in this video we're going to examine digestion in the small intestine so the kim from the stomach already contains partially digested carbohydrates and proteins however the fats remain largely undigested so the kim doesn't really look like anything that we've consumed it looks radically different from the food that was ingested a short while before so this material is going to spend about 3 to 6 hours in the small intestine again depending on the content the fat content it's closer to 6 hours if it's a lipidrich material uh it's closer to the three if it's more of a carbohydraterriich uh meal so the majority of the water is going to be absorbed as well as almost all of the nutrients in the food we ate so the stomach like the small intestine has absolutely no role in ingestion or defecation its major role is going to be in chemical digestion and the absorption of material and so one of the adaptations that we have with digestion that occurs in the small intestine is that we have very slow delivery of acidic and hypertonic hypertonic kime now keep in mind that the kim is very nutrientdense there's lots of solute there which could draw water out of the surrounding tissue and circulation and this would be very problematic in circulatory water balance if a huge amount of kim was deposited into the small intestine at once now there are adaptations that help to mitigate the acidity so that is mitigated by accessory glands so the pancreas for example releases a bicarbonater pancreatic juice that helps to neutralize the low pH kind additionally because there's a lot of fluid in the pancreatic juice that sort of eliminates the hypertonicity of the kind making it closer to being isotonic now the bile helps to uh emulsify the fat so it helps to facilitate along with pancreatic uh enzymes the breakdown of lipids via the lipaces but a lot of our focus in this video is going to be on the mechanical uh digestion that occurs the mixing that occurs that facilitates that digestion of the kime now earlier we had talked about paristtoalsis with the esophagus and with paristtoalsis there's sort sort of this rhythmic contraction and relaxation that occurs uh along the length of the esophagus and that propels the movement of material down the esophagus so you have this rhythmic contraction that occurs here while everything is relaxed and then you have contraction here and above it and below it you have relaxation so paristoalsis does happen in the small intestine but we're going to focus on segmentation first so segmentation is going to be the most common motion in the small intestine and it has an intrinsic rate set by pacemaker cells and what segmentation is is you're going to have regions and I'm going to switch colors here where you'll have contraction here you'll have contraction here and contraction here but in between you have relaxation and what that does is it causes this kim to be pushed into sort of 5050 adjacent regions that are relaxed and this allows for mixing of material now the rhythm itself is not really changed but the intensity of contraction can change and this is going to be due to parasympathetic activity that increases motility uh sympathetic so the fightor-flight response doesn't uh increase uh motility and we'll talk about some hormones uh that facilitate motility now there is a little bit of paristtoalsis and this is more for sort of uh what we like to call housekeeping where we sort of uh clear out the small intestines of material from the previous meal so this is going to be initiated by a rise of a hormone called motilin that is released late in the intestinal phase remember the intestinal phase was that the last part of um gastric digestion so this is occurring roughly about 90 to 120 minutes after the meal reaches the stomach so what happens in this case is paristalsis begins let's say here but only lasts for a short bit and then we progressively initiate paristtoalsis more distally each time and this takes about 2 hours for material to move from the dadnum to the ilium and it allows the removal of remnants of the meal bacteria and any debris into the large intestine so some textbooks will refer to this as the migrating motor complex um but basically it's this paristtoalsis that arises um from waves that start distal to the previous initiation so motility of the small intestine is going to be controlled by local and teric neurons so since the parasympathetic nervous system is the dominant um system for motility coneric or acetylcholine uh base sensory neurons are going to activate uh contraction so contraction is going to occur via circular muscles in the small intestine as well as using those longitudinal muscles and this helps to force the kim along the GI track now eventually the the default position is the ilioal sphincter is contracted so it's preventing the material moving into the large intestine but the ilioal sphincter is going to relax and admit kim into the large intestine under two circumstances the first one is the gastroilial reflex so this enhances the segmentation that occurs in the ilium that propels the movement of material into the large intestine and second the hormone gastrin which is released in the intestinal phase that's going to increase the motility of that ilium now the ilioal valve it's going to close when material is deposited into the large intestine and that backward pressure is going to cause the flap to close and this is going to prevent material from the large intestine going back into the small intestine