Video Transcript
In this video, we’ll learn how to
describe the structure of the liver, first on a macroscopic scale as we could see
with the naked eye and then on a microscopic scale. We’ll also learn about the major
roles the liver plays, which can include detoxification of harmful substances and
excretion.
Did you know that your liver is one
of the only organs in your body that’s able to regenerate itself? This means that up to 60 percent of
it can actually be removed and donated to someone else, in a process called a
living-donor liver transplant. Following the transplant, both
livers immediately start to regenerate, both in the donor’s body and the section
which has been transplanted. Within only eight weeks, both
livers should have almost completely regenerated, showing that this is a pretty
phenomenal organ.
This diagram shows the position of
the liver in the human digestive system. As you can see, the liver is a
fairly large organ made up of two main lobes and is found in the abdomen of
vertebrates like humans. The liver is sometimes known as one
of the accessory organs of the digestive system. This means that as food passes
through the digestive system, it does not actually pass through the liver itself,
but the liver does assist in the digestive process by producing a substance called
bile.
Bile is useful in the digestion of
lipids in the small intestine. The liver also plays a key role in
detoxification of harmful substances and in excretion. Though the liver has many
functions, in this video, we are going to focus on its role in detoxification and
excretion.
Excretion is a process that occurs
in almost all cells of the human body, in which the waste products of the metabolic
reactions are removed. The waste products, for example,
could be carbon dioxide, which is produced in cellular respiration in muscle cells
and needs to be removed from the cells. The process of these metabolic
waste products being removed from cells is called excretion.
There are many other waste products
produced in the various cells of the body, such as urea. They require excretion to prevent
them from causing damage. Many metabolic waste products which
are removed from cells by excretion are also broken down by the liver before they’re
removed from the body. These waste products are excreted
from body cells and into the bloodstream, where they can be transported easily to
the liver. Once these substances reach the
liver, it carries out a process called detoxification. Detoxification is the process by
which these harmful or toxic substances are broken down or neutralized by the
liver.
We’ll learn a little bit more
detail about excretion and detoxification in the liver later on in the video. But first, let’s look at the
macroscopic structures of the liver, which are those that are visible to the naked
eye. We’ve changed the color of the
liver slightly here so that its associated to blood vessels can be distinguished
from it more easily. As we learned earlier, the liver
plays a key role in the digestive system by producing bile.
After being produced by the liver,
bile is transported to an organ called the gall bladder, which is connected to the
liver via vessels called bile ducts. Bile is stored in the gallbladder
until food containing lipid is ingested, at which point the bile is needed in the
duodenum, which is the first section of the small intestine. Bile emulsifies lipids that we
ingest as part of our food once it’s present in the duodenum. So when food containing lipid is
eaten, bile is transported out of the gall bladder along the bile ducts and into the
duodenum.
Let’s see what happens in the
duodenum to these lipids when they interact with bile. Lipids are insoluble in water, so
when they’re present in the small intestine, they form large globules that look a
bit like this. Lipase enzymes, represented by
these blue arrows, therefore only have a small surface area on which they can act to
break down the lipids, as lipases are water-soluble. So they can only surround these
large lipid droplets. However, when we introduce bile
into the mix, bile emulsifies the lipids, which means that they spread out into
smaller, dispersed droplets. This provides a larger surface area
of lipid upon which the lipase enzymes can act to break down the lipids efficiently,
which increases the rate of lipid digestion.
In addition to the bile ducts,
there are many blood vessels that are associated with the liver. So let’s add these into our diagram
now. These structures shown in blue are
branches of the hepatic vein, while those shown in red are branches of the hepatic
artery. In case the liver wasn’t looking
colorful or complex enough for you, there’s another vessel that supplies the liver
called the hepatic portal vein, shown here in pink. You might have noticed that this
term hepat- forms the prefix of each of these vessels. This is because the term hepat-
comes from the Greek word for liver and also explains why the cells of the liver are
called hepatocytes, as hepat- means liver and -cyte means cell.
Hepatocytes have many functions and
are therefore very active cells, using up to 20 percent of the total energy in the
body. They require a lot of glucose and a
lot of oxygen in order to carry out cellular respiration to release the energy they
need for their functions. We’ll look in more detail at the
structure of a hepatocytes soon. But first, let’s investigate what
all these different blood vessels are responsible for, and how the hepatocytes get
such a large supply of glucose, oxygen, and metabolic waste products.
Arteries always carry blood away
from the heart. In fact, the hepatic artery
originates from a vessel called the aorta, which branches out of the heart and
carries oxygen-rich blood to the body tissues, including to the liver by the hepatic
artery, where the oxygen can diffuse into the hepatocytes. The hepatic artery not only
delivers oxygen to the hepatocytes in the liver but also to adjacent organs, such as
the small intestine, the pancreas, and the gall bladder. So in summary, we can see the
hepatic artery carries oxygen-rich blood from the heart to the liver and its
adjacent organs.
Veins tend to carry blood away from
the body tissues towards the heart. The hepatic vein transports
deoxygenated blood away from the liver by a larger vessel called the inferior vena
cava. This blood can then enter the heart
via the inferior vena cava to be pumped off and away to the lungs so it can be
oxygenated again. The hepatocytes of the liver will
have produced carbon dioxide in aerobic respiration, which moves into the blood. So it’s carbon dioxide that’s
transported in the deoxygenated blood from the hepatic vein back to the heart. So in summary, the key function of
the hepatic vein is to carry deoxygenated blood from the liver to the heart.
But you may have noticed that
there’s another vein associated with the liver, the hepatic portal vein. So what does this vein do? The liver receives two blood
supplies, one rich in oxygen from the hepatic artery and the other rich in nutrients
and waste products from the hepatic portal vein. The hepatic portal vein carries
this blood rich in nutrients, waste products, and even toxins from many different
organs including the pancreas, the intestines, the spleen, and the gall bladder to
the liver. This is because the liver is the
main organ of detoxification in the body, but it also requires nutrients to
function.
The blood in the hepatic portal
vein carries lots of the products of digestion to the liver. So let’s have a closer look at
these to see which in particular we’re talking about. Following a meal, the blood in the
hepatic portal vein will be rich in glucose, amino acids, cholesterol, vitamins, and
minerals. The useful substances such as
vitamins and minerals will be stored in the hepatocytes until they’re needed, at
which point they’ll be released into the blood to be transported to the body cells
as required via the hepatic vein.
The hepatic portal vein also
transports substances which need to be detoxified, neutralized, and excreted such as
excess proteins and carbon dioxide. Furthermore, the hepatic portal
vein transports hormones such as insulin and glucagon from the pancreas to the
hepatocytes. The volume of blood delivered to
the liver via the hepatic portal vein is actually three times larger than the volume
coming from the hepatic artery as it’s carrying so many nutrients and toxins.
Let’s erase some of these other
structures and organs so that we can take a closer look at the liver itself. Each of the two lobes of the liver
contains around 100,000 hexagonal lobules, which are each a small lobe, some of
which you can see magnified here. As we will be observing structures
that are not visible for the naked eye anymore, we are no longer looking at the
macroscopic structure. Instead, we’re looking at the
microscopic structure, which is that that would be visible with a microscope.
Each of the lobules contains
several branches of the bile duct, which you can see here in green; several branches
of the hepatic portal vein, shown in pink; branches of the hepatic artery, shown in
red; and a large central branch of the hepatic vein, shown in blue.
Let’s take a look at one segment of
the lobule more closely. Branches of the bile duct, which is
shown in green, delivers bile which is synthesized in the hepatocytes from the liver
to the gall bladder where it’s stored. Branches of the hepatic portal
vein, which is shown in pink, bring waste and the products of digestion to the
liver, so they can enter the hepatocytes themselves. Branches of the hepatic artery,
shown in red, supplies the hepatocytes with oxygenated blood. Once the hepatocytes have used
their nutrients as required, a branch of the hepatic vein, shown in blue, delivers
the deoxygenated blood back to the heart.
As you can see, the contents of the
hepatic artery and the hepatic portal vein mix together in an area called a
sinusoid. The hepatocytes surround the
sinusoid so the contents of the blood can be transported into them. So each sinusoid will contain a mix
of nutrients and waste products.
Each of the hexagonal lobules
contain many hepatocytes, which make up about 80 percent of the total mass of the
liver. So let’s take a closer look at one
of these hepatocytes themselves. Most hepatocytes have a large
central nucleus, a prominent endoplasmic reticulum, and many mitochondria. A remarkable feature of hepatocytes
is that some of them have more than one nucleus. The reason why continues to inspire
scientific research. But it’s believed to make more gene
copies available for protein synthesis, as these cells are very active. It might also provide more
protection from DNA damage and cell death, especially as these cells are submitted
to toxic substances.
Hepatocytes have a prominent
endoplasmic reticulum as they are active in synthesizing proteins and lipids to be
exported to other body cells. As the hepatocytes are highly
metabolically active, they have many mitochondria to carry out respiration and
release sufficient energy.
Now that we know more information
about the structure of the liver and its microscopic components, let’s learn more
about some of the key functions of the liver. One of the roles the liver plays is
in breaking down harmful, excess metabolic waste products. Some substances which might be
produced by or ingested by the body are toxic and need to be removed.
For example, alcoholic drinks
contain a substance called ethanol. If too much ethanol is ingested,
it’s toxic to cells. This is because it interferes with
the phospholipid bilayer in the cell surface membrane themselves. In fact, ethanol can even dissolve
the phospholipids. To avoid the disruption of the cell
surface membrane, and therefore the destruction of the cell itself, ethanol can be
transported to the liver. The cells of the liver can convert
ethanol into a less toxic form, which can then be excreted and eventually removed
from the body.
The liver works hard to carry out
this function, but it also takes the majority of the damage caused by this
toxin. In fact, excessive and continuous
overconsumption of alcohol can cause damage to hepatocytes to the extent that it
causes irreversible liver cirrhosis, which is scarring of the liver. This diagram shows an example of
what a liver with cirrhosis might look like compared to the healthy liver above.
Let’s look at another function of
the liver, deamination. Not all of the amino acids which
are formed during the digestion of proteins can be stored by the human body. In fact, excess amino acids are
transported to the liver by the hepatic portal vein from the digestive system. Once an amino acid has entered a
hepatocyte, an amino group can be removed from it. The amino group is shown in this
pink box as part of a typical amino acid. The removal of the amino group
converts the amino acid into an organic acid and a substance called ammonia.
While the organic acid can be
useful to different cells of the body, the by-product ammonia is highly toxic. So it needs to be converted into
another form to be excreted safely. Ammonia is detoxified by the
hepatocytes of the liver, partly in their mitochondria. The process of ammonia
detoxification is called the ornithine cycle or sometimes the urea cycle. Let’s look at this next.
It’s sometimes called the ornithine
cycle, as it involves three amino acids which include ornithine, citrulline, and
argintine. The reason why it’s sometimes
called the urea cycle is because it involves the conversion of toxic ammonia into a
relatively harmless substance called urea. The ornithine cycle also involves
using the carbon dioxide that’s released in cellular respiration, these three amino
acids, and a number of different enzymes to convert ammonia into urea and water. The urea that’s produced is then
transported to the kidneys to be removed from the body as a part of urine.
Now that we know some more
information about the structure and function of the liver, let’s have a go at a
practice question.
The diagram provided shows a
simplified outline of the ornithine, urea, cycle that occurs in the liver. What compound has been replaced by
X?
To work out which compound is
missing, let’s first learn a bit more about the ornithine cycle itself. Not all of the amino acid which are
formed during the digestion of proteins can be stored by the body. Excess amino acids are delivered to
the cells of the liver via a vein called the hepatic portal vein that runs from the
digestive system. The amino group, shown here in
pink, is removed from the amino acid in the liver, which produces an organic acid
that can be used by body cells and a toxic substance called ammonia.
As ammonia is toxic, it cannot be
stored by the cells of the human body, so it needs to be converted into another form
to be excreted safely. The cells of the liver carry out
this conversion through a process called the ornithine cycle, or sometimes called
the urea cycle.
The ornithine cycle involves carbon
dioxide, which is produced as a product of cellular respiration, and three amino
acids: ornithine, citrulline, and arginine. These substances and a number of
enzymes are used to convert toxic ammonia into a relatively harmless substance
called urea. It also produces water as a
by-product. Urea can then be transported to the
kidneys to be removed from the body as a part of urine. As it is a substance which needs to
be detoxified, we can deduce that compound X is ammonia.
Let’s recap some of the key points
that we’ve covered in this video. The hepatic artery delivers
oxygenated blood to the liver, while the hepatic vein removes deoxygenated blood
from the liver. The hepatic portal vein brings
blood rich in the digestive products of the small intestine to the liver and also
toxic substances for it to detoxify. The hepatocytes, which make up the
majority of cells in the liver, are adapted to their functions by having large
nuclei and prominent endoplasmic reticulum and many mitochondria to release
energy. Some of the main roles of the liver
are detoxification, deamination, and excretion.
