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.