Lesson Video: The Liver Biology

In this video, we will learn how to describe the structure of the liver and the role of the liver in excretion.


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.

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