Liver

Nestled under your diaphragm, above the stomach and right kidney is the largest visceral organ of the human body – the Liver. It has many different functions and plays many different roles from metabolism to digestion to circulation. Traditional Chinese Medicine compares the liver to a military general. Considering its many functions and overall importance to life this analogy is very fitting.

The liver has structural characteristics that are not found in any other internal organ of the human body. One abnormal characteristic is the liver’s regenerative abilities. Pieces of the liver can be cut off and it will regenerate new hepatic tissue almost like a lizard’s tail! All a liver needs to regenerate to its original size is 25% of the original tissue. However, the shape of the new regenerated liver will not be the same and it may not function as well as the original liver.

During surgeries, if the liver is cut for any reason, the cut will seal itself at a very fast rate. These regenerative attributes only add to the amazing characteristics that can be found in the liver!

Location and Structure of the Liver

The liver is around the size of an American football at about 16 cm. Weighing in at around 1.5 kg in men and 1.2 kg in women the liver accounts for about 1/32 of total adult body weight. The fetal liver is considerably larger in proportion to the rest of the fetus. In a fetus, the liver accounts for about 5% of the weight of the body. The large size of the liver seems to correlate to its importance in maintaining the quality of life.

The liver resides in almost the entire length of the upper abdomen. While the greatest portion sits in the right hypochondriac region, it extends past the epigastrium and over into the left hypochondriac region. Neighboring this vital organ we see the right kidney, the gallbladder, pancreas, and intestines. These organs, being in close proximity, are positioned perfectly to work together for processing blood and carrying out digestive processes.

From the front or the anterior view of the liver, we see several different features. Two lobes and numerous ligaments:

• Left Lobe: The smallest of the two lobes.
• Right Lobe: This is the largest lobe. This lobe is divided into four sections. The hepatic vein divides the right lobe into front and back sections while the portal vein divides it into upper and lower sections. If a section of the right lobe is damaged, the rest of the liver continues working and the person may not even know that there is a problem.
• The Diaphragm: Resting directly on top of the liver.
• Falciform Ligament: This ligament separates the left and right lobe when looking at the front.
• Coronary Ligament: Coronary ligaments fork off the top of the falciform.
• The Left and Right Triangular Ligaments: These ligaments are at the end of either side of the coronary ligaments. They are small and like their names suggest, triangular in shape.
• The Fundus of the Gallbladder: The gallbladder is nestled underneath the liver. Peeking out from the inferior border of the liver is the gallbladder.

Examining the liver from a posterior position, we see that there are in fact four different lobes. Visual Anatomy divides the lobes by surface features. Viewing the liver anteriorly, the left and right lobes are divided by the Falciform Ligament. The Falciform ligament connects the liver to the front abdominal wall.

Functional Anatomy divides the lobes in a different way. From a functional perspective, the lobes are divided by their position in relation to the bile duct, hepatic portal vein, and hepatic artery.

From a posterior perspective, we see the two additional lobes as well as a few other very important structures:

• Caudate Lobe: This small lobe is found right next to the inferior vena cava. The caudate lobe is found in the right lobe.
• Quadrate Lobe: Also found on the right lobe, the quadrate lobe is right next to the gallbladder.
• Inferior Vena Cava: This is a very large vein that carries deoxygenated blood from the lower body back up to the heart. There are three hepatic veins that carry deoxygenated blood from the liver and dump it into the vena cava, aptly named the left, right and middle hepatic veins.
• Proper Hepatic Artery: This is where the liver receives its oxygenated blood. Shooting off of the aorta, the proper hepatic artery branches into smaller arteries. Two arteries travel to the right side of the liver while one carries blood to the left side.
• Portal Vein: The portal vein is part of the large portal system. This is a fascinating system that carries materials from other organs into the liver. A portal system of veins are veins that carry blood from one bed of capillaries to another before the blood reaches the heart While some organs obtain all nutrients from oxygenated blood, the liver receives various materials from the stomach, the small intestine and the gallbladder. The liver decides what to do with the materials it receives and filters blood before it reaches the heart.
• Common Bile Duct: Also noticeable from the posterior side of the liver is the common bile duct. This is the large duct that collects bile from the livers many smaller bile ducts and flows down to the gallbladder.
• The Gallbladder: Almost appearing built into the liver we find the gallbladder. It has its own small indention of the liver that it rests in. It receives bile from the liver through the common bile duct.

Liver Development

The liver begins forming when an embryo is about 4 weeks old. During this time we see the formation of what is called the hepatic diverticulum. Also called the hepatic bud, because of its resemblance to a flower bud, this structure is part of a larger structure called the foregut which develops in the endodermal layer, or the inside layer, of a fetus.

The endoderm is one of three germ layers of an embryo. The structure of the foregut is the beginning of what will be the stomach, pancreas, gallbladder, and kidneys. The endodermal layer of an embryo is where internal organs form.

While the embryo develops, it begins to split off into left and right buds. These buds begin resembling the final shape of the liver. In utero, the fetus receives blood and nutrients from the mother. Blood enters the fetus through the umbilical vein. This vein bypasses the liver. Because of this, the fetal liver does not have the same function as a more developed liver. Instead of filtering blood, while the fetus is in utero, its main function is to create T-cells and T-lymphocytes that will aid in the fetus’s immune system. After the baby is born, the umbilical vein begins to close and turns into the round ligament of the liver which helps to keep the liver attached to the individual’s umbilicus.

Histology of the Liver

The liver lobes are made up of tiny hexagons. These hexagonal sections are called Hepatic lobules, the centers of function. Hepatic lobules are made from liver cells called hepatocytes. Hepatocytes come together to form the foundation of the lobule by forming thick hepatic plates. The borders of the lobules contain a branch of the hepatic artery (arterioles), a branch of the hepatic portal vein (venules) and bile ducts.

Both the hepatic arterioles and hepatic portal venules release their contents into the sinusoids of the lobule. Sinusoids act as a mixing station of the oxygenated blood and the materials delivered by the portal vein. Since the sinusoids are exposed to whatever the stomach and large intestine have absorbed, the risk of exposure to harmful substances is very high. Enter the Kupffer cells. These cells line the walls of the sinusoids and protect them from most of the harmful substances they are exposed to. Sinusoids create a space in between hepatocyte walls called the space of Disse. While in the sinusoids the mixture is filtered and nutrients are either stored in the liver or passed on. The leftover material is made into bile and sent down to the gallbladder and large intestine. After the blood has been through this filtration process it is passed out of the central vein of the lobule and runs into the inferior vena cava where it continues in circulation.

There are two main types of cells seen within the lobules:

1. Parenchymal Cells: These types of cells are responsible for the structure of an organism. They are often described as the foundation or ground cells for which more structures can build from. Both plants and animals have parenchymal cells. In the liver, the parenchymal cells are called hepatocytes. Parenchymal cells account for around 60% of the liver’s structure.
2. Non-parenchymal cells: These cells include other types of functional cells. These cells include the Kupffer cells, endothelial cells, and hepatic stellate cells.

Liver Functions

The liver is both an endocrine and an exocrine gland. Endocrine functions of the liver include secreting products like bile to other organs. The liver also filters blood and secretes substances into the bloodstream which makes it an exocrine gland. Like a General, the liver has many responsibilities and runs many different aspects of body processes.

Functions of the liver include:

• Breaking down of old blood cells. These blood cells are changed into bile.
• Storage for glycogen
• Detoxifying and converting medicines and poisons into useful substances
• Storage of vitamins and minerals
• Blood storage
• Processing hormones
• Urea production
• Bacteria filtration
• Secretion of plasma proteins that aid in blood clotting
• Processing fats and cholesterol

Diseases and Disorders associated with the Human Liver

Since the liver has so many functions and responsibilities, a liver disorder can be a serious problem that could turn fatal. Often, symptoms of liver problems manifest themselves in other areas of the body. One such example is the yellowing of the eyes and skin. This symptom is called jaundice and may be one of the first signs of a problem. Jaundice is the effects of the liver not properly filtering dead or broken down red blood cells from the bloodstream.

Some common diseases of the liver include:

• Hepatitis: Inflammation of the liver tissue. There are five different types of Hepatitis; A,B,C,D, and E
• Cirrhosis: Cirrhosis of the liver is the irreversible scarring caused by constant liver problems.
• Fatty Liver Disease: Build up of fats in the liver. Can be caused by alcohol consumption.
• Iron Overload: Hemochromatosis, or iron overload, may cause cirrhosis of the liver.
• Epstein-Barr Virus: A member of the herpes virus family, EBV may infect the liver and cause hepatitis. This is the most widespread form of the herpes virus infecting around 90% of adults. People with EBV can go their whole lives and not notice any symptoms.

Early detection of any liver disease is key in the likelihood of successful treatments. Jaundice is one symptom of liver disease but an individual may also experience fatigue, persistent pain in the abdomen, distended abdomen, changes in urine color, yellow stools, and flaky skin.

The liver is an amazing organ. It plays multiple roles as an exocrine and endocrine gland, it can regenerate itself, and it plays roles in many different body processes. From digestion to blood filtration and buffering the liver is truly one of the most important organs we have.