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Digesting Food

Get an overview of three of the key macromolecules of life (proteins, fats, and carbohydrates), and how they get digested and absorbed. These videos do not provide medical advice and are for informational purposes only. The videos are not intended to be a substitute for professional medical advice, diagnosis or treatment. Always seek the advice of a qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read or seen in any Khan Academy video. Created by Stanford School of Medicine.

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Video transcript

Voiceover: One of the keys to understanding child nutrition is understanding what happens to the food our children eat once it's entered their mouths. Let's start by drawing a very basic model of digestion. Now the food our children eat can be broken down into three catergories and their categories of what we call macro nutrients. All of these macro nutrients are digested in slightly different ways, so I'm going to color code them so that we can sort of follow what happens to each of these categories of macro nutrients. The major macro nutrients are carbohydrates, I'll write carbs for short, fats and proteins. When we use the term micro nutrients, we're talking about things like vitamins and minerals, and the other important components in the food that we eat. Now the digestive tract is actually just a long tube that extends from the mouth all the way down to the anus. We're going to follow that tube and trace the path of food after it enters the mouth and as it passes down this long tube that leads all the way through the digestive system. Digestion actually begins in the mouth with the active chewing that's an important part of digestion that we sometimes overlook because it makes big pieces of food, it breaks them down into smaller, more manageable pieces that can be attacked by the different enzymes. Some of those actually are secreted directly into the oral cavity or the mouth by accessory glands called salivary glands. The salivary glands secrete saliva, surprisingly. In that saliva are a couple of different enzymes and I'm going to color code the enzymes as well. The first enzyme that's in the saliva is called amylase. Amylase works to begin the digestion of carbohydrates that's why I've done it in purple, we call that salivary amylase because there's also another kind of amylase that comes into play later on in the digestive tract. The other enzyme found in saliva is called lipase. We also call that salivary lipase because there's a different kind of lipase that comes into the picture later on. Lipase begins the digestion of fats, that's why I've drawn it in yellow. Food is chewed in the mouth and then swallowed down in the form of what we call a bolus. That bolus has been mixed up with these salivary juices or the salivary enzymes and it passes down a tube called the esophagus. This is the esophagus. The esophagus leads down into a dilated part of this tube that we call the stomach, so this is the stomach. There's actually a sphincter here or it's kind of like a valve that we call the lower esophageal sphincter that guards the entry way to the stomach. If that valve becomes leaky then food can actually leak up from the stomach and give us heartburn, that's where heartburn happens. Once food enters the stomach, it gets mixed with a bunch of other things that are secreted by the stomach. Things like hydrochloric acid or stomach acid and I'll just write HCl because that's the chemical abbreviation for hydrochloric acid. The stomach also secretes a digestive chemical called pepsin. Pepsin I'll write in green because pepsin, you guessed it, it digests proteins. The other thing that the stomach secretes is a chemical called lipase. This is gastric lipase, gastric means it comes from the stomach and just like the salivary lipase, the gastric lipase continues the work of digesting the fats that have entered the stomach. Now the stomach is really muscular, so it kinds of churns all this food around. The food that's come down the esophagus as a bolus and what ends up leaving the stomach is more of a liquid. We call that liquid, chyme, that's what leaves the stomach. It leaves the stomach through another valve or sphincter that's the pyloric sphincter. That's a good word for a spelling bee. Once the chyme has passed through the pyloric sphincter it enters into the first part of the small intestine and that part of the small intestine is called the duodenum. I'll just write that in, duodenum. Some really important things happen in the duodenum or the duodenum as it's sometimes called. What happens there is that the liver secretes a chemical called bile. That bile is stored sometimes when it's not needed immediately. It's stored, so it's created in the liver. I'll just draw it kind of coming out of the liver. It actually is stored in a small organ called the gallbladder. That's over here so the gallbladder's going to store the bile and I'm just going to write that in for you, that's our gallbladder. This is our liver, and then when the bile is needed and I'll just write the bile in here in yellow, so you can probably guess what the bile is used to digest. When the bile is needed it's secreted out of the gallbladder and it enters the duodenum, right about here. There's another accessory organ that also sort of dumps it's products into the duodenum, right about there at the same spot. I'll draw that one in blue and this accessory organ is called the pancreas. The pancreas, and the pancreas is really important. It secretes probably the most important digestive enzymes into the small intestine and those enzymes are amylase, and this is now pancreatic amylase. It also secretes pancreatic lipase, another way of digesting fat. Then it secretes two enzymes that are used in the digestion of protein. One is called trypsin and it's cousin chymotrypsin. All of that exciting stuff happens in the duodenum and after that, the small intestine continues and it continues on for anywhere from about 15 to 30 feet. It's really long and coiled up on itself. I'm drawing it kind of spread out, these coils of small intestine are actually all curled up on top of each other. The second part of the small intestine and I'll just try and give you a sense of how long this part of the tube is. The second part of the small intestine is called the jejunum. Jejunum. Then the last part of the small intestine is called the ileum. Basically the small intestine is where big particles of food are broken down into their absorbable units and absorbed in this tube the small intestine. What happens next is that the small intestine joins the large intestine. The large intestine is sort of like a storage and drying unit, if you will. It kind of curls around and there's an ascending portion, a transverse portion, and the one I'm drawing now, the descending portion of the large intestine. It ends in an S shape part that we call the sigmoid colon. This would be your ascending portion of the large intestine, the transverse, and the descending here, I'll just write des for lack of space. Now what's absorbed in the large intestine are things like excess water, so the longer feces because this is ultimately going to be feces that's secreted out once it passes through the sigmoid colon. This is our end product of digestion and feces is going to pass out through the anus. The longer the feces sits in the large intestine, the drier it's going to get because water is being absorbed and some solutes are also being absorbed in that tube. The food we eat is going to be chewed up in the mouth and swallowed in the form of a bolus. The bolus is going to pass down the esophagus and through the lower esophageal sphincter into the stomach where it's going to be mixed with stomach acid and things like pepsin and lipase and turned into chyme. That chyme is going to pass through the pyloric sphincter and it's going to head down into the duodenum. In the duodenum, bile from the liver and the gallbladder is going to be secreted into this part, as well as the pancreatic digestive enzymes like amylase, lipase, trypsin, and chymotrypsin. While this food is digesting into it's smaller absorbable parts it's also being absorbed as it passes through this long tube and you can see how much time there is here to absorb all of the nutrients because we don't want food to pass through here without enough time for those important nutrients to be absorbed. Then it gets here to the large intestine and this valve I didn't put this in but this is called the ileocecal valve because it's between the ileum and the cecum. Interestingly in case you're interested here, this is where the appendix is. It's a little kind of pocket extension of the cecum and if for example, you eat a grape and you swallow all of the seeds, and you have pretty bad luck then one of those seeds might get stuck in here. This appendix might get clogged and fill up with pus and then you've got yourself an appendicitis. If you're lucky and that doesn't happen, then the food that couldn't be either digested or absorbed passes into this ascending colon and slowly moves up through the transverse colon and down the descending colon. As it's kind of progressively dried and stored. Then once it enters the sigmoid colon then a bowel movement is triggered and feces are secreted through the anus. That is sort of the big picture of how digestion works. Now, I just want you to have just kind of a window into what it looks like if we took, let's say a cross section through this small intestine. Let's say I took a slice, like this, out of the small intestine. What you would see, would be a muscular tube like this. Let's say this is the chunk we've just cut out. That tube on the inside is going to have this kind of finger-like projections and this finger-like projections are called villi, so this is a villus. I'll write the plural term which is villi and I'll just make this arrow pointed two of them so you understand that that's the plural term. The villi, what they do is they increase the surface area for absorption of all of those important nutrients that are going to be found in the lumen. The proteins, the fats, and the carbohydrates that are going to be passing through this lumen. Better yet to further increase the surface area there are this kind of small, they look like hairlike projections, projecting off of the villi and this is what we call the brush border. These are called micro villi or smaller villi. The micro villi make up the brush border. What that does is it further increases the surface area for absorption in the small intestine. Now just to give you another view of what the small intestine looks like, what happens if I were to give you another cross section that looks like this. Through the wall of the small intestine. What if we were to kind of slice it open. What you would see, you have your muscular wall here and here are your finger-like projections called villi. Because we've zoomed in a bit, you can actually see that the brush border or the micro villi are actually made up of cells called enterosites. Each of these cells has their own nucleus and they're actually selective cells that can aid in the absorption of all of the important nutrients in the small intestine. These guys are called enterosites. Now inside each of this villi, this finger-like projections is something called a central lacteal. The central lacteal is actually sort of a projection of the lymphatic system. The central lacteals are continuous with the lymphatic system. Because I'm drawing them in yellow, you might guess that the fats that are going to be in this lumen and they've been emulsified by the bile. They've been broken down into smaller pieces by the gastric lipase and the salivary lipase and the pancreatic lipase. All of this small units of fat are going to be absorbed into the central lacteal and passed into the lymphatic system. Now there's also in each of this villi there is a little capillary network. Each of this villi, each of this central lacteals is surrounded by a network of capillaries that bring blood toward the villus, and then also take blood away. As capillary networks do, they carry blood away from an area. The proteins and carbohydrates, so these are our carbohydrates and these are our proteins, and these guys are going to be absorbed directly into the bloodstream, into the capillary networks that sort of surround these central lacteals. There are carbohydrates going in, so you can see that this is how the small broken down particles of the food that we eat actually get into our bodies, get into our bloodstream. Next we're going to be looking at how are body uses those building blocks for either energy, for growth, or for storage in the human body.