the digestive system
energy and nutrients
Food—delicious, mouthwatering food—contains energy and nutrients that our bodies need to survive. We’ve talked a lot about the importance of energy for our bodies. If you need a refresher, the Cell Respiration lesson is a good place to start. Just about everything that our bodies do—whether that’s an active process that we think about, like running or playing in the park, or something just happening behind-the-scenes that we don’t really think about, like breathing, fighting germs, repairing cuts and bruises, or regulating bodily processes—requires some input of energy. Since we can’t do photosynthesis like plants, that means we need to take in energy through food.
We’ve also talked a lot about the different molecules that are important in our bodies: things like proteins, carbohydrates, nucleic acids, fats, electrolytes, and vitamins. If you need a refresher, the Biochemistry lesson is a good place to start. All of these molecules ultimately come from our food, and we call the good molecules that we need to survive nutrients. You are what you eat! (On a molecular level).
We’ve also talked a lot about the different molecules that are important in our bodies: things like proteins, carbohydrates, nucleic acids, fats, electrolytes, and vitamins. If you need a refresher, the Biochemistry lesson is a good place to start. All of these molecules ultimately come from our food, and we call the good molecules that we need to survive nutrients. You are what you eat! (On a molecular level).
the digestive system
So, how does food go from being food to being you? That’s the job of your digestive system. First, your digestive system breaks food into its itty bitty, molecule-sized components (nutrients) through digestion. Then, your digestive system absorbs these nutrients into your bloodstream so that the nutrients can reach your cells. Finally, water is reabsorbed from the remaining waste food and you poop it out, because we want nothing to do with the stuff we can't digest. Or, to summarize further: digestion, absorption, and elimination. Digest food into little bits (nutrients). Absorb nutrients. Eliminate waste.
(Note that I used the word elimination, not excretion. Excretion is a special word used to describe moving waste from your bloodstream to the outside of your body: We’ll talk about this when we talk about the excretory system. Nothing in poop was ever in your bloodstream. It’s just the leftover junk that we don’t want from digestion. We get rid of it—we eliminate it—but it isn’t excretion.)
Now that you get the big picture, let’s go through the digestive system in a little more detail. (*Go through*—get it? Hehe.) You may want to refer back to this diagram to help you with the wording:
(Note that I used the word elimination, not excretion. Excretion is a special word used to describe moving waste from your bloodstream to the outside of your body: We’ll talk about this when we talk about the excretory system. Nothing in poop was ever in your bloodstream. It’s just the leftover junk that we don’t want from digestion. We get rid of it—we eliminate it—but it isn’t excretion.)
Now that you get the big picture, let’s go through the digestive system in a little more detail. (*Go through*—get it? Hehe.) You may want to refer back to this diagram to help you with the wording:
gi tract and accessory organs
You’ll note from this diagram of the digestive system that there’s basically one really long tube that passes all the way from your mouth to your bum, with lots of convoluted twists and turns in between. This is called the gastrointestinal tract, or GI tract (not to be mistaken for the GI Joe tract, which does not exist). This structure really makes a whole lot of sense: In order to get food into our bodies, we need an in-hole (a mouth). In order to get food out of our bodies, we need an out-hole (an anus). And, in order to get food from our mouth to our anus, we need a tube! (That’s the rest of your GI tract).
You’ll also see these things we call accessory structures. These are the things that do not have food directly pass through them but do help a lot with digestion. They include your salivary glands, your pancreas, and your liver. Mostly, accessory structures make enzymes that digest your food. As you’ve learned, enzymes are special molecules that speed up digestion (and other processes). The ones used for digestion are like molecular "scissors" that cut up molecules in food. The Biochemistry lesson is a great reference if you need a reminder of what enzymes do.
You’ll also see these things we call accessory structures. These are the things that do not have food directly pass through them but do help a lot with digestion. They include your salivary glands, your pancreas, and your liver. Mostly, accessory structures make enzymes that digest your food. As you’ve learned, enzymes are special molecules that speed up digestion (and other processes). The ones used for digestion are like molecular "scissors" that cut up molecules in food. The Biochemistry lesson is a great reference if you need a reminder of what enzymes do.
digestion
Digestion starts in your mouth. Your mouth chews the food so you can swallow it, using your teeth to tear and your tongue to mush food. Physically breaking up food into smaller pieces is called mechanical digestion. While the mouth is chewing, salivary glands secrete saliva (spit), which contains enzymes that break down carbohydrates. The digestion of food into nutrients using enzymes is called chemical digestion.
The mushed up food from your mouth (the bolus) travels down the esophagus and winds up in your stomach. The esophagus is mainly just a tube that connects your mouth to your stomach.
The stomach doesn't actually digest food as much as you might think. It's more of a sack to hold the food. However, mechanical digestion continues as your stomach squeezes itself to break the food into smaller pieces. This is called churning. The stomach also starts the process of chemically digesting proteins into peptides (short proteins that are about 6-20 amino acids in length). Proteins are really hard to break up, so, in addition to enzymes, you also need acid. You may have heard of stomach acid before if you’ve ever had heartburn or tasted the sour taste in your mouth after vomiting. Your stomach has special mucus cells that protect it from this acid. This video provides a good overview of stomach acid:
The liquidy stuff from the stomach (chyme) goes to the small intestine. Most digestion occurs in the small intestine. Your pancreas releases digestive enzymes into the small intestine. These enzymes chop up carbs, sugars, peptides (from the proteins that were broken up in the stomach), and fats into itty bitty, molecule-sized bits. Fats tend to glob up in water, making them hard to digest. To solve this problem, a substance called bile separates the fat molecules into smaller globs. Enzymes can digest these smaller globs. Bile is made in the liver and is stored in the gallbladder.
Absorption
Now that we have all of our food digested into little bits, we need a way to to get these little bits into our bloodstream so that we can use them. This process is called absorption. During absorption, molecules physically cross through the cells of the small intestine and into capillaries. This is done by active and passive membrane transport. Since most nutrients are large and polar, most absorption relies on membrane channels and pumps. Absorption occurs in the small intestine, but further down from where we digested our food.
In order to carry out absorption, the small intestine needs a lot of surface area. Why? Pretty simply, all nutrients have to physically cross through the walls of the small intestine into the bloodstream, and the more wall of small intestine is available to cross, the more nutrients will get into the body. You can think of this like a bunch of cars (nutrients) trying to cross over a bridge (the wall of the small intestine). If you have one bridge with one lane, cars will cross very slowly. Traffic will be miserable, and few cars will be able to get across in a certain amount of time. If you have 10 bridges each with 10 lanes, cars will be able to get across very quickly, and a lot of cars will be able to get across in that same amount of time. More surface area = more stuff getting across. Two main features of the small intestine give it a lot of surface area for nutrients to cross. First, it is a very long, thin tube that loops back and forth in the abdomen several times, so that we can fit more length of small intestine into the body. Second, it has many tiny microscopic folds called villi and microvilli that give us more wall to cross per length of small intestine. This video discusses villi and microvilli a bit more:
Fun Fact! If you stretched out and flattened the small intestine, it would have the same area as a tennis court! Yet, it fits in your body because of all the folding of villi and microvilli.
water reabsorption and elimination
Now that we've gotten all of the useful nutrients out of our chyme, we're ready to move into the large intestine. The large intestine reabsorbs water from the chyme. Eventually, so much water gets reabsorbed that the chyme becomes a solid mass that gets pooped out. If your large intestine isn’t reabsorbing water very well (like if you’re sick), you’ll get diarrhea. If your large intestine is reabsorbing more water than we want it to (like if you’re dehydrated, you haven’t eaten enough fiber, or food is moving very slowly through the large intestine), you’ll get constipation.
What’s left is the solid—this is your poop. We all know where that goes next! (P.S., the answer is the toilet—hopefully.)
Each of these organs of the GI tract are separated by bits of muscle called sphincters. Sphincters are pieces of muscle that "tie off" the sections of your GI tract, so that food only flows when and where you want it to. It’s easiest to see the importance of this for the sphincter you think about mot often: Your anal sphincters close off your anus until you’re ready to use the bathroom. Thanks, sphincters!
If you’d like to learn more about sphincters and their many uses in your body, this video gives a great overview:
If you’d like to learn more about sphincters and their many uses in your body, this video gives a great overview:
Summary
This video gives a great overview of the most important ideas that we’ve discussed:
You should understand:
- That food is first digested and then absorbed. After that, water is reabsorbed from what’s left and the waste is eliminated as feces.
- That digestion involves mechanical digestion plus some chemical digestion in the mouth and stomach (carbs for the mouth and proteins for the stomach, because acid is needed), but that most digestion occurs in the small intestine.
- That digestion is carried out by enzymes that are made by accessory structures. The pancreas makes the majority of digestive enzymes.
- That absorption occurs in the small intestine. The small intestine has specialized structures called villi and microvilli that give it a lot of surface area for absorption.
- That the large intestine reabsorbs water from your feces, which then exits the body through the rectum.
Learning Activity
Contributors: Emma Moulton, Kathleen Yu, Suzanne Xu