Lesson Explainer: The Liver Biology • Second Year of Secondary School

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

The liver is an organ found in the human body. Did you know that your liver is the only organ in your body that can regenerate itself? This means that up to of it can be removed and donated to someone else, in a process known as living-donor liver transplantation. Following transplantation, the liver immediately starts to regenerate, both in the donor’s body and in the body of the person who received the transplant. Within only weeks, both livers should have almost completely regenerated.

The liver is a large organ made up of two lobes. It is found in your abdomen, and you can see in Figure 1 from its position in the human body that it is part of the digestive system. It is one of the accessory organs of the digestive system. This means that food does not directly pass through it, but it rather assists in the digestion of food by producing bile. Although the liver has many functions, in this explainer, we are going to focus on its role in detoxification and excretion.

Excretion is a process that occurs in the cells, in which the waste products of their metabolic reactions are removed. These waste products may, for example, be carbon dioxide produced during respiration in muscle cells. A common misconception is that excretion and egestion are the same thing. While excretion removes metabolic waste formed by the cells, egestion refers to the final removal of undigested waste products as feces.

Many metabolic waste products removed from the cells by excretion are broken down by the liver, before being egested or removed from the body. These waste products are excreted from other body cells and transported via the bloodstream to the liver. As some of these substances can be toxic, the liver then breaks down and neutralizes them. This process is called detoxification.

Let’s look at the macroscopic structures, which are those visible to the naked eye, of the liver and its associated blood vessels. These are shown in Figure 2.

The liver plays a role in the digestive system by producing bile. Bile is used in the small intestine to emulsify the many lipids that we ingest in our food. Emulsification increases the surface area of lipids, which lipase enzymes can act on, to increase the rate of lipid digestion. After being produced by the liver, bile is transported to the gallbladder, which, as you can see in Figure 2, is connected to the liver by bile ducts. Bile is stored in the gallbladder until food containing lipids is eaten and bile is needed in the small intestine.

The prefix hepato- comes from the Greek word meaning “liver.” For this reason, structures in the liver contain the word hepatic. For example, the three major blood vessels in the liver are called the hepatic artery, the hepatic vein, and the hepatic portal vein, and the cells of the liver are called hepatocytes.

Hepatocytes have many functions and are, therefore, very active cells, using up to of the total energy in the body. They require lots of oxygen to carry out aerobic respiration to release the energy needed for these functions.

Arteries always carry blood away from the heart. The hepatic artery carries oxygen-rich blood from the heart to the liver, where it then diffuses into the hepatocytes. The hepatic artery delivers oxygen not only to the hepatocytes in the liver, but also to adjacent organs, such as the stomach, duodenum of the small intestine, pancreas, and gallbladder.

Veins always carry blood to the heart. There are two different veins associated with the liver: the hepatic vein and the hepatic portal vein.

The hepatic vein carries deoxygenated blood from the liver to the heart, traveling via a larger blood vessel called the inferior vena cava. Hepatocytes produce carbon dioxide during aerobic respiration, which moves into the blood. The blood, which is now deoxygenated, is carried by the hepatic vein back to the heart. From the heart, this blood travels to the lungs to be oxygenated, before cycling around the body once more.

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 travels from the intestines, spleen, pancreas, and gallbladder to the liver. It carries blood that is rich in not only nutrients but also metabolic waste products and toxins to the hepatocytes. This is because the liver is the main organ of detoxification in the body. The volume of blood that the hepatic portal vein carries to the liver is three times the volume arriving via the hepatic artery, as it is carrying so many nutrients and toxins.

The blood in the hepatic portal vein carries lots of the products of digestion. For example, especially following a meal, the blood in the hepatic portal vein becomes rich in glucose, amino acids, cholesterol, vitamins, and minerals. Useful substances, such as vitamins and minerals, are stored by the hepatocytes until they are needed. At this point, they are released into the bloodstream to travel to the body cells as required.

The hepatic portal vein also transports substances that 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.

Example 2: Identifying the Key Blood Vessels of the Liver

The diagram provided is a basic outline of the human liver, with the names of some key blood vessels removed.

1. What blood vessel is indicated by X?
2. What blood vessel is indicated by Y?
3. What blood vessel is indicated by Z?

Answer

The hepatic artery carries oxygenated blood from the heart to the liver, where it then diffuses into the hepatocytes (liver cells). Hepatocytes require a lot of oxygen to carry out aerobic respiration to release the energy needed for their many functions. The hepatic artery delivers oxygen not only to the hepatocytes in the liver, but also to adjacent organs, such as the stomach, duodenum of the small intestine, pancreas, and gallbladder.

The hepatic vein carries deoxygenated blood from the liver to the heart, traveling via a larger blood vessel called the inferior vena cava. Hepatocytes produce carbon dioxide during aerobic respiration, which moves into the blood to be carried by the hepatic vein back to the heart. From the heart, this blood travels to the lungs to be oxygenated, before cycling around the body once more.

The hepatic portal vein travels from the intestines, spleen, pancreas, and gallbladder to the liver. It carries blood rich in nutrients, metabolic waste products, and toxins to the hepatocytes. The blood in the hepatic portal vein carries many products of digestion. For example, especially following a meal, the blood in the hepatic portal vein becomes rich in glucose, amino acids, cholesterol, vitamins, and minerals. The hepatic portal vein also transports substances that 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.

Blood vessel X is traveling from the gastrointestinal tract into the liver. Therefore, it will be rich in digestive products and is, therefore, the hepatic portal vein. As blood vessel Y is leaving the liver toward the inferior vena cava, it is the hepatic vein. Blood vessel Z branches off the aorta, which is a major blood vessel carrying oxygenated blood from the heart to the body cells. Z is, therefore, the hepatic artery.

So, our correct answers are as follows:

1. Hepatic portal vein
2. Hepatic vein
3. Hepatic artery

Each lobe of the liver is made up of around 100‎ ‎000 hexagonal lobules. A lobule is a small lobe, one of which you can see magnified in Figure 3. Each of these lobules contains a branch of the bile duct and each of the blood vessels discussed above. You can see the branches of the bile ducts delivering bile from the liver, where it is synthesized, to the gallbladder to be stored. Branches of the hepatic portal vein that bring waste and products of digestion to the liver, as well as the hepatic artery that supplies hepatocytes with oxygenated blood, are also visible, along with a central hepatic vein that transports the deoxygenated blood from the liver back to the heart.

Each of these hexagonal lobules contains many hepatocytes. Hepatocytes make up about of the mass of the liver. The contents of the hepatic artery and hepatic portal vein mix in an area called a sinusoid, shown in Figure 4. The hepatocytes surround the sinusoids so that the contents of the blood can be transported into them. The blood leaves the sinusoid via the hepatic vein. You can also see branches of the bile duct and Kupffer cells in Figure 4. These are specialized phagocytes that are unique to the liver and function to engulf and digest pathogens.

Let’s have a closer look at the hepatocytes themselves. Most hepatocytes have a large nucleus, a prominent endoplasmic reticulum, and many mitochondria.

Their nuclei, which contain their genetic material, are mostly round. A remarkable feature of the liver is that about of the hepatocytes (in a normal liver) have more than two sets of homologous chromosomes. Sometimes they even have more than one nucleus, as you can see in some hepatocytes in the micrograph below.

The reason why hepatocytes tend to have large or multiple nuclei continues to inspire scientific research. This feature is believed to make more gene copies available for protein synthesis, as these cells are very active. It would also offer more protection against DNA damage and cell death, especially when the cells are exposed to toxic substances, for example.

Hepatocytes have a prominent endoplasmic reticulum, as they are active in synthesizing proteins and lipids to be exported to other body cells. Hepatocytes are highly metabolically active, so they have many mitochondria to carry out respiration and release a sufficient amount of energy.

The liver plays an essential role in breaking down harmful or excess metabolic waste products.

Some substances that are produced or ingested by the body are toxic and need to be removed. For example, alcoholic drinks contain ethanol, which is toxic because it dissolves the phospholipids in the cell membranes, causing them to break down. If ethanol is ingested, the liver works hard to convert it into a less toxic form to be excreted.

The liver sustains the majority of the damage caused by this toxin. Excessive alcohol consumption can damage the hepatocytes to the extent of irreversible liver cirrhosis, which is scarring of the liver. An illustration outlining the difference between the appearance of a healthy liver and that of a liver with cirrhosis is shown in Figure 6.

Let’s look at another function of the liver: deamination. Not all of the amino acids that are formed during the digestion of proteins can be stored by the human body. Excess amino acids are delivered to the hepatocytes via the hepatic portal vein. The amino group is removed from the amino acid, which converts it into an organic acid that can be used by the cells, releasing ammonia as a by-product.

Ammonia, which is produced during deamination, is highly toxic to humans, 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, sometimes called the urea cycle, and is summarized in Figure 7.

The ornithine cycle converts the toxic ammonia into the relatively harmless urea. The ornithine cycle involves using the carbon dioxide produced during cellular respiration and three amino acids: ornithine, citrulline, and arginine. These, and a number of enzymes, are used to convert ammonia into urea and water. The urea is then transported to the kidneys to be excreted from the body as part of the urine.

Example 5: Explaining the Importance of the Ornithine Cycle

A product of deamination is ammonia, which is converted into urea via the ornithine (urea) cycle. Why is this conversion important?

Answer

Not all of the amino acids that are formed during the digestion of proteins can be stored by the body. Excess amino acids are delivered to the hepatocytes (liver cells) via the hepatic portal vein. The amino group is removed from the amino acid, converting it into an organic acid to be used by the cells, releasing ammonia as a by-product. Ammonia is highly toxic to humans, so it needs to be converted into another form to be excreted safely. The hepatocytes do this through a process called the ornithine cycle, sometimes called the urea cycle.

The ornithine cycle converts the toxic ammonia into the less toxic urea. This cycle involves using the carbon dioxide produced during cellular respiration and three amino acids: ornithine, citrulline, and arginine. These, and a number of enzymes, are used to convert ammonia into urea and water. The urea is then transported to the kidneys to be excreted from the body as part of the urine.

Therefore, this conversion is important as ammonia is highly toxic and cannot be stored in the human body.

Example 6: Identifying the Reactants in the Ornithine Cycle

The diagram provided shows a simplified outline of the ornithine (urea) cycle that occurs in the liver. What compound has been replaced by X?

Answer

Not all of the amino acids that are formed during the digestion of proteins can be stored by the body. Excess amino acids are delivered to the hepatocytes (liver cells) via the hepatic portal vein. The amino group is removed from the amino acid, converting it into an organic acid that can be used by the cells, releasing ammonia as a by-product. Ammonia is highly toxic to humans and cannot be stored in the body, so it needs to be converted into another form to be excreted safely. The hepatocytes do this through a process called the ornithine cycle, sometimes called the urea cycle.

The ornithine cycle converts the toxic ammonia into the less toxic urea. This cycle involves using the carbon dioxide produced during cellular respiration and three amino acids: ornithine, citrulline, and arginine. These, and a number of enzymes, are used to convert ammonia into urea and water. The urea is then transported to the kidneys to be excreted from the body as part of the urine.

Therefore, compound X is ammonia.

Let’s recap some of the key points that we have covered in this explainer.