Breaking it Down - Chemical and Mechanical Digestion_Final

>> Welcome back. Are you ready to learn more about digestion? This complex process can be a lot to swallow, so we're gonna break things down further. You've learned that the process of digestion can be broken down into three phases. The syphilis or the oral phase, which is when you put food in your mouth and it travels down your esophagus, the gastric phase, which is when your stomach breaks down food and kills any contaminants, and the intestinal phase which is when nutrients and water are absorbed and any waste is excreted. In this lecture, we're going to reexamine these three phases to determine whether the digestion occurring in each phase is mechanical and or chemical. We'll also cover the role of auxiliary organs which you can think of as the helpers that produce enzymes necessary for chemical digestion. So first of all, what do we mean by mechanical and chemical digestion? Mechanical digestion is the physical action of mashing up food and breaking it into smaller parts so that there's more surface area for enzymes to attach themselves to. It includes any movement that involves muscles breaking the food into the smaller parts. Mechanical digestion includes mastication which is a fancy word for chewing and peristalsis. Chemical digestion is the chemical transformation of food into smaller molecules by digestive enzymes. There are eight digestive enzymes, but we'll only be covering three main enzymes. It wasn't until the 1800s that we learned whether digestion was mechanical or chemical. The medical community sat on both sides of the camp until one day in 1822, when a shotgun was accidentally fired inside of a fur trapping store. A fur trapper fell to the floor. It looked pretty certain he would die and his shirt caught on fire, his rib was poking out, and there was a hole in his stomach where his last meal spilled out. The only doctor on the island happened to be William Beaumont, a self-taught army surgeon. The man shot was named Alexis St. Martin. William was able to save his life by feeding him nutritional enemas. So the wound healed, but a hole in his stomach about the size of a quarter, didn't. So for decades, St. Martin participated in experiments, led by William Beaumont to understand the human digestive process. As a result of their efforts, we now know that digestion is both mechanical and chemical. So let's go over what happens in each phase of digestion. In phase one, the oral phase, our teeth masticate and grind the food by clamping down. The tongue pushes it against the hard palate and rolls it around breaking it down further. This process is the start of mechanical digestion. As we mentioned, the goal is to increase the surface area for enzymes to attach and break down the food further. The process of chemical digestion begins with the salivary glands. We have three pairs of salivary glands, the sublingual, under the tongue, the parotid glands, by the cheek, and the submandibular glands, under the jaw. These glands produce two types of saliva, serous and mucus. The serous cells in the mouth secrete a watery fluid and the mucus cells, just like they sound, secrete mucus. Our salivary glands really kick into gear when we're chomping away, triggering the release of more mucous, water, and enzymes to break down starches and moisten our food for its journey down the hollow tube that leads to the next phase of digestion. The salivary glands also release our first enzyme, amylase, which begins to break down starches into sugars. Fun fact, we seem to genetically carry extra copies of the gene for amylase. Some believe that we developed this as a result of the Industrial Revolution gaining the ability to break down newly cultivated grains. However, that theory is still up for debate. Once the mashed-up food is heading down the esophagus, we have our next instance of mechanical digestion, peristalsis. As we learned in the last module, peristalsis is the action of the muscles contracting and relaxing to move food or bolus through the esophagus. So to recap, for phase one, the oral phase, includes both mechanical and chemical digestion. In phase two, the gastric phase, again, there is both, mechanical and chemical digestion taking place. On its way to the stomach, your food must pass through a sphincter which relaxes to let food through and on a good day tightens to keep acids out. A sphincter is a ring of muscle at the opening of a tube. Every organ in the digestive tract has a sphincter on both sides that operate like keys, opening and relaxing for food to enter and leave. The sphincter between the esophagus and the stomach is called the lower esophageal sphincter. When food or bolus enters the stomach, the stomach muscles act like a meat grinder, pushing it around, breaking it up, even smaller. As the stomach expands, it sends a signal to the brain that it's time for hydrochloric acid to be released. Hydrochloric acid released by the parietal cells in the glands of the stomach, lowers the PH and kills off any unwanted bacteria or potential pathogens. Remember, we're taking in all sorts of things from the outside world, so our digestive process is designed to protect us. This acid helps to unwind the proteins and begin their digestion. Parietal cells also secrete intrinsic factor which is essential for the absorption of B12 which occurs later in the process. As PH levels go down, cells in the stomach called "chief cells" trigger the release of a combo of enzymes that produce pepsin. This is the digestive enzyme that attaches to the unfolded proteins. So proteins are made up of hundred of linked amino acids, and when broken down, we can use these to build muscle and tissue. Cool, huh? The stomach also produces an alkaline mucus that is filled with sodium bicarbonate to buffer the low acid in the stomach and prevent the stomach from digesting its own muscle lining. That was one of the questions that William Beaumont was able to examine, so why doesn't the stomach digest itself when it produces such a strong acid? But as it turns out, 30 to 36% of our protein does come from digesting our own dead cells and mucus. Feel your stomach turning now? As we move into the next phase and the stomach slows down, the PH raises back up and pepsin becomes ineffective. Every enzyme requires a different PH to function and pepsin loves the low PH of an acidic stomach. Now you may be wondering how long this part of the process takes. Food or bolus stays in the stomach for around two hours if it's a carb and up to four hours if it's a fat, protein fall somewhere in between. Entering the next phase, the food, now mostly liquid, is released by more peristalsis into the first chamber of the small intestine, the duodenum, through the pyloric sphincter at the other end of the stomach. Here in the duodenum, we begin the third phase of the digestive process, the intestinal phase. The job of the duodenum is first to neutralize the acid with more bicarbonate from the pancreas. When the PH is higher and the small intestine expands with food, the pancreas, liver, and gallbladder know they can start to deliver their own set of digestive enzymes, the enzymes that like a higher PH, and continue the digestion of fats, carbs, and proteins. An enzyme is a substance produced by the body that acts as a catalyst for sparking a specific biochemical reaction. Let's take a moment now and go over the three main digestive enzymes and their functions. We have amylase, which breaks down carbs into sugars, protease, which breaks down proteins to amino acids, and lipase, which breaks down fat into fatty acids and glycerol. These enzymes are mostly released by the pancreas, but some come from the walls of the small intestine. There are also enzymes that attach to the wall of the small intestine and break food down further as it passes by, subsequently helping to activate additional enzymes. Picture it like a car wash, except these enzymes aptly called "brush border enzymes" stay in one place. The pancreas has a big job of releasing major enzymes, but what do the other helper organs do, the liver and the gallbladder? Let's find out. The liver produces bile, a green-colored digestive enzyme that helps break down fats. Now remember, our end goal is to create water soluble nutrients, fats are not water soluble and they tend to clump up and cause problems. Bile acts like an emulsifier surrounding fat molecules to prevent them from clumping. Emulsifiers help keep things in solution and prevent them from touching or reacting, and in the case of bile, they're keeping fat from becoming an impenetrable ball. The bile produced by the liver is stored in the gallbladder until it gets where that there's fat to be digested, at which point it's released into the duodenum. There in the small intestine, it's mixed with lipase which is produced by the pancreas and the small intestine, and assists in breaking down fat. The primary job of the rest of the small intestine is to finish the chemical job of digestion and absorb the resulting nutrients and minerals. Peristalsis, mechanical action, we mentioned earlier, also happens here. Peristalsis moves the chyme forward, turns it over and over like a washing machine so that it's evenly covered in enzymes. Contracting muscles move the chyme along until it reaches the large intestine. Intestinal goblet cells release more mucous for protection and lubrication along the way, they're called goblet because they're actually goblet shaped. Makes sense, right? The majority of digestion and absorption happens in the small intestine. The large intestine has the less glamorous role of taking out the trash. To do this job, your large intestine houses a whole community of bacteria. You can think of them as a garbage compactor, chomping on our indigestible fibers, creating another chemical reaction. These thrifty bacteria transform our leftovers into vitamins, minerals, and fatty acids so we don't miss a thing. Resourceful little guys, no wonder they evolved with us. The muscles of the large intestine move the debris along with the contractions that occur three to four times a day, until it reaches the rectum and triggers the anal sphincter. But the final move to relax and release must be a conscious choice when the time is right. So now that we've reached the final destination, this wraps up our exploration of mechanical and chemical digestion. The only thing left is for ways to leave the body, and then it's time to do it all over again in the next meal. So much of what we know about digestion can be attributed back to William Beaumont and his brave subject, Alexis St. Martin, the man with a hole in his stomach. To recap, the process of digestion breaks down food into fuel or tiny particles that are water soluble that can be absorbed mostly in the small intestine. We want to break down fats, proteins, and carbs into sugars, amino acids, and fatty acids. The body does this through two main processes, mechanical and chemical digestion. Mechanical digestion is the physical act of breaking out food into smaller pieces, chemical digestion uses enzymes to give the food a chemical bath and break it down further. These enzymes are produced in the mouth, the stomach, the pancreas, and the liver. At the end of this process, we're left with nutrients that our cells can absorb and used to fulfill our highest potential. I am so happy you were able to walk through this process with me. I hope you learned a lot. Now you know all about how optimal digestion works. You have a solid foundation to explore gut health. Join us over the Facebook page for discussion and to share your insights. Thanks for watching. Bye for now.