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Lesson Video: Absorption and Defecation Biology

In this video, we will learn how to describe how products of digestion are absorbed by the small intestine, outline how the small intestine is adapted to do this, and explain how food that cannot be digested is removed from the body by defecation.

16:24

Video Transcript

In this video, we will learn how to describe how the products of digestion are absorbed by the small intestine. We’ll investigate the adaptations the small intestine has to do this so that our body cells can get the nutrients they need. We will also explain how food that cannot be digested is removed from the body by defecation.

Did you know that food can take from two to five days to make it all the way through our digestive system? Up to 40 hours of this time can be spent just in the large intestine. In spite of this, the large intestine is actually much shorter than the small intestine. While the small intestine is around seven meters long, the large intestine is only around 1.5 meters long. Both intestines form part of the human digestive system, but they carry out very different roles. Let’s take a brief look at the journey food takes through the digestive system to reach the intestines.

The alimentary canal, which is shown in this diagram in pink, is a very long tube that food passes through, all the way from the mouth to the anus. Let’s see which other organs form part of the alimentary canal. Food is first placed in the mouth, where it is mashed into a ball called a bolus, which travels down the esophagus to the stomach. Once it reaches the stomach, the food is mixed with gastric juices and is now called chyme. The chyme then moves into the small intestine.

The small intestine has a major function of breaking down and absorbing important nutrients from this food that we eat for the body cells to use. The process of breaking down the large nutrients and food using biological catalysts called enzymes is called chemical digestion. Chyme then passes into the large intestine, which reabsorbs water and salts from the undigested food to form feces. Feces is the usually solid, undigested food waste matter, which is egested from the large intestine following digestion. Feces is then stored in the rectum before it is removed from the body from the large intestine via the anus. This process is called egestion.

Let’s take a look at why digestion is so important to the human body before we look in more detail at the structure and function of the two intestines. Digestion is a process by which large molecules in our food are broken down into smaller molecules, which can be absorbed and used by the body cells. Though the majority of digestion will occur in the small intestine, there are various other organs of the digestive system, such as the stomach, which also break down food molecules into smaller subunits. This process is really important, as it means that the molecules are now small enough to be absorbed into the bloodstream.

The blood is the main fluid that transports nutrients to the cells of the body that require them. Nutrients are the substances that the body requires for energy, building materials, and controlling body processes. The main nutrients required by humans are water, carbohydrates, proteins, lipids, vitamins, and minerals. Once they reach the body cells, these small molecules can be built up into many different larger molecules to play various vital functions. Let’s focus on the structure and function of the small intestine first, as this is where the majority of proteins, carbohydrates, and lipids are digested and are subsequently absorbed into the blood.

The small intestine is made up of three main parts. The food first enters the duodenum. Then it passes into the jejunum. And finally the food moves into the ileum before it can enter the large intestine. Until the stage where food enters the small intestine, it has been traveling through the digestive tract but has not entered any body cells yet. By looking at a cross section of a part of the small intestine, we can see that it is a hollow tube-like organ lined with epithelial cells. The small intestine absorbs the products of digestion into these cells of the intestinal wall so that they can then be transported to other body cells to carry out key functions.

For example, proteins are digested into amino acids, which can then be built up into an array of different proteins within the body cells themselves to be used for growth and repair. The simple sugars, which are produced from carbohydrate digestion, will be used as an energy store. For example, glucose can be used in cellular respiration to release energy. Or larger carbohydrates, like glycogen, can store this energy to be used at a different time. Lipids are broken down into fatty acids and glycerol. These can then be built up into different lipid molecules within the body cells to be used as a different type of energy store, for protection, for insulation, or even to make membranes.

The small intestine is adapted for efficient absorption in several ways. Its huge length provides a very large surface area across which nutrients can be absorbed. Another adaptation that provides a large surface area in the small intestine are the finger-like protrusions along the wall of the ileum, the final section of the small intestine. So, let’s magnify these and see how they are adapted more closely. These protrusions are called villi, or a singular villus. Villi increase the surface area of the small intestine even more for really efficient nutrient absorption. With these villi, the small intestine wall is about 10 square meters in its surface area. Let’s take a closer look at the villi to see the other adaptations they have to assist in the absorption of nutrients into the intestinal cells.

Aside from providing a large surface area, the villi are also constantly moving back and forth in a swaying motion to help the food move and mix with enzymes. They also provide a rich blood supply to the intestinal cells via the capillaries, which are shown here in red. Capillaries are small blood vessels that connect arteries to veins and form networks around body tissues to carry out exchange of gases and other materials. This means that the nutrients from food digestion can diffuse or be actively transported across the intestinal cells and into the bloodstream. The blood supply traveling away from the ileum of the small intestine will include water, mineral salts, some water-soluble vitamins, glucose, and amino acids. The blood will initially be carried to the liver and then to the heart to be pumped to the other cells in the body, delivering all of the nutrients that they require.

Villi also contain lacteals, which are represented in this diagram in green. Lacteals are connected to the body’s lymphatic system. And they absorb digested lipids that are too large to enter the blood directly into the lymph instead. Lipids are digested into fatty acids and glycerol in the small intestine. These fatty acids, glycerol, and some undigested lipids will be absorbed into the lacteals. The lacteals also absorb vitamins A, D, E, and K into the lymph. Once these nutrients have been absorbed by the lacteals, they’re transported around the lymphatic system to eventually be emptied into the bloodstream at a large junction to be returned to the heart. The heart will then deliver them to the body cells in the organs that require them.

Let’s take a closer look at one of the epithelial cells that line each villus to see the adaptations that they have to increase the rate of absorption of nutrients. You can see in this diagram that each intestinal epithelial cell contains lots of small projections on its surface called microvilli. The prefix micro- refers to very small structures, and you can see that it is named as these sort of look like very small villi. Microvilli are microscopically small finger-like protrusions of the cell surface membrane at the intestinal epithelial cells. They further increase the surface area across which nutrients can be absorbed. Once the useful nutrients have been absorbed into the capillaries or lacteals of the small intestine, the remainder of undigested chyme passes into the large intestine.

The primary function of the large intestine is to absorb water and salts from this undigested food. The wall of the large intestine, like the small intestine, is highly folded to increase the rate of absorption. Undigested food from the small intestine first enters the large intestine into a region called the cecum. The residue remaining from chyme in the large intestine is semisolid feces. There are many bacteria in the large intestine that help break down this undigested food, which might include those high in fiber, like grains and even some fruit and vegetables. Many of these bacteria are egested as part of feces and are responsible for its unpleasant odor.

A diet high in fiber is helpful to the passage of undigested food through the large intestine, as it adds some bulk to feces, making them larger but softer. This means that feces passes through the large intestine faster and defecation is easier. Feces are stored in the rectum before being egested from the body via the anus, through a process called defecation. Defecation occurs when muscular contractions happen in the rectum and a ring of muscle in the anus called the anal sphincter relaxes.

The process of egestion is aided by the mucus secreted by mucosa of the large intestinal wall, lubricating the feces as it passes out of the anus. This provides us with another key function of the large intestine, which is defecation of feces.

Let’s see how much we’ve learned about absorption and defecation by applying our knowledge to some practice questions.

Undigested food is passed to the large intestine. What is the main function of the large intestine? (A) To absorb water and salts from undigested food. (B) To absorb digested food into the bloodstream. (C) To break down carbohydrates and proteins into their monomers. (D) To emulsify fats. Or (E) to release the digestive enzymes.

The answer options give us a lot of different functions of various components of the digestive system. However, the question is asking us specifically about the function of the large intestine. So, let’s have a look at the different functions of various parts of the digestive system to see which one is correctly identifying the large intestine. This diagram shows us a simplified view of the human digestive system, with the large intestine, our organ of interest, shown in pink.

Food initially enters the mouth, into which digestive enzymes are secreted. These enzymes start to catalyze the breakdown of the large nutrients in food. In the mouth specifically, this will include breaking down large carbohydrates into smaller molecules. Food then passes down the esophagus and into the stomach. Different enzymes are secreted into the stomach to those that acted in the mouth. The enzymes in the stomach are responsible for starting the breakdown of proteins in food. The food then passes from the stomach into the first section of the small intestine. At this point, even more enzymes are secreted from an organ called the pancreas into the small intestine. These enzymes secreted by the pancreas can break down proteins, carbohydrates, and lipids in the small intestine.

Let’s label the other structures that we’ve mentioned so far. By this point in digestion, all of the enzymes that will be acting to break down food will have already been released, but we haven’t yet reached the large intestine. This is because by the time food reaches the end of the small intestine, all of the chemical digestion, which is that using enzymes, has already occurred. So, we can eliminate one of our answer options as we know that the large intestine is not responsible for releasing digestive enzymes, as none of them will be acting there. We can also therefore tell that it is not the job of the large intestine to break down carbohydrates and proteins into their monomers.

There are still a couple of organs in our diagram that we haven’t explored yet. This organ is called the liver. One of the functions of the liver is to produce a substance called bile, which is secreted into the small intestine. The role of bile is to emulsify fats, which makes them easier for digestive enzymes to break down. This process will be occurring in the small intestine, but not in the large intestine. So, we can eliminate another one of our answer options.

We’ve already learned how one of the roles of the small intestine is to break down nutrients using digestive enzymes. Another role of the small intestine is to absorb the subunits that are formed as a product of this digestion of large nutrients. This digested food is absorbed into the bloodstream. By the time food reaches the large intestine, most of the nutrients have already been absorbed into the bloodstream. Therefore, we can deduce that this is not the main function of the large intestine.

Instead, the role of the large intestine is to absorb water and salts from undigested food into the blood. It’s also responsible for the defecation of any undigested food that remains after the food has passed through the large intestine. Therefore, we’ve worked out that the main function of the large intestine is (A), to absorb water and salts from undigested food.

Let’s have a go at another question together.

How are the majority of waste materials removed from the digestive system? (A) By excretion, (B) via respiration, (C) through reabsorption in the large intestine, (D) as urine, or (E) as feces.

In order to understand how to answer this question, let’s see what some of the key terms that have been used in the answer options mean. Excretion is a process that occurs in almost every single body cell, in which the waste products of their metabolic reactions are removed. These waste products might, for example, be carbon dioxide, which is produced in cellular respiration in muscle cells and needs to be removed from them.

A common misconception is that excretion and egestion are the same thing. This is not the case, however. While excretion removes metabolic waste products formed by cells, egestion refers to the final removal of undigested waste products from the large intestine, which occurs via a process called defecation. Therefore, we can deduce that excretion is not how the majority of waste products are removed from the digestive system.

Cellular respiration is a process that occurs in cells by which carbon-containing compounds, like glucose, are broken down to release energy. Though we’ve already learned that the carbon dioxide produced in cellular respiration will be excreted from cells, we know that excretion is not the same as egestion. So, this answer must also be incorrect.

The main function of the large intestine is to absorb water and salts from undigested food into the bloodstream. Waste materials are not useful to body cells, however, and they can even be harmful. So, waste materials will not be reabsorbed in the large intestine, which also shows us that this option is incorrect.

This diagram shows us the kidneys, which are responsible for forming urine. Urine is a waste product that is formed by the kidneys and is stored in the bladder before it’s removed from the body during urination. Urination does remove waste materials and harmful products from the body, just as defecation from the large intestine does. But the kidneys and bladder are not technically a part of the digestive system, which is what this question concerns. Urine is produced by different organs and is removed from the body via a different tube, the urethra, instead of via the anus. So, we can conclude that this option is also incorrect.

The digestive system ends with the large intestine, where semisolid feces are formed after water and salts have been absorbed into the bloodstream. And as you might recall, these feces will be removed from the body by defecation through the anus. Therefore, we can confirm that the majority of waste materials are removed from the digestive system as (E) feces.

Now, it’s time for us to review some of the key points that we’ve covered in this video. We now know that the small intestine’s main functions are digestion and absorption of nutrients and food. The small intestine is well adapted to carry out absorption due to its large surface area, which is increased by villi and microvilli in the intestinal wall. Capillaries and lacteals surrounding these villi carry digested nutrients around the body via the blood and lymphatic systems. The large intestine’s main functions are absorption of water and salts into the blood and defecation. The large intestine is well adapted for its functions due to its fairly large surface area and the presence of mucus-secreting cells in its wall to aid this defecation process.

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