Let's talk about the digestive system. All right.

 First a little review about cells.

 So cells are the basic unit of life. 

And all living things are made up of cells. 

The cell theory says that new cells arise from pre-existing cells.

 So all of the cells in your body came ultimately from the fertilized embryo when the sperm met the egg.

 That was a single cell.

 And it divided over and over again in the same way that this cell is dividing to create you.

 Most textbooks have a generic picture of an animal cell with the primary organelles, the nucleus, where we store the genetic information, the endoplasmic reticulum, which makes the lipids and the proteins, the Golgi apparatus, which takes the products of the endoplasmic reticulum and decides where they're going to go inside the cell or outside of the cell. , the mitochondria, which are going to take the heat energy yielding molecules like carbohydrates and lipids and proteins when needed to release energy. And,  , all of this is bounded by a plasma membrane. So cells are highly specialized inside multicellular organisms. You are a multicellular organism. So while every textbook ever that is introductory biology shows that generic animal cell in the real world, your body at maturity, none of your cells look like that cell. They differ in size, shape, and even in their organelles. So for example, the red blood cells, which are the smallest cells in the body, have this big concave disk shaped sort of like a hockey puck. And that's to maximize surface area for oxygen absorption. But they also have no organelles at maturity. At maturity, they have nothing but hemoglobin more or less inside. And they've lost all their organelles. We can contrast that with the nerve cells, which can be very, very long. All their organelles are in this central part called the cell body. And then they have these long extensions called dendrites and axons, which is part of what helps them do their job which is transmitting information. And then the last example is the cartilage cells, which are very sort of spread out, [have], here's the cell itself. And they're in these little kind of caves called lacunae. And the cell itself is secreting all of these extracellular proteins and matrix that make up, for example, in this case the cartilage.

As mentioned in the prior slide, we are multicellular organisms. It is estimated that there's between 28 and 36 trillion cells in a human adult. About 200 types of cells are found. Although some people think there may be up to 400 types, red blood cells account for the majority of the cells by number in our body, although by mass it's a certain type of cell involved in making bone. We also have a lot of friends that live with us. And so there's somewhere in the number of 40 trillion bacterial cells, and these are living primarily on our body surfaces, like all over your skin, as well as in the spaces inside your digestive tract from your mouth to your anus. There are also some bacteria that are found in the urethra and in the vagina of biological females. So you may have heard this statistic that there are ten bacterial cells for every one human cell. Somebody went and quantified that a couple of years ago and found that, no, that was a vast exaggeration. And of those, it's actually about a 1 to 3 ratio. So one human cell for every three bacterial cells in and on us, those bacteria are primarily mutual- lists where we're getting something from them and they're getting something from us, such as the bacteria in our large intestine that help synthesize vitamin K, and some of them are also commensal in that they don't really give us anything great other than taking up space so that harmful bacteria cannot get a foothold in our bodies. So cells are organized into tissue. So basically a tissue is a group of specialized cells that are doing the same job. And we have four major categories of tissue in animals. Those are nervous tissue which is responsible for transmitting and receiving signals and stimulating responses. We have epithelial tissues which cover and line our body surfaces and our organs, as well as makes up the glands that secrete things like hormones, the muscle tissue there's cardiac in our heart, smooth muscle in our organs, including those of our digestive tract and skeletal muscle, which is the muscle that you workout when you go to the gym, are used for movement. We also have connective tissue, which can be very hard as a bone or very liquidy as in blood; and then some of it's kind of in between, like the fat is a connective tissue.

Those tissues then are organized into organs. So an organ is defined as a structure that has two or more tissues that make it up, and we're primarily focused on the GI tract or the digestive system today. The GI tract or gastrointestinal tract is a long, continuous tube that connects the mouth with the anus. Just a fun fact: if it were fully extended, it would be about 9m or 30ft long in a human adult. And as you can see on the slide, it is made up of its primary organs, which is the mouth, the esophagus, stomach, small intestine, appendix, large intestine, rectum and anus, and the accessory organs, which are your teeth, tongue, salivary glands, liver, gallbladder and pancreas. So we're going to take a look at the specific roles of each of these. So don't get stressed out if you feel like that went really fast.

Okay, so the digestive process has a number of steps in it. And I'm at a very briefly speak of these now and then I'll revisit them as we're going through the organs and the structures. The first thing that has to happen is you have to ingest food. So you put it in your mouth. That's all ingestion is. And then digestion is where we're taking those big chunks of food that we've consumed and breaking them down into tiny, tiny, tiny, tiny molecules. Sort of like taking a beaded necklace and pulling the string out, so all the beads are loose. And digestion can be chemical, which uses enzymes which are specialized proteins to break big things down into small things in this case, or it can be mechanical, such as what happens in your mouth and your stomach, where we're physically [like]grinding and and chopping the food up into smaller bits. Once it leaves our mouth, it enters a series of smooth, muscled organs and is moved through our body in this process of movement. We'll explore that more when we look at the small intestines. Then when it gets to our small intestine primarily is where we're going to have this process of, of absorption, which is moving those small digested molecules into the intestinal cells and then ultimately into the bloodstream and the lymphatic system. Elimination is the elimination of solid waste. That's what occurs in our digestive tract. It is not the same thing is what happens with urination, which also gets rid of waste. But it's the liquid waste. And it's a little bit different.

All right. So let's start with our mouth. So our mouth is the site of ingestion and digestion. There is a little tiny tiny bit of absorption but it's pretty minor. So we're not really going to consider it too much right here. So inside your mouth you have your tongue, which is a big muscle, and your salivary glands. And the job of your tongue is to work with your teeth, to push the food around so that it can get in between the teeth and get ground up into smaller and smaller bits, and to get it liquefied using fluids that are being secreted by the salivary glands in order to, make it something that can be swallowed and passed through the esophagus. Of our teeth, we have three major types. We have the incisors, which are these slicing teeth up front. These are, like think of when you bite into an apple that your incisors are playing a big job with that. And then your bicuspids, which are your premolars and molars towards the back of your mouth. These are going to take that apple and grind it up into smaller and smaller bits. You also have canines which are the pointy teeth on the top and the bottom. And  , in humans, the canines are greatly reduced for what's thought to be a variety of evolutionary reasons. But in general, canines are considered like a ripping or a shredding tooth.

From our mouth. The food is going to move into our esophagus, which is a long, muscular tube that connects our mouth to our stomach. There is no digestion or absorption in our esophagus. It is just a place to move food from one location to the next. So at the back of our mouth, there is this big open space which we call the pharynx. And in the pharynx you have something called the epiglottis, which is a little flap of tissue that operates to close over our windpipe when we swallow, to avoid letting food go down into our trachea. Choking is a major cause of accidental death because the food goes into the esophagus. I mean, goes into the windpipe instead of our esophagus. So please, here's a pro tip: Eat slowly, chew slowly, take small bites, drink lots of fluids as you're eating to try and minimize your risk of choking. Don't (sounds of shoving food in your mouth) your food. I know everybody's in a hurry, as you've said in your discussion boards, but you've got to be careful with choking. So the esophagus is bounded at the top and the bottom between by what are called esophageal sphincters. The upper esophageal sphincter controls the flow of food from the mouth into the esophagus, and the lower esophageal sphincter controls the flow of food from the esophagus into the stomach. The lower esophageal sphincter also tries to stay closed and prevent reflux, or the backflow of digesting food from the stomach, as well as stomach acid from going up into the lower esophagus. When people experience heartburn or GERD or reflux, which are all very related but slightly different problems where that stomach acid and partially digested food is going up and irritating the lining of your esophagus. The food, as it passes from your mouth to your stomach is called a bolus, which is a semi-solid chunk of food.

In our stomach. The food is going to pass from the esophagus into the stomach, where it will undergo both mechanical and chemical digestion. The mechanical digestion is because the muscles of the stomach run in three different directions their circular, diagonal, and longitudinal, and those three different directions of the muscles work together by contracting and relaxing to sort of squish the food up and further liquefy it. Our stomach is also full of stomach acid and which has a very low pH. High acidity solution that will hopefully take care of any pathogens we might have inadvertently swallowed with our food. But it also serves to unravel some of the big proteins that we're eating and taking them from being, like, clumped up things and opening them up so that they're like long strings that enzymes can begin to act on. So we do see a little bit of chemical and mechanical digestion in the stomach. There is no absorption in the stomach. Your stomach, the food in your stomach, by the time it leaves your stomach, it is called chyme. So that's basically your liquefied food, and there is a sphincter, another circular muscle at the bottom of your stomach called the pyloric sphincter, whose job it is to control the flow of the products of the stomach into your small intestine.

From your stomach into the small intestine, which is all of this stuff. That is really when you say you have a stomachache and people like pat their bellies, that's probably actually an intestinal ache that's either in the small intestine, which is sort of middle of your trunk, or your large intestine, which runs,  more or less around your small intestine, and it is right under your stomach. So the small intestine has three parts: these are just regions of the small intestine called the duodenum, jejunum and ilium. And then we also have some accessory organs that get involved with what's going on in the small intestine as the primary site of digestion and absorption. So your accessory organs are the liver, and the liver's job is to make bile. Bile is a substance that is secreted from the liver and sent to the gallbladder where it is stored. And bile sort of acts like a detergent that surrounds fats that we're digesting. And makes it so that we can absorb them. So it's a pretty important role. The gallbladder also secretes, like an antacid agent to neutralize the acid that were is coming from our stomach at a pH of about 1.2 that into our small intestine, which has a pH of about eight. So we've got to neutralize that acid. And so our gallbladder also helps with that. And finally our pancreas; it also plays a key role in that it's going to, secrete juices through a duct into this top part, the duodenum.  That contains a lot of the enzymes that break down fully, break down the proteins, carbohydrates and lipids that we're consuming.

So we'll get into this in some more detail. But this is just a little preview. So when you eat, your food is in this mostly in what's called a polymer form, which are these great big chains of smaller subunits that are called monomers. So for example, carbohydrates are generally these big, long, huge molecules. And most carbohydrates that we consume are broken down into individual glucose molecules. Lipids are broken down into fatty acids and glycerol and proteins are broken down into what are called amino acids. So these on this side are all small enough to be able to go from the space of digestion inside the intestine and be absorbed into the intestinal cells and ultimately into the bloodstream or lymphatic system, depending on the molecule, the processes that are going to move that food from the esophagus, through the stomach, through the small and large intestines are called peristalsis and segmentation. So peristalsis is basically circular waves of smooth muscle contraction that force the food to go forward and segmentation is going to slosh that food back and forth, breaking it up into smaller and smaller bits mechanically so that enzymes can act on it and break it down into its ultimately tiny, tiny bits.

So our duodenum is primarily a site of digestion, whereas our jejunum and ilium are the sites of absorption. Let's take a little closer look at absorption now. So this,  , process of absorption is primarily occurring in the ilium, and it's about a five meter long stretch of your small intestine. And if you look at the wall of the ilium, you'll see that it is very, very, folded and that's to maximize surface area. So you may have heard, you know, sort of fun facts, like if you took the intestine out and spread it out to be about the size of a tennis court, and that's really what this area is. It just has this huge amount of,  , surface area for absorbing all of those digesting nutrients. If you take a look at these,  , folds, you'll see that they have lacteals, which are the lymphatic, vessels as well as arteries and veins. And so these are going to take those digested nutrients that are going to pass through these intestinal cells and then either again go into the lymphatic system or the bloodstream where they will be taken first to the liver, which also decides where everything's going to go once it's out of the digestive tract. And  , and then from there it'll be shipped out to wherever it's needed.

Our final organ is the large intestine, also known as the colon, at the top of the colon, where it meets your small intestine, there is another valve that's called the ileocecal valve. This is another sphincter that controls the flow and tries to keep it going in one direction;  so it keeps the waste of the large intestine going out and not backing up into the small intestine. We also have this tiny finger called an appendix that is right at the base of our large intestine. And for a long time, people thought that was sort of a very useless, a useless organ, "vestigial". But we now think that it's probably acts as like a refuge for bacteria and other parts of our microbiome so that when we do get sick and have big diarrhea, for example, that we have this little pocket of microbes that we can use to recall and ize our gut when we're healing. The other job of the large intestine major job is to remove the water from all of the digested food. So remember, our stomach and small intestine were involved in liquefying and using liquids. Well, we want to reabsorb all that liquid and and then compact the indigestible stuff into our feces. So that's going to happen in our colons. We also reabsorb some of our   mineral nutrients from the in the colon, such as sodium, because we don't want to excrete that unless we really don't need it. So we have huge amounts of beneficial bacteria  and when we talk about the microbiome later in the quarter, will investigate those more closely. But their job is to help break down some of the foods that we're not capable of breaking down. And we do get some energy and vitamins and minerals from that process. So there's some absorption that does happen in the large intestine. This is also the place where those bacteria may be breaking foods down and producing methane, nitrogen gases and other gases. And that's why we get gas. And I've put a little video for fun in one of the lecture sites about the science of farts, if you're interested. And then we also make some of those bacteria make B1, B2, B7, B12, and K vitamins. So they are pretty important. And we will again explore those in more detail later.