Largest Internal Organ in The Human Body

Introduction

With a weight of about 1.5 kg, the liver is the biggest organ within the body and also the largest gland. Accounting for about 2-3% of the body weight, the liver performs various important functions, such as storage of glucose, synthesis of proteins via amino acids, production of bile, removal of toxic wastes, production of blood clotting proteins, cholesterol, etc. Given its indispensable importance in digestion and regulation of the metabolic process, disorders of the liver pose serious threats to healthy bodily functioning.

Anatomical Location

Anatomically the liver is situated in the abdominal cavity. It occupies the upper right and some parts of the left abdominal quadrants and is located just below the diaphragm. It sits above the kidneys, the stomach and the intestines.

Histology

• The liver is a reddish brown organ that comprises four parts or lobes. Each lobe further consists of several smaller lobules.

• The structural and functional unit of the liver is called the hepatic lobule, roughly hexagonal.

• Each lobule is covered with a thin layer of connective tissue or a sheath, which is known as Glisson’s capsule. The peritoneum covers this capsule.

• Each hepatic lobule is made up of many cells known as hepatocytes which are parenchymatous.

• The capsules of Glisson also form the stroma of the liver, made up of connective tissue and contains blood vessels.

• Capillaries called sinusoids run between the hepatocytes.

• Perisinusoidal spaces between hepatocytes and the sinusoids are present (spaces of Disse). They contain the vitamin A storage hepatic stellate cells (aka Ito cells). These cells also secrete lots of extracellular matrix during wound injury, therefore are actively involved in hepatic fibrosis.

• The sinusoids also contain specialised cells known as Kupffer cells, which phagocytose any antigens or damaged RBCs.

• The lobules of the two lobes of the liver are connected to ducts which eventually converge together as the left and the right hepatic ducts, which together form the common hepatic duct.

Parts of Liver

The liver consists of the right and the left lobe which are the larger lobes of the organ, and the caudate and the quadrate lobe, which are the smaller lobes.

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Functions of Liver

• Storage of glucose in the form of glycogen and the conversion of glycogen to glucose to maintain the blood glucose levels.

• Detoxification of blood by removing and metabolizing toxic substances.

• Storage of iron and vitamins including vitamins A, D, E, K and B12.

• Production of plasma proteins such as albumins, prothrombins, and fibrinogens.

• Production of bile which carries away waste substances via the common hepatic duct into the gallbladder and the duodenum.

• Production of cholesterol.

• Production of urea after degradation of amino acids.

• Kupffer cells located in the sinusoids of the liver play a phagocytic role.

Disorders of Liver

• Hepatic fibrosis − It also known as liver fibrosis, this condition results from damage inflicted to the liver and subsequent excessive accumulation of collagen and other components of the extra-cellular matrix. This disease results from excessive alcohol consumption, hepatitis virus infection, and other chronic liver diseases. The accumulation of the ECM results in scarring of the liver and eventually leads to the late stage of liver scarring, known as liver cirrhosis, which results in improper functioning of the liver.

• Hepatitis − It is the infection of the liver by a virus, which may be acute or chronic.

• The causal agents of hepatitis are hepatitis A, Hepatitis B, and Hepatitis C viruses.

• Sources of infection vary and may include faeces-contaminated food and water or blood and other body fluids.

• While an effective vaccine exists for hepatitis A and hepatitis B, there is currently no vaccine available for hepatitis C. Hepatitis A is often fully cured, while persons infected with hepatitis B and C are often subject to chronic liver disease.

• Some forms of hepatitis are non-viral, including autoimmune hepatitis and toxic hepatitis.

• Non-alcoholic fatty liver disease (NAFLD)− NAFLD is characterised by excess fat in the liver. As the name suggests, this disease affects people who don't (or rarely) consume alcohol. Risk factors include type 2 diabetes, heavy body weight, high cholesterol, etc. NAFLD typically does not have any prominent symptoms and hence is also referred to as a silent disease. NAFLD may be of two kinds −

• Non-alcoholic steatohepatitis (NASH) −It causes inflammation of the liver, which may lead to fibrosis or even cirrhosis.

• Non-alcoholic fatty liver (NAFL) −It inflammation of the liver doesn’t occur. Fat build-up can be reduced by weight loss.

Regeneration of Liver

The liver of vertebrates has a remarkable capacity to replace its tissues and is the only visceral organ that can do so. The regenerative process is triggered by either the removal of liver tissue (hepatectomy) or by some injury imparted to the liver. Regeneration involves various genes and signalling pathways that regenerate the liver tissue through three stages −

• The priming stage involves the activation of certain genes and overall preparation for regeneration.

• The proliferative phase involves growth factors like the hepatocyte growth factor (HGF), epidermal growth factor (EGF), and tumour necrosis factor (TNF).

• The termination phase is regulated by the transforming growth factor (TGFbeta), which is a strong inhibitor of hepatocyte proliferation.

The period required for the liver to regenerate 50-60% of its weight is about one month. Chronic liver diseases are characterised by the loss of the ability of the liver to regenerate itself.

Conclusion

• Being the largest internal organ of the body, the liver weighs 1.5 kg and accounts for almost 3% of the total body weight.

• It is divided into four parts– the left and right lobes and the caudate and quadrate lobes.

• The functional unit of the liver is the hepatic lobule which is made up of parenchymatous cells called the hepatocytes.

• The liver also contains immunological phagocytic cells known as Kupffer cells.

• The liver performs several important functions including storage of glucose as glycogen, glycogenolysis, detoxification, bile production, production of proteins and storage of iron, among many others.

• When the liver is subjected to certain disorders that affect its functioning and overall homeostasis of the body.

  Example − liver fibrosis and cirrhosis, hepatitis and NAFLD

• The liver has an amazing ability to regenerate.

FAQs

Q1. What is the composition and function of bile?

Ans. Bile is largely composed of water, bile pigments (bilirubin and biliverdin), bile acids, some enzymes, cholesterol, phosphatidylcholine, vitamins, and other substances. Bile is important in the emulsification and absorption of lipids.

Q2. What are the different forms of iron stored in the liver?

Ans. Ferritin, transferrin, haptoglobin, and hemopexin.

Q3. What are the two sources of the vasculature of the liver?

Ans. The liver is vasculated by the hepatic portal vein and the hepatic artery.

Q4. What are the symptoms of hepatitis?

Ans. Common symptoms include fever, loss of appetite, nausea, fatigue, joint pain, dark-coloured urine, and jaundice.

Q5. When is a liver transplant performed?

Ans. A liver transplant is the last option under the condition of chronic liver failure due to last-stage alcohol cirrhosis or hepatitis C infection.