Gastrointestinal system introduction
- [Voiceover] I think we have a pretty good idea of how the liver works. One of the things I kinda touched on when we talking about our hepatic lobule is that we have bile that's produced in the liver and flows out through the common hepatic duct. Now bile is composed of two things. One are just bile pigments. Bile pigments are just sort of things that make the color or are not necessarily important for the function of bile. And the other thing that we have are bile salts. Now this is the most important part. This is what actually helps us emulsify, remember that term, emulsify which just means to organize to eventually absorb our fat. So bile salts help us emulsify fat. They allow us to turn fat into micelles, which can then be absorbed in the ileum. So let's take a step back and get a better idea of how our biliary tree really works. Remember the liver synthesizes bile but it doesn't necessarily store it. The bile that's made here actually goes through what's called the common hepatic duct. So this is the common hepatic duct that comes out of the liver. So bile that leaves from the liver after it's produced has to go somewhere to be stored. Where is it stored? One of the first places that it's going to go is this duct right here. And this guy is called the cystic duct. This is the cystic duct. And this is the next place that we see bile flow after the common hepatic duct. The cystic duct will then lead our bile to be stored momentarily in an organ that sorta sits as a blind pouch. This guy is our gallbladder. The gallbladder has one and one purpose only. This is the dude that stores our bile. This is where our bile hangs out until it's time to be released. So when bile just comes into our gallbladder and is just sitting there, what causes it to suddenly decide to leave and go somewhere else? As I'll mention in detail in a separate video the hormone that causes bile to be released from the gallbladder is called sort of a mouthful cholecystokinin. Cholecystokinin. Which is often abbreviated just CCK. Now this term might be a little crazy but if you ever heard of the surgical procedure called a cholecystectomy. Cholecystectomy, that's just the removal of the gallbladder. So cholecystokinin will cause our gallbladder to contract. And when the gallbladder contracts, remember it's a blind pouch, so this is just going to squeeze all the bile that's just sitting here right back out to flow through the cystic duct and now go somewhere else. The next place you're gonna have bile flow through is going to be another duct altogether. Now this guy is called the common bile duct. The common bile duct. And this is sorta the last part of the biliary tree we're talking about here. The last part of the biliary tree that conducts our bile until it finally reaches the GI tract. And the part of the GI tract that bile is released into is the duodenum. The duodenum or the first part of the small intestine. In the duodenum the bile is finally going to get to see some fat and start doing its job by emulsifying or making the fat easier to absorb. Now the fat isn't absorbed in the duodenum. In fact the bile salts with our new emulsified fat from our food will travel along to the ileum. Now you remember the ileum's the last part of our small intestine. And this is where we're going to have our bile salts and our fats that were just emulsified absorbed. So this is where the absorption process occurs. The ileum is where absorption of our bile salts and our fats occur. The duodenum is where bile is secreted into the GI tract. So bile is secreted into the GI tract. Now we talked about what happens to fats once they're absorbed in the ileum when we talked about the small intestine. But what happens to our bile salts? Well the bile salts, after they've been absorbed in the ileum, are actually gonna circulate right on back to the liver to undergo this process all over again. So if we were to review all the different pit stops that our bile takes in this process. Number one is that bile is made in the liver. Then once it's made number two it's going to go through the common hepatic duct and then flow to the cystic duct to be stored in the gallbladder until we receive cholecystokinin as a signal to tell the gallbladder to contract and then from there bile is squeezed out of the gallbladder, re-enters the cystic duct. I guess you can say this is the fifth place it would go. Re-enters the cystic duct and goes into the common bile duct. The common bile duct is the last part of the biliary tree which will then release its contents into the duodenum. And so finally our bile has reached the GI tract. The bile salts will then do their job to emulsify the fats that we've eaten. And then we'll have our bile salts get re-absorbed in the ileum before it heads back to the liver where it gets reused. So we reuse some of the existing bile salts in the liver. And that's how our biliary tree works.