It is no secret that I love the platypus. Many know that the platypus is this cool mammal that can lay eggs. And then you might explore further to find out that, wow, the males can have venomous spurs on their legs?! What? Okay, now you may be thinking, isn’t this a video on digestion and not platypuses? Playtpi. I’ve heard it both ways. This is a video on digestion. Which brings me to this question: do you also know that if you do define stomach as an organ that contains gastric glands and produces gastric juice- then the platypus would be considered to not have a stomach? Fascinating! We’ve included some links to science articles and papers on this topic to learn more and why they may have evolved this way. But when I mentioned this about the platypus, was your first thought, well then, do they not have a digestive system? Because a lot of the time, when we think about digestion in mammals- whether it be a platypus or a human- we tend to think about just the stomach. In fact, maybe that’s most of all we think about with the word digestion. But we shouldn’t, because digestion in mammals is so much more than a stomach. For both a platypus and a human. What do I mean? Petunia, it’s time to take a tour of major digestive structures: and this tour, specifically, will be the digestive system of another mammal: the human digestive system. In the human digestive system, we’re going to focus on its ability to accomplish four major tasks: ingestion, digestion, absorption, and elimination. Ingestion is the taking in of food, eating. Digestion is when biomolecule polymers – you remember the four major biomolecules- are broken down physically or chemically into their building blocks. In absorption, they are absorbed. Absorption is important as nutrients will eventually be delivered with the help of other body systems to cells as cells need them in order to survive. In elimination, waste which includes anything that wasn’t digested, will be removed. With ingestion in humans, food is brought into the mouth. Your mouth can release saliva even in thinking about food. Digestion actually starts in the mouth, because your saliva contains enzymes that can break down some biomolecules-salivary amylase for example breaks down some carbohydrates. This is a form of chemical digestion. But you also have mechanical digestion happening in the mouth! Mechanical digestion is when you have something physically breaking the food down. That would be your teeth. Your teeth help grind that food up. Saliva is a very underrated thing. Not only does it have enzymes in it, but it has buffers to counteract acidity in your mouth---a good thing as that can help prevent tooth decay. In fact, a condition known as “dry mouth” can be hazardous for your teeth. Saliva also helps lubricate food, which is awesome, because your tongue will help shape that food into a little ball called a bolus. The bolus will be swallowed, and when it is, it will travel down the esophagus. Your body has this really cool flap called the epiglottis which blocks your windpipe other wise known as your trachea when swallowing food. You don’t want food to go down the trachea and it so happens that your trachea and your esophagus are in real close proximity so that epiglottis is indispensable. Peristalsis, which are these wave-like movements made by smooth muscle, happens in the esophagus and helps moves that food down…down down down to the stomach. An adult human stomach can store an estimated 2 liters of food and liquid. It’s acidic in here; chemical digestion definitely occurs here with the stomach’s gastric juices – which includes HCL - and the enzymes such as pepsin which break down proteins. There’s mechanical digestion too, the churning of the stomach which helps bathe the contents in those gastric juices. The resulting substance is known as chyme. A few interesting things about the human stomach. There’s a sphincter that separates the esophagus from the stomach and there’s also another sphincter that separates the contents in the stomach from the intestines. We’re about to go there now, but are you wondering why doesn’t the stomach digest itself? That’s specialized cells for you: you have these amazing cells that line the stomach that divide often and make a protective mucus layer. Ok, so the chyme, it’s going to leave that second sphincter and on to the intestines! And digestion is what the small intestine does best! All three major parts of it: the duodenum, jejunum, and ileum. By the way, since they’re not in alphabetical order, I like to use DJ Ivan…a little mnemonic to remember it. ANYWAY! Here there will be significant chemical digestion of all four biomolecules: carbs, lipids, proteins, and nucleic acids. There are enzymes involved here and also digestive juices that are released here from other organs such as the gallbladder, the liver, and the pancreas. There will also be mechanical digestion too because peristalsis moves the chyme through this segmented space. But after finishing digestion here, it’s time for absorption. Most nutrient absorption happens in the small intestine. And if you’re wondering, what exactly are some things that could be absorbed: just a few examples could be glucose and fructose- those are sugars- or amino acids- those are building blocks of proteins. It could be electrolytes and vitamins and lipids…I could go on. Absorption occurs through the lining of the small intestine which has projections called villi, and the villi themselves have microvilli. Shape is so amazingly significant here, as it often is in biology, because the shape here allows them to have lots of surface area for absorbing nutrients. They are also very vascular, meaning they have capillaries running through them so the nutrients can be taken up by these capillaries and transported. As we move along, we eventually enter the large intestine, also known as the colon. The large intestine is home sweet home to a lot of harmless and beneficial bacteria in our bodies: some of these good bacteria even make some of our vitamins! But the major function of the large intestine? It involves water. The large intestine’s major role is to reabsorb water so that the body will not lose all of this precious water involved in digestion. Feces are composed of undigested contents and can contain other substances, and it can include the bacteria that we mentioned. The very end of the large intestine is the rectum, and feces will remain here until they are expelled from the anus. So now we finished that last step of digestion. Elimination. Now obviously there’s more to the digestive system than this. As with most of our videos, we can’t cover all the details. However, there are just a few things I want to squeeze in here. We mentioned that organs like the gallbladder, liver, and pancreas release digestive juices in digestion. While they may be called accessory organs, they are not mere accessories. They have extremely important functions. The liver is your largest internal organ, and it has functions beyond the digestive system. But in the digestive system, it’s involved in carbohydrate and protein metabolism. It also produces bile which helps breakdown lipids. The gallbladder stores the bile. The pancreas produces pancreatic juices that have important digestive enzymes and will neutralize acid chyme. In addition to mentioning these accessory organs, there are an assortment of hormones at work in the digestive system to explore. It’s also likely that you’ve heard of disorders that involve the digestive system. Celiac disease which often involves an issue with the small intestine, diverticulitis which often involves an issue with the large intestine, and heartburn -also known as gastroesophageal reflux disease – which often involves an issue with contents from the stomach entering the esophagus. These all affect the function of the digestive system. You can explore more of these in our video details. Having an understanding of the amazing human digestive system helps us better understand how to help when this system does not function as it should. Well that’s it for the Amoeba Sisters, and we remind you to stay curious.