Liver | bile

 Liver and biliary system

Liver is a dual organ having both secretory and excretory functions. It is the largest gland in the body, weighing about 1.5 kg in man. It is located in the upper and right side of the abdominal cavity, immediately beneath diaphragm.

 LIVER

Hepatic Lobes

Liver is made up of many lobes called hepatic lobes. Each lobe consists of many lobules called
hepatic lobules.

Hepatic Lobules

Hepatic lobule is the structural and functional unit of liver. There are about 50,000 to 100,000 lobules in the liver. The lobule is a honeycomb-like structure and it is made up of liver cells called hepatocytes.

Hepatocytes and Hepatic Plates 

Hepatocytes are arranged in columns, which form the hepatic plates. Each plate is made up of two columns of cells. In between the two columns of each plate lies a bile canaliculi.
In between the neighboring plates, a blood space called sinusoid is present. Sinusoid is lined by the endothelial cells. In between the endothelial cells some special macrophages called Kupffer cells are present.

Portal Triads

Each lobule is surrounded by many portal triads. Each portal triad consists of three vessels:
 1. A branch of hepatic artery
 2. A branch of portal vein
 3. A tributary of bile duct.

Branches of hepatic artery and portal vein open
into the sinusoid. Sinusoid opens into the central vein. Central vein empties into hepatic vein.
 Bile is secreted by hepatic cells and emptied into bile canaliculus. From canaliculus, the bile enters thetributary of bile duct.

Tributaries of bile duct from canaliculi of neighboring lobules unite to form small bile ducts. These small bile ducts join together and finally form left and right hepatic ducts, which emerge out of liver.

BILIARY SYSTEM 

Biliary system or extrahepatic biliary apparatus is formed by gallbladder and extrahepatic bile ducts (bile ducts outside the liver).

Right and left hepatic bile ducts which come out of liver join to form common hepatic duct. It unites with the cystic duct from gallbladder to
form common bile duct. All these ducts have similar structures.

Common bile duct unites with pancreatic duct to
form the common hepatopancreatic duct or ampulla of Vater, which opens into the duodenum.

There is a sphincter called sphincter of Oddi at the lower part of common bile duct, before it joins the pancreatic duct. It is formed by smooth muscle fibers of common bile duct. It is normally kept closed; so the bile secreted from liver enters gallbladder where it is stored.

Upon appropriate stimulation, the sphincter opens and allows flow of bile from gallbladder into the intestine.

BLOOD SUPPLY TO LIVER

Liver receives maximum blood supply of about 1,500 mL/minute. It receives blood from two sources, namely the hepatic artery and portal vein.

HEPATIC ARTERY

Hepatic artery arises directly from aorta and supplies oxygenated blood to liver. After entering the liver, the hepatic artery divides into many branches. Each branch enters a portal triad.

PORTAL VEIN

Portal vein is formed by superior mesenteric vein and splenic vein. It brings deoxygenated blood from stomach, intestine, spleen and pancreas. Portal blood is rich in monosaccharides and amino acids. It also contains bile salts, bilirubin, urobilinogen and GI hormones.

 However, the oxygen content is less in portal blood. Flow of blood from intestine to liver through portal vein is known as enterohepatic circulation.

The blood from hepatic artery mixes with blood from portal vein in hepatic sinusoids. Hepatic cells obtain oxygen and nutrients from the sinusoid.

HEPATIC VEIN

Substances synthesized by hepatic cells, waste products and carbon dioxide are discharged into sinusoids. Sinusoids drain them into central vein of the lobule.

Central veins from many lobules unite to form bigger veins, which ultimately form hepatic veins (right and left) which open into inferior vena cava.

PROPERTIES OF BILE

Volume : 800 to 1,200 mL/day
Reaction : Alkaline
pH : 8 to 8.6
Specific gravity : 1.010 to 1.011
Color : Golden yellow or green.

COMPOSITION OF BILE

Bile contains 97.6% of water and 2.4% of solids. Solids include organic and inorganic substances.

Process of bile secretion

Bile is secreted by hepatocytes. The initial bile secreted by hepatocytes contains large quantity of bile acids, bile pigments, cholesterol, lecithin and fatty acids. From hepatocytes, bile is released into canaliculi.

From here, it passes through small ducts and hepatic ducts and reaches the common hepatic duct. From common hepatic duct, bile is diverted either directly into the intestine or into the gallbladder.

Sodium, bicarbonate and water are added to bile when it passes through the ducts. These substances are secreted by the epithelial cells of the ducts. Addition of sodium, bicarbonate and water increases the total quantity of bile.

STORAGE OF BILE

Most of the bile from liver enters the gallbladder, where it is stored. It is released from gallbladder into the intestine whenever it is required.

When bile is stored in gallbladder,it undergoes many changes both in quality and quantity such as:
1. Volume is decreased because of absorption of
a large amount of water and electrolytes (except
calcium and potassium)
2. Concentration of bile salts, bile pigments, cholesterol,
fatty acids and lecithin is increased because of
absorption of water and electrolytes
3. The pH is decreased slightly
4. Specific gravity is increased
5. Mucin is added to bile

FUNCTIONS OF LIVER

Liver is the largest gland and one of the vital organs of the body. It performs many vital metabolic and homeostatic functions, which are summarized below.
1. METABOLIC FUNCTION
Liver is the organ where maximum metabolic reactions such as metabolism of carbohydrates, proteins, fats, vitamins and many hormones are carried out.

2. STORAGE FUNCTION
Many substances like glycogen, amino acids, iron, folic acid and vitamins A, B12 and D are stored in liver.

3. SYNTHETIC FUNCTION
Liver produces glucose by gluconeogenesis. It synthe-sizes all the plasma proteins and other proteins (except immunoglobulins) such as clotting factors, complement factors and hormone­binding proteins. It also synthesizes
steroids, somatomedin and heparin.

4. SECRETION OF BILE
Liver secretes bile which contains bile salts, bile
pigments, cholesterol, fatty acids and lecithin.
The functions of bile are mainly due to bile salts. Bile salts are required for digestion and absorption of fats in the intestine. Bile helps to carry away waste products and breakdown fats, which are excreted through feces or urine.

5. EXCRETORY FUNCTION
Liver excretes cholesterol, bile pigments, heavy metals (like lead, arsenic and bismuth), toxins, bacteria and virus (like that of yellow fever) through bile.

6. HEAT PRODUCTION
Enormous amount of heat is produced in the liver because of metabolic reactions. Liver is the organ where maximum heat is produced.

7. HEMOPOIETIC FUNCTION
In fetus (hepatic stage), liver produces the blood
cells . It stores vitamin B12 necessary for erythropoiesis and iron necessary for synthesis of hemoglobin. Liver produces thrombopoietin that promotes production of thrombocytes.

8. HEMOLYTIC FUNCTION
The senile RBCs after a lifespan of 120 days are
destroyed by reticuloendothelial cells (Kupffer cells) of liver.

9. INACTIVATION OF HORMONES AND DRUGS
Liver catabolizes the hormones such as growth hormone, parathormone, cortisol, insulin, glucagon and estrogen. It also inactivates the drugs, particularly the fat­soluble drugs. The fat­soluble drugs are converted into water-
soluble substances, which are excreted through bile or urine.

10. DEFENSIVE AND DETOXIFICATION 
 FUNCTIONS
Reticuloendothelial cells (Kupffer cells) of the liver play an important role in the defense of the body. Liver is also involved in the detoxification of the foreign bodies.

i. Foreign bodies such as bacteria or antigens are
swallowed and digested by reticuloendothelial
cells of liver by means of phagocytosis.

ii. Reticuloendothelial cells of liver also produce
substances like interleukins and tumor necrosis
factors, which activate the immune system of
the body.

iii. Liver cells are involved in the removal of toxic property of various harmful substances. Removal of toxic property of the harmful agent is known as detoxification.

Detoxification in liver occurs in two ways:
a. Total destruction of the substances by means
of metabolic degradation.
b. Conversion of toxic substances into non-
toxic materials by means of conjugation with
glu cu ronic acid or sulfates.