Video Transcript
In this video, we will learn to describe the major components of the digestive system and their adaptations. We’ll cover the general purpose of the digestive system, and we’ll trace the changes in a piece of food as it’s digested and absorbed in our gastrointestinal tract. Well, I hope you’ve brought your appetite for science. Let’s get started.
The digestive system is one of the organ systems you may be the most familiar with since it includes lots of recognizable organs, like your stomach, your liver, and your intestines. The digestive system, like other organ systems, is a group of organs that work together to carry out a function, in this case the digestion of food. What do we mean by digestion of food?
Well, the food we eat comes from living sources, mostly plants and other animals. And they’re made of the same things we are, macromolecules, including lipids, complex carbohydrates, proteins, and nucleic acids. And we eat these organisms to use the things they’re made of to build, repair, and power all of the things that we are made of. In order to accomplish this, we have to break our food down into the smallest molecular components possible and then absorb it so that it can be used by ourselves.
So digestion actually occurs in three steps: the digestion step, which is the mechanical and chemical process of breaking down food into usable components, and the absorption step, which is getting those nutrients into our bloodstream. And then there’s the additional final step of safely removing the leftovers and waste from the body when this process is all done.
One of the things that makes the human digestive system so interesting is that humans are generalists, which means that we can adjust our diet to our surroundings to make use of the available sources of nutrition, in contrast to an organism like a bee, which is adapted to only one source of food. So our digestive system is designed to digest many different types of foods. It’s basically ready for anything we throw at it.
No matter what we choose to eat, our food travels through a long, complex, continuous tube called the gastrointestinal tract or the alimentary canal. The GI tract consists of the mouth, the esophagus, the stomach, the small intestines, the large intestines, and the rectum. Because this tube passes through our body and is open at each end, it’s considered an external environment, which is good because many things within our GI tract could harm our other internal organs.
The digestive organs that are not a part of the alimentary canal are referred to as accessory organs. And these include the liver, the gallbladder, and the pancreas. In order to investigate our digestive system, we’ll follow a meal as it travels through the alimentary canal and is broken down and absorbed by our digestive system. And we’ll investigate the role of each of these organs along the way.
So let’s start by imagining that you’ve just taken a bite of your very favorite meal. The food enters your mouth, the first stop on the alimentary canal and where digestion begins. Your teeth break the food down into smaller and smaller particles as you chew. The food is mixed with saliva produced by your salivary glands. And the saliva contains enzymes that start to break down starches or complex carbohydrates into simple carbohydrates like sugars. When you’re finished chewing, your tongue pushes the food back towards the pharynx where it enters the esophagus.
Chewing is a very important step in the process, not just because it makes your food easier to swallow, but also because it increases the surface area of whatever it is you’re eating, which makes it easier to digest. Imagine trying to dissolve a huge chunk of salt in water. It would probably take quite a while, even with constant stirring. Now, imagine breaking that chunk into tiny pieces first. The salt would dissolve much more quickly. Increasing the surface area of the salt crystal makes it easier to dissolve in water, in the same way that increasing the surface area of your food makes it easier to digest by the enzymes in your digestive system.
From the mouth, the food moves into the esophagus. The chewed food mixed with saliva is now referred to as a bolus. The esophagus is a long, lubricated, muscular tube that pushes the bolus from the mouth to your stomach. The process of mechanical digestion to increase surface area continues in the stomach. The bolus arrives, delivered by the esophagus, and it’s mixed with a liquid full of chemicals and enzymes, which is referred to as gastric juice. The gastric juices are very acidic and have a pH of about two. The stomach is lined with mucus that protects it from the acidic environment.
While the acid is dangerous to our tissues, the enzymes in the gastric juices find this pH to be optimal. And they get the work, mostly on breaking down proteins. The stomach is a muscular sac that clenches and churns, breaking apart the food in the bolus and increasing the surface area even more. At this point, the mixture of food, mucus, and gastric juices is referred to as chyme. And little by little, the chyme is passed into the small intestine.
The first section of the small intestine is important because it’s where the chyme is deacidified. This section is called the duodenum, and it’s one of my very favorite anatomical terms. Here, the chyme mixes with bile and other neutralizing agents to raise the pH back to around neutral. This way, the delicate lining of the rest of the small intestine is not damaged. And a new set of enzymes can do their job in their optimal conditions.
This bile is produced by the liver and stored in the gallbladder before it’s released into the small intestine. Bile not only neutralizes the acids found in gastric juices. It also helps to break down lipids. More digestive enzymes are pumped into the small intestine. These enzymes are produced by the pancreas and added to the small intestine in a mixture called pancreatic juice.
Bile and the enzymes in gastric juice are able to break down lipids, proteins, and carbohydrates. And most absorption takes place in the small intestine. Chyme moves through this narrow tube slowly, being constantly mixed, until the components are small enough to enter the bloodstream.
The small intestine is the primary site for absorption in our digestive system. In absorption, like in digestion, surface area plays an important role. The inner lining of the small intestine is covered in circular folds. Completely covering these folds are tiny, one-millimeter finger-like protrusions called villi. The villi are filled with capillaries and covered with just one layer of epithelial cells. And these epithelial cells themselves are covered with small protrusions called microvilli.
Here, we see two adaptations that allow our small intestines to absorb nutrients efficiently. The first is that nutrients only need to travel a short distance, the width of one cell, to leave the small intestine and enter the bloodstream. And the second is that the folds, villi, and microvilli, increase the surface area available for absorption. The small intestine has a surface area that’s between 30 and 40 square meters, about as big as a two-car garage. This large surface area allows the small intestine to pass as much nutrition from our food as possible into our bloodstream through the surrounding capillaries.
After passing through the small intestine, digestion and absorption of nutrients is complete. Next, the chyme enters the large intestine, which is shorter but larger in diameter than the small intestine. The large intestine helps our body to conserve resources by absorbing almost all the water from the chyme. Along with water, the large intestine also absorbs electrolytes back into the bloodstream. The chyme then becomes more dense and more solid as it passes through the large intestine.
After completing its journey through the large intestine, this final waste product is known as feces. And it’s stored in the rectum until it’s removed from our bodies. We tend to think of feces as a mass of undigested food, but it’s mostly composed of bacteria, living and dead, from the bacteria that live in our digestive system. It also has a significant amount of our own cells, removed from the lining of the gastrointestinal tract during digestion.
This entire track from mouth to rectum can take anywhere from 24 to 72 hours, depending on the individual and their diet.
Before we wrap up this lesson, let’s take a moment to digest the things we’ve learned. In this video, we learned how food is digested or broken down into substances which can be absorbed by the blood. And we traced the changes that occur in our food at each step as it passes through the digestive system. We learned about the structure and function of the organs of the digestive tract, also called the alimentary canal. These organs include the mouth, the esophagus, the stomach, the small intestine, the large intestine, and the rectum.
We also learned about the functions of the digestive organs that are not a part of the gastrointestinal tract. We refer to these as accessory digestive organs. And they include the liver, the gallbladder, and the pancreas.
