Video Transcript
In this video, we will learn how to
describe the digestive process in the human body and why digestion is so
important. We will investigate the different
types of digestion that occur as food moves through the digestive system from the
mouth, which is the site of buccal digestion, through the stomach, where gastric
digestion occurs, and into the small intestine, where intestinal digestion
occurs. We will also learn about the
important roles of various enzymes that are involved in chemical digestion and the
role of muscles and other body structures in carrying out mechanical digestion.
The human digestive system works
tirelessly, whether we are standing upright or balancing upside down, when we’re
awake, and even when we’re asleep, by using chemicals, muscles, and other structures
to break down the food we eat into smaller pieces. Digestion is a process by which
large molecules are broken down into smaller molecules. This is essential, as we need to
break down the large nutrients in the food we ingest into a form small enough to be
absorbed into the bloodstream.
But how does this actually
happen? These large molecules are broken
down into their smaller constituent subunits by chemicals called enzymes, which are
specific to the type of substrate that they break down. The enzymes in the digestive system
mostly function to break bonds within a substrate molecule to form products, which
are smaller. Most of these products are now
small enough to move into capillaries, which surround certain parts of the digestive
system. The blood within these capillaries
can then transport the products away around the body to all the cells that might
require them.
This diagram shows an outline of
some of the major organs in the human digestive system. Keep in mind that they have been
color-coded here, but in reality they mostly all appear a similar fleshy pink or
brown color. We can use this diagram throughout
the video to pinpoint the different organs that food travels through, which is known
as the alimentary canal or the gastrointestinal tract. We can also use it to identify the
different organs which food does not directly travel through so are not part of the
alimentary canal but are still useful for the digestive process. These organs, which are known as
accessory organs, have been color-coded here in blue, and we’ll look at them in more
detail as we go through each stage of the digestive process.
So let’s get started. Say that you’ve just taken a bite
of a tasty chicken sandwich. The first place the food enters is
the mouth, where the food is turned into a ball that is now referred to as a
bolus. The mouth is the site of buccal
digestion. The word buccal refers to the mouth
itself. And buccal digestion includes many
different structures and chemicals within the mouth and in the areas surrounding
it.
Let’s take a closer look at the
mouth and these surrounding areas, so we can see how buccal digestion occurs in more
detail. Human teeth are excellently adapted
to chew different types of food, as we are biologically adapted to be omnivores,
eating both animal and plant products. Our incisors in the front of our
jaw cut our food, while the adjacent fairly sharp canines grip and tear the food
into smaller pieces. At the back of the jaw, the molars
and premolars grind down food to give it a larger surface area. We mostly think of our tongue as a
tool to taste food, but it’s actually also a muscle. And it helps to play an important
role in moving food and helping the teeth to grind it down.
These processes are all examples of
mechanical digestion, using muscles in the jaw to grind down food into smaller
chunks. Mechanical digestion results in the
same volume of food being broken down to have a larger surface area. This makes it easier for enzymes,
which are represented here by green arrows, to digest food, as it increases the
surface area that these enzymes can act on to break down the large nutrients and the
food even further.
Generally, mechanical digestion
includes hard structures like teeth or muscles like the tongue, and it occurs in
other regions in the digestive system, too. But let’s finish off buccal
digestion first.
Aside from mechanical digestion,
chemical digestion also occurs in the mouth, as this term is generally used to refer
to the role of enzymes in digestion. This diagram shows a close-up side
view of the mouth and some surrounding areas. We’ve already looked at the
functions of the tongue and the teeth in mechanical digestion, but let’s see how
some of these other structures play a role in chemical buccal digestion.
You may recall that we refer to the
blue structures in the diagram as accessory organs. In this close-up of the mouth, we
can see three out of the three paired salivary glands, which are our first example
of accessory organs. The role of the salivary glands is
to secrete saliva into the mouth. Saliva contains mucus, which
softens food by making it more fluid. Saliva also contains enzymes called
amylases.
Let’s see how amylases work. Amylase starts off the breakdown of
carbohydrates, such as large starch molecules in food, into smaller sugars, such as
the disaccharide maltose. This occurs through a process
called hydrolysis, which literally means breaking a molecule down using water. Maltose will eventually be broken
down itself into smaller monosaccharides, called glucose, by another enzyme called
maltase. But this doesn’t happen till later
on in the digestive tract.
Amylase works best in slightly
alkaline conditions, and its optimum pH is around 7.4. It’s important to note that an
optimum pH is very specific to each particular enzyme, so it will differ depending
on which enzyme we’re talking about. Amylase is just one example of an
enzyme involved in chemical digestion in the digestive process, more of which will
come later.
At the back of the mouth, there are
two tubes, one of which is the trachea that leads down to the lungs, and the
esophagus which leads to the stomach. Usually a flap of tissue called the
epiglottis is held open, which allows gases like oxygen to move down the trachea and
into the lungs. To prevent food from entering the
trachea, when the food bolus reaches the pharynx at the back of the mouth, the
trachea and the larynx, which is otherwise known as the voice box, rise upwards,
which causes the epiglottis to close over the entrance to the trachea. So no food and drink enters the
trachea, only the esophagus. If the epiglottis did not close,
food or drink might enter the lungs and cause choking.
Once food is in the esophagus, it
needs to move down to the stomach. So let’s see how the esophagus
manages this. The esophagus is a long tube with
glandular cells in its lining that secrete mucus onto the bolus. The esophagus lining also contains
circular muscles. These circular muscles rhythmically
contract and relax to move the bolus through the esophagus to the stomach. This process is called peristalsis,
which is the rhythmic contraction and relaxation of these circular muscles, which
can move food and sometimes can mix up the bolus with digestive juices in certain
organs of the body, though this does not occur in the esophagus. Peristalsis also occurs in the
stomach, the small intestine, and the large intestine. As muscles are involved, this shows
us a nice example of mechanical digestion.
Let’s look at gastric digestion
next, which refers to processes occurring in the stomach. We will need to see a closer view
to understand this clearly. So let’s see a larger image of the
stomach where it meets the bottom of the esophagus. When the food bolus approaches the
end of the esophagus, it reaches a thick muscular ring called the cardiac
sphincter. The cardiac sphincter is usually
closed but opens when the bolus reaches it, allowing peristaltic movements to push
the bolus out of the esophagus and into the stomach.
The stomach is a muscular sac which
is able to move food via peristalsis. And like the mouth, mechanical
digestion also occurs in the stomach. Once the bolus reaches the stomach,
it mixes with gastric juice and becomes more watery. At this point it is no longer
referred to as the bolus, and instead it is known as chyme.
Gastric juice, which is sometimes
known as stomach acid, contains strong hydrochloric acid. This is helpful for two main
reasons. Firstly, if pathogens, which are
disease-causing biological agents such as bacteria, were to enter the digestive
system as a result of being ingested as part of food, the hydrochloric acid in the
stomach would kill most of them. This prevents them from entering
the rest of the digestive system, or worse the bloodstream. Hydrochloric acid also provides the
acidic conditions which are optimal for the enzymes that function in the
stomach.
Gastric juice also contains a
substance called pepsinogen, which is secreted by the cells lining the stomach. Pepsinogen is an inactive form of
an enzyme. The acidic pH provided by the
hydrochloric acid allows pepsinogen to be activated and converted into an enzyme
called pepsin.
Let’s take a closer view as to how
pepsin functions as an enzyme in the stomach. Pepsin is an example of a protease
enzyme. The stem “prote” shows us that
protease enzymes break down proteins. Pepsin specifically catalyzes the
hydrolysis of proteins into smaller polypeptides. These polypeptides will eventually
be broken down by other protease enzymes in the small intestine into amino
acids. The optimum pH of pepsin is between
1.5 and 2.5, so it functions very effectively in the acidic stomach environment.
The cells in the wall of the
stomach itself are protected from being digested by pepsin by mucus, which lines the
inside of the stomach. Digestion of proteins into
polypeptides is another example of chemical digestion.
At the base of the stomach is
another ring of muscle called the pyloric sphincter. The pyloric sphincter is also
usually closed, but a peristaltic contraction causes it to open briefly, allowing
some chyme to pass from the stomach into the first section of the small intestine,
which is called the duodenum.
The final part of the digestive
process is intestinal digestion, which primarily occurs in the small intestine. The small intestine is where the
majority of chemical digestion occurs in the digestive tract as proteins,
carbohydrates, and lipids will all be digested by enzymes there. Most digestive enzymes are secreted
into the small intestine by an accessory organ called the pancreas. By adjusting the location of the
other organs slightly, we can see how the pancreas attaches to the duodenum of the
small intestine more clearly.
This green structure running
through the pancreas represents the pancreatic duct, which is responsible for
secreting pancreatic juice, which contains all these enzymes into the duodenum,
where pancreatic juice and intestinal juice will mix.
Let’s take a closer look at the
enzymes that pancreatic juice contains that will be acting in the small
intestine. Pancreatic juice contains many
carbohydrases that catalyze the hydrolysis of carbohydrates, which you might
remember began in the mouth with amylase. In the small intestine, for
example, carbohydrases like maltase can break down maltose into molecules of
glucose. Pancreatic juice will also contain
lipases, which catalyze the hydrolysis of lipids in food into fatty acids and
glycerol.
Pancreatic juice also contains
proteases, which, you might recall, break down proteins. Trypsinogen is secreted as a part
of pancreatic juice and, like pepsinogen, is an inactive form of a protease
enzyme. Trypsinogen is activated by another
enzyme, which is present in intestinal juice, called enterokinase. When trypsinogen enters the small
intestine and comes into contact with enterokinase, it’s converted into its active
form, trypsin. Trypsin is a protease enzyme that
catalyzes the hydrolysis of polypeptides and peptides into smaller units and
eventually into the monomer subunit amino acids.
Some gastric juice from the stomach
will enter the small intestine. But pancreatic juice contains
another substance called sodium bicarbonate, which neutralizes the acidic gastric
juice entering the duodenum from the stomach. This means that the pH at the start
of the duodenum is around six, but the pH increases as the small intestine continues
to provide an optimal pH for many of the enzymes that will be acting there.
The liver is another accessory
organ that is involved in the digestive system. By magnifying our summary diagram
and adjusting the organs slightly, we can see where this interaction occurs more
clearly. The duodenum, the first part of the
small intestine shown here in pink, is connected to the pancreas, which is one of
the accessory organs that we already explored. And it’s also connected to the
liver. The liver is described as an
accessory organ of the digestive system, as it produces a substance called bile.
Bile is stored after its production
by the liver in another accessory organ called the gall bladder before being
transported along a duct called the bile duct, which joins up with the pancreatic
duct so that both pancreatic juice and bile are both secreted into the duodenum.
Now that we know how it gets there,
let’s see how bile can be helpful in digestion in the small intestine. Bile emulsifies lipids, which means
that it disperses them into smaller droplets. These smaller globules are
sometimes called emulsion droplets. Lipids are insoluble in water, but
the enzymes that break them down, lipases, are soluble in water. So the two don’t naturally mix
well. Emulsification by bile, however,
means that these droplets of lipids are more interspersed throughout water, which
means that the lipases have a larger surface area upon which they can act, as shown
by these green arrows. This increases the efficiency of
lipid hydrolysis in the small intestine.
Once the nutrients have been broken
down sufficiently by all of these different enzymes, the simple sugars, amino acids,
fatty acids, and glycerol are absorbed across the wall of the small intestine. The diagram on the right shows us
how the small intestine wall is highly folded. This gives the small intestine a
really large surface area for nutrient absorption. Amino acids and simple sugars are
small enough that they can move directly across the cells that line the wall of the
small intestine into the blood capillaries, which are shown here as these dense, red
networks of blood vessels. Once they’re in the capillaries,
these essential nutrients can be transported via the blood to the body cells that
require them.
As fatty acids and glycerol are
larger and are not water-soluble, they cannot pass directly into the
capillaries. Instead, they move into structures
called lacteal vessels, which sit in the same area as the capillaries just outside
the cells lining the wall of the small intestine. The lacteal vessels transport these
fatty acids and glycerol and any undigested lipids into the lymphatic system, where
they can enter the bloodstream at a larger junction.
Let’s summarize the processes that
we’ve covered so far. First, food is placed in the mouth,
where it is mixed with saliva and mashed into a ball called a bolus. This is called buccal digestion,
and it involves amylase enzymes breaking down starch in food into smaller sugars,
such as the disaccharide maltose. From the mouth, the food passes
down the esophagus to the stomach. This is where gastric digestion
occurs and proteins in food are broken down into polypeptides.
Once the bolus reaches the stomach,
it mixes with digestive juices and becomes more watery. The solid pieces break down, and
they have a lower surface area. And the bolus is now known as
chyme. Chyme can then move into the small
intestine, where intestinal digestion will occur. This includes the majority of
chemical digestion, where proteins, polypeptides, and peptides will be broken down
into amino acids by protease enzymes. Larger carbohydrates like maltose
can be broken down into comparatively smaller simple sugars like glucose by
carbohydrase enzymes. And lipids are converted into fatty
acids and glycerol by lipase enzymes.
The chyme then passes into the
large intestine, which reabsorbs water, any remaining vitamins, and salts to form
solid feces. Feces is then stored in the rectum
before it is passed out of the body by egestion via the anus.
Let’s have a look at the key points
that we’ve covered in this video. Digestion is the process by which
large food molecules are broken down into smaller and generally more soluble
molecules that can be absorbed into transport systems like the blood to be
transported to the body cells that require them. The digestive process begins with
buccal digestion in the mouth, which involves the teeth, the tongue, and the
salivary glands.
Gastric digestion occurs next in
the stomach and involves gastric juice containing hydrochloric acid and protease
enzymes. Finally, intestinal digestion in
the small intestine uses pancreatic juice, bile, and additional enzymes to carry out
the majority of chemical digestion. And it’s also the site where the
smaller nutrients will be absorbed into the bloodstream.
