23.669 its shape yet again, further lowering its affinity for oxygen. 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 00:07:27.979 --> 00:07:30.410 with all this CO2 that they need to get rid of. 00:07:30.410 --> 00:07:32.780 Your red blood cells ride the vein-train back to the lungs, 00:07:32.780 --> 00:07:35.800 where they encounter a new wave of freshly inhaled oxygen. 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 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. 00:07:45.520 --> 00:07:50.060 So the hemoglobin drops its carbon dioxide, which moves down its partial pressure gradient 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. 00:07:54.980 --> 00:07:58.100 Now if that isn’t enough to make you hyperventilate, I’m not sure what is. 00:07:58.100 --> 00:08:02.360 But this brings us back to that unfortunate episode you had before your big presentation. 00:08:02.360 --> 00:08:06.540 This whole complex code of chemical signals that I just described? Well, it assumes that 00:08:06.540 --> 00:08:09.360 what your cells and tissues are telling each other is actually true. 00:08:09.360 --> 00:08:14.600 But as we all know, sometimes our bodies don’t mean what they say. Thanks, body. 00:08:14.600 --> 00:08:17.400 Like, when you’re freaking out about your presentation, your sympathetic nervous system 00:08:17.400 --> 00:08:21.500 makes your heart race and your breathing increase, to prepare you to fight or flee. 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 00:08:25.820 --> 00:08:29.850 actually doing anything, so they’re not using all the extra oxygen you’re breathing in. 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. 00:08:34.770 --> 00:08:38.740 So when you start to exhale CO2 faster than your cells release it, its concentration in 00:08:38.750 --> 00:08:43.390 your blood drops. And with less carbonic acid around, your blood’s pH starts to rise. 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 00:08:47.270 --> 00:08:51.480 hemoglobin to deliver oxygen, high pH causes vasoconstriction. 00:08:51.480 --> 00:08:54.820 Normally, this is supposed to divert blood from the parts you’re not using during times 00:08:54.820 --> 00:08:57.960 of stress, like your digestive organs, to the parts that you are using. 00:08:57.970 --> 00:09:01.899 But when you hyperventilate, this constriction happens everywhere, which means less blood 00:09:01.899 --> 00:09:04.570 is delivered to your brain, which makes you light-headed. 00:09:04.570 --> 00:09:07.820 Luckily, that trick with the breathing into the paper bag -- it really does work. 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 00:09:12.180 --> 00:09:16.500 the partial pressure of carbon dioxide in the bag is higher, which forces that CO2 into 00:09:16.500 --> 00:09:19.850 your blood, which lowers its pH, and you get back to homeostasis. 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. 00:09:25.780 --> 00:09:30.340 If you were able to remain calm today, you learned how your blood cells exchange oxygen 00:09:30.350 --> 00:09:35.010 and CO2 to maintain homeostasis. We talked about partial pressure gradients, and how 00:09:35.010 --> 00:09:39.620 they, along with changes in blood temperature, acidity, and CO2 concentrations, change how 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. 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 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 00:09:53.460 --> 00:09:57.360 help us keep making videos like this one, you can go to patreon.com/crashcourse. 00:09:57.360 --> 00:10:01.180 This episode was filmed in the Doctor Cheryl C. Kinney Crash Course Studio, it was written 00:10:01.180 --> 00:10:05.660 by Kathleen Yale, the script was edited by Blake de Pastino, and our consultant is Dr. 00:10:05.660 --> 00:10:10.270 Brandon Jackson. It was directed and edited by Nicole Sweeney; our sound designer is Michael 00:10:10.270 --> 00:10:12.790 Aranda, and the graphics team is Thought Cafe.