 Hey everybody, Dr. O here. Welcome to chapter three about digestion, absorption, and transport. So this is a hugely important topic, right? This whole idea that you are what you eat. I mean truly when you eat, your body will mechanically and chemically digest and break down the food you eat into the simplest building blocks, the raw materials that will be absorbed into your bloodstream or into your lymphatic system and then to your bloodstream. And they're going to be transported through your body where they can be used or they can be stored. I mean it's a huge deal, right? You truly are what you eat. I won't go into as great a detail as you probably need to know in this video because I've done an entire video playlist, an entire series about the digestive system already in my anatomy and physiology video series. So and then also the next three chapters over carbs, lipids, and proteins, we will talk about the digestion, absorption of each of those there. So we'll just do more of a high level overview here and then I'll have plenty of resources for you in the course for where you can get more information. But I'll cover it in pretty good depth here as well in case this is the only resource that you have. All right, so that's the big picture of what we're going to be talking about here, digestion, absorption, and transport. And I'd also like to add then elimination, right? So we have to get rid of the things that aren't digested and absorbed. Think about fiber and other undigestible food types. All right, let's go ahead and dive in. So an ice breaker, the digestive system plays a critical role in our ability to break down foods into nutrients. Why is it important to eat foods that are easy for the digestive system to break down? Why are some foods easier for our digestive system to break down than others? So that's obviously a huge topic. First thing I would say is, I don't remember where I heard this, but I love this saying, don't eat what you like, eat what likes you, right? So everyone's going to be different. Everyone's going to have foods that they can consume and digest and not have problems with, and there's going to be some other foods that probably bother them a little bit. Think about if someone's lactose intolerant, then drinking a glass of milk is going to be a whole different situation than someone that isn't. So don't eat what you like, eat what likes you. Your diet should be built around foods that you can easily digest and break down and use for fuel, because that's what food's really for. Anyways, all right, so why is it important to eat foods that are easy for the digestive system to break down? Well, what's the point of eating, right? If you, why eat something that doesn't have a function? And the food you eat needs to be digested and absorbed before it actually has a function. So another little tagline I like to use, yes, you are what you eat, but I always like to say, you're not what you eat, you're what you absorb. Because if you consume foods that pass through you and aren't digested and absorbed, then they're not going to help you. Now fiber's a whole different story, right? We have different types of fiber that we'll talk about in this chapter and the chapter on carbohydrates. But some fibers are just roughage. They are designed that their function is to pass through you. So I'm not saying that every single thing you eat needs to be absorbed, because in soluble fiber, that's its function, is to kind of clean out your gut and pull toxins with it as it's being passed through your body. And then soluble fibers, they're digested, some of them are, but not by you, right? They're digested by the microbes that live inside of you. So I cover that in a lot more detail in microbiology classes, but we will definitely talk about the role of your gut microbiome, right? That a pretty decent percent of the calories that you digest and absorb today are going to be digested by microbes living in your gut. And then you will absorb them and you will use them. So they are helping you. You could say five to 15% of the calories that you're going to get today are going to come from gut microbes. And then also those gut microbes are making vitamins for you, like vitamin K and some of the B vitamins, so pretty cool. All right, so why are some foods easier for you to digest a system to break down than others? Just individually, it has to do with enzyme levels and that's genetic. So if you have plenty of the enzyme lactase, then you can drink a glass of milk and not have any problems. If you're lactose intolerant, you don't have enough of the enzyme lactase. And any of these milk sugars that you consume will be difficult for you to digest. Now you can avoid those things or you could take things like lactate. You could buy milk that has the enzyme lactase added to it. Raw dairy, for example, already has the enzyme lactase in it, but whatever. So that'd be one example. And then from an evolutionary standpoint, over time, we have tons of copies of genes that allow us to break down starches. So we can consume starches in a way that other species cannot. What else? Your microbiome will change as well. So people that live in parts of the world where they can or people that consume a lot of sushi, for example, or let's just say seaweed. People that consume a lot of seaweed, over time, their microbiome can change to a microbiome that can actually help them digest seaweed. If I had seaweed, I wouldn't have the microbes in me to properly digest it. But if I consumed it all the time, I might. So your microbiome can impact digestion, your genetics impact digestion, and then just the types of foods. But as a species, one of the things you need to know is we have an unbelievable capacity to digest foods. Think about how many animals live on really specific diets where they eat like a handful of things. Well, humans don't have to be like that. If you travel around, one of the reasons that we've kind of conquered the planet is no matter where we went, we found foods that we could eat. So some cultures survive on coconut products. Other cultures survive on seal blubber and everything in between. So we have a really adaptive metabolism. So that's a pretty cool way to introduce a topic. But just back to this idea of digestion, when you think about it. Let's say yesterday I was having some chicken breast after I worked out and I'm like, I am trying to take the protein in that chicken breast and turn it into the protein in my muscles. That's basically what we're doing when we consume food. We're trying to pull the nutrients out of the food so that we can use them. We're trying to pull the macronutrients out of food so we can use them. And we're trying to pull the calories out of food so we can fuel our own bodies. I said this would be a little less of a deep dive, but I have a hard time doing that. So learning objectives. Identify the actions of the organs, muscles, and secretions. So we'll talk about all the key players involved in digestion. Describe the anatomical details of the intestinal cells that facilitate nutrient absorption. So I will cover that in good depth. But if you want a real deep dive, go find my digestive system playlist on the YouTube channel. Number three, explain how nutrients are routed in the circulatory systems from the GI tract into the body. And identify which nutrients enter the blood directly and which must first enter the lymph. So you'll see that most nutrients are digested and absorbed straight into the bloodstream. But your fats, your large fats, and your fat-civil vitamins, they are actually going to be carried into the lymphatic system first, and I'll explain why. Number four, describe how bacteria, hormones, and nerves influence the health and activities of your GI tract. So we'll talk about the microbiome and then again the hormones and the nervous component involved in digestion. So let's start with, so we really have, we'll cover digestion, then absorption and transport, and then we'll talk about elimination. So digestion is just the beginning of the process. It is the mechanical and chemical breakdown of the food into small enough pieces that can be absorbed. So think about like starting with, you know, you have a stake. Start by just chewing it, right? You're mechanically breaking it down into something that's small and manageable enough to swallow, and then it's going to be broken down further in your stomach. And then enzymes are going to start to take the proteins that were in that stake and break it down into individual amino acids. So the end product of digestion of protein is individual building blocks, the amino acids. Same thing with carbs. If you eat a potato, the starches are going to be hundreds of glucose units long, and you're going to mechanically and physically and then chemically break it down into individual glucose units that will be absorbed. Fats aren't fully digested like that, which is actually why they have to travel through your lymphatic system. Fat is a triglyceride. Think of a glycerol backbone with the three tails, tri, glyceride, and it'll be broken down. You'll lop a couple of the tails off and then you'll bring them in. But you're not fully digesting fat in the same way you do carbs and proteins, which is why these big fat chunks can't be in your bloodstream, so that's why they're going to be carried in your lymphatic system first, and they're going to be packaged because, remember, also oil and water don't mix, so that's another reason why we use the lymphatic system. So that's digestion. You're going to be breaking food down into small of pieces so you can absorb it. Imagine you were trying to carry something that was really, really heavy, way too heavy for you to lift. Well, you take one piece of it at a time and carry one piece of it at a time. That's what digestion is. You break these things down into manageable pieces, bite size if you want to use that pun. All right, the digestive process. So a digestive system, the organs and glands associated with the ingestion and digestion of food. So ingestion is the deciding what you put in your mouth. Digestion is the mechanical and chemical breakdown. So the organs and glands, so the GI tract itself from your mouth to the anus. So you have your mouth, the pharynx, which is your throat, the esophagus, stomach, small intestine, large intestine there into the rectum and the anus. That is your GI tract, your gastrointestinal tract. So that's going to be your primary digestive organs. Anything that helps the GI tract is going to be a secondary organ of digestion. So that would be your, think about like the salivary glands, your pancreas, your liver, gallbladder. Those would be the best examples there. So what is digestion? We've already talked about it. You're breaking food down so you can absorb it both mechanically and chemically. So think chewing and churning of food in your stomach is mechanical and then chemicals going to be the acid in your stomach, breaking things down and then your digestive enzymes. And then what's absorption? The uptake of nutrients by cells of the small intestine for transport into either the blood or the lymph depending on if they're fat or not. What is true digestion? So you see there are a lot of options here but it's going to be breaking down food into nutrients. It happens in way more places than just the stomach and small intestine. So digestion is the breakdown of food into nutrients which can then be broken down further for energy. And that happens, that's your metabolism. That happens inside your cells. All right, this is a great picture. This is a overview of the entire system. So I'll kind of run through it here now and then we'll cover some more details as we go. But remember, we're also going to come back and look at the system again and we're just going to look at the role it plays in digesting absorbing carbs and then digesting absorbing fats and protein. So we're going to review all of this later. So we start with, and I've already done a really good overview of this entire system on the YouTube channel. But all right, so we're going to start with the mouth. The digestion really begins, it's called the cephalic phase of digestion but digestion truly begins when we start to think about food or when we smell food or when these types of things. So your body basically prepares for digestion before you start to eat, at least if you give it enough time. So imagine like hearing your food sizzling like a fajita, I always think about those commercials or smelling food. These things, thinking about food, these things will start to trigger the digestive process. But so the mouth is though is where ingestion occurs where you decide what to put in your mouth. And then the primary function of the mouth is to break down the food and mix it with saliva so that it can be swallowed and travel into the throat and the esophagus. So as we're chewing your food, it's being clipped and cut and gnashed and torn and ground up by your teeth. Your tongue is coordinating all this movement and moving everything around. And then while, so your food's being broken down so you can swallow it but then it's also being mixed with saliva and saliva has enzymes in it, it has immune components in it, it lubricates it, all these types of things. So that's the real basic function of the mouth is like it says there to chew and mix food with saliva and then you're going to swallow it. With swallowing, the food's going to travel them through your pharynx, through your throat and then down the esophagus into the stomach. So that's the pharynx and then we talked about the role of saliva there. Next on the list there, we have the epiglottis. So the trachea is your airway, your windpipe. The trachea is going to, what carries air from the throat to the lungs. So when you swallow, the epiglottis will cover the trachea. The glottis is called the opening of the trachea or the opening of the trachea is called the glottis. The epiglottis will cover the trachea so that when you do swallow, it doesn't go to the lungs and it travels down the esophagus instead. All right, we talked about the esophagus. It's basically about 12 inches long hollow muscular tube where so when you initiate swallowing that's the last conscious thing you do is you decide to swallow. But once you start the swallowing process, the esophagus goes from voluntary muscle, skeletal muscle to involuntary smooth muscle. So you initiate swallowing then the esophagus takes over and uses what are called peristaltic waves to carry food down into the stomach. The sphincters, we'll talk about them when we get a better look at them, but that we do have an upper and lower esophageal sphincter. You can see here on the image that these are normally closed. So the trachea is always open. The esophagus is normally closed. It only opens when you're using it. This lower esophageal sphincter, also known as the gastroesophageal sphincter is a big deal because if this sphincter doesn't stay closed when it's supposed to you can develop GERD or gastroesophageal reflux disease which is heartburn, where stomach contents will splash up into the esophagus. As long as this sphincter stays closed, that shouldn't happen. All right, the diaphragm you see there, the diaphragm is what separates the thoracic cavity from the abdominal cavity and you'll notice the esophagus has to travel through it. So this is important if someone, let's say because of obesity or pregnancy or something else, people can develop hiatal hernias and what that is is where part of the stomach will actually protrude up through that opening into the esophagus and that can cause problems for some people. So the stomach, a basic function of the stomach, this is where protein digestion begins. The stomach is a big bag full of acid and the primary function of the acid is to denature proteins, to take the large proteins that you eat and unravel them so that they can be digested. And then the stomach is also where we have our first enzyme for digesting proteins. So the digestion of carbohydrates begins in the mouth with an enzyme called salivary amylase. The digestion of fats also begins in the mouth with an enzyme called lingual lipase, but the digestion of proteins does not begin until the stomach. All right, so the stomach's gonna turn your food up and mix your food up with stomach acid and other stomach contents like these enzymes and it's gonna turn. So when you're done chewing your food and you go to swallow it, it's called a bolus. When your stomach is done with food, it basically turns it into a syrup called chime, C-H-Y-M-E. All right? And then we have then next on the list there, you see the pyloric sphincter, that's the sphincter that keeps the stomach closed until it's ready, materials ready to move through it. It basically squirts this chime, these stomach contents into your small intestine, about a teaspoon at a time. So the pyloric sphincter plays a big role there. Just going down the list, then we see the liver and the gallbladder. So remember the liver does tons of metabolic things with the blood, but its primary function right here is the liver is where we make bile. When we talk about bile, you often talk about the gallbladder, but the liver makes bile. It uses cholesterol to make bile salts and then it produces the bile. It is stored and concentrated in the gallbladder. It's not made there. So the gallbladder stores and concentrates bile. Why does that matter? Well, if you don't have a gallbladder, you're still making bile. So you can still digest and absorb fat, just maybe not as much as you used to. I know when I had my gallbladder out, I'd eat like a thing of yogurt, you know, maybe have like six grams of fat in it. And it was about all I could tolerate at that time, but your body kind of gets used to it. So the liver is where we make bile, the gallbladder is where it is stored and concentrated. And then we have the bile ducts. So the bile ducts are, they're going to be carrying this bile into the small intestine. And then also you'll see that the gallbladder, yeah, it's the liver and gallbladder are going to dump into the bile ducts there. All right, that's the left-hand side. The right side, the appendix. So, you know, a lot of times you'd see that people would say the appendix doesn't do anything, but we've known for quite a while that it does. And that's a good description there. The appendix houses bacteria and lymphatic cells. So the appendix plays a role in the maturation of your immune system. And also the reason it houses bacteria, it appears that after diarrhea or after antibiotic use, when your microbiome was kind of decimated and partially destroyed by one of those reasons or the other, the microbes in the appendix will come out and help to repopulate. That's why your microbiome recovers from antibiotics pretty well and recovers from diarrhea. Next, we have the small intestine. So this is where 90% of digestion, absorption of food takes place. The small intestine is the most important organ of your GI tract. Everything prior to this has really just prepared the food to be fully digested and absorbed. And then everything after it is kind of like catching a little bit of digestion products and reabsorbing water, things like that. So the small intestine, as you'll see later, is broken down into three parts. It's called the duodenum, the jejunum, and the ilium. We'll come back to that. So at the end of the small intestine, we have the iliocecovalve. You can find it there on that image. This is the valve between the ilium of the small intestine and the cecum, which is the beginning of the large intestine. So the iliocecovalve determines when food contents are gonna move from the small intestine to the large intestine. Another reason the iliocecovalve's important is you don't want stuff from the large intestine leaking back into the small intestine. If this stays closed, then most of your microbiome, most of your microbes will stay in the colon. There are millions and millions of bacteria in the small intestine, but there are billions and trillions of them in the large intestine. All right, the pancreas. So when you think about the pancreas, you almost always think about insulin, at least most people do, but only 1% of the pancreas's function is endocrine producing hormones, like insulin which lowers blood sugar and glucagon which raises it. We'll cover that in the next chapter. 99% of the pancreas is exocrine and it's a digestive organ. So it produces bicarbonate, which is bicarbonate is a buffer. Remember how acidic the stomach is. When stomach contents leak into the small intestine, the acids have to be neutralized by bicarbonate, which is a buffer, and then also the pancreas makes most of your key digestive enzymes. It makes pancreatic amylase, which breaks down carbohydrates, pancreatic lipase, which breaks down lipids or fats, and then the proteases, which break down proteins. So very, very important. Once those digestive enzymes have gone into the small intestine and had a crack at your food, it's almost fully chemically digested. The small intestine itself will have some enzymes right in the lining that will finish off digestion before things are absorbed. Okay, the pancreatic duct, that's gonna carry these pancreatic juices to the small intestine. Almost done with this slide. The large intestine or the colon, its primary job is to reabsorb water. You use about six or seven liters of water a day for digestion. You couldn't afford to lose that much fluid. You basically always have diarrhea. The large intestine will actively reabsorb that water and then it's gonna temporarily store fecal material before you defecate. And then the microbiome is there, which I don't wanna downplay that. This is not a microbiology class, but the microbiome's a huge deal. It's making nutrients for you, helping digest your food, stimulating immune system, et cetera. The rectum is where we temporarily store feces before defecation, which is how we eliminate it, and the anus is going to be where we eliminate our food. All right, so that's gonna be everything from ingestion, deciding what to put in your mouth, mechanical digestion, chemical digestion, absorption, and elimination. That's the 10,000 foot view. All right, challenges of digestion. So the multiple tasks of the mouth, we just talked about all the main things your mouth is doing. It's cutting and grinding and tearing your food, but it's also lubricating it. It's exposing your food to your immune system, vice versa, et cetera, et cetera. Passage to the diaphragm, I mentioned hiatal hernias causing problems for some people. I had a patient years ago that had one, but he could basically control it. When he felt it slide, when he felt his stomach sliding up through the diaphragm, he would drink a whole bunch of water. This is not medical advice, but he'd drink a whole bunch of water to fill his stomach, and then he would hop down the stairs, like trying to use gravity to get the stomach to slide back down, and it would work sometimes. So movement through the GI tract, it's very important that things keep moving. That's one of the main functions of fiber to keep you regular and keep your intestines constantly churning away. Lubrication of food, we talked about saliva there. Also, we can drink water and stuff while we're eating. That's why I actually recommend you don't drink a ton of water while you eat. I actually like drinking most of my fluids between meals. I don't drink a ton of liquid with meals, because if I do, I feel like I'm like swallowing my food like a snake, right? Like I take big bites and just kind of swallow it down. If you don't drink a lot of water, it forces you to properly chew and lubricate your food more naturally. The digestive enzymes, we've already talked about a few, but we'll cover them more later, and then getting rid of waste products. So the mouth, what do we do? Obviously, you see there, we taste in the mouth. So that's obviously a big deal, because we want to eat foods that we like. We've covered that already. But from a digestive standpoint, the function of the mouth is to chew the food, to break it up, and then mix it with saliva, which means that it's gonna be lubricated so you can swallow what's called a bolus from the back of your mouth down into your throat. And then it also, like I said, there's lysozyme, there's antibodies, like secretory IgA antibodies, so your immune system is playing a role there in the mouth. And then you have digestive enzymes. So carbohydrate digestion begins in the mouth, with a chemical digestion, begins in the mouth, with an enzyme called salivary amylase, and then fat digestion begins in the mouth as well, lingual lipase, we talked about that. All right, so taste, aroma, texture, and temperature also affect flavor. I think the smell of your food actually has a much bigger impact on its flavor than you'd imagine. Taste is purely a combination of sweet, salty, sour, bitter, and umami, which is a savory flavor. But flavor is, some would say 90% of it is things like aroma and texture. It's all these things rolled into one. One of the neat things is while you're chewing your food, you're smelling it from the inside of your nose. Obviously you smell food from the outside of your nose too, but as food is in your mouth being chewed up, those smell particles, if you wanna call them that, their odorants, whatever you wanna call them, they're actually entering your nostrils internally and they're reaching your nose that way, so kinda cool. All right, then we talked about swallowing already. So we're now through the mouth, we're through the pharynx, now we're in the esophagus. So the esophagus is gonna use what's called peristaltic waves of contraction. So the smooth muscle of the esophagus is gonna squeeze food down into your stomach. We talked about those finkters and their importance because the esophagus should be closed and it's only opened with swallowing. We called our food a bolus before, so we talked about that, so that bolus of food. Now in the stomach, it's gonna be syruplified as the term I like to use. The hydrochloric acid in the stomach acid is gonna denature proteins, unraveling them, and it's gonna liquefy your food and turn it into this syrupy mass called chyme, so they call it a semi-liquid mass, but I think syrup is a good analogy. And then that pyloric sphincter I've already mentioned, it will pop open and let that chyme be squirt from the stomach into the small intestine. So the intestines, we have both small and large, so what you're seeing here on the right side would be the large intestine. So the small intestine does have three segments and they are called the duodenum, which is about the first 10 inches or so. I call that the mixing bowl. It's where the chyme from your stomach is gonna be mixed with the bicarbonate, the buffer from the pancreas to neutralize the acids. It's gonna be mixed with all the digestive enzymes from the pancreas, which will break down carbs, lipids, and proteins. And it's gonna be mixed with bile coming from the liver and gallbladder. So those four things are gonna be mixed together in the duodenum and then it's gonna be churned up. That's called segmentation. Peristalsis is when you move things from one area to another. Segmentation is when things get churned and mixed together. And then we have the jejunum. So I mentioned earlier that 90% of digestion and absorption takes place in the small intestine. Well, 90% of that takes place in the jejunum. So the jejunum is where most digestion and absorption takes place. Then the third portion, the ilium, that's gonna be the longest portion of the small intestine. And its primary job is just to catch digestion products. It also, it reabsorbs bile. That's gonna be one of the functions of the ilium. It reabsorbs bile. So bile is normally gonna be used and then reabsorbed. But if you don't want that to happen, remember bile is made of cholesterol. So if your cholesterol is high, you actually would want some of this bile to escape out of your body. Then your liver would have to use new cholesterol to make bile. That's why high fiber diets that capture this bile and carry it through you into your fecal material, they will lower your cholesterol. It's kind of cool. All right, so the duodenum, jejunum, and ilium, that's the small intestine. The large intestine, you see here, we have the cecum is the pouch that captures material from the small intestine with the little appendix dangling off a bit. Then we have the ascending colon up the right side, transverse colon across the top, descending colon, which then turns into the sigmoid colon because of its S shape there. And then the rectum and anus, rectum being where we temporarily store feces and the anus is how we get rid of it with elimination. Okay, we talked about how the large intestine reabsorb all that water, so we're good there, we've covered that. Okay, I've covered peristalsis. Remember that's the multiple layers of smooth muscle will squeeze material from one location to another. Imagine like squeezing the bottom of a tube of toothpaste. That's peristalsis. It's forcing the toothpaste in one direction. Segmentation is more of this churning action. And we've talked about that already as well. So we'll, I think we've said everything we need to say there. Okay, sphincters, we've talked about them. This would be that lower esophageal sphincter. Your entire stomach could be full of stomach acid. And your stomach can tolerate that because it has a layer of mucus that protects it. But nowhere else can, right? That's why acid has to be neutralized as it enters a small intestine and you don't want acid splashing in your esophagus. If you've ever had heartburn, that is acid irritating the lining of your esophagus. It doesn't irritate the lining of your stomach that way, but because of the mucus. So this, as long as this sphincter's working correctly and it stays closed, you should be good. But if this sphincter is open, when it shouldn't be, that's when stomach contents can splash up into the esophagus and cause heartburn. And if you have chronic persistent heartburn, that's called GERD or gastroesophageal reflex disease. All right, the secretions of digestion. So we see the salivary glands are gonna produce saliva. The stomach produces gastric juice, which is a combination of acid, stomach acid, and the pepsinogen, which becomes pepsin and enzyme that digest proteins. The pancreas, we've already said the pancreas produces bicarbonate, which is a buffer, and then pancreatic amylase to digest carbs, pancreatic lipase to digest lipids or fats, and the proteases to break down proteins. The liver makes gallbladder, gallbladder doesn't, it makes bile, bile salts, which become bile. And the small intestine is going to have juices and mucus, but then also the small intestine is going to have in what's called the brush border at the lining of the small intestine. There's going to be the enzymes that finish off that last little bit of digestion before we absorb our food. All right, we talked about saliva. Saliva is almost 100% water. It's got lots of other things in it too, but water's a big part of it. We make, again, six, seven liters of water a day is needed to digest our food, and we've talked about enzymes now. All right, pancreatic juice and intestinal enzymes. We've talked about them several times now. We've talked about bicarbonate being a buffer, so we've hit all that. Let's talk about bile. We've talked about it, but we haven't said what it does. Bile doesn't actually digest anything. Bile emulsifies fats. So remember, oil and water don't mix. So imagine if you look at like a salad dressing, how the oil and water separate. Well, that would happen in your gut without bile. So bile is an emulsifier, kind of like soap that you use shampoo in your hair. So what it does is it takes these large, fat globules and it emulsifies them, which means it temporarily mixes them with water. So instead of having one huge fat droplet, imagine having 1,000 tiny ones. Well, the benefit of that is the surface area of those 1,000 tiny droplets is so big that the digestive enzymes can act on it, can digest it. So instead of having all this fat that would just pass through you, it's temporarily broken down and emulsified so that it can be digested. So that's a really important point. Remember, bile does not digest fat. Bile makes fat digestible. Bile emulsifies fats so that your digestive enzymes, your pancreatic lipases, for example, can actually digest it. So emulsification, that's a really important word. All right, we talked about how it's made by the liver, stored in the gallbladder. So if you have a gallbladder, you will have more bile and it will be more concentrated, more effective. So if you have a functioning gallbladder, you can eat quite a bit of fat. If you don't have one, so like if you've ever had your gallbladder removed, you'll notice that that's usually an issue. So if you don't have a gallbladder and you try to eat too much fat, then a whole bunch of it will pass through you and that leads to pain and abdominal pain and bloating and gas and diarrhea and those kind of things. But your body does seem to get used to it. Like I said before, I can eat a fatty meal now without any trouble. But when I first had my gallbladder out, which was, I don't even know, maybe 10 years ago now, that was not the case. All right, that's the function of bile. So the pH scale, you just see the key there is that gastric juice is a pH of two. Your stomach can tolerate that because of that mucus lining that protects it. But no other part of your GI tract can, which is why we have to keep gastric juice out of our esophagus or else you get heartburn. And we have to neutralize gastric juice with bicarbonate, that buffer, when it enters the small intestine. Okay, quick summary here. I think we've said like everything here. It makes you read this slide, but yeah, I don't, there's nothing that we haven't mentioned here now. So we'll go ahead and go over that. The final stage reflection activity. So match these. We have, just go ahead and look at them. So bacteria, they ferment some fibers. So remember, one of the functions of the microbiome is that we eat foods that we can't digest, but the bacteria and other microbes living inside of us can. So soluble or fermentable fibers is a perfect example of that. Where we consume, like if you eat beans or something, well, some of the fiber in there is a soluble fiber that you can't break down. You don't have the enzymes to digest it, but the microbes inside of you will ferment it. So why do we want that to happen? Well, as these microbes ferment these fibers, they actually make fats. They make short chain fatty acids, like butyrate's a good example. But then those fats actually will feed the cells of your large intestine. And the cells of your large intestine will use that fat for the energy they need to make mucus. And that leaves a nice beautiful mucus layer around the cells of your intestines. So if you consume fiber, your intestines will directly be healthier. And then those bacteria will also make other vitamins for us and do all sorts of cool things, but that's a big deal. All right, fiber retains water. That's why you don't wanna just eat fiber. If you're constipated, you will probably want to increase your fiber consumption, but I never tell someone to increase fiber consumption without also increasing water, drinking more water. So as your fiber intake goes up in your diet, your water intake should go up too, because you take the fiber and it's gonna pull water with it. And that's actually what helps keep you regular. Undigested residues, they exercise your intestinal muscles. So again, fiber and things that aren't digested, they are actually a form of exercise for your gut. That's what keeps you regular, keeps things moving, keeps your intestines healthy. That's why a low fiber diet and constipation will weaken the lining of your intestine. And it makes you more likely to develop certain intestinal disorders, like, we'll be a good one. So like diverticulitis or diverticulosis, you get these weak little pockets in your large intestine and they can get inflamed and irritated and that can cause a lot of problems. All right, then the colon, the function of the colon we set is to absorb water. That's its primary function and then it recycles the dissolved salt. So like minerals and things that were used in your gut are gonna be reabsorbed here. Okay, so the fate of a sandwich. Let's go through this kind of quickly because we've covered this already quite a bit and each of these separate things, carbs, protein, fat and fiber, we're gonna cover in great detail in later chapters. But carbs, so the carbs from your sandwich, all of them are gonna be mechanically broken down in the mouth, but then the enzyme salivary amylase will begin the digestion of carbohydrates in your mouth and then you'll turn that food into a bolus, you'll swallow it. Carb digestion doesn't happen in the stomach because the stomach acid destroys the enzymes. Then when you get into the small intestine, the pancreatic amylases will further break down the carbs from that sandwich and then they'll be absorbed and all of those should be absorbed so there won't be any excretion. Let's skip fiber. So protein, again, you break it down in the mouth but there's no enzymes in the mouth for digesting proteins. The stomach is remosed, the work occurs. You see little dots kinda showing remosed the activity is. In the stomach, you're gonna denature those proteins and also begin the digestion with an enzyme called pepsin. And then once you get to the small intestine, the proteases will take over, you'll break down those proteins and individual amino acids and absorb them. Then with fat, you see very little occurs in the mouth with an enzyme called lingual lipase. Not much occurs in the stomach and then when we get to the small intestine with the help of bile, the pancreatic lipases will break down the lipids and then we'll absorb them but absorption's gonna be a lot different here. We'll cover this more in the chapter on fat but the fat's gonna be absorbed into the lymphatic system instead of into your circulatory system. So into lymph fluid instead of blood. And then fiber is not gonna be digested. When you get into the colon though, when you get to the large intestine, some of the fibers will be fermented by gut bacteria and the rest will pass through you. You know, maybe 70% of the dry weight of your fecal material is like dead bacteria and fiber and these kinds of things. All right. So group activity, we obviously won't do this but you should, this is a great way to practice. I always recommend putting all the organs of the GI tract in order and then make sure you kind of review, maybe use flashcards or something and kind of review the function of each of the structures. All right, so now we've digested our food. Let's absorb it. So I've already mentioned the small intestines where most absorption takes place. It's around 10 feet long, maybe a little longer. We talked about the three parts, the duodenum, the mixing bowl, the jejunum where most of this takes place and the ilium which reabsorb its bile and is the longest part. Large surface area, absolutely. The small intestine has folds in it and then each of the folds have things called villi that look like fingers and then each of the cells that line those fingers have microvilli or microscopic fingers. So huge surface area. How I like to look at it this way. If your small intestine was just a hollow tube, right with no wrinkles, no fingers, you would absorb depending on who you ask, you would absorb 600 to 3000 times less of your nutrients. So this huge surface area, very, very important. All right, the different types of absorption, I won't make a huge deal out of this, but there are different ways that things are gonna be absorbed. You see simple diffusion, things just move from an area of high to low concentration. Facilitated diffusion, you're gonna need carriers that actually will carry things in through your intestinal lining. And then we have active transport. So diffusion is a passive process. It just doesn't require energy. Active transport, you see a little arrow there. Active transport will require energy. You physically have to carry some things in, which means that it costs some energy to digest and absorb your food. We'll cover that later with metabolism. It's called the thermic effect of food. So between 10, maybe 10 and 25 or 30% of the calories in any meal is gonna be needed to digest and absorb and transport those nutrients. And that's called the thermic effect of food. And it's around 10% of your daily energy expenditure that most people would say. All right, so you can read through here about different types of transport or are needed for absorption. So not a massive deal there. All right, we mentioned this before. So you look at this picture here just to see the massive surface area we're talking about here. So the intestines has little folds. You can see right there in itself. And then each of those folds is lined with these fingers called villi. And then each of the cells is lined with microvilli. So we have folds with fingers, with microscopic fingers, huge surface area. So we've talked about all that. All right, goblet cells we haven't mentioned. So goblet cells are going to be, so not all of the cells that the intestine are there to absorb things or secrete things. Goblet cells are gonna make mucus. Mucus is important because it keeps things lubricated and keeps things moving through our GI tract. But it also forms a barrier that keeps those trillions of bacteria and other microbes living inside of you away from you directly, right? So this is something that, it's kind of hard to wrap your head around. But your GI tract is a hollow tube running through your body. It's not, it's actually, it's a surface. It's not inside of you. It's almost like a really dirty skin, right? You think about your skin out here, but you have this internal skin, this hollow tube running through you. And there's a barrier between what's inside that and actually you. So it's a hollow tube running through you. That's why you have, you know, depending on which yogurt commercial you watch, you know, 70 or 80% of your immune system is lining your intestines. Well, that's why. There's trillions of microbes in there inside your gut and we don't want them inside our body. So it's that we keep them in a hollow tube running through our body. So this mucus protection is actually a pretty big deal. Okay, so we talked about villi being the fingers that increase surface area and they regulate absorption. Microvilli, just, you know, that's microscopic little fingers on each of the cells. We talked about mucus producing cells and obviously we have cells that produce enzymes, cells that their primary job is absorption. Okay, so now the absorption. So, you know, in almost everything, right? Your water soluble vitamins, carbohydrates, proteins, and small fats, right? Not all fats are large, right? We have what are called small chain. We'll cover this in the chapter on fat, but we have small chain fatty acids like butyrate that I mentioned earlier, that are tiny. We have medium chain fats. Maybe you've heard of like medium chain triglycerides. You can buy those at supplement stores or coconut oil has a lot of these medium chain fats. Those are absorbed. They don't need, those don't need bile. Those don't need to use a lymphatic system. So, carbs, proteins, small fats, and water soluble vitamins are gonna be absorbed right into your blood and they're gonna travel to your liver where your liver decides what to do with them. Should I use them, store them, get rid of them, etc. Large fats, your large chain fatty acids and your fat soluble vitamins, which are vitamins A, D, E, and K, they are gonna be carried in your lymphatic system. And I mentioned why earlier, they have to be put in these huge packages, which are called kylo-microns and you don't want these big fatty packages swimming through your bloodstream right away. So the kylo-micron is gonna travel through your lymphatic system while these fats are gonna be kind of distributed and then what's left over is gonna be dumped into your bloodstream and then it's gonna be an easier, easier task for your body to handle. Okay, we won't talk about this a ton here. Again, this is just the entire circulatory system. We're just focusing on that adjusted system here. This is, if you've taken my anatomy classes, you've learned all this already, but we're gonna focus on the liver. So let me show you, I think there's a picture up here. All right, so let's look at the, we can go back and define them first, but the liver is interesting, right? With most organs, you have about half of the blood traveling to it is arterial blood to feed the organ and then half of it's gonna be venous blood, which is carrying that used blood back, but the liver isn't like that. About 80% of the blood flowing through the liver is venous blood, meaning that it's on its way back to the heart, but that's not a bad thing, right? So you've got, let's look at the different players here. So we have the hepatic portal vein, which I'll show you on that picture. We have hepatic arteries and then we have the hepatic veins. So the hepatic, just like other organs, the hepatic artery carries blood to the liver to keep the liver alive and the hepatic vein is gonna carry this blood back from the liver to the heart, but the hepatic portal vein, the portal system is what's special. So let me show you that here. So it's not 50-50 because remember what the liver, the liver is going to get all the blood coming from your GI tract. So almost all of it, not here at the tail end, but so everything that's gonna be absorbed from your GI tract, from your small intestine, for example, into your blood is gonna travel to the liver. So the liver gets the first crack at everything. That's why there's so much venous blood. It's not like it's this used blood. It's full of nutrients. It's full of everything that you just absorbed from your GI tract, but it goes to the liver on the weight of the heart because the liver is where we decide what to do with this. Oh, okay, here's some glucose. I'll turn it into glycogen. Here's an amino acid. I'll turn it into a different amino acid, right? Here's a fat when it finally gets there. I'll use it for fuel or I'll store it. Oh, we have too much glucose and our glycogen is already full. Well, I'll turn that into fat. Oh, here's a toxin. I'll detoxify it. So the liver gets the first crack of everything traveling into your GI tract and that's what this portal system, that's what makes the portal system special. All right, so you can read to the rest of that if you want, but okay, so the lymphatic system, I'm not gonna, again, there's entire chapter on this in anatomy, but the lymphatic system is like a second, I call it a second circulatory system because students know what I mean, but it's not a circulatory system because a circulatory system is it pumps away and then back. The lymphatic system all travels in one direction. So there's not a pump, there's not a second heart, right? So it's just the movement of your muscles in your body that actually pump lymphatic fluid and all lymphatic fluid is traveling towards your heart. So we talked about why we use that with the, why we use that with fats because they can't just be dumped straight into the bloodstream. Okay, gastrointestinal microbes, we talked about these quite a bit already, but factors influence bacteria. I mean, first of all, like the biggest factors happened as you were born and early in your life, right? What kind of microbes were you exposed to? It takes about three years to develop an adult microbiome. So those first three years are critically important. Did you need antibiotics? Did you play in the dirt, right? Were you born vaginally or BSC section? Were you breastfed or not? These are all factors that, what was your parents' microbiome, right? What was the microbes in your home? So all these things will influence which microbes are inside your gut. But then as an adult, we can still impact them for sure. It's kind of like the old, you know, where many of you are from Iowa, so the whole, if you build it, they will come, right? From the movie was I called Field of Dreams. Your microbiome is going to change and adapt to the way you feed it, you know? And not just that, right? Sleep, stress, all these things impact your microbiome, but what you eat is a big, big component. So you see here probiotics and prebiotics. So with your diet, so probiotics, you've probably all heard of probiotics, but probiotics are living organisms that you consume, right? You eat yogurt, you take a probiotic pill, you eat kimchi, these kind of things. Those are probiotics when you actually consume living organisms to try to impact your microbiome. Prebiotics are different. Prebiotics are when you feed the microbes their food, which is mainly fiber. So probiotics would be eating living organisms. Prebiotics would be consuming fiber, especially soluble fibers or fermentable fibers because then they will feed the microbes that are already there. Now, if you asked me which is better, I would generally say for most people, prebiotics, right? Actually, because if you think about you eat yogurt, most of those microbes are going to be killed by your stomach, but nothing wrong with probiotics and thankfully we can do both. But if you had to choose one or the other, generally speaking, I think most people are better off just eating a diet that feeds the good microbes that are already there rather than just adding more microbes. Sorry, my nose is itching. All right, so that's it. So a probiotic is living organisms. Prebiotics would be food for those organisms and you should do both. So you eat probiotic foods as often as possible. This is not medical advice, this is just generally good advice for most people. Consume soluble fiber. I recommend trying to make sure you get at least six grams a day of soluble fibers. And if you do those things, that should improve your gut health. All right, so we talked about what the microbes do. They, you know, about five to 15% of the calories that you absorb in a day are gonna come from digestion of these things. They ferment fibers and they make those healthy fats for us. They do all sorts of cool things. They make vitamins for us. The big one would be vitamin K, but then some of the B vitamins as well. All right, so microbes in the GI tract are influenced by prebiotics and probiotics. So if you build that they will come, they will respond to what you eat. I mentioned earlier that if you eat seaweed every day, sooner or later, the microbes in your gut will adapt and be able to digest seaweed better. If, you know, obviously they don't just come from nowhere, you would have had to have been exposed to them, probably from the seaweed you were eating, but that, those kinds of things will happen. So over time, your microbiome will change. I think that's one of the reasons that it takes a while. If you try to do a completely different diet, your body has to change enzyme levels, your microbiome has to change. It takes time. If you shift your diet, it can take some time for sure. All right, we talked about, so I've already defined probiotics and prebiotics. You can read that. All right, the hormones and nerve pathways that are needed, this is just basically like everything else in your body. Your body, you know, the hormones are basically going to coordinate activities of your GI tract for digestion. They're gonna turn it on, turn it off. Same thing with the nervous system. So your body's trying to maintain homeostasis, which basically means that, you know, if there's food in your gut, you should deal with it, so you can return back to a normal situation where it's kind of empty. I'm a big believer that we should have periods where we eat and periods where we don't for sure. But so some of the hormones you see here, we'll talk about them in the next slide, but gastrin, secretin or secretin and colocysticinin are the key ones here. But notice what happens when there's food, when there's food, when your stomach is filling, it's gonna trigger the release of the hormones that are needed to stimulate digestion so we can deal with it. And we can get back to a situation where your stomach's empty. That's how our body uses negative feedback to maintain homeostasis. So these three key hormones here. So gastrin is, anytime food is in your stomach, it's gonna trigger the release of gastrin. And notice what it does. It tells the stomach, generate more acids. There's food sitting here, we need to break it down. Secretin or secretin, I generally call it secretin. This is gonna be triggered by when chyme is entering the small intestine and that's gonna tell the pancreas, release bicarbonate, before this acid eats a hole in the duodenum. So secretin is gonna tell the pancreas, dump your juices into the small intestine so that we can bring the pH up and digest the food. And then cholecysticinin is gonna respond to fat or protein in the small intestine and it's gonna cause the release of bile and it's gonna cause the release of digestive enzymes to digest our food. So that means that this starts with your stomach acid. So food entering your stomach causes the stomach to become acidic. That acidity triggers the signaling that's gonna lead to these other hormones being released. So it's basically, you wanna have a nice acidic stomach because it stimulates digestion of all the macronutrients. But those are the three key hormones, not the only ones involved, but those are the key ones involved. All right, the system edits best. So sensitive and responsive to the environment. Immunity against intestinal diseases and defense against foreign invaders. What does that mean? If you have a really healthy GI tract, then it's really intact. You don't have what's called a leaky gut or intestinal permeability. You have a beautiful thick lining of mucus that's protecting your gut from its contents and your immune system is there and it's monitoring things. You don't have bacteria and toxins entering your bloodstream. So that's why you want a good, healthy gut. And there's lots of things that can impact our gut health. Medications, diet, stress, exercise, all those kind of things. All right, so the health of the GI, how do you get a healthy GI tract? Keeping the blood flowing, obviously. So being physically active exercise helps a healthy gut for sure. Lifestyle factors, don't smoke, manage your stress, get plenty of sleep, eat well, all the regular ones. Don't take drugs or medications that impact it unless you need to. The types of food eaten, that's one of the main things we can control. Right, if someone tells me I want a healthier gut, then I would tell them, for most people, you need more water, you need more fiber, you need more fermentable fibers, you need probiotic foods, the things we've been covering in this whole lecture. And then if your diet is all the other things, balanced, moderate, adequate, variety, make sure you're getting all the nutrients you need, then that should be how you have a healthy intestine. All right, which of the following helps to coordinate all the digestive processes in the body? That would be both your enteric system and nervous system. That's just the entire body, is controlled by hormones and nervous system. The nervous system is really in charge. You have a whole division of your nervous system here called the enteric nervous system that subconsciously controls your gut. And then we have other systems like your parasympathetic nervous system plays a big role in digestion. So the nervous system is really in charge, but then the hormones control kind of the region by region activities, I guess you'd say. But nervous system does that as well. Think about an example of a gastrocolic reflex. When you eat, oftentimes within 30 to 60 minutes after eating, you have to defecate, you have to go to the bathroom. Well, that's reflexes. That's the stomach saying, hey, I'm full. Let's clear room ahead of us because this food's gonna be coming by. So that's an example of how your nervous system's involved as well. Okay, summary. Now that the lesson has ended, you should have learned how to identify the actions of the organs, muscles, and secretions. We covered all those. Describe the anatomical details of the intestinal cells that facilitate nutrient absorption. Also talked about that. Explain how nutrients are routed in the circulatory system from the GI tracts. We talked about how the liver gets a crack and all the stuff coming from the blood. We talked about how fat, large fats, sorry, and fats of the vitamins have to be carried through the lymphatic system. And then we talked about the bacteria quite a bit, hormones and nerves we just did. Okay, so this isn't everything about the digestive system. Go into the YouTube channel, look for the digestive system playlist. That'll be a big help and then we'll cover metabolism once we've absorbed these things later. But that's a really great start. So I really hope this helps and really think about this. When you eat, eat the foods that like you. Eat the foods that you can properly digest and absorb. Make sure you're feeding your microbiome. Make sure you're eating foods that aren't irritating your gut and you will be healthier. Your entire body will be healthier, right? I firmly believe, and this apocrates said it, all diseases begin in the stomach or the gut. But I truly believe that you can only be as healthy as your GI tract. You can only be as healthy as your intestines. All right, so I do hope this helps. Have a wonderful day. Be blessed.