After the stomach, the food, or as we should call it now, the chyme, will enter the small intestine through the pyloric sphincter. The small intestine is a long tube in which digestion is going to occur, and also, this is very important, absorption. This is where you get to finally get the payoff for this and absorb those nutrients. And, the small intestine is actually going to be assisted by the pancreas and the liver, who are going to provide some nice secretions that will help with the digestive process. Now, small intestines often lead to some misconceptions because it's actually longer than the large intestine; it just has a smaller diameter. And in fact, it's really long, it's like 6 meters long, which, you know, if you think that a meter is, like, approximately, like, a little more than 3 feet, you're talking about roughly 20 feet here. Obviously, humans aren't 20 feet long, and that's because our small intestine is all wrapped up to minimize the amount of space they take up, but because they're super long, they have a lot of surface area. And this surface area is actually even longer, due to some cool modifications that the small intestine has. Now first, if we look at the small intestine here, you can see that the tissue on the inside of the intestine has these folds in it. Right? That's what these little lines are. Basically, the tissue itself is made to fold to increase the amount of surface area on the inside of the small intestine. Now this fold, if you zoom in, is actually covered in what are called villi. So basically, this fold has all these little projections on it, these villi, and these villi are in turn covered in what are called microvilli. So the villi, the cells of the villi rather, the enterocytes have these little hair-like structures on their tips called microvilli. This is going to result in a crazy amount of surface area. Right? We have the tissue itself folded, those folds are covered in villi, and those villi are covered in microvilli. So we are maximizing surface area here. Now villi actually surround blood vessels, as you can see in this image here, and they also have what's called a lacteal in them. And, this structure here, it's a lacteal, and basically, it's a lymphatic vessel. The lymphatic system is going to be a system involved in both circulation and immune functions, and we'll talk about it at a different time. Just know that it has projections into these villi, as do blood vessels. Now the duodenum is what we call the start of the small intestine. That's where the chyme is going to enter. And the duodenum secretes hormones in response to the chyme. Two of these hormones are secretin, which is a hormone that's going to stimulate bicarbonate release from the pancreas, and that's super important because remember, the chyme is filled with acids. And you don't want all that acid sitting in your poor small intestines, so they're going to ask for some bicarbonate from the pancreas to help neutralize the acid. The other hormone is cholecystokinin (CCK). That's what they call it in med school and stuff; that's what everyone calls it; it's a lot easier. So CCK is going to be the hormone that stimulates the pancreas to secrete digestive enzymes. See, not only does the pancreas provide a nice bicarbonate solution for the chyme to cool off, so to speak, it's also going to secrete a bunch of enzymes that digest the food coming from the stomach. Now these include things like nucleases, which break down nucleic acids, pancreatic amylase, which, similar to salivary amylase, is going to break down carbohydrates into maltose and dextrins, as well as pancreatic lipases that break triacylglycerides into two fatty acids and a monoacylglyceride. Now, the pancreas will also secrete proteases. And two of the important ones to take note of are trypsin and chymotrypsin. And these are proteases that break down proteins into smaller polypeptides. It should be noted that they actually will only break specific polypeptide bonds. That is peptide bonds between specific types of amino acids. So just like other enzymes, they are very specific as to what they will act on. What's cool about these is they're actually also zymogens, like pepsinogens, so they're released as trypsinogen and chymotrypsinogen. And this enzyme called enterokinase, which is produced by the small intestine, is what will activate trypsinogen, and that leads to the activation of other proteases, which will help break down those polypeptides. Now, with that, let's flip the page.
39. Digestive System
Digestion
Video duration:
6mRelated Videos
Related Practice
