Video Transcript
In this video, we will learn how to
describe the structure and function of the pancreas. We will first discover where the
pancreas is located in the human body and how it can function as both an exocrine
gland, by releasing enzymes, and also an endocrine gland, by releasing hormones into
the bloodstream. We will look at some of the
different types of cells that make up pancreatic tissues on a microscopic scale. Some of these cells release insulin
and glucagon, important hormones involved in maintaining our blood sugar levels. We will also investigate what might
happen to the body when this maintenance does not occur as it should.
The pancreas is located just behind
the stomach in your abdomen. As we can see, the pancreas is an
elongated organ, around 15 centimeters long. Let’s magnify this image of the
pancreas so that we can see what’s happening inside this organ more clearly. The pancreas is a gland. You may recall there are two main
types of glands: exocrine glands and endocrine glands. As it has both endocrine and
exocrine functions, the pancreas is often described as a mixed gland.
Let’s look at the different
functions of the pancreas in a little more detail, starting with the pancreatic
exocrine functions. Exocrine glands are groups of
specialized cells that secrete substances, like enzymes, via a duct onto a body
surface that is either external or internal. The pancreas functions as an
exocrine gland, as it secretes digestive enzymes via the pancreatic duct into the
upper part of the small intestine, which is called the duodenum. Most of the pancreas is made up of
exocrine tissue. This exocrine tissue contains
acini, which themselves contain clusters of pancreatic acinar cells that are
responsible for producing and secreting these digestive enzymes. These enzymes contained within a
substance called pancreatic juice are transported through the pancreas along the
pancreatic duct, at the end of which these enzymes are secreted into the
duodenum.
The pancreas is sometimes referred
to as an accessory organ in the digestive system. This is because as food passes
through the digestive tract, it heads through the stomach and bypasses the pancreas
directly into the small intestine. But the enzymes that are secreted
by the pancreas are still essential to digestion, as they break down the large food
nutrients in the small intestine.
Let’s zoom in on a section within
the duodenum to see how these different enzymes act there. There are three main groups of
enzymes that are secreted as part of pancreatic juice to digest food once it reaches
the duodenum: proteases, amylases, and lipases. Proteases, such as trypsin, break
down proteins into polypeptides and peptides, which can then be broken down into
smaller units called amino acids. Amylases continue to break down
starch that began to be digested in the mouth into simple sugars, like glucose. And lipases break down lipids into
fatty acids and glycerol.
The smallest subunits that are the
product of enzyme digestion should now be small enough to be absorbed by the cells
lining the small intestine to enter the body’s transport systems, such as the blood,
to be moved to our other body cells as required. All of these enzymes require a
specific pH in order to function properly. Some hydrochloric acid from the
stomach enters the duodenum, which would make it too acidic for the enzymes to
function effectively. To counter this, pancreatic juice
also contains an alkaline substance called sodium bicarbonate. This neutralizes the stomach acid,
making the pH of the duodenum more optimal for these digestive enzymes. Between approximately 200 to 800
milliliters of these secretions that make up pancreatic juice is secreted every
single day, which is up to around a pint and a half. These secretions support the
chemical reactions that are necessary to efficiently digest food and help the
intestines to absorb the nutrients that we need.
Let’s investigate the endocrine
functions of the pancreas next, which occurs in tissues interspersed throughout the
pancreatic acinar cells called the islets of Langerhan. Endocrine glands are made up of
groups of specialized cells that secrete hormones directly into the blood. The endocrine function of the
pancreas is how it secretes hormones, such as insulin and glucagon, into the
bloodstream.
Let’s magnify one of these islets
of Langerhan so we can see this in more detail. This diagram represents the
structure of a single islet of Langerhan that has been magnified. They are called islets as they look
a bit like islands dispersed throughout a sea of pancreatic acinar cells. The pancreas is supplied and
drained by a dense network of capillaries, which are very small blood vessels that
maintain the functions of the pancreatic cells. The blood vessels are located close
to the islets of Langerhan so that the hormones produced there can be transported
from the endocrine pancreatic cells to their specific target tissues elsewhere in
the body.
Let’s see some specific examples of
these endocrine cells in the islets of Langerhan and how the hormones they produce
can trigger events that control blood glucose concentration. A normal and healthy concentration
of blood glucose ranges between 80 and 120 milligrams per deciliter. This means there is between 80 and
120 milligrams of glucose per 100 centimeters cubed of blood, which is like a
quarter of a teaspoon of sugar diluted in one liter of blood, about 100 times less
than the concentration of sugar in a soda.
When blood glucose drops to a value
lower than this range, for example, after a long workout that has used up a lot of
your blood sugar stores, alpha cells produce a hormone called glucagon, which is
secreted into the bloodstream. Glucagon has many different target
cells, for example, cells in the liver, which glucagon stimulates to break down
glycogen stores into glucose and produce glucose from amino acids and fats. This newly produced glucose is
secreted into the bloodstream and increases the blood glucose concentration back to
a normal range.
When blood glucose increases above
the normal range, for example, after eating a meal rich in carbohydrates, beta cells
in the islets of Langerhan produce a hormone called insulin, which is secreted into
the bloodstream. Insulin also has various target
cells, which include the skeletal muscle cells that are encouraged to take up more
glucose and liver cells. Insulin stimulates the liver to do
the reverse of glucagon: convert more glucose into glycogen to store it, thereby
reducing blood glucose concentration back to the normal range. The rate of glycolysis, the first
step in cellular respiration, is also increased to use that more glucose among the
many other functions of insulin.
You may be wondering why we need
the pancreas to control the levels of glucose in the blood. Let’s look at what can happen if
the body cannot maintain a fairly constant blood glucose level to answer this
question.
Type one diabetes is a condition
that often arises as a result of the beta cells of the pancreas not functioning
correctly to control blood glucose concentration. When someone has type one diabetes,
the pancreatic beta cells tend to not produce enough, if any, insulin. This means that their blood glucose
concentration can become very high if it’s not controlled appropriately. This can result in numerous
damaging effects on the body’s different systems and organs.
For example, an excessively high
blood glucose concentration can damage nerve cells, reducing the efficiency of
nervous transmission throughout the body. It can also damage blood
vessels. High blood glucose concentrations
can also lead to a person even to urinate more frequently. As more glucose will be present in
this urine, urine tests can be used as an indirect way to monitor blood glucose
levels in a person with diabetes or to indicate that a person who has not yet been
diagnosed might have diabetes or another underlying health condition. The increased frequency of
urination means that lots of water will also be removed from the body, leading to
excessive thirst.
Did you know that the physicians of
ancient Egypt already knew about this disorder in 1500 BC? This is because they noticed that
ants were more interested in the urine of people with diabetes. This is because the urine of people
with diabetes contained more glucose, and so it was sweeter.
This graph shows us how the blood
glucose of a person with diabetes, shown in orange, compares with a person without
diabetes, shown in pink. The vertical 𝑦-axis shows a
concentration of blood glucose in each person, while the horizontal 𝑥-axis shows
the time in hours, following each person both consuming the same meal, which
occurred at hour zero. You can see that, at the moment of
the meal, the blood glucose is already higher in the person with diabetes. You can see that, over the first
hour following the meal, the blood glucose level in both diabetics and in
nondiabetics increased, as sugars in their food are absorbed into their
bloodstream.
As the person with diabetes has a
higher blood glucose concentration to start with, their blood glucose concentration
also rises to a comparatively higher peak than the person without diabetes. This is because as you might
recall, their beta cells will not be producing the insulin that they require to
start bringing down the blood glucose concentration to a normal range. This is also why it takes longer
for the blood glucose concentration of a person with diabetes to return to a lower
level overall. To prevent the condition’s damaging
effects, people with type one diabetes must monitor their blood glucose closely and
treat the symptoms with daily insulin injections. Pancreas transplants can sometimes
be administered to people with type one diabetes to replace their nonfunctioning
beta cells. But this is a very invasive
procedure and organ transplants are really challenging to source.
There is the potential for use of
stem cells in treating diabetes. Stem cells could be triggered to
differentiate into beta cells and injected into a person who cannot produce insulin
themselves. The development of the use of stem
cells in medicine is a hope for people with diabetes and many other conditions in
the years to come.
Let’s see how much we’ve learned
about the pancreas by having a go at a practice question.
Why is the pancreas referred to as
a mixed gland? The pancreas contains endocrine
cells that secrete hormones and exocrine cells that secrete enzymes. Or the pancreas contains exocrine
cells that secrete hormones and endocrine cells that secrete enzymes.
Let’s approach this question by
first defining some of the key terms that’ve been used. Endocrine glands secrete hormones
directly into the bloodstream, and exocrine glands secrete substances via ducts,
usually onto body surfaces, such as the skin or the inside of the digestive
tract. This diagram represents the human
pancreas. The pancreas contains endocrine
cells within regions called the islets of Langerhan, which are shown here in
pink. These endocrine cells in the islets
of Langerhan produce and secrete hormones, such as insulin and glucagon, releasing
them into the bloodstream. Once in the blood, these two
hormones can travel to their target cells in order to have an effect, which is to
increase or decrease blood glucose concentration to maintain it within a normal
healthy range.
Most of the pancreas is made up of
exocrine tissue, which consists of clusters called acini, which contain pancreatic
acinar cells, which we can see here in orange surrounding this green structure,
which is the pancreatic duct. The acinar cells within the acini
secrete digestive enzymes into this pancreatic duct, which then empties into the
small intestine, where these enzymes are responsible for breaking down proteins,
lipids, and carbohydrates in the food that we eat. As the pancreas has both endocrine
and exocrine functions, it is sometimes referred to as a mixed gland.
Let’s use this information to
answer the question. We know now that endocrine cells
secrete hormones and that the exocrine cells of the pancreas secrete enzymes. So this option must be the correct
answer.
Let’s review some of the key points
that we’ve addressed in this video. We now know that the pancreas is
referred to as a mixed gland due to its endocrine and exocrine functions. The majority of the pancreas is
made up of exocrine tissue, consisting of clusters of acinar cells in regions called
acini. These acinar cells secrete
digestive enzymes into the small intestine via the pancreatic duct. The islets of Langerhan are the
endocrine tissues of the pancreas. Each islet contains alpha cells
that secrete a hormone called glucagon, which raises blood glucose levels when
they’re too low, and beta cells that secrete a hormone called insulin, which lowers
blood glucose levels when they’re too high. Type one diabetes is a condition
where beta cells cannot produce insulin in sufficient quantities to control blood
glucose, if they are able to produce it at all.
