From Food to Finish
Ingestion, Digestion, Absorption, and Elimination
We've all been there before. You've spent hours mindlessly surfing the interwebs. You started off wanting to know what gremlins were, and eventually found yourself buying another Furby. They'll never really go out of style, and you know it. You feel that little rumble in your stomach, demanding food. You get up, slink over to the kitchen, and are faced with a tough choice: potato chips or pretzels?Believe it or not, most of the animal kingdom is not agonized by these heart-wrenching choices. In fact, it's relatively mindless for most. See food, eat food. End of equation. The only hard part is getting the food to hold still, if you're a carnivore.
Getting the food to the animal's mouth (or ingestion) is the easy part, one that the animal actually has some sort of control over. The hard part comes when organs and enzymes actually have to digest the food. That means the body has to decide what nutrients get to stay for the feature film (or absorption) and those that leave after the reminder to turn off all cell phones (or elimination). The stakes are high: gain a pass from the body and you get to play an integral role in keeping this living entity going. Fail to get the nod and you'll be unceremoniously dumped out of the body as a byproduct.
Broadly speaking, the digestive process for animals follows the same basic path. Food starts as food, gets worked over by an animal's digestive system to extract nutrients and other good stuff, and comes out the other end as something that is definitely not food. It sure doesn't look or smell like BBQ-flavored potato chips anymore. We're quite content to just assume the taste isn't quite the same either.
We're going to focus on general mechanisms of digestion. Keep in mind that each animal has its own unique way of wringing out nutrients from food depending on what they like to eat. For example, cows' stomachs are actually broken down into four chambers to help break down plant material, which can be quite resilient.
Ingestion: Here we have the first step of this crazy journey, ingestion. What is ingestion, you ask? It's getting the food that's not in your mouth into your mouth (or whatever you have that is a substitute for a mouth) so you can start routing it through the digestive system. The actual mechanics for ingestion varies widely among the world's countless species. Apes have plenty of dexterity in their hands that make bringing food to their mouths pretty easy. Other animals have developed other creative ways to ingest their food. Take leaf miners, for example. These bugs pretty much eat their habitat; they live in tree leaves, and munch the leaf tissue. Just think how little you'd use your thumbs if the walls of your house were made of beef jerky. You'd just bite a big hunk off whenever the moment struck you. Who needs opposable thumbs?
Eating their way through leaves, leaf miners wreak havoc on their own environment. Image from here.
Animals, bugs, and all other heterotrophs have all sorts of ways to ingest food. No matter what, they all should remember one important lesson: chew with your mouths closed! What would a leaf miner's mother say if she saw him just chomping away, showing off to the world his half-eaten leaf matter? Tsk, tsk.
Digestion: Digestion is the process where food is broken up into small compounds that can be absorbed into the blood stream or used by cells. Digestion starts at one end of the alimentary canal. The digestive tube runs from the mouth all the way to…the other end. In fact, the digestion process actually begins as soon as teeth, or whatever else, begin to break down the food. That's because the mouth is where salivary glands secrete enzymes, such as salivary amylase, that break carbs down into more simple parts. If a hypothetical hippopotamus eats a hypothetical strawberry shortcake for dessert, these enzymatic reactions transform it into a glob of smaller polysaccharides (or groups of sugar molecules), while fats and proteins remain relatively intact.
When our hypothetical hippopotamus (we'll call him Henry) swallows, his dessert mush moves through his pharynx and esophagus, and into the stomach. The stomach is where an animal stores its ingested food, and also where it secretes acidic gastric juices that start to break down proteins. Think of it as the world's least appealing hot tub. Since Henry's strawberry shortcake probably looks super gross at this point, biologists gave an equally nasty name for the partially broken down food setting up shop in the stomach: chyme.
Table 2. The Who/What/Where of Digestion.
| Nutrient | Location for Digestion | Enzyme |
| Protein (amino acids) | Stomach Small intestine | Pepsin Pancreatic enzymes, trypsin & chemotrypsin |
| Carbohydrates (sugars) | Mouth Small intestine | Salivary amylase Pancreatic amylase |
| Lipids (fatty acids) | Small intestine | Bile; Pancreatic Lipase |
The stomach is lined with two cell types that do all the hard work: parietal and chief cells. Parietal cells secrete hydrogen (H+) and chloride (Cl-) ions to make hydrochloric acid (HCl) molecules, which create a really acidic environment (in the neighborhood of pH = 2–3). The stomach's chief cells join this acidic pool party by secreting a compound called pepsinogen. When pepsinogen enters an acidic solution (like, say, the inside of a stomach), it is converted into the protein digesting enzyme, pepsin. Since pepsin functions best in low pHs, it's able to break down our the strawberry shortcake goo even further. In case you wanted to know, bolus is the more technical term for a glob of partially digested food working its way through the body. Bit by bit, enzyme by enzyme, that tasty piece of cake is looking less and less like cake, but more and more like nutrients.
Here's a drawing of the parietal and chief cells in the stomach.
By now, you might be scratching your head. "Self," you say to yourself, "I've seen my fair share of spy movies. I know that the evil mastermind always chooses to explain his dastardly plan to take over the world when he's got the spy where he wants him—dangling over a shark tank, laser field, or even a vat of acid! Well hang on just one second, how does something that could dissolve James Bond exist in our stomachs and everything stays A-Okay?"
You've got to give your body some credit. Since secreting something that will end up hurting you is just silly, a thin layer of mucus prevents all this acid from harming these parietal and chief cells lining the stomach. If the mucus gets too thin or deteriorates altogether, these acids wreak havoc on the stomach lining, and cause painful ulcers.
After the acid works its wonders on the chyme, the stomach churns through a series of muscle contractions and relaxations called peristalsis, and the artist Formerly Known as Food makes its way to the small intestine. Now this is where the magic happens.
Once inside the duodenum (a.k.a. upper part of small intestine), our hippo's acidic chyme stimulates the release of bicarbonate (that's HCO3-) -based digestive juices from the pancreas that will buffer the lumen (or the area inside any tube, including the intestine) to a more neutral pH (roughly pH = 7). "Acidic? Neutral? Make up your mind, digestive system," is what the chyme would say if it could talk.
As a safety precaution, a thin layer of mucus prevents any HCO3--escaping acidic chime from doing any major damage at the small intestine.
Here's Henry and Harriet, our Hippos, most likely fighting over strawberry shortcake. Image from here.
The pancreas tends to be an overachiever. Perhaps it just has bills to pay. The pancreas works an extra job on the side, producing other enzymes to complete this digestion job. Since the pancreas is so close to the small intestine, it makes sense that it would pitch in and help when needed. Pancreatic amylases are secreted into the small intestine where they will continue the hard work that was started in the mouth. They'll break sugars down even further, to the disaccharide (now they are two sugar molecules big) level. There are also pancreatic enzymes that specifically break bonds in polypeptides, resulting in various individual amino acids.
For all their hard work, pancreatic enzymes just can't get the hang of breaking down fats. That's where bile comes in. Everyone loves some good bile. It's made in the liver, but stored in the nearby gallbladder. That way, everyone gets in on the bile action. Bile digests big fat globules into smaller fat globules that a pancreatic lipase (which breaks down lipids, or fats) can handle. Before you can say, "Hungry, Hungry Hippo," fats become smaller fatty acids.
At this point, Henry's strawberry shortcake, once haute cuisine, has been ravaged by the body's extensive arsenal of gut-based firepower that degraded it to the simple states of basic compounds, ready to be absorbed and put to use by the body.
Absorption: Also occurring in the small intestine, absorption is the process where molecules pass through the walls of the alimentary canal and into the body to be used in various biological processes. Everything up until now has really been a precursor to the main event. When absorption starts, we finally have food (or what used to be food) actually moving from the digestive system into the body as a whole to be put to good use.
Along the wall of the small intestine, epithelial cells are covered in wave-like structures, called villi. On top of each villi are microvilli, making these cells appear bumpy (like our tongue's taste buds). All those little bumps and valleys created by the undulating waves of villi means there's plenty of surface area – a lot more than you would get with just a smooth intestinal lining. What's so great about high surface area? The more surface area there is, the more places there are for the intestine to absorb nutrients and pass them off to the body.
This image shows all the important parts to the alimentary canal, starting with the mouth and ending with the anus.
Nutrients pass through these epithelial cells in the intestine and into the smallest type of blood vessel, capillaries. Some nutrients, like fructose (a simple sugar), will be absorbed from the lumen and into the epithelial cells with passive transport. Other molecules, including amino acids, require active transport into these cells. People who say there's no free lunch have never heard of passive transport—it allows for the movement of molecules with no energy cost. Not everything in life is free, though, and that's where active transport comes in. It takes a little bit of energy to get this done, but the upside is that active transport improves absorption by building up the nutrient concentration and speeding the process along. On the other side of the epithelial cells lie small capillaries, and the one-time intestinal nutrients find a new home in the bloodstream. They head on to the liver, and are portioned out from there, providing nutrients wherever they're needed.
Fatty acids don't follow the rules, and confuse these processes a bit. Once they are absorbed by the epithelial cells, they link onto each other, form triglycerides, and get packed into water-soluble chylomicrons. These globs of fats are too large to pass into the small capillaries at the other end of the cell. Don't mention it to the chylomicrons—they're sensitive about it and insist it's a glandular problem. Chylomicrons have to take a detour through the lymphatic system before they can be absorbed by larger blood vessels later on.
Elimination: Obviously not everything an animal eats is jam-packed with nutrients. There's a lot of filler, and some not-so-great foodstuff, too. Fortunately, these unsavory bits of food aren't absorbed into the epithelial cell. They are too large, and bigger isn't always better. Instead, they'll travel to their next destination, the large intestine. Fun fact: The large intestine is actually shorter than the small intestine. It's just got a larger diameter. Confusing? Yes. Don't blame us—we didn't name them. This is the beginning of the end for these waste products since the large intestine is comprised of everyone's favorite body parts: the colon, cecum, appendix, and rectum.
The first job for the large intestine is to suck up any water and other digestible material that's grouped together with the waste. At this point, about 90% of the nasty-looking liquid-y stuff that's in the alimentary canal is water, and it is reabsorbed thanks to osmosis. An animal will have diarrhea if too little water is absorbed at this step, and constipation if the converse is true. The rectum is where everything is packed and stored, until the next time nature calls.
Just like that, Henry the Hippo is no longer hungry. His strawberry shortcake, once a delectable dessert with whipped cream on top, has now undergone an extreme makeover. Now, it's just nutrients and feces. Remarkable.