hello everybody my name is Iman welcome back to my YouTube channel today we're going to be discussing the digestive system in this chapter we're going to cover the following objectives first we'll talk about the anatomy of the digestive system as a whole second we will focus on the different organs of the digestive system and talk about their roles in ingestion and digestion third we will briefly touch on the accessory organs of a digestion and then last but not least we will end the chapter with an explanation on absorption and defecation what a great note to end on isn't it all right so let's go ahead and get started with the first objective anatomy of the digestive system now the food we eat is complex it's composed of meats grains vegetables and fruits dairy products and sugars and the role and job of the digestive system is to take these complex foods and turn these large macromolecules into smaller and simpler monosaccharides fatty acids and amino acids now in order to cleave all of these bonds the body requires a complex system of mechanical and chemical agent these compounds can then be absorbed from the gut transported to the tissue by the circulatory system and used by cells and so in this chapter we will consider the organs that make up the digestive system as well as the processes by which the food we eat become the fuel we need for both energy growth development and maintenance so with that let's begin there are two types of digestion that occur all right first we have intracellular digestion all right as a part of metabolism involves the oxidation of glucose and fatty acids for energy however our diets don't consist of just pure glucose and fatty acids rather these substances have to be extracted from the food we eat the processes by which these nutrients are obtained from food occur within the Lumen of the elementary canal and it's known as extra cellular digestion this is technically quote-unquote outside the body as it is outside the cell borders now this alimentary canal runs from the mouth to the anus and it's sectioned off by spinters or circular smooth muscles around the canal that can contract to allow compartmentalization of function now the human digestive tract has specialized sections with different functional roles and the most basic functional distinction distinction is between digestion and absorption now digestion involves the breakdown of food into its constituent organic molecules lipids into free fatty acids and glycerol starches and other carbohydrates into monosaccharides and proteins into amino acids now digestion can be subdivided into mechanical and chemical processes mechanical digestion is the physical breakdown of large food particles into smaller food particles but it does not involve breaking chemical bonds all right chemical digestion on the other hand is the enzymatic cleavage of chemical bonds such as the peptide bonds of proteins and the glycosidic bonds of starches all right so that's digestion that's one basic function of the human digestive tract the other functional role of the digestive tract is absorption and absorption involves the transport of products of digestion from the digestive tract into the circulatory system for distribution to the body's tissues and cells now that we have a understanding of that let's start talking about how things move or progress in the digestive system the digestive tract begins with the oral cavity aka the mouth followed by the pharynx which is a shared pathway for both food entering the digestive system and air entering the respiratory system from the pharynx food enters the esophagus which transports food to the stomach then from the stomach food travels to the small intestine and then the large intestine finally waste products of digestion enter the rectum where feces are stored until appropriate time of release now in addition to the actual organs of the digestive tract the salivary glands the pancreas the liver and the gallbladder all help to provide the enzymes and the lubrication necessary to Aid the digestion of food now believe it or not neurons also play an important role in the gastrointestinal system the enteric nervous system is a collection of 100 million neurons that govern the function of the gastrointestinal system these neurons they're present in the walls of the digestive tract and they trigger this thing called peristalsis or the rhythmic contraction of the gut tube in order to move material through the system and the system can function independently of the brain and the spinal cord although it is heavily regulated by the autonomic nervous system now actually the para sympathetic division it's involved in stimulation of digestion activities increasing secretion from the exocrine glands and promoting this peristalsis and then the sympathetic division that's involved in inhibition of these activities the fact that so often we feel sleepy and lethargic after eating a big meal is due in part to our parasympathetic activity now that now that we have introduced the general anatomy of the digestive system as a whole let's start to focus on the different organs of the digestive system and let's talk about their role and roles in ingestion and digestion so to supply the body with nutrients we must ingest or eat you know food now several hormones are involved with the feeding behavior and digestion digestion begins in the mouth the oral cavity and then it continues in the stomach and the first part of the small intestine known as the duodenum all right so let's talk about every part of that process all right we're going to talk about every part of the process now we're gonna focus on ingestion and digestion all right ingestion taking food into the body digestion breaking down food then in the latter half in the latter half of this chapter specifically that last objective will cover will focus on absorption so moving food into cells and elimination removing unused food now multiple hormones regulate feeding behavior all right multiple hormones regulate feeding Behavior which uh including antidiuretic hormone and aldosterone these promote thirst glucagon and ghrelin which promote hunger and leptin and Colo cystokinin which promotes fullness now the first part all right of us in understanding ingestion and digestion all right we start with the oral cavity the oral cavity plays a role in both mechanical and chemical digestion of food mechanical digestion in the mouth that's going to involve the breaking up of large food particles into smaller particles by using the teeth the tongue and the lips all right this is the process this process is called chewing right we know this also called mastication all right chewing helps to increase the surface area to volume ratio of the food that's going to allow for more surface area for enzymatic digestion as it passes through the gut tube now it also moderates the size of the food particles that are going to be entering the Lumen of the alimentary canal food particles that are too large they can create an obstructive risk in the tract which is why this first step is important all right chewing your food is important now chemical digestion begins the breakdown of chemical bonds in the macromolecules that make up the food all right this relies on enzymes from the saliva produced by the three pairs of salivary glands saliva also AIDS mechanical digestion by moisture moistening moistening and lubricating food the salivary glands like all glands of the digestive tract are are innervated by the parasympathetic nervous system the presence of food in the oral cavity it triggers a neural circuit that ultimately leads to increased parasympathetic stimulation of these glands salvation Salvation can also be triggered by signals that you know food is near like making eye contact with a juicy burger on the kitchen top or the smell of your mom cooking really delicious food in the in the kitchen that all can be triggering signals all right and so you start to produce saliva in your mouth in preparation of ingesting food and then you know once it enters the mouth the oral cavity digestion can begin all right so even in preparation of ingesting food salivation can be triggered by signals that food is near now saliva contains salivary amylase also known as Thailand but that's spelled with a p in the beginning let's spell it out all right and lipase now salivary amylase this is capable of hydrolyzing starch into smaller sugars like maltose and dextrins while lipase catalyzes the hydrolysis of lipids the amount of chemical digestion that occurs in the mouth is minimal though because the food doesn't stay in their mouth for very long right you don't you're not chewing your food for like five minutes you're chewing your food for probably a minute minute and a half tops our muscular tongue forms the food into a ballis which is forced back to the pharynx and swallowed alright so after you're done chewing your food all right the mechanical digestion and a little bit of the chemical digestion that happens to the mouth then you move it back you swallow it and it goes the food will make its way to the pharynx all right so that's the next part we started with the oral cavity now we're gonna make it to the pharynx the pharynx is the cavity that leads from the mouth and posterior nasal cavity to the esophagus the pharynx connect not only to the esophagus but also to the larynx which is a part of the respiratory tract the pharynx it can be divided into three parts the nasopharynx that's right behind the nasal cavity the orphal pharynx this is at the back of the mouth and the laryn Joe pharynx all right and that is going to be above the vocal cords now food is prevented from entering the larynx during swallowing by epiglottis which is a like a cartilage structure that folds down to cover the uh the laryngeal Inlet if this mechanism Fails Like failure of this mechanism can lead to choking on your food all right but we have things in place that prevent that from happening all right so for first the oral cavity then the food has made it to the pharynx now from the pharynx we make it to the esophagus the esophagus is a muscular tube that connects the pharynx to the stomach the top third of the esophagus it's composed of skeletal muscle the bottom third is composed of smooth muscle and the middle the middle part is kind of a mix of both what does this mean in terms of nervous control well while the top of the esophagus is under somatic or voluntary motor control the bottom and most of the rest of the gastrointestinal tract for that matter is under autonomic or involuntary nervous control the rhythmic contraction of smooth muscle that propels food towards the stomach is called peristalsis all right now under normal circumstances peristalsis proceeds down the digestive tract however certain conditions like exposure to chemicals or infectious agents or physical stimulation in the posterior pharynx and even cognitive stimulation can lead to the reversal of peristalsis in the process of vomiting all right so uh things that certain conditions can cause hiccups in this process of peristalsis and that could lead to vomiting the food that you have ingested now swallowing is initiated in the muscles of the oropharynx which constitutes the upper esophageal spinster peristalsis it squeezes pushes and then propels the ball is towards the stomach and as the ballis approaches the stomach a muscular ring known as the lower esophageal spinster all right or also known as the cardiac spinster relaxes and it opens to allow the passage of food so that the food can pass through the esophagus and make it now to the stomach all right so again we started at the oral cavity the mouth all right then the food made it to the pharynx then from the pharynx we made it to the esophagus and from the esophagus we make it to the stomach now the stomach is a central organ in the digestive process it's capable of both mechanical and chemical digestion it's located in the upper left quadrant of the abdominal cavity this muscular organ has a capacity of about two liters and it's structured to withstand harsh acidic environment its location and capacity really makes it ideal for its primary role which is the initiation of protein digestion and the continuation of carbohydrate and fat digestion that began in the mouth now the anatomical structure of the stomach consists of four main divisions you have the fundus the body the antrum and the pylorus all right so those are the four main divisions of the stomach now the internal lining known as the mucosa features gastric and pyloric glands as well as folds called rugai that increase surface area now gastric glands are activated by the vagus nerve part of the parasympathetic nervous system which itself is triggered by sensory inputs such as this site and the smell of food now cellular specialization within the gastric glands facilitates its digestive functions essentially mucous cells they produce bicarbonate Rich mucus to protect the stomach lining from the acidic environment within parietal cells they secrete hydrochloric acid which serves multiple functions including microbial defense protein protein denaturation and the conversion of pepsinogen to its active form pepsin Chief cells they produce this pepsinogen which is activated by the acidic environment to become pepsin all right and uh and this this is a unique enzyme that's most active at low pH levels pepsin actually Cleaves proteins into smaller peptide fragments and that helps initiate their digestion so from that notably the stomach produces six essential products all right it produces pepsinogen all right which is cleave to pepsin in the stomach this is an enzyme that partially digests proteins it secretes mucus which protects the mucosa it produces hydrochloric acid which kills microbes denatures proteins and also converts pepsinogen into pepsin it produces bicarbonate which also protects the mucosa all right it secretes water this is dissolves and dilutes some of the ingested material and also it also secretes this thing called intrinsic factor which is required for normal absorption of vitamin B12 all right so those are the six main products that the stomach secretes now the pyloric glands that are located in the antrum and the Polaris region these they contain these cells they're called G cells that secrete gastrin gastrin is a peptide hormone it regulates Hydrochloric secretion and stomach contractions it also AIDS in the mechanical breakdown of food and the end product of this phase of digestion is chime or chyme an acidic or semi-fluid mixture that moves into the small intestine for further nutrient absorption and so the point here the summary for the stomach part right because there's a lot that's happening in the stomach the summary is that the stomach is a highly specialized organ with functions that are tailored to its acidic environment and it plays a very very crucial role in the early stages of digestion and it's aided by specific cellular components that produce a variety of enzymes and other substances these compounds they work together to break down food into smaller components setting the stage up for further digestion and absorption in the subsequent in the in the subsequent segments of the gastrointestinal tract and with that note all right from the stomach we move into the dude enough all right moving forward in the digestive tract The partially dissolved food or or chime leaves the stomach and it enters the duodenum all right the first of three segments of the small intestine now this segment is integral to the chemical digestion process all right producing enzymes and hormones that are really essential for the breakdown of food and the regulation of other digestive organs now this one of three parts of the small intestine despite its minor role in absorption it really sets the stage for nutrient uptake in the subsequent segments of the small intestine now upon entry into the duodenum the presence of chime triggers the release of things called brush border enzymes like disaccharide disaccharide Aces and peptides peptid aces all right as well as hormones like secretin and cck now disaccharide Aces including maltase isomaltase lactase sucrase all break down disaccharides into their constituent monosaccharides whereas peptide peptidasis such as aminopeptidase and dipeptidase they cleave peptides into free amino acids or smaller peptides that can then be absorbed across the intestinal wall now another important thing to mention is anteropeptidase this is another critical enzyme that's secreted by the duodenum and it's responsible for activating trypsinogen to trypsin which in turn activates other pancreas pancreatic proteases this Cascade of enzyme activation is really crucial for the effective breakdown of proteins into absorbable units then secretin and cck hormones um are released by the duodenum they regulate the secretion of pancreatic juices and bile into the digestive tract so secretin reduces gastric acid production it increases bicarbonate secretion from the pancreas and it also slows gastrointestinal motility and that allows more time for the fats to be digested now on the other hand cck is released in response to the presence of amino acids so that was secretin cck is released in respond in response to the presence of amino acids and fats in the chime promoting the release of bile and pancreatic juices now bile salts components of bile are really essential for the emulsification of fats it's a necessary step for their subsequent enzymatic digestion digestion in addition cck acts in the brain to promote the feeling of fullness now pancreatic juices stimulated by cck they they're going to contain enzymes that are really capable of digesting all three main nutrient types that's carbohydrates fats and proteins the this bicarbonate Rich alkaline solution neutralizes the acidic chime that comes from the stomach and provides an optimal environment for these enzymes to play their role in the in in the duodenum all right and so with that We've Ended objective too we've covered the first objective and we've covered the second objective we're going to stop here all right let's quickly summarize what we just learned in these first two objectives all right we talked about the anatomy of the digestive system we talked about intracellular digestion and how it involves the oxidation of glucose and fatty acids to make energy we also talked about extracellular digestion which occurs in the Lumen of the elementary canal all right we talked about two types of digestion mechanical and chemical mechanical digestion is the physical breakdown of large food particles into smaller ones and chemical digestion is the enzymatic cleavage of chemical bonds such as the peptide bonds of proteins or the glycosidic bonds of starches we talked about the pathway of the digestive tract its oral cavity then pharynx then esophagus then stomach followed by the small intestine and then the large intestine and then finally the rectum we talked about how the accessory organs of digestion are going to be the salivary glands the pancreas the liver and the gallbladder and actually the third objective focuses focuses on these accessory organs of digestion and then we also mentioned that the enteric nervous system is in the wall of the elementary canal and it controls peristalsis then we moved into the second objective where we talked about ingestion and digestion all right we said that multiple hormones regulate feeding behavior all right and regulate the feeling of fullness we talked about um we started at the beginning in the oral cavity that's where we start we ingest food and then through chewing it we start the mechanical digestion of food while other enzymes like salivary amylase and lipase start the chemical digestion of food then food is formed into a bolus and swallowed from the oral cavity moves into the pharynx which connects the mouth and the posterior Naval nasal cavity to the esophagus then we move into the esophagus where it propels food to the stomach using peristalsis food enters the stomach through the lower esophageal spinster and then the stomach we enter the stomach the stomach has four parts the fundus the body the antrum and the Polaris the stomach has a um many important parts that contribute to digestion we have the mucous cells that produce bicarbonate Rich mucus to protect the stomach we have chill cheap cells that secrete pepsinogen a protease activated by the acidic environment of the stomach we have parietal cells that secrete hydrochloric acid and intrinsic factor which is needed for vitamin vitamin B12 absorption we have G cells that secrete gastrin which is a peptide hormone that increases Hydrochloric secretion and gastric motility all right we covered all of those important uh um parts of the stomach all right now after mechanical and chemical digestion in the food the food particles are now called chyme or chime and the food passes into the duodenum through pyloric svenster um through the pyloric's Finster now in the duodenum this is the first part of the small intestine it's primarily involved in chemical digestion we have disaccharidases and these are brush border enzymes that break down things like maltose isomaltose lactose and sucrose into monosaccharides we also have brush border peptidases including aminopeptidase and dipeptidase then we have enteropeptidase which activates trypsinogen and other things like pro-carboxy peptidases which initiates an activation Cascade then we also have secretin which stimulates the release of pancreatic juices into the digestive tract and slows motility and then we have cck which stimulates the bile release from the gallbladder release of the pancreatic juices and also plays a role in you feeling full after you've eaten a meal all right so those are all the important points from the first two objectives let me know if you have any questions comments concerns Down Below in the next video we'll continue and finish up the chapter other than that good luck happy studying and have a beautiful beautiful day future doctors