0:00 So, in this video, we're going to look at vitamin B12 absorption, and we begin 0:13 by drawing 0:13 here the stomach and the swollen testin. 0:16 Here is the duodenum of the swollen testin. 0:19 So, you know, here's the stomach and then the esophagus. 0:23 Now, here's the pancreas. 0:26 The swollen testin is divided into three parts - the duodenum, the jejunum, and 0:30 the ilium. 0:31 The ilium connects to the large intestine, but we won't really talk about the 0:35 large intestine 0:36 in this video. 0:38 Anyways, other important structures and cells are the salivary glands here. 0:45 And within the stomach are two important types of cells that we must talk about 0:50 . 0:50 These are the chief cells which produce your pepsinogen and your parietal cells 0:56 which 0:56 secrete hydrochloric acid. 0:59 Okay, so where do we get vitamin B12 from? 1:12 Well we can get it from a variety of foods such as egg, milk, fish, and red 1:19 meat. 1:20 So here is a red meat which contains vitamin B12. 1:25 Vitamin B12 is known formally as cobalamin. 1:31 So the red meat is eaten and travels to the stomach. 1:35 It has already been partially digested by the mouth. 1:43 The salivary glands secrete an important substance for vitamin B12 absorption 1:48 called haptocorin, 1:49 also known as our binder. 1:52 To make it a little bit more confusing, it is also known as transcobalamin 1. 1:58 Keep note of that, transcobalamin 1. 2:02 Anyway, so this also travels to the stomach. 2:05 In the stomach the chief cells secrete pepsinogen which gets converted to peps 2:09 in in the presence 2:10 of hydrochloric acid. 2:13 Hydrochloric acid is secreted by the parietal cells of the stomach. 2:19 Hydrochloric acid, together with pepsin, help in the digestion of meat, 2:24 liberating the vitamin 2:25 B12. 2:27 The liberated vitamin B12 has high affinity for the R binder within the stomach 2:33 . 2:33 And so they bind, forming the R binder vitamin B12 complex. 2:39 And this complex then travels to the duodenum of the small intestine. 2:44 Aside from producing hydrochloric acid, the parietal cells also secrete an 2:49 important substance 2:50 for vitamin B12 absorption. 2:53 This molecule is intrinsic factor. 2:56 The intrinsic factor released in the stomach will travel to the small intestine 3:02 as well. 3:03 Now within the small intestine, the pancreas secrete its juices, the amylases, 3:08 the lipases, 3:09 as well as proteases which will break down protein. 3:14 proteases break down to the R binder which will liberate the vitamin B12 once 3:20 again. 3:20 In the small intestine, the liberated vitamin B12 has high affinity for the 3:26 intrinsic factor. 3:28 And so they form the vitamin B12 intrinsic factor complex. 3:33 This complex then will travel through the small intestine towards the end of 3:39 the small intestine, 3:41 the ileum. 3:43 Now within the ileum, there are receptors for intrinsic factor. 3:48 So here if we zoom into the section, we can see that there's ileal cells. 3:54 And on the basal side of these cells are the, of these columnar ileal cells are 4:02 , is the 4:03 portal vein which will return blood to the liver. 4:06 Anyways, it's on the apical surface of these cells where we find intrinsic 4:11 factor receptors. 4:12 So when the intrinsic factor carrying the vitamin B12 comes along, it will bind 4:18 onto 4:19 the intrinsic factor receptor. 4:21 And then this whole, this whole thing will get endostized. 4:24 So when the vitamin B12 is absorbed, it will be released into the plasma. 4:30 And in the plasma, the vitamin B12 will bind onto plasma proteins called transc 4:37 obalamin2. 4:39 20% of vitamin B12 binds to transcobalamin2 in the plasma. 4:49 However, interestingly, the majority of vitamin B12 is bound to transcobalamin1 4:55 , remember also 4:57 known as the R-binder. 5:01 Transcobalamin2 is responsible for delivering the vitamin B12 to body tissues. 5:08 Whereas transcobalamin1 also circulates and binds approximately, as I mentioned 5:14 , 80% of 5:15 the circulating vitamin B12, rendering it unavailable for cellular delivery by 5:22 the transcobalamin2. 5:25 So transcobalamin2 delivers the vitamin B12 to body tissues. 5:29 And that's the important one. 5:32 The vitamin B12 is important for DNA and RNA synthesis, red blood cell 5:37 production, as 5:39 well as lipid synthesis, which is important for maintaining healthy nerve cells 5:46 . 5:46 Because by lipids, I mean myelin sheath, which are on your nerve cells. 5:55 So if you have vitamin B12 deficiency, you can see signs and symptoms such as 6:00 anemia, 6:01 fatigue, numbness, tingling, cause of the unhealthy neurons, slower healing, 6:07 shortness 6:07 of breath, muscle weakness, unsteady movements, again because of the nerves, 6:14 and also an increase 6:15 in homocysteine levels. 6:18 The increased homocysteine levels is due to the chemical reaction that are 6:25 inhibited 6:25 when there is vitamin B12 deficiency. 6:28 And we really won't go into that now. 6:33 Okay, so what sort of problems can give you this vitamin B12 deficiency? 6:41 Well we can see, well we can see this, these signs and symptoms, if someone 6:46 doesn't eat 6:47 any food containing vitamin B12, so dietary problems. 6:53 They could probably also have an infection which will prevent them from 6:56 absorbing vitamin 6:57 B12, and there can also be an inflammatory reaction in the GIT, damaging your 7:04 stomach 7:05 or your intestine. 7:07 But a major cause of vitamin B12 deficiency is a condition known as pernicious 7:12 anemia. 7:13 Now pernicious anemia is an autoimmune disorder where antibodies attack your 7:17 parietal cells 7:18 in the stomach. 7:19 This results in vitamin B12 deficiency because no intrinsic factor is produced. 7:26 So let us look at pernicious anemia where the parietal cells are attacked. 7:33 So here we are zooming into the parietal cell. 7:35 We can also find tissue dendritic cells in this area. 7:40 So looking at it from a step-by-step process, the dendritic cells clear apopt 7:46 otic parietal 7:47 cells produced during normal turnover of gastric mucosa. 7:55 On the parietal cells you have hydrogen-potassium ATP transporters which are 8:02 responsible for 8:02 producing or secreting the hydrogen ions that is necessary for hydrochloric 8:09 acid production. 8:10 During the turnover, the dendritic cell somehow takes in the ATP transporter 8:16 and deems it 8:17 as foreign. 8:18 And so the dendritic cell will present this hydrogen-potassium ATPase antigen 8:25 to naive 8:25 CD4 T cells in the lymph node. 8:29 So here is the lymph node with a naive CD4 T cell. 8:33 When the naive CD4 T cell is stimulated to become a T cell, a T helper cell, it 8:38 will 8:39 stimulate B cells to produce antibodies against this antigen. 8:45 The CD4 T cell will also become hydrogen-potassium ATPase pump reactive and 8:52 basically cause an 8:53 immune response towards the parietal cells which contain these hydrogen-pot 8:58 assium ATPase 8:59 pumps. 9:01 So the immune system attacks these parietal cells. 9:04 Okay, in pernicious anemia you can have antibodies, autoantibodies, and immune 9:11 cells attacking 9:13 either parietal cells, the intrinsic factor themselves produced by the parietal 9:20 cells, 9:21 or the intrinsic factor receptors on the ileal cells, which is in the small 9:28 intestine. 9:29 We just looked at an example of when the immune system attacks the parietal 9:35 cells. 9:36 When antibodies, however, attack either of these things, the result will be a 9:40 decrease 9:40 in vitamin B12 absorption. 9:43 And so we see the signs and symptoms of vitamin B12 deficiency.
