[0:07]In this video, we're learning about the digestive system. So, we'll cover the role of the digestive system and all of the organs involved. Let's begin by looking at the role of the digestive system. You can think of the digestive system as having two main roles: digestion and absorption. Digestion is the process by which we break down the large food molecules that we eat into much smaller molecules. For example, break down carbohydrates like starch into glucose, break down proteins into amino acids, and break down lipids like oils and fats into glycerol and fatty acids. Once everything has been broken down like this, it then moves on to the second role of absorption, which is the process by which we absorb these small molecules, along with vitamins, minerals, and water, from the digestive tract into the bloodstream, so that they can be sent around the body and used for whatever we want, like rebuilding large molecules or in respiration to release energy. Next, let's take a look at all of the organs involved in the digestive system. The key to understanding the digestive system is to think of it as a journey with food starting at one end and then slowly passing through all the tubes and organs. So, slowly being broken down, with any nutrients being absorbed, and then what's left over being excreted at the end. So let's now explain what happens at each stage. Starting at the very beginning, we have the mouth, which contains two important organs: the teeth and the salivary glands. The role of the teeth is to physically break down the food by chewing. This increases the surface area of the food so that it is easier for enzymes to break it down, which we'll learn more about later. But it also makes the food easier to swallow. Meanwhile, the role of the salivary glands is to release saliva, which is a watery mix that contains the enzyme amylase. The water part wets the food, which makes it easier to swallow, while the amylase digests starch into maltose. So it breaks down big carbohydrates into smaller ones. We call enzymes like amylase biological catalysts, which basically means they reduce the energy that's needed for reactions. So the nutrients in our food can be broken down faster.
[2:53]Next, you swallow the food, and it passes down a muscular tube known as your oesophagus, or sometimes called the gullet. Very quickly though, it passes through your stomach, which does three important things. First, it contracts its muscular walls, which push the food around and mix it all up. Second, it produces pepsin, which is a protease enzyme that breaks proteins down into amino acids. And third, it produces hydrochloric acid, which both kills any bacteria we might have swallowed and provides the right pH for the pepsin enzyme to work in. After a couple of hours or so, the stomach squeezes really tight and pushes the mixture into the small intestine, which is basically a long tube. Before we look at what it does though, we need to look at two other organs: the pancreas and the gallbladder. At roughly the same time as the food mixture moves to the stomach, the pancreas releases pancreatic juices into the small intestine, and the gallbladder releases bile into the small intestine. So, there's now a mixture of food, pancreatic juices, and bile in the small intestine together. Pancreatic juice is basically a liquid mixture that contains loads of digestive enzymes, including carbohydrases like amylase that break down carbohydrates, as well as proteases and lipase enzymes. So once it's in the small intestine, they can help digest the food. Bile on the other hand has two roles. One is that it neutralizes the acid from the stomach, making the pH more ideal for the digestive enzymes to function. And it can do this because the bile itself is alkaline. Its other role is emulsifying lipids, which basically means that it helps break the big blobs of fat and oil down into tiny droplets. And this helps because it gives the lipids a much larger surface area for digestive enzymes to work on. And one extra thing to know is that bile is actually made in the liver. It's only stored in the gallbladder until it needs to be released.
[5:15]Now, the small intestine itself is arguably the most important part of the entire digestive system. This is because, one, it's where most of the digestion takes place, and two, it's where the nutrients are absorbed from the intestines into the bloodstream. If we start with its role in digestion, you might remember from earlier that carbohydrates are partially digested in the mouth, and proteins are partially digested in the stomach. However, the large majority of digestion takes place in the small intestine. And to help with this, the small intestine releases digestive enzymes, just like the pancreas did. So, more carbohydrases, lipases, and protease enzymes. At this point in our journey, all of the nutrients are broken down. So, next, let's look at how they get absorbed across the intestinal lining and into the bloodstream. If you were to look inside one of the tubes that makes up your small intestine, you'd see that the surface is covered in these tiny finger-like projections called villi, or a villus if you're just talking about one. Villi have a few important features that you need to know about. One is that they have a huge surface area over which diffusion can take place. Second, they only have a single layer of surface cells, which means there's only a very short distance for diffusion to take place across. Third, they have a really good blood supply, which maintains the concentration gradient for the absorption of nutrients. And finally, the cells of the villi also have their own tiny finger-like projections called microvilli, which further increases the surface area.
[7:11]Okay, so let's now imagine that all of the useful nutrients have been absorbed into the bloodstream. At this point, there's still a lot of material left in the small intestine, because a lot of the food that we eat isn't actually digestible, and it's still going to be a bit watery. So, next, it passes into the large intestine, which has the role of absorbing most of the excess water that hasn't been absorbed yet. This leaves behind a relatively dry mixture that we technically call faeces. And these are stored in the rectum until they're ready to be removed at a convenient time through the anus.
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