1.5 Role of Liver

Introduction

The liver is the largest internal organ and one of the most important organs in the human body. It is both an organ and a gland. It performs hundreds of functions, but in the context of the digestive system, its role is especially critical. The liver produces bile, processes absorbed nutrients, stores energy, and detoxifies harmful substances. This topic explains the major roles of the liver.

Definition

The liver is a spongy, wedge-shaped, reddish-brown organ and gland located on the right side of the body, under the ribs. It is the largest gland in the human body and performs many vital functions related to digestion, metabolism, storage, and detoxification.

Main Idea

The liver has many functions. Some of the most vital roles of the liver are:

• The liver cells manufacture bile.
• Excess quantity of glucose is stored by the liver as glycogen.
• Liver converts glycerol and amino acids to glucose molecules.
• Liver removes the amino group from amino acids. This is called deamination.
• Ammonia is converted into urea in the liver.
• Majority of the plasma proteins are synthesized in the liver.
• It destroys old red blood cells.
• It stores fat-soluble vitamins (A, D, E, and K) and mineral ions such as iron.

Core Concept

The Liver's Role in Digestion and Metabolism

Detailed Explanation

1. Manufacture of Bile

The liver cells manufacture bile.

What is bile?

• A greenish-yellow liquid.
• Contains bile salts, bile pigments, cholesterol, and other substances.
• Stored in the gallbladder.

What bile does:

• Bile emulsifies fats, breaking them into small droplets.
• Emulsification increases the surface area for lipase action.
• Bile also helps in the absorption of fat-soluble vitamins (A, D, E, K).

Simple explanation: The liver produces bile, a detergent-like substance that helps break down fats in the small intestine.

Important point: Bile does not contain enzymes. It only emulsifies fats (physical breakdown).

2. Storage of Glucose as Glycogen

Excess quantity of glucose is stored by the liver as glycogen.

What happens:

• After a meal, blood glucose levels rise.
• The liver takes up excess glucose.
• The liver converts glucose to glycogen (glycogenesis).
• Glycogen is stored in the liver.

What happens when blood glucose falls:

• The liver breaks down glycogen to glucose (glycogenolysis).
• Glucose is released into the blood.
• This helps maintain normal blood glucose levels.

Simple explanation: The liver is like a battery that stores extra glucose and releases it when the body needs energy.

Why this is important:

• The brain and red blood cells depend on glucose for energy.
• The liver ensures a steady supply of glucose between meals.

3. Conversion of Glycerol and Amino Acids to Glucose

The liver converts glycerol and amino acids to glucose molecules.

What happens:

• Glycerol (from fat digestion) can be converted to glucose.
• Amino acids (from protein digestion) can be converted to glucose.
• This process is called gluconeogenesis.

Simple explanation: When the body needs glucose but not enough carbohydrate is available, the liver can make glucose from other nutrients.

Why this is important:

• It ensures that the body always has glucose for energy.
• It allows the body to use different types of food to maintain blood sugar.

4. Deamination of Amino Acids

The liver removes the amino group from amino acids. This is called deamination.

What is deamination?

• The removal of the amino group (–NH₂) from an amino acid.
• The remaining part of the amino acid (carbon skeleton) is used for energy or converted to glucose/fat.
• The amino group is converted to ammonia (NH₃).

Simple explanation: Amino acids are like building blocks with a special tag (amino group). The liver removes this tag so the rest of the molecule can be used for energy.

Why this is important:

• Excess amino acids cannot be stored in the body.
• Deamination allows the body to use excess amino acids for energy.
• The amino group is converted to urea for excretion.

5. Conversion of Ammonia to Urea

Ammonia is converted into urea in the liver.

What happens:

• Ammonia (NH₃) is toxic to cells.
• The liver converts ammonia to urea (CO(NH₂)₂).
• Urea is less toxic and can be safely transported in the blood.
• Urea is carried to the kidneys and excreted in urine.

Simple explanation: Ammonia is a poisonous waste product. The liver converts it to urea, which is safer, and the kidneys remove it from the body.

Why this is important:

• Ammonia is highly toxic and can damage cells.
• Converting ammonia to urea protects the body from harm.
• Urea is the main nitrogenous waste in urine.

6. Synthesis of Plasma Proteins

Majority of the plasma proteins are synthesized in the liver.

Which plasma proteins:

• Albumin — maintains osmotic pressure of blood.
• Fibrinogen — essential for blood clotting.
• Globulins — transport lipids and fat-soluble vitamins, and help in immunity.
• Prothrombin — essential for blood clotting.

Simple explanation: The liver makes many proteins that circulate in the blood. These proteins help in clotting, transport, and maintaining blood pressure.

Why this is important:

• Without plasma proteins, blood cannot clot properly.
• Without albumin, fluid balance in the body would be disturbed.

7. Destruction of Old Red Blood Cells

The liver destroys old red blood cells.

What happens:

• Red blood cells have a lifespan of about 120 days.
• Worn-out red blood cells are destroyed in the spleen and liver.
• The iron from haemoglobin is stored in the liver.
• The breakdown products of haemoglobin are used to make bile pigments (bilirubin and biliverdin).

Simple explanation: The liver is like a recycling center. It breaks down old red blood cells and recycles the iron and other components.

Why this is important:

• Iron is essential for making new red blood cells.
• Bile pigments are needed for bile production.

8. Storage of Vitamins and Minerals

The liver stores fat-soluble vitamins (A, D, E, and K) and mineral ions such as iron.

What is stored:

• Vitamin A — for vision and immune function.
• Vitamin D — for calcium absorption and bone health.
• Vitamin E — antioxidant.
• Vitamin K — for blood clotting.
• Iron — for haemoglobin synthesis.
• Glycogen — for energy storage.

Simple explanation: The liver is like a storage warehouse. It keeps vitamins and minerals until the body needs them.

Why this is important:

• The body cannot make most vitamins, so they must be stored.
• Iron is essential for oxygen transport in the blood.

Intermediate Level

The liver performs the following major functions:

• Manufactures bile.
• Bile emulsifies fats.

• Stores excess glucose as glycogen.
• Converts glycerol and amino acids to glucose.
• Releases glucose when blood sugar falls.

• Deaminates amino acids.
• Converts ammonia to urea.
• Synthesizes plasma proteins (albumin, fibrinogen, globulins, prothrombin).

• Stores fat-soluble vitamins (A, D, E, K).
• Stores mineral ions such as iron.
• Stores glycogen.

• Breaks down worn-out red blood cells.
• Recycles iron.
• Produces bile pigments.

Important Points

• - The liver is the largest gland in the body.
• - The liver manufactures bile.
• - Bile emulsifies fats.
• - Excess glucose is stored as glycogen in the liver.
• - The liver converts glycerol and amino acids to glucose.
• - Deamination removes the amino group from amino acids.
• - Ammonia is converted to urea in the liver.
• - Majority of plasma proteins are synthesized in the liver.
• - The liver destroys old red blood cells.
• - The liver stores fat-soluble vitamins (A, D, E, K) and iron.

Remember

• - Liver = largest gland
• - Bile = produced by liver, emulsifies fats
• - Glycogen = stored form of glucose
• - Deamination = removal of amino group
• - Urea = formed from ammonia in liver
• - Plasma proteins = synthesized in liver
• - Old RBCs = destroyed in liver
• - Vitamins A, D, E, K = stored in liver
• - Iron = stored in liver

Common Mistakes

• 1. Thinking the gallbladder produces bile.
• - The liver produces bile. The gallbladder only stores and concentrates it.
• 2. Believing bile contains enzymes.
• - Bile does not contain enzymes. It only emulsifies fats.
• 3. Confusing deamination with transamination.
• - Deamination = removal of amino group.
• - Transamination = transfer of amino group.
• 4. Thinking ammonia is excreted directly.
• - Ammonia is toxic. It is converted to urea in the liver and then excreted by the kidneys.
• 5. Forgetting that the liver stores glycogen.
• - The liver stores excess glucose as glycogen and releases it when needed.
• 6. Mixing up the roles of the liver and pancreas.
• - Liver = bile, glycogen storage, deamination, urea formation.
• - Pancreas = insulin, glucagon, pancreatic juice.
• 7. Believing the liver only has one function.
• - The liver has hundreds of functions, including digestion, metabolism, storage, and detoxification.

Quick Revision

60-Second Revision

• Liver = largest gland in the body
• Bile = produced by liver, emulsifies fats
• Glycogen = stored form of glucose
• Deamination = removal of amino group from amino acids
• Urea = formed from ammonia in liver, excreted by kidneys
• Plasma proteins = synthesized in liver (albumin, fibrinogen, globulins, prothrombin)
• Old RBCs = destroyed in liver
• Storage = vitamins A, D, E, K and iron
• Warning: Do not confuse liver (produces bile) with gallbladder (stores bile).