In this video, we're going to examine the gross and micro ananatomy of the small intestine. So, the small intestine is actually the major organ of both digestion as well as absorption that occurs throughout the GI tract. So, if we were to stretch out the small intestine, its length would be anywhere between 2 to 4 m. That's roughly the equivalent of about 20 ft. Now that's assuming that all the muscles are completely relaxed. Uh while a person is living, the muscles have tonis. So there's some slight contraction. So you wouldn't be able to extend it that full 20 ft. Now we're measuring the small intestine from the pyloric sphincter where the stomach is going to dump contents into it and then the ilioal valve where the contents of the small intestine go into the larger uh large intestine. So there are three major subdivisions in the small intestine. The dadum is the first subdivision. It's going to be the one where the bulk of the absorption is going to occur. It is retroparitinal. So it doesn't have a sarosa. Instead, it has an adventicia. The jugum is going to be attached along with the illium through the mezzent and they are going to be um paritinal or intraparitinal excuse me. So let's look at the dadum first. So the dadnum is going to curve around the head of the pancreas. It's going to be the shortest part of the small intestines. So if you were to measure it, it's about the length of about 12 fingers. So it's about 25 cm if we were using the metric system. uh it's going to have content from both the liver and the pancreas be deposited into it. So here we see the liver and there's the bile duct and then here we have the major duct system of the pancreas. They will meet at this region called the hpatop pancreatic ampula. So this region here is the hpetto pancreatic ampula and there's a sphincter here that controls the movement of both bile as well as pancreatic juice into the dadum. Now this sort of volcanoike structure here this is known as the major dadinal papula. So that is going to be the pathway through which both bile as well as pancreatic juices are going to be added to the kim that is in the dadum. So in the eugenum and ilium the eugenum is going to extend from the dadum to the illiam. So its length is about 2 1/2 m which is approximately the equivalent of about 8 ft. And then the illiam is going to join the large intestine. Um, and it's also flanked by the eugenum. And its length is about 3.6 mters or approximately 12 ft. When we look at the nervous system stimulation as well as the blood supply, again, we're not going to be heavily emphasizing this. The parasympathetic is going to be via the cranial nerve 10, the vagus nerve. uh and the sympathetic is going to be from the thoracic splangic nerves uh and that's going to serve the entire small intestine. With regards to blood flow, the majority of the blood is going to come from the superior mezentary artery. uh and the nutrientrich blood from the small intestines is going to be drained into the superior mezentary veins and then from there it goes into the hpatic portal vein where that nutrient-rich blood is going to be processed by the liver. Now probably the most important aspect to understand in the small intestine are the specific structural modifications that are present there that enhance the efficiency of digestion. So number one there's an increase in surface area and this is predominantly going to be in the proximal part of the small intestine. So the dadnum and the first part uh of the uh ginoum. So there are three major uh modifications. The first modification is that we have to understand that the small intestine is not a hollow tube. Okay? So we're not just propelling material down like we did in the esophagus. Right? In the esophagus, we had paristtoalsis and the material didn't really hang out long at a particular region. So the first structural modifications are called circular folds or plea circularis. So these are going to be permanent folds. uh they represent uh these little ridges that are about 1 cm deep and they're going to force the kim to spiral through the lumen and not just progress straight down. So again, if we're considering this as a straight tube, material would be moving down like this, sort of like the way it moved down the esophagus. But because of these sort of circular folds, the material actually moves like this down the esophagus. And so this dramatically is going to increase the contact that the digested material makes with these cells that are along these little finger-like projections, which we call villi. So villi are another structural modification. These finger-like projections are about 1 millm high. They're a component of the mucosa. And if we actually zoom into each of these finger-like projections, we see that it is densely packed with capillaries. There are lactal there. So, there's a lot of fluid exchange, which is going to be very important for fat absorption. Uh, and along these villi, you have these specialized absorptive cells. So their name says what they do. At the very sort of tips of the villi, you have these modifications called microvilli. So we're further increasing the surface area of the individual cells through the microvilli. And at the tips of the microvilli, they're sort of encrusted with a little bit of mucus, but you have embedded these enzymes called brush border enzymes. So these are the main enzymes that are going to be found in the intestines that are intestine specific, small intestine specific. Keep in mind that there are going to be other enzymes via the pancreatic secretions, the pancreatic juices. All right. Now at the base of many of the villi you have these indentations called intestinal crypts shown right over here. So the intestinal crypts there are three main classes of sort of cells that we need to be aware of. Uh the first of these cells are going to be the entendocrine cells. So these intereroindocrine cells are going to be the ones that are going to secrete the interogastone. So we mentioned uh gastrin with the positive feedback um earlier. That's not going to be one of the enzymes. We're primarily focusing here on secretin and colly uh collystokinine or cck that are going to sort of inhibit gastrin secretion. they have that inhibitory effect so that we're not dumping more material into the deadum that we can neutralize and process. You also have intraepithelial lymphosytes. So these are going to release uh special signaling molecules called cytoines that facilitate uh the killing of infected cells. Uh and then you have at the very base and it's kind of not shown on this particular image here you have a group of cells called penith cells and so these panith cells they secrete specialized antimicrobial peptides that target specific types of bacteria. So defensins um and lysosyme lysosyme we've already mentioned was a compound that was also found in your tears and in your saliva. So they specifically inhibit the growth of what we call gram positive bacteria. So you're sort of regulating potentially what bacteria are later capable of seeding the large intestine. So when we look at these intestinal crypts at the very base of them, we have these epithelial stem cells that divide pretty rapidly. And then there's a migration and differentiation that occurs as these cells progress up the villi. And by the time you reach the sort of the top of the villi, those cells are a little bit older and they shed. So they're they're sort of renewed every two to four days. So just on this image here, um I kind of failed to point out. So here are the microvilli. So the absorptive cell. So you see the progre uh the projection of the cell membrane of the individual cell to increase the surface area. We have mucus granules and along here at the very tips you have these uh brush border enzymes sort of embedded in the membranes of the microvilli. So what are some homeostatic imbalances that occur in the small intestine? Well, clearly with these mucosal absorptive cells, there's a high rate of uh replication. So any type of treatment that targets rapidly dividing cells has the potential of impacting the small intestine. So chemotherapy, for example, that targets cancer cells which are rapidly dividing is going to kill off these sort of rapidly dividing cells. So as a consequence, the GI tract epithelium is not going to be properly renewed and this can result in things like nausea, vomiting as well as diarrhea. All right. So if we dive a little bit deeper into the mucosa specifically in and around the lamina propria uh you can have these uh specialized structures uh that are key for immune responses. Obviously the foods we eat are not going to be sterile. Uh so we want to prepare our bodies for any potential pathogens. So we've already mentioned the mucosa associated lymphatic tissues the malts. uh you have another sort of grouping of uh lymph tissue called payers patches and they're going to sort of help protect the distal part of the small intestines against bacteria and sometimes these payers patches can protrude into the sub mucosa. If we look at the belymphosytes, uh the belymphosytes can actually leave the intestine, enter the blood and they will secrete IgA antibodies. So these are specialized antibodies that help to target various pathogens uh potentially that might have breached the area. And then lastly and I kind of we'll jump back to this particular figure here. You see the sub mucosa here. There's the dadinal gland. So that dadinal gland is otherwise known as the Bruner's gland and its job is to secrete an alkaline mucus which is predominantly going to be water but there is a little bit of bicarbonate ion and uh mucus to help facilitate the motion of material throughout the GI tract or specifically through the small intestine. So the last thing that I want to touch on is the intestinal juice. So this is going to be primarily via these bruner glands. So about 1 to two lers are secreted daily and the trigger is going to be the distension of the small intestine as well as irritation through the kim the acidic kim uh coming in from the stomach as it relates to the mucosa. So it's going to be isotonic, mostly made up of water. We're not secretreting enzymes in the intestinal juices. The bulk of the enzymes are going to be via at least the bulk of the intestine intestinal enzymes are from those brush border uh cells.