WEBVTT 1 00:00:00.610 --> 00:00:05.990 Your digestive system is, in the quietest and most literal sense, 2 00:00:07.309 --> 00:00:16.039 a tube. A long, winding, astonishingly complex tube that runs 3 00:00:16.109 --> 00:00:19.579 from your mouth all the way to the other end 4 00:00:19.620 --> 00:00:23.980 of you. If you were to take that tube and 5 00:00:24.059 --> 00:00:28.539 stretch it out flat, it would be roughly nine meters long. 6 00:00:30.010 --> 00:00:36.310 Nine meters. That is about 30 feet of biology. Coiled 7 00:00:36.399 --> 00:00:40.640 and folded inside a space. No bigger than your torso. 8 00:00:42.539 --> 00:00:50.159 And inside that tube, right now, something is moving. Digestion 9 00:00:50.200 --> 00:00:53.880 does not wait for you to be awake. It does 10 00:00:53.939 --> 00:00:58.320 not pause when you sleep. It does not take a holiday. 11 00:01:00.200 --> 00:01:03.640 from the moment you were born until the moment your 12 00:01:03.700 --> 00:01:10.750 life ends, your digestive system is always working, always squeezing, 13 00:01:11.569 --> 00:01:17.989 always churning, always quietly transforming whatever you give it into 14 00:01:18.069 --> 00:01:23.920 something your body can use. There is something almost meditative 15 00:01:24.000 --> 00:01:30.049 about that. A system so faithful, It never asks for 16 00:01:30.079 --> 00:01:36.709 a break. Let us start at the very beginning. Before 17 00:01:36.829 --> 00:01:41.629 food ever reaches your stomach, before it travels anywhere at all, 18 00:01:42.769 --> 00:01:48.349 the process of digestion has already started. It starts in 19 00:01:48.370 --> 00:01:55.370 your mouth. And it starts, remarkably, before you even take 20 00:01:55.409 --> 00:02:00.980 a bite. The moment you smell something cooking, or see 21 00:02:01.019 --> 00:02:04.739 a plate of food across the room, or even just 22 00:02:04.939 --> 00:02:10.460 think about eating, your brain sends a signal, and your 23 00:02:10.539 --> 00:02:16.629 mouth begins to water. That watering is not just a reflex. 24 00:02:18.409 --> 00:02:25.530 It is preparation. Your body is getting ready. Saliva is 25 00:02:25.590 --> 00:02:30.819 being produced. In small glands tucked beneath your tongue and 26 00:02:30.879 --> 00:02:36.460 along the inside of your jaw, and it is flowing gently, softly, 27 00:02:37.460 --> 00:02:42.189 into the space of your mouth, you produce somewhere between 28 00:02:42.370 --> 00:02:45.849 one liter and one and a half liters of saliva 29 00:02:46.810 --> 00:02:53.729 every single day. That is quietly remarkable. All of it 30 00:02:54.090 --> 00:02:58.129 made by your body. All of it. serving a purpose. 31 00:03:00.469 --> 00:03:05.939 Saliva is not simply water. It is a carefully assembled 32 00:03:06.159 --> 00:03:14.099 liquid containing enzymes, proteins, minerals, and a compound called amylase, 33 00:03:15.280 --> 00:03:20.419 which begins breaking down carbohydrates the instant they touch your tongue. 34 00:03:22.400 --> 00:03:25.900 So that piece of bread you chewed this morning, the 35 00:03:25.939 --> 00:03:31.469 digestion of it In the very first second, it was 36 00:03:31.500 --> 00:03:36.870 in your mouth. The stomach had not even been involved yet. 37 00:03:38.550 --> 00:03:44.069 It was your saliva doing the first quiet work. Chewing, 38 00:03:45.060 --> 00:03:49.180 which we tend to rush through without thinking, is also 39 00:03:49.340 --> 00:03:54.360 far more important than it might seem. Every time you chew, 40 00:03:55.409 --> 00:04:00.210 you are doing something your stomach cannot do. Your stomach 41 00:04:00.590 --> 00:04:06.439 can churn and squeeze and mix food with acid, but 42 00:04:06.500 --> 00:04:12.520 it cannot grind. That grinding, that mechanical breaking down of 43 00:04:12.599 --> 00:04:17.259 food into smaller and smaller pieces, is the job of 44 00:04:17.300 --> 00:04:23.029 your teeth and your jaw. And the more thoroughly you chew, 45 00:04:24.050 --> 00:04:29.750 the less work everything downstream has to do. Your digestive 46 00:04:29.790 --> 00:04:35.410 system is, in a quiet way, grateful when you chew slowly. 47 00:04:37.920 --> 00:04:43.220 Once food is chewed, moistened with saliva, and shaped by 48 00:04:43.240 --> 00:04:47.339 your tongue into a soft little bundle, it is called 49 00:04:47.800 --> 00:04:53.160 a bolus. That word comes from a Greek word meaning lump, 50 00:04:53.649 --> 00:04:59.360 or clod. Not the most elegant name, but that small, 51 00:05:00.100 --> 00:05:06.540 soft lump is what you swallow. And swallowing, when you 52 00:05:06.600 --> 00:05:10.019 stop and think about it, is one of the most 53 00:05:10.079 --> 00:05:15.550 coordinated acts your body performs. In the fraction of a 54 00:05:15.629 --> 00:05:20.610 second it takes to swallow, over 30 muscles work in 55 00:05:20.730 --> 00:05:25.889 perfect sequence. Your tongue pushes the bolus to the back 56 00:05:25.939 --> 00:05:30.100 of your throat. A small flap of tissue called the 57 00:05:30.220 --> 00:05:37.480 epiglottis folds down over your windpipe to make absolutely sure 58 00:05:38.399 --> 00:05:42.519 the food does not go the wrong way. Your throat 59 00:05:42.620 --> 00:05:48.550 contracts in a wave and the food slides smoothly and 60 00:05:48.610 --> 00:05:53.740 efficiently into the tube. that connects your mouth to your stomach. 61 00:05:55.560 --> 00:06:01.589 That tube is called the oesophagus. The oesophagus is roughly 62 00:06:01.589 --> 00:06:08.529 25 centimeters long. It sits just behind your windpipe, tucked 63 00:06:08.829 --> 00:06:13.550 in the chest between your heart and your spine. And 64 00:06:13.610 --> 00:06:17.860 it does something that surprises many people. When they first 65 00:06:17.980 --> 00:06:23.170 learn it, It does not simply drop food downward with gravity. 66 00:06:24.610 --> 00:06:30.720 It actively squeezes food toward the stomach using rhythmic muscular waves. 67 00:06:32.620 --> 00:06:38.560 These waves are called peristalsis. And peristalsis is the fundamental 68 00:06:38.639 --> 00:06:45.879 movement of your entire digestive system. Peristalsis happens whether you 69 00:06:45.959 --> 00:06:53.589 are standing, sitting, or lying completely flat. Astronauts in orbit 70 00:06:53.689 --> 00:06:58.889 can eat and swallow normally, because their oesophagus does not 71 00:06:58.949 --> 00:07:04.569 rely on gravity at all. The food moves because of muscle, 72 00:07:06.180 --> 00:07:10.839 because of rhythm, because of a quiet and reliable biological 73 00:07:10.899 --> 00:07:15.500 wave that has been moving through you your whole life. 74 00:07:17.990 --> 00:07:21.920 The journey from your mouth to your stomach takes only 75 00:07:21.939 --> 00:07:26.500 a few seconds. Just 5 to 10 seconds for a 76 00:07:26.560 --> 00:07:30.959 soft piece of food, up to about 30 seconds for 77 00:07:31.000 --> 00:07:37.519 something harder and drier. And then the food arrives at 78 00:07:37.560 --> 00:07:44.300 a remarkable muscle called the lower esophageal sphincter. A sphincter 79 00:07:44.620 --> 00:07:48.370 is a ring of muscle that can tighten and relax 80 00:07:48.470 --> 00:07:53.449 on command. And this one is the gatekeeper of your stomach. 81 00:07:55.589 --> 00:07:59.730 It stays closed most of the time. It opens only 82 00:07:59.790 --> 00:08:04.800 to let food in, and then closes firmly again, to 83 00:08:04.860 --> 00:08:10.819 keep the stomach's contents from flowing back upward. When it 84 00:08:10.860 --> 00:08:15.699 does not close firmly enough, you get what we call heartburn. 85 00:08:17.430 --> 00:08:22.610 not a heart problem at all. Just that gatekeeper, not 86 00:08:22.709 --> 00:08:28.050 quite doing its job. And then there is the stomach itself. 87 00:08:30.089 --> 00:08:34.269 Most people picture the stomach as a large, hollow bag 88 00:08:34.600 --> 00:08:38.200 somewhere in the middle of the abdomen. But when it 89 00:08:38.279 --> 00:08:42.399 is empty, the stomach is actually not much bigger than 90 00:08:42.419 --> 00:08:49.259 your fist. It collapses into folds, like a deflated balloon, 91 00:08:50.720 --> 00:08:57.389 with walls layered and ridged, waiting. And when food arrives, 92 00:08:58.409 --> 00:09:04.090 it expands. The stomach can stretch to hold about one 93 00:09:04.149 --> 00:09:09.049 litre comfortably and up to four litres. If you push it, 94 00:09:11.009 --> 00:09:15.559 four litres. That is roughly the same as four large 95 00:09:15.639 --> 00:09:22.210 bottles of water. fitting inside that one small space. The 96 00:09:22.269 --> 00:09:26.809 stomach is, in its own quiet way, one of the 97 00:09:26.870 --> 00:09:33.320 most chemically aggressive environments in the human body. The lining 98 00:09:33.340 --> 00:09:38.559 of your stomach produces hydrochloric acid, the same kind of 99 00:09:38.659 --> 00:09:44.269 acid used in some industrial cleaning processes. It is strong 100 00:09:44.330 --> 00:09:50.500 enough to dissolve metal, if given enough time. And it sits, 101 00:09:51.600 --> 00:09:57.629 right now, inside you. The acid kills most of the 102 00:09:57.669 --> 00:10:04.470 bacteria that enter your digestive system with food. It unravels proteins. 103 00:10:05.970 --> 00:10:11.809 It activates enzymes. It begins breaking food down into a 104 00:10:11.889 --> 00:10:21.379 semi-liquid mixture called chyme. You might wonder quite reasonably, why 105 00:10:21.399 --> 00:10:26.629 your stomach does not simply digest itself. And the answer 106 00:10:26.750 --> 00:10:32.629 is that it tries to. Every few days, the entire 107 00:10:32.710 --> 00:10:37.950 lining of your stomach renews itself. New cells replace the 108 00:10:38.029 --> 00:10:43.909 old ones before the acid can eat through. The stomach 109 00:10:44.220 --> 00:10:48.159 is in a constant quiet war with its own contents, 110 00:10:49.470 --> 00:10:54.379 and it wins by shedding and regrowing faster than it 111 00:10:54.419 --> 00:11:00.200 can be damaged. It is, in the most literal sense, 112 00:11:01.419 --> 00:11:08.549 continuously remaking itself. Your stomach also does something beyond chemistry. 113 00:11:10.309 --> 00:11:15.450 It churns. The walls of your stomach contain three layers 114 00:11:15.529 --> 00:11:21.470 of muscle arranged in different directions, and they squeeze and 115 00:11:21.529 --> 00:11:29.259 mix and roll the food from all angles simultaneously. This 116 00:11:29.440 --> 00:11:36.080 mechanical mixing turns solid food into that soft, acidic, partially 117 00:11:36.139 --> 00:11:42.220 digested liquid, the chyme, which is then slowly released into 118 00:11:42.259 --> 00:11:48.070 the next part of the journey. The stomach usually empties itself, 119 00:11:49.460 --> 00:11:53.799 Over the course of two to four hours, depending on 120 00:11:53.899 --> 00:12:00.519 what you ate, a light meal passes through quickly. A heavy, 121 00:12:01.480 --> 00:12:08.710 fatty meal stays longer. The stomach is deliberate. It does 122 00:12:08.769 --> 00:12:14.389 not rush. The exit from the stomach leads into the 123 00:12:14.429 --> 00:12:21.809 small intestine. And here the word small is almost comically misleading. 124 00:12:23.669 --> 00:12:28.450 The small intestine is called small only because it is 125 00:12:28.529 --> 00:12:34.220 narrow in diameter, not because it is short. In length, 126 00:12:35.220 --> 00:12:39.059 the small intestine is the longest part of your digestive system. 127 00:12:41.000 --> 00:12:43.299 It is somewhere between 6 and 7 meters long. 6 128 00:12:43.299 --> 00:12:51.519 to 7 meters. folded back and forth, coiled upon itself, 129 00:12:52.669 --> 00:12:59.049 packed into your abdomen, in loops and curves. If you 130 00:12:59.110 --> 00:13:02.909 unraveled it, it would stretch the length of a small room. 131 00:13:05.289 --> 00:13:09.200 The small intestine is where the real work of digestion happens. 132 00:13:11.220 --> 00:13:15.159 By the time food leaves the stomach, it is partially 133 00:13:15.220 --> 00:13:20.269 broken down, But it is in the small intestine that 134 00:13:20.289 --> 00:13:26.940 the useful parts are actually separated out and absorbed into 135 00:13:26.980 --> 00:13:37.519 your body. Nutrients, vitamins, minerals, glucose, amino acids, fatty acids. 136 00:13:39.360 --> 00:13:42.049 All of them pulled through the wall of the small 137 00:13:42.090 --> 00:13:48.559 intestine and into the bloodstream. What enters the small intestine 138 00:13:48.860 --> 00:13:55.350 is food. What leaves it going into the blood is you. 139 00:13:58.250 --> 00:14:03.190 To absorb nutrients as efficiently as possible, the small intestine 140 00:14:03.269 --> 00:14:06.649 has a surface area that would stagger you if you 141 00:14:06.710 --> 00:14:12.230 saw it. The inside wall of the small intestine is 142 00:14:12.289 --> 00:14:19.250 not smooth. It is covered in tiny finger-like projections, called villi. 143 00:14:20.870 --> 00:14:26.750 And each villus is covered in even tinier projections, called microvilli. 144 00:14:28.649 --> 00:14:33.980 So many layers of surface. So many folds within folds. 145 00:14:35.639 --> 00:14:39.200 When you account for all of them, the total surface 146 00:14:39.299 --> 00:14:43.529 area of the inside of your small intestine is roughly 147 00:14:43.580 --> 00:14:50.299 the size of a tennis court. A tennis court, folded 148 00:14:50.600 --> 00:14:58.379 inside your abdomen. These villi are not decorative. Each one 149 00:14:58.620 --> 00:15:02.690 contains a network of tiny blood vessels, and a special 150 00:15:02.750 --> 00:15:07.850 channel for carrying fat. When nutrients pass through the wall 151 00:15:07.889 --> 00:15:12.580 of the villus, they step directly into this network, and 152 00:15:12.639 --> 00:15:16.080 are carried away in the blood, to wherever the body 153 00:15:16.179 --> 00:15:21.590 needs them most. To your liver first, where they are 154 00:15:21.629 --> 00:15:27.990 processed and sorted, then out through the body, to your muscles, 155 00:15:29.029 --> 00:15:35.019 your organs, your brain. The meal you ate hours ago 156 00:15:36.019 --> 00:15:41.799 is already becoming part of your thoughts, your movements, your heartbeat, 157 00:15:44.279 --> 00:15:49.100 The small intestine also receives help from two other organs 158 00:15:50.299 --> 00:15:57.659 that live just nearby. The pancreas and the liver. The 159 00:15:57.700 --> 00:16:03.299 pancreas is a long, soft organ tucked behind the stomach. 160 00:16:04.980 --> 00:16:11.090 It produces a remarkable fluid containing digestive enzymes that flow 161 00:16:11.139 --> 00:16:17.289 into the small intestine. through a small duct. These enzymes 162 00:16:17.389 --> 00:16:24.500 break down proteins, fats, and carbohydrates that the stomach's acid 163 00:16:24.580 --> 00:16:30.980 could not fully dissolve. The pancreas also neutralizes the acid 164 00:16:31.820 --> 00:16:35.889 that arrives from the stomach, making the environment of the 165 00:16:35.950 --> 00:16:43.009 small intestine far gentler, safer for the delicate wall's doing 166 00:16:43.049 --> 00:16:49.340 the absorbing. The liver is the largest internal organ in 167 00:16:49.379 --> 00:16:55.570 your body. It sits beneath your right ribcage, weighing about 168 00:16:55.710 --> 00:17:01.629 one and a half kilograms, dense and dark red. The 169 00:17:01.669 --> 00:17:10.700 liver performs over 500 different functions. 500. Among them, it 170 00:17:10.759 --> 00:17:17.369 produces bile, Bile is a greenish-yellow fluid that is stored 171 00:17:17.470 --> 00:17:21.809 in a small pouch beneath the liver called the gallbladder. 172 00:17:23.730 --> 00:17:28.970 And when fat enters the small intestine, the gallbladder squeezes, 173 00:17:30.039 --> 00:17:34.519 releasing bile into the intestine to help break the fat 174 00:17:34.539 --> 00:17:40.900 apart into tiny droplets that enzymes can then digest and 175 00:17:43.019 --> 00:17:48.369 Bile is why your stools have color. Without it, they 176 00:17:48.410 --> 00:17:54.809 would be pale and gray. Even something so ordinary has 177 00:17:54.869 --> 00:18:00.210 a story behind it. After the small intestine has done 178 00:18:00.289 --> 00:18:07.460 its work, what remains continues onward. And what remains at 179 00:18:07.519 --> 00:18:13.789 this point is not much that your body can use. water, 180 00:18:14.970 --> 00:18:20.049 fiber that cannot be digested, some dead cells shed from 181 00:18:20.089 --> 00:18:27.130 the intestinal lining, and a vast living community of microorganisms. 182 00:18:29.430 --> 00:18:34.799 All of this passes into the large intestine, which is, 183 00:18:35.819 --> 00:18:41.640 despite its name, shorter than the small intestine. The large 184 00:18:41.700 --> 00:18:44.980 intestine is only about one and a half meters long. 185 00:18:46.829 --> 00:18:53.230 But it is wider, and its job is different. The 186 00:18:53.289 --> 00:19:00.170 large intestine's primary task is to absorb water. The chyme 187 00:19:00.210 --> 00:19:04.859 that arrives from the small intestine is still quite liquid. 188 00:19:06.539 --> 00:19:11.099 As it moves slowly through the large intestine, water is 189 00:19:11.200 --> 00:19:17.380 pulled back into the bloodstream. What was liquid becomes softer, 190 00:19:18.279 --> 00:19:23.539 then firmer, then firmer still, as more and more water 191 00:19:23.599 --> 00:19:29.450 is reclaimed. The large intestine is efficient in this way. 192 00:19:31.150 --> 00:19:36.710 It recycles, it takes back what the body gave, and 193 00:19:36.769 --> 00:19:42.660 returns it to circulation. the large intestine is also home 194 00:19:43.460 --> 00:19:49.950 to a remarkable population of organisms. Roughly 100 trillion bacteria. 195 00:19:51.039 --> 00:19:58.690 live in your gut. 100 trillion. That number is so large, 196 00:19:59.690 --> 00:20:04.869 it is almost meaningless when spoken aloud. But imagine this. 197 00:20:06.130 --> 00:20:12.910 Your body contains roughly 37 trillion human cells. And the 198 00:20:13.009 --> 00:20:18.849 number of bacteria in your gut alone outnumbers those cells 199 00:20:19.789 --> 00:20:25.269 by nearly three to one. You are, in a very 200 00:20:25.369 --> 00:20:34.279 real numerical sense, more microorganism than human. These bacteria are 201 00:20:34.319 --> 00:20:43.420 not invaders. They are residents. Partners. They have evolved alongside 202 00:20:43.500 --> 00:20:48.420 humans for millions of years, and both sides of that 203 00:20:48.480 --> 00:20:54.430 relationship have come to depend on the other. The bacteria 204 00:20:54.509 --> 00:21:01.109 help digest things. Your own enzymes cannot break down. Certain 205 00:21:01.170 --> 00:21:06.630 types of fiber, for example, pass through the small intestine 206 00:21:06.799 --> 00:21:13.250 untouched and arrive in the large intestine intact. And there, 207 00:21:14.230 --> 00:21:19.990 the bacteria feast on them, fermenting them, and producing short-chain 208 00:21:20.049 --> 00:21:24.309 fatty acids that feed the cells of the colon wall. 209 00:21:26.529 --> 00:21:32.250 They are, in a sense, farming themselves a meal out 210 00:21:32.289 --> 00:21:38.890 of your leftovers. The bacteria in your gut also produce vitamins. 211 00:21:40.670 --> 00:21:46.119 Vitamin K, which is essential for blood clotting, is partly 212 00:21:46.200 --> 00:21:52.019 synthesized by gut bacteria. So is a type of vitamin B, 213 00:21:52.880 --> 00:22:00.849 called biotin. Your gut microbes are, among other things, a 214 00:22:00.930 --> 00:22:08.940 tiny internal pharmacy, quietly producing things your body needs. Without them, 215 00:22:09.900 --> 00:22:14.799 you would be less than you are. With them, you 216 00:22:14.880 --> 00:22:21.480 are more. The collection of all these microorganisms is called 217 00:22:21.880 --> 00:22:28.779 the gut microbiome. And in recent decades, scientists have begun 218 00:22:28.839 --> 00:22:35.910 to understand just how influential it is. Your gut microbiome 219 00:22:36.309 --> 00:22:41.549 affects your immune system. It influences how your body responds 220 00:22:41.630 --> 00:22:47.450 to inflammation. It may affect your mood. It may even 221 00:22:47.529 --> 00:22:53.220 play a role in how clearly you think. The gut 222 00:22:53.559 --> 00:22:58.259 and the brain are connected by a long pathway called 223 00:22:58.279 --> 00:23:02.900 the vagus nerve and signals flow in both directions along it. 224 00:23:05.119 --> 00:23:10.690 What happens in your gut is never entirely separate from 225 00:23:10.730 --> 00:23:16.140 what happens in your mind. This connection between the gut 226 00:23:16.519 --> 00:23:22.559 and the brain is deep and ancient. Your gut contains 227 00:23:22.640 --> 00:23:27.720 its own nervous system. It is called the enteric nervous system, 228 00:23:28.839 --> 00:23:35.690 and it contains roughly 100 million nerve cells. 100 million. 229 00:23:37.369 --> 00:23:42.490 More than the spinal cord. The enteric nervous system can 230 00:23:42.589 --> 00:23:48.980 operate independently, without any instruction from the brain. It is 231 00:23:49.019 --> 00:23:55.069 sometimes called the second brain. And it is always listening, 232 00:23:56.569 --> 00:24:03.730 always adjusting, always reading the environment inside you, and responding. 233 00:24:05.849 --> 00:24:09.299 Have you ever felt butterflies in your stomach when you 234 00:24:09.319 --> 00:24:16.690 were nervous? That is the gut-brain connection in action. Stress 235 00:24:16.730 --> 00:24:21.779 signals from the brain travel down the vagus nerve and 236 00:24:21.859 --> 00:24:27.079 cause the gut to change its behavior. Blood flow shifts 237 00:24:27.180 --> 00:24:35.029 away from digestion. Muscle contractions change. The gut, in a 238 00:24:35.089 --> 00:24:40.430 state of anxiety, behaves differently than the gut at rest. 239 00:24:42.670 --> 00:24:47.119 And signals travel the other way too. The state of 240 00:24:47.160 --> 00:24:55.789 your gut influences your emotional state. A healthy, thriving microbiome 241 00:24:56.930 --> 00:25:02.490 has been associated with better mood, lower anxiety, and calmer 242 00:25:02.529 --> 00:25:09.150 responses to stress. Your gut is, in a very real way, 243 00:25:10.390 --> 00:25:18.470 feeling things. About 95% of the body's serotonin is produced 244 00:25:18.869 --> 00:25:24.150 in the gut. Not in the brain. In the gut. 245 00:25:26.089 --> 00:25:31.910 Serotonin is the neurotransmitter most associated with feelings of well-being 246 00:25:32.190 --> 00:25:37.279 and calm. The fact that the majority of it is 247 00:25:37.339 --> 00:25:43.349 made in your digestive system says something quietly profound. about 248 00:25:43.390 --> 00:25:47.809 the relationship between how you eat, how you live, and 249 00:25:47.849 --> 00:25:53.279 how you feel. Your gut is not just digesting your food. 250 00:25:55.000 --> 00:26:00.660 It is, in some measure, shaping your experience of being alive. 251 00:26:03.619 --> 00:26:09.019 Let us return to the journey. The large intestine moves 252 00:26:09.119 --> 00:26:16.519 material slowly, Slowly is the right word. The small intestine 253 00:26:16.559 --> 00:26:21.759 moves food relatively quickly, but the large intestine takes its time. 254 00:26:23.480 --> 00:26:27.579 It can take anywhere from 10 hours to 50 hours 255 00:26:28.859 --> 00:26:31.779 for material to travel the length of the large intestine. 256 00:26:31.779 --> 00:26:39.789 50 hours. The food you ate yesterday or perhaps the 257 00:26:39.829 --> 00:26:45.259 day before may still be completing its journey inside you 258 00:26:45.700 --> 00:26:53.299 right now. The large intestine is divided into sections. It 259 00:26:53.339 --> 00:26:58.140 begins with the cacum, a small pouch where the small 260 00:26:58.180 --> 00:27:07.069 intestine connects. Attached to the cacum is the appendix. For 261 00:27:07.109 --> 00:27:11.950 a long time, the appendix was dismissed as a vestigial organ, 262 00:27:13.170 --> 00:27:17.609 a leftover from an earlier stage of evolution with no 263 00:27:17.690 --> 00:27:24.400 meaningful function. But more recent thinking suggests it may play 264 00:27:24.460 --> 00:27:29.039 a role as a kind of reservoir for beneficial bacteria, 265 00:27:30.740 --> 00:27:37.180 a backup supply. If the gut is severely disrupted, perhaps 266 00:27:37.259 --> 00:27:42.309 by illness or infection, the appendix may help repopulate it 267 00:27:43.190 --> 00:27:50.930 with the right microorganisms. An organ once thought useless quietly 268 00:27:50.990 --> 00:27:56.130 turned out to have a purpose after all. From the caecum, 269 00:27:57.069 --> 00:28:00.259 the large intestine rises on the right side of your body, 270 00:28:01.430 --> 00:28:07.700 crosses over to the left, and descends again. This path 271 00:28:08.200 --> 00:28:13.599 is called the ascending colon, the transverse colon, and the 272 00:28:13.619 --> 00:28:20.289 descending colon. It ends in the sigmoid colon, a gentle-shaped 273 00:28:20.390 --> 00:28:26.549 curve that leads to the rectum. The rectum holds waste 274 00:28:27.539 --> 00:28:32.759 until the body is ready. And then, in the most 275 00:28:32.900 --> 00:28:39.710 uncelebrated step of the entire digestive process, The waste exits 276 00:28:39.789 --> 00:28:46.000 the body. The whole journey from first bite to final 277 00:28:46.160 --> 00:28:52.799 exit takes somewhere between 24 and 72 hours on average. 278 00:28:54.740 --> 00:28:58.950 Your body has been quietly running this process every day 279 00:28:59.869 --> 00:29:06.630 of your entire life. But let us go back because 280 00:29:06.690 --> 00:29:11.269 there is so much more To say about the small intestine. 281 00:29:13.210 --> 00:29:15.680 About the way it coordinates with the rest of the body. 282 00:29:17.420 --> 00:29:23.380 About the hormones it releases. About the way it knows, somehow, 283 00:29:24.619 --> 00:29:27.789 what it is receiving and what to do about it. 284 00:29:29.970 --> 00:29:37.089 Your small intestine produces hormones. This surprises many people. who 285 00:29:37.130 --> 00:29:42.480 think of hormones as something produced only by glands. But 286 00:29:42.519 --> 00:29:46.900 the digestive system is one of the largest hormone-producing systems 287 00:29:47.259 --> 00:29:53.880 in the body. When food enters the small intestine, specialized 288 00:29:53.920 --> 00:29:57.930 cells in the lining detect what kind of food it is. 289 00:29:59.759 --> 00:30:05.920 Is it fatty? Sugary? Full of protein? Based on what 290 00:30:06.000 --> 00:30:12.789 it detects, the small intestine releases different hormones to orchestrate 291 00:30:12.849 --> 00:30:20.710 the response. Cholecystokinin tells the gallbladder to release bile. Secretin 292 00:30:20.930 --> 00:30:26.599 tells the pancreas to send more digestive enzymes. And other 293 00:30:26.660 --> 00:30:30.759 hormones travel all the way to the brain To say, 294 00:30:31.769 --> 00:30:37.829 in the quietest way, we have had enough. The sensation 295 00:30:38.029 --> 00:30:43.789 of fullness is partly generated by the gut, not the brain. 296 00:30:45.490 --> 00:30:50.980 It takes about 20 minutes, after you begin eating, for 297 00:30:51.000 --> 00:30:56.299 the gut hormones to reach the brain and signal satiety. 298 00:30:56.299 --> 00:31:04.480 20 MINUTES which is why eating slowly so often leads 299 00:31:04.859 --> 00:31:11.799 to eating less. The gut is communicating. It just needs 300 00:31:11.920 --> 00:31:17.950 time for the message to arrive. There is also a 301 00:31:17.990 --> 00:31:23.670 hormone called ghrelin. Ghrelin is produced primarily in the stomach, 302 00:31:24.730 --> 00:31:27.690 when it is empty, and is one of the main 303 00:31:27.750 --> 00:31:34.859 drivers of hunger. It rises before meals and falls after eating. 304 00:31:36.559 --> 00:31:40.700 It signals to the brain, it is time to eat. 305 00:31:42.579 --> 00:31:47.529 Ghrelin also increases when you are sleep deprived. This is 306 00:31:47.609 --> 00:31:52.230 part of why, when you have not slept enough, you 307 00:31:52.269 --> 00:31:58.910 feel hungrier than usual. Your gut and your sleep are 308 00:31:58.970 --> 00:32:05.319 in conversation. Even right now, as you drift toward rest, 309 00:32:06.519 --> 00:32:13.819 your digestive system is adjusting, slowing in some ways, continuing 310 00:32:13.859 --> 00:32:20.609 in others, never truly still. Let us think about the 311 00:32:20.670 --> 00:32:25.970 stomach for a moment more. The stomach produces roughly 2 312 00:32:25.970 --> 00:32:31.710 litres of gastric acid every day. 2 litres of acid 313 00:32:32.640 --> 00:32:39.529 carefully managed, carefully contained. The lining of the stomach is 314 00:32:39.609 --> 00:32:43.630 protected from this acid by a thick layer of mucus 315 00:32:44.950 --> 00:32:50.369 about half a millimeter thick. That half millimeter of mucus 316 00:32:51.369 --> 00:32:55.009 is the only thing standing between the acid and the 317 00:32:55.069 --> 00:33:02.700 tissue beneath. And it is renewed constantly. Stomach cells that 318 00:33:02.740 --> 00:33:07.569 produce mucus are among the fastest dividing cells in the body. 319 00:33:09.470 --> 00:33:15.569 They have to be. The acid would win otherwise. When 320 00:33:15.589 --> 00:33:19.970 that mucus layer is compromised, perhaps by a certain type 321 00:33:20.009 --> 00:33:26.859 of bacteria, called Helicobacter pylori, or by long-term use of 322 00:33:26.960 --> 00:33:33.930 certain pain medications, the result is an ulcer. a painful 323 00:33:33.970 --> 00:33:38.490 erosion of the stomach lining, where the acid reaches tissue 324 00:33:39.450 --> 00:33:45.740 it was never meant to touch. For decades, doctors believed 325 00:33:45.839 --> 00:33:51.859 stomach ulcers were caused by stress and diet. Then, in 326 00:33:51.880 --> 00:33:58.650 the 1980s, an Australian researcher named Barry Marshall proposed that 327 00:33:58.809 --> 00:34:05.450 many ulcers were actually caused by a bacterial infection Nobody 328 00:34:05.490 --> 00:34:10.630 believed him. So he drank a solution of the bacteria himself, 329 00:34:11.789 --> 00:34:18.869 to prove the point. He developed gastritis within days, treated 330 00:34:18.909 --> 00:34:24.599 himself with antibiotics, and eventually shared the Nobel Prize in 331 00:34:24.659 --> 00:34:31.000 Medicine for the discovery. Sometimes the most important ideas in 332 00:34:31.079 --> 00:34:36.159 science begin with someone willing to be proven right. in 333 00:34:36.199 --> 00:34:42.309 the most uncomfortable way possible. The digestive system is also 334 00:34:42.389 --> 00:34:48.610 deeply involved in your immune response. About 70% of the 335 00:34:48.650 --> 00:34:56.559 body's immune system is located in the gut. 70%. This 336 00:34:56.639 --> 00:35:00.380 makes sense when you consider that the gut is one 337 00:35:00.400 --> 00:35:06.409 of the main entry points for the outside world. Everything 338 00:35:06.449 --> 00:35:11.340 you eat and drink arrives from an environment full of pathogens. 339 00:35:13.039 --> 00:35:16.239 The gut has to be able to tell the difference 340 00:35:16.300 --> 00:35:24.280 between food, helpful bacteria, and harmful invaders. And it does 341 00:35:24.360 --> 00:35:30.550 this with remarkable precision. Specialized immune cells line the gut wall, 342 00:35:31.710 --> 00:35:41.940 constantly sampling what passes through Learning. Remembering. Building a record 343 00:35:42.860 --> 00:35:48.820 of what belongs and what does not. Your gut is 344 00:35:48.900 --> 00:35:53.449 also responsible for the gradual education of your immune system 345 00:35:54.570 --> 00:35:58.630 from the very beginning of your life. When a baby 346 00:35:58.710 --> 00:36:06.590 is born, their gut is essentially sterile. In the first hours, days, 347 00:36:07.719 --> 00:36:12.300 and weeks of life, bacteria begin to colonize the gut. 348 00:36:14.260 --> 00:36:20.510 Which bacteria settle in first depends on many things. How 349 00:36:20.570 --> 00:36:26.170 the baby was born, whether they were breastfed, what environment 350 00:36:26.570 --> 00:36:33.119 they are in. And this early microbiome has consequences that 351 00:36:33.219 --> 00:36:38.239 ripple through decades of life. The foundation of the gut's 352 00:36:38.320 --> 00:36:44.210 immune education is laid in the first months. The bacteria 353 00:36:44.269 --> 00:36:50.429 you carry now are, in part, a legacy of those 354 00:36:50.530 --> 00:36:58.599 early days. There is something called the hygiene hypothesis. It 355 00:36:58.639 --> 00:37:03.219 suggests that children who grow up in environments that are 356 00:37:03.320 --> 00:37:09.269 too clean Too free of bacteria and minor infections may 357 00:37:09.309 --> 00:37:14.309 develop immune systems that have not had enough to practice on. 358 00:37:16.489 --> 00:37:21.679 Without early exposure to a wide variety of microorganisms, the 359 00:37:21.739 --> 00:37:27.199 immune system may become confused. It may begin to overreact 360 00:37:28.159 --> 00:37:35.340 to things that are not actually dangerous. Pollen certain foods, 361 00:37:36.789 --> 00:37:42.349 its own tissue. The rise in allergies and autoimmune conditions 362 00:37:43.369 --> 00:37:48.190 in the modern world may be, at least in part, 363 00:37:49.429 --> 00:37:53.570 a story about a gut microbiome that has not been 364 00:37:53.650 --> 00:38:01.210 given enough to do. The gut, in some ways, needs 365 00:38:01.269 --> 00:38:08.039 to be challenged to become wise Let us consider fiber 366 00:38:08.320 --> 00:38:13.300 for a moment. Dietary fiber is the part of plant 367 00:38:13.380 --> 00:38:19.670 food that your own digestive enzymes cannot break down. It 368 00:38:19.750 --> 00:38:26.010 passes through the small intestine without being absorbed. And when 369 00:38:26.070 --> 00:38:31.289 it reaches the large intestine, the bacteria there are ready 370 00:38:31.369 --> 00:38:37.119 for it. They ferment it. They use it as fuel. 371 00:38:38.829 --> 00:38:44.050 And in doing so, they produce compounds that protect the 372 00:38:44.110 --> 00:38:50.320 lining of the colon, reduce inflammation, and even influence genes 373 00:38:51.619 --> 00:38:57.219 in ways that are still being studied. Fiber also acts 374 00:38:57.300 --> 00:39:02.659 like a slow-moving broom. It adds bulk to the contents 375 00:39:02.739 --> 00:39:08.590 of the intestine, and helps move things along. Without enough fiber, 376 00:39:09.769 --> 00:39:16.239 the gut contents slow down. They linger. And the longer 377 00:39:16.280 --> 00:39:21.340 they linger, the more water is absorbed, and the more 378 00:39:21.420 --> 00:39:29.869 compacted things become. Constipation is at its simplest. What happens 379 00:39:30.269 --> 00:39:35.099 when the gut's contents stay too long. And it is 380 00:39:35.219 --> 00:39:44.280 largely preventable with enough fiber and enough water. Water, in fact, 381 00:39:45.519 --> 00:39:51.889 is essential to every part of digestion. Saliva is mostly water. 382 00:39:53.610 --> 00:39:58.630 Stomach acid is dissolved in water. The enzymes of the 383 00:39:58.670 --> 00:40:05.360 small intestine work in a watery medium. The large intestine 384 00:40:05.539 --> 00:40:14.969 absorbs water. Without adequate hydration, the entire system slows. Food 385 00:40:15.190 --> 00:40:25.519 moves less smoothly. Absorption becomes less efficient The microbiome is affected. Water. 386 00:40:26.420 --> 00:40:31.170 Plain and quiet. Is perhaps the single most important thing. 387 00:40:32.030 --> 00:40:37.210 You can give your digestive system. More than any supplement 388 00:40:37.289 --> 00:40:45.940 or special diet. Just water. Steady and consistent. Throughout the day. 389 00:40:48.400 --> 00:40:51.920 Now let us talk about the liver. a little more deeply. 390 00:40:53.800 --> 00:40:56.599 Because the liver is not simply a producer of bile. 391 00:40:58.260 --> 00:41:05.309 It is a metabolic command center of extraordinary complexity. After 392 00:41:05.329 --> 00:41:10.769 the small intestine absorbs nutrients and they enter the bloodstream, 393 00:41:11.949 --> 00:41:17.630 almost everything goes to the liver first. The liver inspects everything. 394 00:41:19.570 --> 00:41:24.329 It processes glucose and decides whether to use it immediately, 395 00:41:25.329 --> 00:41:30.900 store it as glycogen, or convert it to fat. It 396 00:41:30.980 --> 00:41:36.900 processes amino acids and builds proteins from them, including the 397 00:41:36.980 --> 00:41:41.380 proteins that carry substances through the blood and the proteins 398 00:41:41.500 --> 00:41:49.920 that help clot it. The liver also detoxifies. Drugs, Alcohol, 399 00:41:50.610 --> 00:41:56.489 toxins that are absorbed through the gut wall, must pass 400 00:41:56.710 --> 00:42:02.889 through the liver before entering the general circulation. The liver 401 00:42:02.949 --> 00:42:11.139