Human Physiology - Regulation of Pancreatic Juice and Bile Synthesis & Secretion

  we’re specifically going to be focusing on pancreatic juice and the bile, and what’s regulating its secretion. All right, so let’s go ahead and look at the synthesis and secretion of pancreatic juice in the bile. So what I wanted to do is just remind you when we went back to—oh, the third week of class and we talked about the accessory organs, right, of the digestive tract. And so accessory organs are the pancreas and the liver, for example, and that’s what we’re focusing on here. So remember, it’s the acinar cells that are going to synthesize and secrete into this duct. And as that fluid passes through the duct, it’s going to get modified, okay? And then it will get dumped into the GI tract. So let’s start with the acids in the duodenum. So we know, first off, why there’s an increase in the acidity in the duodenum. That’s because the chyme just moved from the stomach into the duodenum. So once that acid gets into the duodenum, that gets detected by the chemoreceptors. And once it gets detected by the chemoreceptors, they’re going to generate some short and long reflexes. Ultimately, we’re going to stimulate the endocrine cells of the duodenum and the jejunum, and that’s going to cause the synthesis and secretion of secretin. Remember, secretin is another one of your hormones. So secretin is secreted by the duodenum and the jejunum. So secretin then—since it's a hormone, it enters the blood, right? It goes into the plasma, and it’s going to target the pancreatic duct cells. And when it targets the pancreatic duct cells, then what that’s going to do is it’s going to cause the synthesis and secretion of bicarbonate, HC03. Bicarbonate, then, can move into the duodenum to neutralize the acids, and therefore, the acidity of the duodenum becomes less. All right, it’s less acidic. So when bicarbonate moves from the pancreatic duct into the duodenum, it has to pass through what structures? I’ll let you think about that one or go back and look that up in your notes. So again, we’re going to start up here in the duodenum. Once again, we know why there’s an increased acidity in the duodenum because the chyme just moved from the stomach into the duodenum. We also know why there are high protein digestion products in fat. Okay, again, because the chyme has moved from the stomach to the duodenum. So these are your stimuli; let’s follow the high acidity content first. So that’s going to stimulate the synthesis and secretion of secretin from the duodenum and the jejunum, which once again travels out into the blood, but is also going to target the liver. And when secretin targets the liver, that’s going to increase bile secretion, and then bile can enter the duodenum. So same question as before: what tubes and what structures does bile pass through to get from the liver to the duodenum? So ultimately, we end up with a high amount of bile in the duodenum that neutralizes the acids, helps emulsify fats, right? That’s the function of your bile. So now let’s look at the stimuli where you have a high protein digestion product. So in other words, you have a lot of amino acids, and you have fats. So that’s going to stimulate the synthesis and secretion of CCK from the duodenum and the jejunum. So CCK is this hormone, travels out into the blood, right, gets into the plasma. And it’s going to target the gallbladder, the sphincter of Oddi, as well as the acinar cells of the pancreas. So three targets for CCK. All right, when CCK targets the gallbladder it’s going to cause the gallbladder to contract, and we know the gallbladder stores bile. So bile then enters into the duodenum, and then we know the bile is going to neutralize the acids and emulsify the fats. CCK is also going to target the sphincter of Oddi, and that’s going to cause relaxation of the muscle. All right, so if you relax the muscle, then bile can easily enter into the duodenum. Now the other thing that can allow when that relaxes—and let’s go up here too, to CCK targeting the acinar cells of the pancreas— and that’s going to cause enzyme secretions, the synthesis, and secretion of your enzymes. And those enzymes then, remember, break down fat, they break down protein, they break down carbs, they break down nucleic acids. So those enzymes are going to get secreted into the duodenum, and the only time they can do that is when the sphincter of Oddi is relaxed. So not only is relaxing the sphincter of Oddi allowing bile to enter the duodenum, it’s also allowing the enzymes from the pancreas to enter into the duodenum so that we can break down fats, we can break down protein, we can break down carbs, and we can break down nucleic acids. What is potentiation? So I want you to think about potentiation in terms of secretin and CCK, and what it means—what it means to have both of these hormones being released at the same time.