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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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