Digestive System 4, mechanical digestion

so we know that the process of digestion is breaking down large complex food molecules into much simpler molecules which can then be absorbed into the lymphatic and into the blood system of the body to be transported to organs and tissues where the nutrients are required to facilitate physiological processes and digestion can be broken down into two components really a lot of people when we talk about digestion the immediately think of chemical digestion this is the action of the enzymes chemically breaking down large molecules into small molecules by facilitating these catabolic breaking down reactions but there's also another very important aspect of digestion that can be overlooked and that is mechanical digestion the process of mechanical digestion the physical breaking down so this is the chemical breaking down this is the physical breaking down and as we'll see both are essential for the digestive process now here we have the model of the digestive system that were used to the diagram of the digestive system that were used to and I think we'll just draw a little bit more on we'll start with the mouth up on top so this goes up to the pharynx at the back there then the mouth will come off at the mouth will be here but the mouth then this of course will go on forming the the nasal cavities so this will really these will be the nasal cavities here this will be the oral cavity here so the lips will be at the front of the oral cavity just here and the tongue will be on the floor of the overall cavity just there like that so we'll start off thinking about what's going on in the oral cavity first of all and the first thing we can think about is water and mucus softening food so water and mucus basically the the parasympathetic branch of the autonomic nervous system is going to cause the production of saliva in the mouth this is a parasympathetic autonomic activity now the converse of this is sympathetic activity and very often sympathetic activity does the opposite of parasympathetic activity so if parasympathetic activity is going to moisten the mouth by causing the production of saliva from the saliva glands then sympathetic activity is going to do the opposite of that and I think we can all relate to this if you're anxious or afraid you get a dry mouth because the sympathetic activity is not going to cause the production of saliva and as you get increase in sympathetic activity you get a corresponding reduction in parasympathetic activity so you get a dry mouth when you're anxious but physiologically the parasympathetic nervous system is stimulating the production of saliva and survivor is actually comes from the saliva glands and it's stimulated by the seventh cranial nerve which is the facial nerve and the ninth cranial nerve which is the glossopharyngeal nerve these components of the autonomic nervous system and physiologically saliva is going to be produced on an ongoing basis and we actually produce about a liter to 1.5 liters of saliva per day and that keeps the mouth moist but it also runs down the esophagus and keeps the esophagus moist as well so the esophagus is also moistened by saliva important that the esophagus is kept moist and well lubricated so that food can pass down its and travel down it easily as the peristaltic waves of contraption carry food down the esophagus it's also worth mentioning that if someone is systemically dehydrated especially if it's as bad as hypovolemic if someone's dehydrated or hypovolemic then the if saliva will go down so it's always worth inspecting the mouth and if the mouth is dry that's the possible sign of dehydration and if the mouth is dry very dry that's a possible sign of hypovolemia so they're quite a useful clinical indicator looking at the level of moisture in the mouth and if you're wondering what the normal levels are the best thing to do of course is just compare it to your own and see how moist the patient's mouth is compared to your own mouth so we have this water and mucus the mucus softening the food and then we have this process of mastication going on in the mouth mastication mastication just means chewing I'm not quite sure why we don't just say chewing but is it it's mastication I suppose there's a bit more to it than that because you've got the teeth coming together crushing food tearing food you've got the tongue regulating the activity of the food putting the food just between the teeth so it can be appropriately chewed up you've got the muscles in the cheek again putting the food in just the right position so it's actually an awful lot of complex neurological control going on in this process of mastication and most of the time we get it right and of course it's absolutely infuriating and paper when you get it wrong because that's when you bite your tongue when it's not filiz could be very painful when the tongue gets in between the teeth but the fact that that happens very rarely indicates just how good that the level of neurological control is of this process of mastication and what will happen here is the food will be formed into a bolus of food like that ready to be swallowed a nice swallowing sized ball of food mixed up with saliva so this is going to contain the food it's going to be moistened by the water moistened and lubricated by the mucus from the saliva ready to be swallowed and the process for swallowing for some reason is called deglutition so a deglutition or deglutition is the process of swallowing this bolus of food so it can then pass down the pass down the esophagus towards the stomach now the next anatomical area of interest when we're talking about this process of mechanical digestion is the stomach itself there's a lot of mechanical digestion going on in the stomach because what we have in the stomach are these these mixing waves the stomach has mixing waves and the mixing waves are going to churn the food up in the stomach and that's going to form chime so chime is the next stage after the bolus goes into the stomach becomes chime because in the stomach what you get is rippling peristaltic movements every 15 to 25 seconds so there's these rippling movements in the wall of the stomach as the muscular wall that the smooth muscular wall of the stomach contracts and that's going to macerate and mix the food masa red means it makes it wet so it's going to be well mixed up the saliva there as well but it's also going to be mixed up with gastric secretions in addition mixing the food with gastric secretions now these stomach mixing waves don't occur very much in the top of the stomach in the fundus so that's the fundus of the stomach at the top that's largely involved in storage so when you eat a large meal that will that will distend and they'll be storage your food in there waiting to go into the lower parts of the stomach but other parts of the stomach we get these stomach mixing waves producing this semi liquid chime now in the gastric pits in the lining of the stomach there's a special sort of cell called pol ayatul cells and these parietal cells will produce hydrogen ions and chloride ions h plus and Cl minus and of course that gives rise to the formation of the main acid in the stomach HC hydrochloric acid which is the main acid in the stomach now the hydrochloric acid is obviously a strong acid and that's good because it means that bacteria that come into the stomach can be partly killed by the very acidic medium that they're confronted with when they get into the stomach but the hydrochloric acid also unfolds proteins so proteins are often in a very large sometimes a globular arrangement like this highly folded like that which is good because they form physiological functions as proteins but the hydrochloric acid will help them to unfold and become a longer thinner strand and that's good because it means that the digestive enzymes can access them more easily the unfolding of the proteins is greatly increasing the surface area of the protein over which digestive enzymes can then take effect now I think the next one we'll mention in terms of mechanical digestion is boil so bile of course is produced in the liver it goes down the bile ducts after being stored in the gallbladder goes down the hepatic ducts common hepatic duct into the gallbladder down the common bile duct and into the duodenum and fats are actually emulsified by bile so ball does not digest fat in any way that it emulsifies it so what will happen is a large globule of fatty material will be emulsified down into many smaller globules massively increasing the surface area for the digestive enzymes that break down fat the lipase to access the surface of the the fat and this emulsification process means that it looks at a white creamy color so for example milk is white because it's an emulsion of fat and it's interesting to note that the liver actually produces eight hundred to a thousand by on the day now much less of that goes into the the duodenum because the bile is stored and concentrated in the gallbladder so if we think about where we're going with this digestive process we started with the with the mouth we went on to the stomach mentioned bile we're now in the small intestines in the in the small intestine called small because it has a more narrow lumen than the large intestine and we noticed that one of the thick processes going on here is called segmentation segmentation so what's happening in this process of segmentation well the small intestine is essentially a long tube so here we have a long tube like this and there's longitudinal muscles and the outside but there's also circular muscles and what happens in segmentation firstly is there's contraction of areas of circular muscle let's say there's a contraction here and there's a contraction there and these contractions are largely stimulated by distension so when the chime comes in then these segments will be formed by contraction of the circular muscles and this is controlled by the my enteric plexus so in the walls of the intestine there's a lot of neurological tissue called the my enteric plexus which is controlling this process and in the duodenum these contractions will happen about 12 times a minute in the ileum it's a bit slower maybe eight times a minute so initially we've got this contraption let's say the initial contractions of these three these three areas so this contraction of the circular muscle there they're at roughly the same time and I think you can now see that means we've got two segments hence the name segmentation that's the first stage now the second is that the muscle fibers in the middle of each segment will also contract so we're going to get contraction of the muscles here and the muscles here so now we've got even more segments we've got further segmentation you could call these sub segments if you like then the third thing that happens is the fibers that contracted in the first place that's that one there that one there and that one there the first ones to contract they will relaxed so they're going to be relaxed now relax so what this means is that we've got larger segments again and then this repeats this repeated process and what this actually means is that the material in the small bowel is essentially sloshing back and forward so we tend to think about peristalsis as being a process where we go progressively along there along there along there all the way through the intestine to the large intestine so we tend to the Gipper so as a progressive process and that's true peristalsis is but this muscular segmentation happens before the peristaltic processes propel the material through the small bowel so the aim of segmentation is not to propel material from the stomach towards the large intestine but rather to mix it up and to churn it all together so it's localized mixing contractions and as that happens material is going to be absorbed from the small bowel into the blood supply into the lymphatics therefore as it's absorbed its volume is clearly going to be reduced so there's going to be a reduction in volume as it's washes back and forward and as it sloshes back and forward is going to be well mixed it's going to come in contact with all the right enzymes and of course the mixing is going to be that it comes into contact with the walls of the small intestine which is where the absorption takes place from a segmentation process is helping the digestive process and it's helping the absorptive the absorption process as well so that's this process of segmentation and then when that process of segmentation is finished and there's been a lot of absorption then there's migrating motility complex complexes of muscular contraction and these will also be contractions but they occur much more sequentially in waves there's a lot piece paper so we imagine that this is one length of the small intestine here they'll be contraction there that's contraction there and what's like what that's going to do when the area contracts you can probably see that that's going to push material along now material will be pushed back to some extent as well but that will be stopped from going back into the stomach by the pyloric sphincter which will be closed off so the material is going to be pushed along a bit and then if we imagine the same length of bowel a short time later that bits going to a relaxed and this bit will over that this bit of the wall will have contracted like that so that means that the bolus which was there that's been moved along is now was there has been moved along now it's been moved along a bit and it's going to be going to be there and and then this muscle is going to relax and the next part of the muscle is going to contract so we go further along slightly lost my scale here but never mind you get the idea then that part that bar is going to contract so the bolus is pushed further along push further along like this so we get this wave of muscular contraction just like any Paris dancing process really pushing the material further along the lumen of the small intestine as we go along and that's what we call although the the migrating motility complex this wave of contraction going down the small intestine pushing material from the stomach towards the colon and for such a wave of contraction to go all the way along this morning I'm from the base of the stomach to the terminal Illium that's going to take about 90 to 120 minutes and altogether the chime remains in the intestine for about three to five hours before it's absorbed or pushed through to the colon so that was the migrating motility complex now we notice that there is a intestinal juice intestinal juice and the intestinal juice is produced imagine this as a length of the small bowel here so the intestinal juice is going to be produced by the lining of the small intestine it's produced by the lining of the small intestine and it's produced mostly in the lining walls of the first parts of the small intestine the duodenum and the jejunum and about one to two liters of this fluid are produced today so there's quite a lot of it being produced one to two liters a day of being secreted into the lumen of the small intestine and this is very important because it makes everything in here fluid because it's got to be fluid to mix with the digestive enzymes it's got to be fluid to come into contact with the wall of the small intestine to facilitate absorption so this if you like it's kind of a it's a diluting so the intestinal juice is diluting the contents of the small intestine now of course there's a readi pancreatic juice in here so that's going to further dilute the pancreatic juice and as we thought that did I say what 1 to 2 liters of this are produced every day a lot of its reabsorbed the part that's not reabsorbed will go to the colon and probably be reabsorbed nearly all of it reabsorbed in the colon if not before and it's a clear yellowish fluid it contains water and Moo now some people think that this juice contains digestive enzymes it doesn't but of course it does mix with the pancreatic juice that does contain digestive enzymes so when it's produced from the wall of the small intestine it won't produce it won't contain digestive enzymes but when it's all mixed up with the pancreatic secretions then yes there will be digestive enzymes in the fluid within the lumen of the gastrointestinal tract although it doesn't come from this intestinal juice produced by the wall of the small intestine this just contains in it but mostly water and mucus and it's quite alkaline the pH here is about um well we know the small intestine is generally an alkaline environment so it's about seven point six is the pH of this intestinal juice and in the old days this juice used to have a great name it was called the circus in terrace the sucker Sint Erika's but these days we call it Lea the intestinal juice so combining with the pancreatic juice to produce this liquid medium for mixing for coming into contact for absorption so we see that there's quite a few aspects of mechanical digestion all these aspects of digestion we've talked about aren't chemical we haven't chemically broken down the food yet they're all physical processes so we've had water and mucus softening the food about the process of mastication without the stomach mixing waves mixing of the chyme without the hydrochloric acid unfolding the proteins that bile emulsify the fats the segmentation sloshing back and forward in the small intestine the progressive peristaltic migrating motility complex of muscle or complexes of muscular contraction and finally this inter Steindl juice all aspects of mechanical digestion so of course in the next video we want to go on and think about the processes of chemical digestion

Channel: Dr. John Campbell

Views: 13,230

Rating: 4.9361024 out of 5

Keywords: digestive system

Id: PW2i1E16u6Q

Length: 21min 58sec (1318 seconds)

Published: Mon Jul 22 2019