Lesson Explainer: Absorption and Defecation Biology

In this explainer, 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.

Did you know that food can take from 2 to 5 days to make it all the way through your 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 far shorter than the small intestine. While the small intestine is around 7 m, the large intestine is only around 1.5 m.

Key Term: Small Intestine

The small intestine is an organ of the digestive system between the stomach and the large intestine that is essential for the chemical digestion of nutrients in food and their subsequent absorption.

Key Term: Large Intestine

The large intestine is an organ of the digestive system between the small intestine and the anus that reabsorbs water and salts from undigested food to form feces.

Both intestines form part of the human digestive system, but they carry out very different roles. The small intestine has a major function of breaking down and absorbing important nutrients for the body cells to use. The large intestine plays a role in reabsorbing water and salts into the body from food before it leaves the body as feces.

Key Term: Feces

Feces are the solid undigested food waste matter egested from the large intestine following digestion.

Key Term: Nutrients

Nutrients are 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.

Digestion is the process by which large molecules are broken down into smaller ones, which can be absorbed and used by the body. Although the majority of digestion occurs in the small intestine, there are various other organs of the digestive system that also break down food molecules into smaller subunits. This process is really important, as it means that the molecules are in a form small enough to be absorbed into the bloodstream. The blood is the main fluid that transports nutrients, such as simple sugars and amino acids, to the cells of the body requiring them. Once they reach the body cells, these small molecules can be built up into many different larger molecules to play various vital roles.

Definition: Digestion

Digestion is the process by which large molecules are broken down into smaller molecules, which can be absorbed and used by the body.

Example 1: Describing the Purpose of Digestion

What is the primary purpose of digestion in the human body?

  1. To transport essential nutrients and oxygen around the body
  2. To break down large molecules into smaller, soluble ones that can be absorbed by the body
  3. To coordinate and control the body’s responses to internal and external stimuli
  4. To regulate cellular metabolism and respiration
  5. To maintain a constant internal environment

Answer

Digestion is the process by which large molecules are broken down into smaller ones, which can be absorbed and used by the body. This process is really important, as it means that the molecules are in a form small enough to be absorbed into the bloodstream. The blood is the main fluid that transports nutrients, such as simple sugars and amino acids, and oxygen to the cells of the body requiring them. Once they reach the body cells, these small molecules can be built up into many different larger molecules to play various vital roles.

It is the role of the nervous system, not the digestive system, to coordinate and control the body’s responses to internal and external stimuli.

There are many enzymes involved in the process of digestion, which act to break down large molecules into smaller ones. There are also enzymes involved in most other metabolic reactions in cells, such as in respiration. The majority of digestion in the human body, however, occurs outside the cells, in the hollow tubes of the alimentary canal that leads from the mouth, where food is ingested, all the way to the anus, where undigested food is egested as feces. Therefore, digestion does not regulate cellular metabolism and respiration.

The process of maintaining a constant internal environment is called homeostasis. This is not a process carried out by the digestive system and so is not the primary purpose of digestion. Instead, digestion provides small soluble molecules for the body to use.

Therefore, the primary purpose of digestion in the human body is to break down large molecules into smaller soluble ones that can be absorbed by the body.

Let’s look at the journey that food takes through the digestive system to reach the intestines. The main organs that carry out the digestive processes can be seen in Figure 1, where the direction that food travels in is marked with blue arrows.

The alimentary canal is a very long tube that food passes through, all the way from the mouth to the anus. Let’s see which organs are part of this alimentary canal.

Food is first placed into the mouth, and then the teeth mash it into a ball called a bolus. This is called buccal digestion. From the mouth, the bolus travels down the esophagus to the stomach. Once the bolus reaches the stomach, gastric digestion occurs, where the food is mixed with gastric juices and is now called chyme. This chyme then moves into the small intestine, where intestinal digestion occurs. The chyme then passes into the large intestine, which reabsorbs water and salts from the undigested food to form solid feces. These feces are stored in the rectum before they are removed from the body by egestion through the anus.

Definition: Egestion

Egestion is the process by which undigested waste is removed from an organism.

You may have noticed that not all of the organs in Figure 1 had food pass through them. Any organ that plays a role in digestion but does not encounter the food itself is called an accessory organ. The main accessory organs in the human digestive system are the salivary glands, pancreas, liver, and gallbladder.

Let’s focus on the structure and function of the small intestine.

The small intestine is a long tube located in the abdomen. The majority of chemical digestion occurs in the small intestine, as proteins, carbohydrates, and lipids are all digested by enzymes there. Until this stage, food has been traveling through the digestive tract, called the alimentary canal, but has not entered any body cells. The small intestine absorbs the products of digestion into the cells of the intestinal wall so they can be transported to the cells of the body to carry out key processes.

For example, the amino acids from protein digestion are used by body cells for growth and repair. The simple sugars from carbohydrate digestion are used as an energy source in respiration. This energy may also be stored in a different form to be used later. The fatty acids and glycerol molecules from lipid digestion can be built up into different lipid molecules to create membranes and insulating materials.

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. There are finger-like protrusions along the wall of the ileum, the final section of the small intestine, that increase the surface area even further to about 10 square metres. The wall is highly folded into these structures called villi (singular: villus), which you can see in Figure 2.

Key Term: Villi

Villi (singular: villus) are finger-like protrusions on the wall of the small intestine that increase its surface area for nutrient absorption.

The villi have adaptations that assist in the absorption of food into the intestinal cells. They provide a large surface area and constantly move to help food move and mix with the enzymes. They also provide a rich blood supply to the intestinal cells via capillaries, which you can see in Figure 2. This means that the digested food can diffuse, or be actively transported, across the intestinal cells and into the bloodstream. This blood supply traveling from the small intestine will, hence, include water, mineral salts, some water-soluble vitamins, glucose, and amino acids. This blood is carried to the liver via the hepatic portal vein. Once it passes through the liver, the hepatic vein carries it to the heart via the inferior vena cava.

Key Term: Capillaries

Capillaries are small blood vessels that connect arteries to veins and form networks around the body tissues to exchange gases and other materials.

The villi also contain structures called lacteals, which you can see in Figure 2 represented by green lines. Lacteals are connected to the body’s lymphatic system, and they absorb digested lipids, which are too large to enter the bloodstream directly and instead enter the lymph. Digested lipids are broken down into fatty acids and glycerol in the small intestine. These fatty acids, glycerol, and some undigested lipids are absorbed into the lacteals. The lacteals also absorb vitamins A, D, E, and K into the lymph.

Once these nutrients are absorbed by the lacteals, they are transported around the lymphatic system to eventually be emptied into the superior vena cava. The blood carrying the other nutrients absorbed by the capillaries in the villi of the small intestine ends up in the inferior vena cava. The superior and inferior vena cava are two of the major veins returning blood to the heart. The heart then delivers them to the body cells in the organs that require them.

Key Term: Lacteals

The lacteals are lymphatic vessels of the small intestine that absorb digested lipids.

Example 2: Identifying the Target Location of the Products of Digestion

The diagram provided shows the structure of the villi that line the small intestine. Their primary function is to absorb most of the products of digestion. Where will these products be transported?

  1. To the stomach to be broken down and absorbed into the digestive tract
  2. To the kidneys to be excreted in the urine
  3. To the skin to be excreted as sweat
  4. To organs that require them via the bloodstream and lymphatic systems
  5. To the large intestine to be removed as feces

Answer

The villi are finger-like protrusions along the highly folded wall of the small intestine.

The villi have adaptations that assist in the absorption of food into the intestinal cells. They provide a large surface area and constantly move to help food move and mix with the enzymes. They also provide a rich blood supply to the intestinal cells via capillaries. This means that the digested food can diffuse, or be actively transported, across the intestinal cells and into the bloodstream, to be carried to the liver via the hepatic portal vein. This blood supply traveling from the small intestine will, therefore, include water, mineral salts, some vitamins, glucose, and amino acids. Once they pass through the liver, the hepatic vein carries the blood to the heart via the inferior vena cava.

The villi also contain structures called lacteals, which you can see in Figure 2. These lacteals are connected to the body’s lymphatic system, and they absorb digested lipids, which are too large to enter the bloodstream directly, into the lymph instead. Digested lipids are broken down into fatty acids and glycerol in the small intestine. These fatty acids, glycerol, and some undigested lipids are absorbed into the lacteals. The lacteals also absorb vitamins A, D, E, and K into the lymph.

Once these nutrients are absorbed by the lacteals, they are transported around the lymphatic system to eventually be emptied into the superior vena cava. The blood carrying the other nutrients absorbed by the capillaries in the villi of the small intestine ends up in the inferior vena cava. The superior and inferior vena cava are two of the major veins returning blood to the heart. The heart then delivers them to the body cells in the organs that require them.

As the villi are located on the walls of the small intestine, which follows the stomach in the digestive tract, the products of digestion are not transported back to the stomach. The products of digestion that are absorbed by the villi are useful to the body, and so they are not excreted as urine or sweat or egested as feces.

Therefore, these products are transported to the organs that require them via the bloodstream and lymphatic systems.

The epithelial cells forming the outer layer of each villus also have adaptations to increase the rate of absorption of nutrients.

Microvilli, as you can see in Figure 3, are finger-like protrusions that line the cell surface membrane of the intestinal epithelial cells. This further increases the surface area across which nutrients can be absorbed.

Key Term: Microvilli

Microvilli (singular: microvillus) are microscopically small protrusions in the cell membrane of the small intestinal wall’s cells that increase the surface area and the efficiency of nutrient absorption.

Once the useful nutrients are absorbed into the capillaries or lacteals of the small intestine, the remainder of the undigested chyme passes into the large intestine. Figure 4 shows the different sections of the large intestine that the chyme passes through before being removed from the body.

The primary function of the large intestine is to absorb water and salts from this undigested food. The wall of the large intestine is highly folded to increase the rate of absorption. Undigested food first enters the cecum of the large intestine from the small intestine. The red arrows in Figure 4 show how the chyme moves through the large intestine.

The residue remaining from the chyme in the large intestine has the form of semisolid feces. There are many bacteria in the large intestine that help break down this undigested food, which may include those rich in fibers, such as grains, and even some fruits and vegetables. Many of these bacteria are egested as part of the feces and are responsible for the feces’ unpleasant odor. Feces are stored in the rectum before being egested from the body via the anus, in a process called defecation. Defecation occurs when muscular contractions occur in the rectum and a ring of muscles in the anus called the anal sphincter relaxes.

Key Term: Rectum

The rectum is the final part of the large intestine where feces are stored before being egested from the body.

Key Term: Anus

The anus is an opening at the end of the large intestine through which solid feces are egested from the body.

Key Term: Sphincter

A sphincter is a ring of muscles that surrounds an opening or closing of a tube, such as the anus at the end of the large intestine.

The process of egestion is aided by the mucus secreted by the mucosa of the large intestinal wall, lubricating the feces as they pass out of the anus. A diet rich in fibers is also helpful for the passage of undigested food through the large intestine, as it adds some bulk to the feces, making them larger but softer. This allows the feces to pass through the large intestine faster and makes defecation easier.

A common misconception is that excretion and egestion are the same thing. While excretion removes metabolic waste formed by the cells, egestion refers to the final removal of undigested waste products as feces via defecation.

Example 3: Describing the Function of the Large Intestine

Undigested food is passed to the large intestine. What is the main function of the large intestine?

  1. To absorb water and salts from undigested food
  2. To absorb digested food into the bloodstream
  3. To break down carbohydrates and proteins into their monomers
  4. To emulsify fats
  5. To release the digestive enzymes

Answer

The primary function of the large intestine is to absorb water and salts from undigested food. Although bacteria are present in the large intestine to help break down the undigested food before it is egested via the anus, water and salt are the main nutrients reabsorbed in the large intestine.

The majority of chemical digestion and emulsification of fats occurs in the small intestine, and some digestion also occurs in the stomach and mouth. By the time the food reaches the large intestine, the majority of digestion has already occurred. Therefore, digestion does not generally happen in the large intestine.

For this reason, the large intestine produces no digestive enzymes, as almost the whole process of digestion has already occurred by the time the food reaches the end of the small intestine.

Therefore, the main function of the large intestine is to absorb water and salts from undigested food.

Example 4: Describing the Method of Removing Waste from the Digestive System

How are the majority of waste materials removed from the digestive system?

  1. By excretion
  2. Via respiration
  3. Through reabsorption in the large intestine
  4. As urine
  5. As feces

Answer

Excretion is a process that occurs in the cells, in which the waste products of their metabolic reactions are removed. These waste products may, for example, be carbon dioxide produced during respiration in muscle cells. A common misconception is that excretion and egestion are the same thing. While excretion removes metabolic waste formed by the cells, egestion refers to the final removal of undigested waste products as feces.

Respiration is a process that occurs in the cells to release energy, so it does not describe how waste is removed from the digestive system.

Water and salt are reabsorbed from food into the bloodstream in the large intestine. Generally, waste materials are not useful to the body cells and can even be harmful. Therefore, waste materials are not reabsorbed in the large intestine.

Urine is a waste product formed by the kidneys, which is stored in the bladder before being removed from the body. It contains waste and harmful products, just like faces do, but is produced by a different organ and exits the body via the urethra instead of the anus.

The digestive system ends with the large intestine, where semisolid feces are formed after water and salts have been reabsorbed back into the bloodstream. These feces are egested from the body via the anus, in a process called defecation.

Therefore, the majority of waste materials are removed from the digestive system as feces.

Let’s recap some of the key points that we have covered in this explainer.

Key Points

  • The main functions of the small intestine are the digestion and absorption of nutrients in food.
  • The small intestine is well adapted to carry out absorption due to its large surface area, which is expanded by the villi and microvilli in the intestinal wall.
  • Capillaries and lacteals carry digested nutrients around the body via the bloodstream and lymphatic systems.
  • The main functions of the large intestine are the absorption of water and salts 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 defecation.

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