The Science of Everything Podcast - Episode 137: The Digestive System

Episode Date: July 23, 2023

A journey through the digestive system, beginning with the mouth and progressing through the esophagus, stomach, small intestine, and large intestine, we consider the structure and function of all key components of the gastrointestinal tract and their roles in facilitating the metabolism of our food. We also discuss the contribution of accessory organs, including the liver, gall bladder, and pancreas. We conclude by considering a few myths about digestion. Recommended pre-listening is Episode 25: Tissues, Organs and Systems. If you enjoyed the podcast please consider supporting the show by making a PayPal donation or becoming a Patreon supporter. https://www.patreon.com/jamesfodor https://www.paypal.me/ScienceofEverything

Hello, you're listening to The Science of Everything podcast, episode 137, the digestive system. I'm your host, James Fodor. In this episode, we're going to have a journey through the human digestive system and talk about how we digest and obtain nutrients from our food. Obviously, this is going to include a coverage of the major components of the digestive system, including the mouth, the tongue, the esophagus, the stomach, the small and large intestines and the accessory organs, including the liver and the gallbladder. Recommended pre-listening for this episode is episode 25, tissues, organs, and systems.

And without further ado, I think we're just going to jump straight in and start talking about the digestive system, because there's a lot to cover. So the digestive system is a very large organ system in terms of the amount of space that it takes up in the body, and it's probably one of the more familiar ones as well. The digestive system is responsible for processing and absorbing. absorbing nutrients from our food and then expelling the waste of the non-degestible products. The complete human digestive system is made up of the gastrointestinal tract plus various accessory organs. So the gastrointestinal tract or the GI tract is a passageway that runs from one end of the body right through to the other.

In fact, technically, the inside space within the gastrointestinal tract is actually topologically at least outside of the body. The food doesn't actually truly enter the body until it's absorbed across the epithelial cells that line the gastrointestinal tract, but we'll talk about that later. The GI tract is made up of the mouth through to the esophagus, through the stomach, small and large intestines, and out to the anus. The accessory organs include things like the tongue, celery glands, the pancreas, liver, and gallbladder. Here in this podcast, we're going to focus more on the GI tract.

We will talk about the accessory organs, but many of the many of the body. those, particularly the liver, have many other functions beyond digestion, which we won't have time to cover in this episode. So before we go through and talk about the structure and function of the different components of the digestive system, let's pause and think about what the requirements are for digestion, what has to happen for food to be digested and taken up into the body. The ultimate aim is to get the food in a simple form, so breaking it up into its component molecules. So the first requirement is to break the food up into small pieces so that the enzymes can come into contact with more surface area of the food and so also that it can be kind of absorbed and move around the

body as needed. And that happens mostly in the mouth and to an extent in the stomach as well where there's churning that occurs. Obviously the teeth and tongue play an important role in that. So just physically breaking it up into smaller pieces. Now the next thing that has to happen is that the complex molecules like your carbohydrates, proteins and so forth need to be broken down into simple components, particularly amino acids, fatty acids and simple sugars. Now that happens via enzymes, and this occurs mostly in the small intestine and also in the stomach. To a small extent it occurs, it begins actually in the mouth as well, as we'll talk about,

but mostly that occurs in the small intestine and the stomach. Now once we've broken down the complex molecules into smaller components, those simple components need to cross an epithelial cell layer, so they actually need to move from the outside of the body, like in the GI tract. cross a cell layer and then actually be in the body proper and enter the bloodstream. That happens in the small intestine, that absorption part of the digestive process. And that's sort of the small intestine's role. So the mouth and tongue is kind of where food is broken up into smaller pieces and then it's

swallowed down. The stomach stores food preparatory to its entering into the small intestine. The stomach also has other advantages in terms of why it's separated from the small intestine. It has a very low pH, meaning it's very acidic. and that helps with certain enzymes, as we'll discuss. Also, the absorption process takes time. It takes time for the enzymes to break down the complex molecules,

and so the stomach stores food preparatory to entering into the small intestine, where it's then absorbed. So far, we've kind of introduced the key things that need to happen and explained how the mouth, stomach, and small intestine interact there. But you might be wondering how the accessory organs kind of relate to this and what the large intestine is for. There's a couple of complications to this story. One is that there's a need to dissolve fats as well as carbohydrates and proteins.

And the thing about fats is that they're not soluble in water. So in order to get around this problem, the body has to find a way of emulsifying fats, essentially surrounding them with molecules that allow them to be, well, not dissolved, but taken up into water. And that occurs through the help of a substance called bile. That's produced by the liver and then stored in the gallbladder until it's not. needed. So those are kind of two key digestive roles of the liver and also the gallbladder. The liver does many other things, but as I said, we won't discuss all of its functions today.

Another problem is that, as I mentioned, the stomach is very acidic, and there's a need to neutralize that acid before the material passes into the small intestine. And that's what the pancreas is for. The pancreas produces a substance which neutralizes the stomach acid. A final problem is that not everything can be digested, particularly there are plant substances, such as cellulose that humans cannot digest, and that and various other things that we don't digest need to be excreted, and we need to extract water and a few other things from it and then excrete it.

And that's the role of the large intestine, essentially, to deal with non-degestable material. So that's how all the components of the digestive system work together. It's sort of fairly logical, as far as these things go in the human body, at least, in terms of why the different organs exist and what their different roles are. Having gone through in very high level

about the different covering the different organs in their roles. Let's now move through and we'll talk about the structure and functions of the GI tract in general and then the different components thereof and we'll come back to the accessory organs briefly at the end. Okay so let's talk about the gastrointestinal tract kind of as a whole and its structure. So fundamentally it's a very long tube although of course there's different sort of components of the tube and different regions of it from the esophagus through the stomach and the small and the large intestines. But all of these different regions, have a common four-layer concentric structure. So think of it as like four tubes inside each other

or layers of the tubes surrounding each other. So there's the mucosa, the sub-mucosa, the muscular layer, and then the cirrhosa. So we'll go through those in turn. By the way, this is going from inside to outside. So the mucosa is the innermost layer and then the serosa is on the very outside. So let's go through those from inside out.

So the mucosa is the innermost layer of the gastrointestinal tract. It surrounds the lumen. So the lumen is the space in the very. middle. The lumen is kind of the part that's sort of outside of the body, technically speaking. The mucosa surrounds the lumen, and it comes into direct contact with digested or partially digested food. So the mucosa has a few layers of its own. There's an epithelium layer, so that's where the digestion and absorption and secretary processes occur, because it's

the very innermost layer. Then below that, inwards to that, is a layer of connective tissue, and then inside of that, again, is a layer of smooth muscle. So the smooth muscle just kind of provides support and structure, and the connective tissue just kind of connect them together. So the overall purpose of the mucosa is to perform many of the direct secretion of enzymes and absorption of food, as well as to just kind of contain the material. Now, the sub-mucosa below that contains the nerves that innovate the gastrointestinal tract, as well as blood vessels, so that's important, obviously, because we need to get the nutrients from the lumen, where they originally are, into the bloodstream.

and so the blood vessels are contained in the sub-nucosa. That's kind of like the second layer. In addition, the sub-ecos also contains elastic fibers like collagen, so that helps to give the whole GI tract flexibility while it's also being able to maintain its shape. So it's kind of elastic. That's very important because unlike other parts of the body, well, obviously, there's the lungs that change in size and shape

and muscles change in to some extent. But the GI tract has to be very flexible because it can go from essentially being empty to being filled with large volumes of food, and the food moves along it sequentially. So it needs to be very flexible, and so collagen elastic fibers which support that in the sub-mucosa. As I mentioned, there's also blood vessels, nerves and lymphatic vessels that run through that layer. The layer below that, the muscular layer, contains multiple layers of smooth muscle. So smooth muscle is muscle that is not under conscious control.

So you may recall there's cardiac muscle. that's special muscles in the heart. Then there's skeletal muscle, that's muscles that are under conscious control. And then there's smooth muscle, those are those that aren't. And smooth muscle is particularly used for bodily support functions, as well as for sort of basal metabolic functions, such as digestion, that we don't have conscious control of it. So you can't consciously decide or voluntarily decide to contract or relax the smooth muscles in your body,

including in the digestive system. But it's still very important because they need to be able to move the food along. So in many parts of the GI tract, there's actually two main layers of muscles in the muscular layer. There's a layer of circular muscles and a layer of longitudinal muscles. Now, we'll talk about more about why we have these kind of two different layers when we get to discussing swallowing and peristelsis. But basically, they perform different functions. So longitudinal muscles are aligned along the length of the GI tract, whereas circular muscles are arrayed around it. So circular muscles can kind of pinch it or widen, like basically turning on a tap to constrict the flow of water or opening it up to allow it through, whereas longitudinal muscles kind of constrict it long ways.

So they serve different purposes, but many parts of the GI tract have both of those separated by a layer of connective tissue so that there's a variety of ways that we can control, not consciously, but that the shape and the diameter and such of the GI tract can be controlled. and that supports the movement of food through the GI tract. Now the final layer, the outermost layer, is the cirrhosa, and that's just a layer of loose connective tissue that helps to connect the whole GI tract to surrounding tissue, and it also provides sort of cushioning. It's coated with mucus as well, so it helps to prevent friction as the intestines move around,

as they change in shape, as food is consumed and then digested, so there's a fair bit of kind of motion of that relative to the rest of the body, and so there's a need for a coating of mucus around the outside to prevent rubbing against other tissues in the body. So just to recap there, we've got the innermost layer of mucosa, and the kind of important part of that is the epithelial layer, which is where the actual key actions of releasing enzymes and absorption and so forth occur. Below that is submucosa, where the blood vessels and nerves are, and that's where the nutrients ultimately going in the blood vessels and the submucosa.

Below that you've got the muscula with the circular and longitudinal muscles that help the food move through the digestive system or the GI tract and then Atomos is the cirrhosa which kind of provides connection to the rest of the body and mucus to prevent friction So that's a quick overview of the basic structure of the GI tract I should mention as well that I'm not going to give the like the proper names and anatomical names of all of the components or regions occasionally I'll give them like mucosa and submucosa, but often I'll just use a more common name or kind of describe what it does, just to make things a bit simple. All right, so, okay, and so we're going to start with the mouth, which is where the GI tract begins. I won't tell you what a mouth is. I assume you

kind of know basically what a mouth is, but the mouth in human serves many different functions, and it's kind of interesting that it actually does serve so many different functions. Here, we're talking about its function for eating and chewing and swallowing food. Let's start by talking about the salivary glands, which are actually not part of the mouth. They're an accessory organ, but I mean, they're located near the mouth, so we'll talk about them here. So the salivary glands are exocrine glands. That means that they produce and excrete a hormone or other substance, in this case enzymes, which are released into the bloodstream and travel sort of a distance from the sight of those glands. In this particular case, they actually produce saliva,

which travels through a system of ducts and then is released into the mouth. So humans produce between like 1 to 1.5 litres of saliva every day, which is kind of a lot, I guess. It's kind of crazy. Saliver is almost entirely water, but there are other things there as well, including electrolytes, mucus, white blood cells, some epithelial cells from the inside of your mouth, and importantly, enzymes. So there are some enzymes in saliva which help to begin the process of digestion, particularly for carbohydrates. We'll come to that in a bit. saliva is also important for taste. If food is not wetted and covered with saliva, it's very difficult to taste anything because we've talked about how taste works in a previous episode, but chemical detectors

on the tongue that mediate the sense of taste essentially require that the chemicals, the taste in food are dissolved in solution in order for them to come into contact with the receptors and then trigger the responses. So entirely dry food, essentially doesn't really taste like anything. Let's talk about the tongue. The tongue is a muscular organ. Maybe we don't think of it as that, but it effectively is. Its purpose is to manipulate food for chewing and swallowing.

And this is another thing that I guess, maybe at least me, we don't really think of it this way, but the tongue is essential for swallowing. You actually use your tongue to swallow, even if it maybe feels like you're just kind of using your mouth or whatever. When you swallow, your tongue actually pushes the food back into your throat and pushes it down. It's also the primary organ of taste, as we've discussed,

before. Now behind and kind of below the tongue are a series of muscles which are used to manipulate and move the tongue. It's a bit hard to sort of describe but they're contained kind of in the lower jaw and kind of the back and lower region of the throat and they're connected at a few different points so that it's quite an intricate arrangement of muscles which are connected to like the mandible bone and the jaw up near the top of the neck. These muscles allow us to have fairly fine control over the tongue and so that we can move it around and use it to manipulate food and so forth. Now, let's talk about the teeth. Teeth are a hard calcified structure, which are found in and kind of grow out of the jaws of many vertebrates, including humans. So teeth are quite interesting

and have an elaborate structure. The outermost layer that we see, which is sort of whiteish in color, is known as the enamel. And the enamel is a very hard substance, which is made mostly of crystalline calcium phosphate. That's why calcium is important for, you know, strong teeth and bones, as they say, because in the teeth case, the enamel is made of calcium phosphate, and it's effectively just a mineralized structure. So there's not any living tissue in the enamel itself, though there is living tissue below that. So below the enamel is a layer of tissue called dentin, and then inside of that is the pulp chamber where the blood vessels and nerves are. And that's why if you have teeth decay, the enamel erodes, and then it exposes

ultimately the nerves below that, and you experience pain. The roots of the teeth are embedded in the gum and keep the teeth in place. And also the roots of the teeth is where the blood vessels and nerves extend outwards from and then join up with the rest of the body. There are four main different types of teeth found in humans. Incisors at the very front, they're used for cutting. Canines just to the side of the incisors, there's the kind of long, pointy teeth, and those are used for sort of tearing. The premolars behind the canines

are used for grinding and crushing, and then the molars, the big ones at the back, they're used for even more kind of intensive chewing and grinding. As you would know, the teeth of an animal tells you a lot about its diet, and the teeth that humans have tells you that we are omnivores, so we have teeth that are designed to sort of cut and tear, which is useful for eating meat or protein-rich substances.

we also have teeth that are useful for grinding out very fibrous substances, such as used for eating plant material. That's the, like the molars and premolars. So we have teeth for a wide range of purposes. Okay, so that's a brief overview of some of the key components of the mouth. Remembering that we're focused on digestion here, and the overall purpose of the mouth is to break the food up into smaller pieces and wet it with saliva so that it can be more easily processed and ultimately absorbed in the small intestine mostly. And so you can, I think, fairly readily. see how the salivary glands, tongue and teeth or contribute to that. That brings us then to talking about the esophagus. So the esophagus is about 25 centimetres long. It's basically just a tube that

connects the back of the mouth to the stomach. Its purpose is to convey food from the mouth where we eat and the stomach where the food is stored until it's processed by the small intestine. The esophagus travels behind the windpipe, the trachea, and behind the heart, and passes through the diaphragm, which is the muscle at the base of the ribcage that helps with breathing, and then it empties into the uppermost region of the stomach. Swallowing, which in a scientific context is called deglutition, which is a word I didn't know, so that's one to remember, is the process in which a human allows a substance to pass from the mouth to the pharynx, which is the back of the throat, and then into the esophagus.

Now this is quite an intricate process, although normally we don't think about it, I suppose unless it goes wrong and we start to choke, as there's quite a few different components that have to come together. So let's kind of talk about what happens when we swallow. First of all, the mass of chewed and wetted food, which is called a bolus, is pushed to the back of the mouth using the tongue. Not the tip of the tongue, but kind of the rear and more sort of basal part of the tongue. So it's sort of pushed to the back. at the back of our mouth there is a little flap of tissue hanging down called the uvula you can see that if you open your mouth and look into the mirror the uvula is there so that the bolus of food

isn't pushed upward by accident and thereby entering the navel passage that can happen if you're sort of not careful particularly with when one is drinking but it's not a very pleasant experience and that's because the nasal passage is directly connected to the back of the mouth, which in turn is also connected not only to the esophagus, but also to the

trachea, which is the windpipe. And this is to what I was saying before, that the mouth serves many functions in humans. It serves the function of eating and tasting, but it also serves a function of breathing, because we can breathe through the mouth, and it's directly connected to the nasal passage and the trachea. It also serves a third purpose of communication and talking. And it's sort of interesting to imagine what it would be like if those three were all separated, if we could breathe, talk and eat all at the same time without the risk of choking. But that's not the way evolution designed us, and because these things are all connected, we need to kind of separate things so that the bolus of food doesn't go into our nose,

or nasal cavity, and also so that it doesn't go into the trachea. So the uveolar, that kind of flap of tissue hanging down, is pushed backwards and blocks the entrance to the nasal cavity so the food doesn't go upwards. When we're swallowing, there's also a need to block off the entrance to the trachea. that the food doesn't go down the windpipe. This is done by another piece of tissue called the epiglottis. The epiglottis is usually not visible, but it is located just below and to the back of the mouth. It consists of elastic cartilage covered with a membrane, and it bends kind of backwards and down to close off the entrance to the trachea so that the bolus of food passes down into the

esophagus instead of down into the windpipe. So when we swallow our tongue pushes the bolus of food back and downwards, and around the same time, the uvula pushes back and blocks the nasal passage, and the epiglottis kind of bends backwards and down to block the entrance to the trachea. The bolus of food then passes behind the epiglottis and then is pushed down into the esophagus. The esophagus then continues to move the food down until it reaches the stomach through a process called peristelsis. So this is a series of rhythmic contractions and relaxation of muscles. So what happens is the smooth muscles in the musculos layer of the GI tract that I talked about. That's the third layer from the inside. So the smooth layer contracts and relax in sequence in an organized way.

It propels the food forwards. The layer of circular muscle kind of just behind the food bolus will contract and that in front of it will relax. And then the longitudinal muscles behind the food bolus will contract to push it forwards. So essentially the circular layers in front of a kind of open up to allow it through. They constrict behind it to kind of form a seal to prevent it from going backwards. And then the longitudinal muscles contract, which shortens the GI tract behind it and pushes the bolsa food forward. And as that happens rhythmically, the bolsa food is moved forwards.

And this is the general process by which food or partly digested food is moved through the entire GI tract. bit, it's probably kind of easiest to visualize in the esophagus since it's essentially just a tube to connect the mouth to the stomach. The entrance to the stomach, which is at the junction between the bottom of the esophagus and the uppermost part of the stomach, that junction is controlled by what's called the lower esophageal sphincter. And generally that remains closed at all times, because you want to control what enters your stomach. It opens up during swallowing, and also during vomiting. So that's when the food goes the other way, shall we say. And that's to prevent the contents of the stomach from entering the esophagus and then being ejected back into the mouth,

which generally you don't want to happen unless you're trying to expel some nasty material. And the sphincter here is essentially just a circular band of smooth muscle as well as the surrounding tissue, which just kind of pinches off at the point of the entrance to the stomach. So it's controlled in a similar way to the surrounding tissue, but it's a particular point of control of entry, essentially. We'll meet some other sphincters as we move through. All right, so that's the mouth and the esophagus covered. Now let's move into the stomach. So the stomach is a muscular hollow organ, which is found in the GI tract. It's located between the esophagus and the small intestine. We know this. The purpose of the stomach, as we also know, is to store food until it can be

processed by the small intestine. But it's not just a storage sack. The stomach also can continues the process of chemical and mechanical digestion which began in the mouth. The stomach can vary greatly in size. It's obviously a feature rather than a bug because one of its main purposes is to store food. So when there's nothing to store, it doesn't need to be very large. When it's empty, the stomach is about the volume of a clenched fist, so that's actually quite small. But it can expand in volume to between one and two litres in size. The structure of the stomach follows that of the rest of the GI tract,

So you've got you four layers that we've talked about. The innermost layer of the mucosa, however, has a specialized structure, which consists of columnar epithelial cells. So epithelial cells are the innermost ones that form kind of a layer. And these columnar epithelial cells have these hollow regions, which are called gastric pits. And they kind of are like that. It's like a little mine shaft that goes downwards and sometimes branches a little bit. So it's an invagination of the epithelial layer, which forms these little pits on the surface of the inside of the stomach.

One of the major purpose of these gastric pits is to contain what are called gastric glands, and these glands are mostly exocron glands that secrete product into the gastric pit above, which then enters the lumen of the stomach. So essentially there's a bunch of these epithelial cells which produce special, mostly enzymes or other substances. We'll talk about what some of them are in a moment. that's produced various chemicals, which are needed for the purposes of the digestive purposes of the stomach. So they are manufactured in these pits and secreted into the shaft region, if you like, of the pits, and thence they enter into the main part of the stomach, kind of out the top of the pits,

and then enter the lumen of the stomach. There are a very wide number, and there are a very wide range of different types of epithelial cells that produce different types of products. Some of the names include foveolar cells, chief cells, parietal cells, G cells, and many more. I'm just mentioning them in case you've came across some of these names before, but I'm not going to worry too much about the different types and why they're called what they are. I'm just emphasizing that there are a wide range of types, and each of them produces a different substance or substances. I'll just talk about a few of the main ones that we're interested in here. So there are cells that produce mucus, which helps protect the inner lining of the stomach, and also by,

bicarbonate. That helps to neutralize the stomach acid when it's in contact with the very edge of the, well, the epithelial cells so that the stomach doesn't digest itself. So the mucus and the bicarbonate both help to protect the inner lining of the stomach from the very low pH. And there are other cells which produce gastric acid, which helps maintain the very low pH environment, about a pH of two in the stomach. The purpose of that, by the way, is to help certain enzymes, particularly proteases, which are enzymes that help chop up proteins, some of those require low pHs to function, and so that's one of the reasons for the low pH. Another reason is that it helps to kill or inhibit the growth of bacteria, which generally don't like low pHs. Of course, the rest of the

body doesn't like low pHs either, which is why there's the need for the mucus and the bicarbonate to protect the inner lining of the stomach. The inner lining of the stomach, though, is still slewed off and will itself be digested periodically and is then replaced. Okay, so we've got the cells to produce the chemicals that protect the stomach, the cells that produce the gastric acid. There's other cells that produce different types of digestive enzymes, such as pepsin, gastric lipase, and different proteases. There's also other regulatory substances that, like, inhibit or stimulate acid production depending on various metabolic needs. So there's a whole different range of these chemicals, including enzymes and acids and such things that are produced in these epithelial

cells in the gastric pits that are just below the surface or invaginations really into the inner surface of the inside of the stomach. So, digestion is occurring in the stomach. The stomach also produces rhythmic waves of contractions, which help to churn up the material in the stomach, which is called chime. That's a mixture essentially of acids and water and partly digested food material. So it's called chime once it's in the stomach. And the stomach engages in periodic kind of contractions, which help to churn up the material and just kind of move it around, mix it up, and help to further the digestive process. As that happens, as the peristaltic wave, as it's called, of the muscles contract and constrict

the stomach, the material is pushed downwards and kind of onwards into the stomach and and towards the rest of the gastrointestinal tube. Just as there's a sphincter at the entrance to the stomach, there's also a sphincter at the exit of the stomach. So that's, again, just essentially a circular ring of muscle and the surrounding tissue, which blocks off and closes up the exit of the stomach. In this case, we're talking about the juncture

between the stomach and the small intestine. This is called the pyloric sphincter. The pyloric sphincter is typically closed to prevent the highly acidic material of the stomach from leaving, basically it keeps the small intestine closed to the stomach material, to the to the chime of the stomach until the small intestine is ready for it, because it can only deal with so much at a time, essentially. The pyloric sphincter, however, in conjunction with these peristeltic waves of contraction that push some of the chime out of the stomach and the

pylorox ficts opens and sort of a little squirt of material comes out, and then the sphincter closes again. So there's little squirts of material of chime that are pushed out of the stomach periodically, then enter the small intestine, and then will be processed by the small intestine. The fundamental issue here is that food is consumed much more quickly than it can be digested. So there's a need for a temporary storage location for that food until the small intestine is ready for it. And that's the purpose of the stomach. And so in order for that to work effectively, the material needs to be released a little bit at a time. And that's the function of these periodic peristaltic waves, which help to push the material out of the stomach. And then the

pyloric sphincter lets in a little bit of chime each time to be digested by the small intestine. And that's our cue then to move and talk about the small intestine. So the small intestine is where most of the absorption of nutrients takes place. It's where the biochemistry of the digestive process really sort of gets going and interesting. I mean, as I said, it sort of starts in the stomach, but it's most intense and most of that occurs in the small intestine. The small intestine is about five or six meters long, and it's highly folded so that it can actually fit in the abdomen because if it was all stretched out, it wouldn't really work. It's called the small intestine, not because it's short, it's actually much longer than the large

intestine, but it's narrower. So it's small in the sense of being narrow, whereas the large intestine is wider, but it is much longer than the large intestine. So as I said, most chemical digestion happens in the small intestine, and as I've also mentioned, that requires enzymes to break up the large complex molecules into smaller, simpler components, particularly amino acids for proteins, simple sugars for carbohydrates, and fatty acid molecules for lipids. Since that requires enzymes, the enzymes have to come from somewhere. They have to be produced and excreted into the lumen. In the case of the stomach, as I mentioned, that occurs in the gastric pits.

But the small intestine doesn't have gastric pits. Instead, its enzymes are largely produced in the pancreas and liver, and then enter the small intestine via a special tube called the pancreatic duct. We'll come back to that when we get to accessory organs a bit more. So for now, we won't worry too much about where the enzymes come from. We'll just take them for granted, and then we'll kind of explain where they came from in a bit. There are three distinct regions of the small intestine, and they all have kind of weird or silly names, all derived from the Latin. The duodenum, the dejunum, and the Ilium. So the duodenum is a fairly short structure. It's only about 20 centimeters in length, and it's the first part

of the small intestine moving from the stomach. It's shaped like a C, so it's sort of bent. It receives gastric chime directly from the stomach, and it also receives digestive juices and enzymes from the pancreas, and it also receives bile from the liver. Remember, bile is this special substance that helps to emulsify fats. You can think of the duodenum as this kind of mixing ground, where the chime comes from the stomach,

the digestive juices from the pancreas, and the bar from the liver. Also entering here are alkaline substances which help to neutralize the acid from the stomach so that the pH of the small intestine is sort of a more reasonable level and not the very low level of the stomach. So the duodenum brings all these components together, brings the pH up to a more standard level, and then the material passes through to the judunum. Now this is the midsection of the small intestine which is about half of its length, about two and half meters. The The genum and the ileum aren't very distinct for our purposes here, so I'm just going to talk about them together.

This part of the small intestine, so the majority of its length, consists of a very specialized structure in the mucosa. So remember, again, mucusa is the innermost layer of the four layers of the GI tract. And we talked about how the stomach has a specialized lumen because of the gastric pits that contain the glands that excrete the various substances needed for the stomach. Well, the small intestine also has a highly specialized mucosa structure. In some sense, it's almost the opposite of the stomachs. In that while the stomach has these pits that contain these cells that excrete substances into the stomach, the small intestine almost has these tentacle-like structures that project into the lumen. These tentacle-like structures are called villi,

and they're small like finger-like projections that extend into the lumen. each of them is about one millimeter long, so they're very small, but you can see them with a microscope, and in turn, each of these little finger-like projections has tiny, tiny little micro-villay that project from the epithelial layers, and they collectively form what's called a brush border, which is this very intricate structure of very fine little pieces of membrane. So just to recap there, the villi structure are these finger-like projections of, the membrane. They're surrounded by epithelial cells, but inside them they also contain blood vessels. And so they're not entirely single cellular. Like there's a, there are several cells.

The microvely, which is when you zoom in even further, the microvely are really just tiny projections of the cytoplasm of individual cells, of the epithelial cells. So there's two different layers of structure here. And I suppose you can think of it a third layer as simply the highly folded structure of the small intestine itself. So all of this long length and all of the different folds combined with the villi, then combined with the microvilli on each individual little epithelial cell that lines all of the vilai and the inner surface of the lumen. The purpose of all of this is to increase the surface area between the substance like the lumen inside the small intestine and the body itself. So that interface needs to have a very high surface area. The reason why there is such an elaborate structure is solely to increase the surface area.

This is important because the absorption of the material that's in the process of digestion has to occur across a surface. And so if it was just a flat surface, if the inside of the small intestine was just flat and there were no vilay and no microvili, the surface area would be far too small and the rate of transfer would be too low. You can think of the surface area as kind of being like an open doorway. Each area of surface is a new doorway via which molecules can be transported from the lumen

into the epithelial cells and then ultimately into the bloodstream. I'll talk a bit more about exactly how that works in a moment, but each little area of surface is a little doorway. But there's a lot of molecules that need to come through, so we need to open lots of doorways. And so to do that, the small intestine gets longer and folds up so that you can have more surface area fitting in the same amount of space.

But then we also have these projections of the mucosa inwards, which are the villi, and then even on each of the epithelial cells within a single villi, there's these tiny little projections of the membrane, which are the microvilly. And so all of that dramatically increases the surface area, so that we can digest our food in a timely fashion, essentially. We can get all these molecules transported across the interface between the lumen and the bloodstream. Let's talk about the process of absorption itself. So what happens is, as I've said, by this point, we've broken down the food into small pieces,

we've chopped it up with enzymes first in the stomach and then even more so in the small intestine. I'll talk a bit more about some of those enzymes in a moment. But by this point, what we've got in the lumen, the small intestine, is a series of fairly small simple molecules, so things like simple sugars like glucose, single amino acids, and simple lipids like fatty acids. You know, there's a few other things as well, but that'll do for the basic underscule. standing, what we now need to do is we need to get those fairly simple molecules from the lumen into the bloodstream. To do that, remember the lumen is still technically, like topologically speaking, outside of the body, meaning it has not crossed a cell membrane yet. It hasn't even crossed

any layer of cells yet. All it's done is pass through a series of tubes and been broken up, but it hasn't crossed over any cell barrier yet, and it needs to do that in order to enter the body proper and be utilized. So what has to happen is that these molecules need to pass over one layer of cell membrane to enter the epithelial cells of the villi, of the mucosa layer of the small intestine, right? They have to cross one membrane layer to enter the villay, the epithelial cells, and then they have to sort of pass through the cell, and then cross over another layer of membrane on the other side. And when it does that, it then can enter the bloodstream. So it's got to cross, It's got to go through two gates, essentially.

It's got to enter the cell and then exit the cell, and then it can enter the bloodstream. So how do the molecules do that? Well, by this point, again, they're fairly small. They're just single amino acids or simple sugars and such. So they're not too hard to transport because they're not very big, but they still need some assistance. So simple sugars, such as glucose, galactose and such, can cross the membrane by active transport. So what this means is that there are special receptors located on both sides of the cell, of the epithelial cells, the outermost cells.

When the molecule, such as glucose, binds to the receptor of the gate or the channel, it changes its shape, allowing it to open and transporting the glucose or whatever other molecule is across the membrane. And usually that will require expenditure of energy, which typically is provided by ATP. I've discussed this in other podcast episodes if you want to look at the detail that at a cellular or molecular level but here we just need to understand the basic process that active transport and sometimes facilitated diffusion and facilitated diffusion is similar it just doesn't require energy but I'm not going to explain the difference

here but basically there's special channels which are selective for particular types of molecules say for simple sugars or for individual amino acids and they allow the transport with use of energy of individual monomers or sometimes small chains like maybe two amino acids together across the membrane. And these special gates or channels allow the molecules like glucose or the amino acids or whatever else to pass into the epithelial cells and then out of the epithelial cells again. So they've crossed two layers of membrane. Now they're actually inside the body proper and can enter the bloodstream. Now I haven't mentioned lipids and that's because fatty acids are hydrophobic. That means that they don't actually need to pass through special gates or channels

in order to pass through the membrane of a cell because they can simply diffuse right through, because as you may recall, the bilipid membrane layer surrounding cells is composed of largely fatty acid substance. I mean, they're technically substances that have a polar end and a non-polar tail, but because most of their width is composed of non-polar tail, that means that short fatty acid chains can just diffuse straight through. Anything that's polar, such as water or also amino acids and sugars, will be repel, it won't be able to go through, it'll be pushed out, just like water and oil don't mix,

water is polar, oil is non-polar, and so if you try to put one and the other, they just sort of repel each other, it's the same thing here. So glucose, amino acids and such, they can't enter cells unless they're assisted, unless they go through the special channels, but the short-tained fatty acids can just diffuse straight through. So those are kind of easy. So that's how everything ultimately gets into the body. It's either actively transported or facilitated diffusion in some cases through special channels and gates, or it just diffuses straight through in the case of short-chain fatty acids. Water can also just diffuse straight through.

All right, so we haven't talked very much about the different enzymes that help break up proteins, lipids, and carbohydrates. I'm going to come back to that when we kind of give an overview of which chemical digestive processes occur in which organs, because it's a little bit hard to sort of keep in your head. But there are multiple different enzymes that carry out different types of metabolic functions here, and we can't even cover a fraction of those. So we're going to move on at this point, and we will come back to talk about some of the enzymes operative in the small intestine,

but at least we have an idea of how the material enters the bloodstream and what the small intestine is doing. Primarily, it's carrying out these enzymatic reactions and providing a very large surface area for the material to actually be absorbed into the bloodstream. Now we move to the large intestine, and as I indicated at the very start, the purpose of the large intestine is to prepare indigestible material for excretion. because by the end of the small intestine, pretty much all of the nutrition that can be extracted by the body has been extracted. With a couple of exceptions we'll get to, but basically we've gotten everything out of the food that we can, and now what's left is essentially waste. And so that waste needs to be prepared and then ultimately excreted.

So the large testin's job is to do that. Now there are several different parts of the large intestine, and technically the colon is only one of those parts, although sometimes the colon is also used interchangeably to refer to all of the large intestine because in terms of length the colon is almost the entirety of the large intestine. So I'll generally talk about the colon separately from the large intestine. If I mean the whole thing, I'll say large intestine, but most of it's the colon. So there's not too much lost here for a bit of imprecision. The other components of the large intestine are the seacum, the colon as mentioned, the rectum,

and the anal canal. The seacum is the very first part of the large intestine. it's a large pouch which is also the widest part of the large intestine and it provides an interface between the large intestine and the small intestine so it receives chime from the ilium the final part of the small intestine and connects to the colon so that the rest of the large intestine now the different parts of the colon have names depending on and describing their sort of relative orientation unlike the small intestine which kind of just sort of squiggles around and has a very complex structure.

The large intestine has a much more defined structure. It sort of goes around the outside of the small intestine where the small intestine is located kind of in the middle of it. Indeed, the large intestine almost forms a complete square around the outside of the small intestine. So if we start from the left-hand side looking towards the body, we have the Seacom, that's again the first part of the large intestine, which connects it with the small intestine,

and then we move up in the ascending column. across in the transverse column, down in the descending colon, and then across back to the left again with the sigmoid colon, but not quite the whole way, because we go part of the way back left and then downwards. This structure is sort of important because ultimately the indigestible material needs to be excreted from the body, which occurs at the anus, and as you would know, the anus is located roughly in the middle of the body. So we need the colon to join up with the anus you know, at the right location. So that requires this sort of complex structure where the large intestine kind of goes up and across and down and then part way back and joins up with the anus

where it needs to be. So all of that up and across and down and back part way across again, those are the different parts of the colon, but functionally they're pretty similar. By the time Chime reaches the colon proper, pretty much all of the nutrients and most of the water have already been absorbed. The large intestine absorbs whatever's left of most of the water, which turns the chime into eventually feces and solidifies it. Much of what remains after the process of digestion is completed in the small intestine, indigestible parts of food, so that's dietary fiber. So much of what's excreted as feces consists of this dietary fiber, so this indigestible material, but it's also mixed with mucus from the small and large intestine, slewed off epithelitis.

As I mentioned, the epithelial cells across the GI tract are periodically replaced, and so slid off epithelial cells form part of the compacted and form part of feces, as well as bacteria. So, as we know, the colon is home to a very large population of bacteria, which are also called the gut flora. There's more cells of bacteria in your colon than there are your own cells in your whole body. It's just they're a lot smaller than your cells, so they don't weigh that much, but there are actually more different cells there than your own bodily cells. The importance of the gut floor has only really been appreciated, I think, in the last few decades. We're learning a lot more about that.

Even every year, it seems, there are new functions and important variations and aspects of that that's discovered, and probably that deserves a whole episode to itself, but we don't have time to go into detail of that here. Essentially, what we need to know is that there are a very wide range of, different species of bacteria that live in the colon and that's entirely good and normal. Indeed, the bacteria basically live in a symbiotic relationship with us in that they contain enzymes that allow them to metabolize some of the fiber in our indigested food. And as they do that, they break down some of this fiber for their own nourishment, so they get that out of it.

And what they excrete as waste products, well, some of their waste products are used and absorbed by the cell lining of the colon for nourishment. So we get useful products from the bacteria, and they get useful products from us, essentially from material that we can't digest with our own cells. However, there is a downside to that. Because the bacterial cells are not part of our body, they're not supplied with oxygen in the same way. The way they metabolize is through fermentation, and that process, because it doesn't use oxygen to accept electrons, as I've discussed in detail in other podcast, it produces a large amount of carbon dioxide. dioxide, as well as some other gases as waste products. So carbon dioxide, as well as hydrogen,

methane, hydrogen sulfide, and this gas in combination is called flattis, and it gives rise to flatulins. So that's the downside of the bacteria providing useful nutrients for us is that they do produce stinky gases as a side effect. The large intestine is not nearly as long as the small intestine, but it's still about one and a half metres long. And as the material is moved along, the large intestine through that typical process of periodic contractions of the smooth muscle that pushes the material along, whatever water remains is gradually removed, solidifying the material, and it's also mixed with, progressively mixed with bacteria and some of the sleut-off cells and so forth, and is transformed from chime, which is what the food is called in the stomach and the

small intestine, into feces. which is what it's turned into by the end of its passage through the large intestine. Feses are stored in the rectum, which is the very final section of the large intestine, and that holds the feces until they're eliminated via defecation. As usual, there is a sphincter which controls the exit of material from the rectum. There's two, actually, the internal and the external anal sphincter, although they're very closely connected. And unlike most of the components of the GI tract,

that's actually under conscious control. We have the ability to control that, or at least we learn that at a very young age, so we can control the time of elimination. Dietary fiber, which is the largest component of the indigestible part of food, that remains when the chime enters the large intestine. Dietary fiber consists mainly of large polysaccharide, so that's these complex long-chain sugar molecules. And there are many types of polysaccharides that we can digest,

in particular we can digest different types of starch, which are contained in many animal products. However, we can't digest some plant polysaccharides, such as cellulose, which you may be familiar with as a key component of the cell wall of plant cells. It might seem odd that of a class of quite similar molecules, some of them we can be digested and some of them can't, but each of these different types of sugars and polysaccharides require different enzymes in order to cut them up, and humans don't possess all of them. And so because of this fact, there is a fairly large range of plant-based polysaccharides, which we can't digest, and so these are called dietary fiber.

Now, there's two different types of dietary fiber, soluble and insoluble, depending on whether it can be dissolved in water. As I mentioned, although human cells do not possess the enzymes needed, or humans, human cells don't project the enzymes needed to digest dietary fiber, bacteria that live in our colon do, and so they ferment the dietary fiber, or parts of it at least, and produce byproducts, some of which can then be absorbed through the wall of the large intestine and be useful to us. We've now completed our journey through the gastrointestinal tract right from the mouth through to the anus,

but as I promised, we will stop back and talk about the accessory organs very briefly, just so we understand a bit better about what their role is. in particular the liver has so many functions. I saw a reference somewhere that said it has at least 500 known functions that it really deserves a whole episode to itself at the very least. Here I'm just going to talk about its primary function in terms of digestion, which is to produce bile. It's also involved in detoxification, metabolism of carbohydrates,

brag down of red blood cells, but here we'll just focus on the fact that it produces bile. So bile, I've mentioned, is a liquid that's composed of mostly water, but also bile salts, which are special compounds that act as emulsifiers. So they consist of steroid molecules, so they're a type of lipid, which are ambipathic, so that allows them to have, well, that means that they have a polar and a non-polar end. They can aggregate around droplets of liquids to form my cells, so their non-polar tails poke towards the lipids and then the polar heads point outwards and in turn are surrounded by water.

So they allow small droplets of lipids, so small droplets of fats, to be embedded or emulsified into a liquid substrate. That's very important because if you have huge globs, well huge on a, at least a microscopic scale, big globs of fat, there's no way for the necessary enzymes to get into interact with the fat to break them down, to cut them up into smaller pieces. That can only happen at the surface interface between the fat itself and the surrounding liquid where the enzymes are coming from. So in order to increase the surface area to dramatically accelerate the rate at which these enzymes can come into contact with fats, they need to be broken down into smaller sections so that the surface area is increased. And this is what bile does. It emulsifies them, surrounds the droplets of lipids,

increasing their surface area relative to the surrounding water, which then allows the digestive enzymes to come into contact with them. Without bile, we would not be able to digest really any of our lipids, and it would just be excreted with the rest of the indigestible material. And that happens in some conditions. So that's the liver and the role of bile. So bile is produced in the liver, and it's carried by the hepatic duct to a small hollow organ called the gallbladder, and that's where bile is stored and concentrated before being released into the small intestine. Remember that the first component of the small intestine, the duodenum, is the region where the bile is excreted and comes into contact with the chime from the stomach.

The last accessory organ that we'll talk about is the pancreas. The pancreas also has a wide range of functions. it helps to regulate blood sugar levels, secreting hormones such as insulin and glucagon. But as part of the digestive system, it functions as an exocrine gland. So it secretes pancreatic juice into the duodenum through the pancreatic duct. And that juice contains a wide range of digestive enzymes necessary for digesting our food. But it also contains bicarbonate, which, as I've mentioned earlier, is an alkaline substance which helps to neutralize the acid from the stomach.

So it was to bring up that pH from the very low level to a more normal pH. So these accessory organs are very important because without the bile and without the digestive enzymes and also without the biocarboneate from the pancreas, we wouldn't be able to effectively digest really anything from the stomach. So we need these accessory organs there to provide these compounds at the right place and in the right amounts for the digestive system. but they're called accessory because they're not part of the main GI tract and they don't engage in digestion directly. They support and facilitate digestion.

Now before we finish up, I did want to give a bit of an overview and a summary of the process because it's a bit hard to keep track of what's happening to different compounds where and what the different components and organs of the digestive system contribute. So here, instead of focusing on the organs, which we've done so far, I'm going to focus on different types of compounds or nutrients. So let's start with proteins. I'm going to talk more in a future episode, by the way, about the detailed biochemistry of what happens in metabolism.

So here we're just going to talk about it at a fairly general level to understand the basics of the process and where it happens. So let's talk about proteins. Proteins are not digested in the mouth. Digestion of proteins begins in the stomach with hydrochloric acid, bringing that pH right down, and that helps to facilitate the activity of, various enzymes which break down proteins into smaller pieces, particularly proteases which chop up

the long protein molecules and breaks it up into smaller peptide chain. So a peptide chain is just a linear sequence of amino acids. By the way, I should also mention that the low pH of the stomach helps to denature proteins. So basically that means it unfolds them so that instead of being this complicated three-dimensional structure, they kind of get unwound. And that helps to bring enzymes into contact with the proteins. So there's a wide range of enzymes that are in the stomach that chop up the proteins into smaller peptide chains, but they're still too big at that point generally to be absorbed directly. The small intestine is where a different set of similar enzymes chop peptide chains up into single amino acids, which are then directly absorbed. So it's a two-stage

process in the case of proteins. Initially they're denatured and chopped up into small chains, and then those small chains are chopped up mostly into single amino acids, and those are then absorbed across the epitheliosols and into the bloodstream in the small intestine. Now let's talk about carbohydrates. The process is even more complex in the case of carbohydrates because it begins not in the stomach, but in the mouth. Salivary amylase is an enzyme that begins the digestion of starch during chewing,

so that actually happens pretty much straight away. Stomach acid inactivates cellaray amylase, and so there's, I don't think, too much digestion of chemical digestion, in the stomach of carbohydrates. However, enzymes kick off again in the small intestine. Many different types of enzymes are responsible for chopping up different types of carbohydrates and reducing them to simple sugars, which are then able to be absorbed, again, across the epithelial layer as discussed. So there's a very wide range of different types of enzymes there, but bear in mind that not all types of carbohydrates can be metabolized by humans,

because we lack some of the enzymes. And those types of carbohydrates that can't be so digested are called dietary fiber. And dietary fiber is not metabolized at any point. Some of it is metabolized in the large intestine, but not by humans. It's digested by the gut flora.

But that's obviously a different biochemical process because that occurs in bacteria, not in human cells. Some of that is broken down, but much of it is excreted. Let's now talk about lipids. lipids are a different beast again because they require emulsification in order to be effectively metabolized so there's no chemical digestion in the in the mouth and limited digestion in the stomach there is an enzyme called gastric lipase which splits lipids into smaller components so you split

split them into fatty acids and monoglycerides a glycerol is a molecule that kind of keeps the different fatty acid chains kind of together there's a there's molecules that has a glycerol and then three fatty acid chains so those are split up in the stomach So they're broken down into smaller pieces, but most of the chemical digestion happens in the small intestine when those fatty acid chains are broken down into simpler components, which can then directly diffuse across the epithelial cell membranes and then enter the bloodstream. And that process, again, requires bile to really help emulsify the fatty acids and ensure that those enzymes can come into contact with it as necessary. Water is progressively absorbed throughout this process. Most of the water is absorbed by the small intestine and the large intestine, particularly the large intestine,

as that's its role to convert the chime that still exists in a small intestine into mostly solid substance ready for elimination. But that can really occur, absorption of water can occur really at any point other than the mouth. There are also certain vitamins. Vitamins are small organic molecules, which cannot be synthesized. and those are absorbed, particularly in the large intestine, because some of them are produced by intestinal flora. We'll talk more about vitamins when we get to the episode on nutrition. There's also minerals, which are inorganic elements like iron or sodium and calcium. Most of those are taken up in the small intestine. So to summarize, most of the chemical digestion occurs in the

small intestine. Digestion of carbohydrates starts with salivary amylase in the mouth, is largely paused in the stomach and then continues in the small intestine. For proteins, it's a two-stage process, it's denatured and chopped up into smaller segments in the stomach, and then those smaller segments are broken down into individual amino acids in the small intestine and then absorbed. And for lipids, not much happens in the mouth, in the stomach, gastric lipase, sort of begins the job of chopping them up, separating the fatty acids from the glyceral molecules, and then in the small intestine, the bile from the liver emulsifies the fat droplets and brings other enzymes into contact with them, which allows them to be

chopped up even further and then pass across the epithelial cells into the bloodstream. Water is mostly absorbed by the large intestine. Fiber is indigestimal material, which is partly broken down by the intestinal flora bacteria in the large intestine, but the rest of it's excreted. And there's various vitamins and minerals which are largely absorbed in the small on the large intestines. So that's a summary of different nutrients about when the digestion occurs. This whole process takes maybe about 24 hours, though there's a lot of variation. Food is usually held in the stomach for two to four hours and in the small intestine for one to five hours or so. The final stages of the compaction in the large intestine might take 10, 12 hours,

something like that. There's a fair amount of variation from person to person. Now, before we finish, there's a couple of sort of digestion myths, I suppose, that I wanted to briefly mention. And one of them relates to this idea of detoxification. I mean, I don't know that there's any precise definition of detoxing, but some people seem to have this idea that there are various toxins or undigested materials or kind of crap that accumulates in your digestive system, and that you can go on special diets or cleanses or whatever to remove this material. Unless you have a very specific medical condition, and probably you'll be in the hospital, because those are quite severe, this just doesn't happen. So you're,

your liver and your kidneys, in particular, do a very good job of detoxifying your body. As for your digestive system, that does a very effective job at digesting material. So there's not really any such thing as material that accumulates there or indigestible substances that accumulate there that need like detoxing. You might feel better after fasting for a few days or going on a very restricted diet, but that's not because you're clearing out any waste that's accumulated there. any type of intestinal blockage is very severe and will almost certainly lead you to be in the hospital very soon. Aside from that, the gastrointestinal tract is very effective at moving all of the material that enters it out of the body within 24-ish hours.

Of course, it can be a bit longer sometimes, but it doesn't take very long. There's nothing that's left over or builds up in there. There are some urban myths about things like chewing gum taking years to digest in the stomach or whatever. Those are all false. there's nothing that accumulates in the stomach, again, other than in, like, very severe medical conditions. Everything that enters the GI tract is processed very efficiently in the stomach and small intestine and is then removed and expelled from the body within a fairly short period of time. Another thing is that the small intestine is very effective at extracting pretty much all of the

nutrients that food has in it. Now, there's maybe a little bit of variation from person to person about this, and there's certainly variation from person to person about gut flora. But for the most part, if you consume food, the nutritional value of that food, like the caloric intake in particular, will be extracted by your small intestine, and anything that's indigestible will be passed into the large intestine, and then ultimately excreted. I've heard people make claims to the extent that it doesn't matter how much they eat because they'll just excrete more or the digestive system is more or less efficient. I'm not aware. of really any evidence for that. To any significant degree there is some amount of variation.

Now, there are specific conditions, of course, where people do not have the enzymes to digest certain types of compounds. Probably the most well-known of those would be lactose intolerance where people lack the enzyme, lactase, to break down lactose, which is a type of sugar. And therefore, they cannot process or they cannot metabolize lactose. And it will be broken down by the gut flora in the large intestine, but that results in discomfort and flatulence, and so it's not very pleasant. So, you know, there are cases like that. There are other types of intolerances that people have where they lack the certain enzymes to process certain nutrients. But pretty much all of those are sort of fairly obvious in the sense that you will get stomach pains

because of the, basically your gut flora dealing with those indigested materials and producing producing a lot of gas and discomfort with that. But that sort of thing aside, you're going to extract pretty much all of the nutrients that exist. Again, unless you have some sort of particular severe condition. There are certain rare types of conditions where people are unable to extract much of them material and they need to eat a lot and so forth. But for the most part, the small intestine is very effective at extracting nutrients. So the summary here is to be pretty cautious about some of these urban myths or misconceptions or pseudoscientific claims that are made. The digestive system is very efficient. If you put something in your mouth, it's going to be, and swallow it,

it's going to pass through the GI tract. It's going to have pretty much all of its nutritional value extracted unless you have a particular intolerance. And it's going to be excreted out the, what remains is going to be excreted out the other end, you know, within a fairly short period of time, a couple of days generally at most. And that's kind of how. works. It's designed to work that way and it works pretty well for that most of the time. So some of the other aspects of the nutritional side of things we will talk about in a future episode. I've deliberately kind of left that out here. And I'm hoping to do a series of at least two more episodes, one where we talk more about the chemical process of digestion and metabolism

and look at some of the biochemistry of that. And another where we talk about nutrition. So we'll see if that gets expanded beyond just two more episodes, but that's the plan at the moment. So hopefully you found that interesting. Thanks everyone for tuning in. If you would like to support the podcast, you can do so in a number of ways. You can leave a favorable review on iTunes or Spotify or wherever else you listen. Those are most appreciated. You can make a financial contribution by becoming a Patreon supporter or by making a one-off donation via PayPal. My email address for PayPal or just getting in touch is Fods12.g.com. That's FODDS1.com. Thanks once again for all of my listeners and a special shout out to those who've been helping me with preparing some videos

to combine picture content with audio of previous podcasts, which I'm going to put up on YouTube. I've got a bit of a decent catalogue now of episodes that my assistants have been putting together for me. So I'm planning to launch those on YouTube. I don't know, within a few months, basically when I get time to do that. So when I do that, I will let everyone know because I'll want to have a bit of a push there and try to get some, try to, try to bring the show to a new audience effectively. So be on the lookout for that. That's coming up on the horizon. Other than that, that's all I have for now. Thanks again for listening and I'll talk to you next time.