The next accessory organ we’re going to talk about inthe GI tract is the liver. The liver, and that’s thisbig fatty guy right here. Now, the liver has to be one of the coolest organs in the body. It’s the only one thatyou can take a piece of, this small chunk right here, and transplant into arecipient that has cirrhosis or liver failure, andit can grow to become a much larger organ thatcan sustain the patient. How wild is that? And so to better understandthe liver, we’ll start by discussing the main functionsit’s responsible for. The first thing we need to think about is that it’s responsible for metabolism, metabolism, and that involvesthe breakdown of nutrients or catabolism, and I’ll justwrite cat right here, catabolism, as well as the buildupof larger macromolecules, and that’s through anabolism. So, catabolism and anabolismare achieved in the liver. The other function theliver is responsible for is storage of these nutrients. \When we’re not breaking themdown or building them up, we need to have a placefor them to hang out, and so, they’reresponsible for the storage of our carbohydrates, so I’lljust write carbs right here, and also mainly ourfats, so these two guys, and they’re stored inthree different ways. First, carbs or fats canbe stored as glycogen. Glycogen is just apolysaccharide, so a linkage of a whole bunch ofmonosaccharides or simple sugars. The other thing that we canstore our carbs or our fats as are lipoproteins, lipo meaning fatty and proteins meaning, well,protein, and then lastly, we can also store theseguys as triglycerides, triglycerides, which isone of the forms of fat that we ingest in our food. Now, these are two ofthe major macromolecules we use for energy. The other major macromoleculeclass that the liver sees are proteins, but these aren’treally stored in the liver. Instead, they’re processedinto molecules like albumin and then cast off into the bloodstream, where they’ll float aroundand carry out other functions, until they need to beretrieved by the liver to be broken down orused for other molecules. The other major process theliver is responsible for is called detoxification, detoxification, and this is one of thoseawesome scientific terms where the function is in the name. This is where we taketoxins and modify them so they can’t harm our body. This is achieved mainly bycytochrome P450 enzymes, cytochrome P450, and theseare kind of funky enzymes, because unlike the otherones you’ve probably heard about before, thesedon’t bind a specific single substrate or molecule.
If you remember an enzyme like phenylalanine hydroxylase,that’s something that will add a hydroxyl group to phenylalanine and only phenylalanine,but cytochrome P450’s will take a whole bunchof different substrates and react with them. That’s something that’s pretty unique to this class of enzymes. Now, because of thesecytochrome P450 enzymes, we have a problem whenwe take medications. In fact, we have adecrease in drug efficacy because of this detoxification process, and what we mean by drug efficacy is the amount of an effecta drug is able to have, and so, if the liver or these enzymes perceive medications that we ingest to be foreign or like toxins,they’re going to try and break them down so they don’tcause an effect to our body.
So, doctors knows this,and because of that, a specific dose must be given to account for the amount of a drug that’s going to bedetoxified by the liver. And finally, as we’ll talkabout in a separate video in better detail, the last thing that the liver is responsiblefor is bile production, bile production, and as you might recall from our discussion on small bowel, bile is needed for theabsorption of fats from our food. Okay, so now that weunderstand what are the main functions of the liver, how does it get all thenutrients that we ingest and absorb from our intestinaltract to be metabolized or stored or detoxified in the liver? Well, let’s take a look atthe blood supply to the liver. So, one of the things thatmakes the liver so unique is that it has twoseparate blood supplies. The first, that comes infrom the intestinal tract, is from the portal vein. The portal venous systemis another term for it, and the portal veinsupplies the liver with nutrient-rich blood, nutrient-rich. So, where do these nutrients come from? Well, food that’s absorbedin the intestinal tract will then go through thecirculation and end up in the portal vein, tobe delivered to the liver for metabolism, and so weconsider this blood to be nutrient-rich, but where doesthe liver get its oxygen from?
Well, that’s a separate blood source, and that’s actually through what’s called the proper hepatic artery,the proper hepatic artery, and as the name suggests,this is arterial blood that will be supplyingoxygen-rich red blood cells. So great, these are the two different sources of blood that go to the liver. Well, what about bloodthat leaves the liver? There’s one main vessel that carries blood out of the liver, and that’scalled the hepatic vein. The hepatic vein is the exactopposite as the two other types of vessels we just talked about. It’s nutrient- and oxygen-poor. Blood that leaves through the hepatic vein from the liver willcirculate back to the heart to receive oxygen, flowpast the intestines to receive nutrients, andthen return to the liver, either through the portal vein or through the proper hepatic artery.
The other output of the liver, that I guess I should mention right now, we’ll talk about in moredetail in a separate video, is bile, and bile mainly leaves the liver through what’s calledthe common hepatic duct, the common hepatic duct, common hepatic duct that will take bile.