Importance of Liver in Poultry
LIVER is the most vital organ of the bird.

Relatively larger liver of poultry when compared to other animal sp. itself indicates its major role in body metabolism.

Liver is the largest and one of the most important organs because liver acts as a clearing house for substances that enter the body. It is the largest gland in the body. It secretes bile; metabolizes proteins, carbohydrates, and fats; stores glycogen, vitamins, and other substances; synthesizes coagulation factors; removes wastes and toxic matter from the blood; regulates blood volume; and destroys old red blood cells. The portal vein carries blood from the gastrointestinal tract, gallbladder, pancreas, and spleen to the liver to be processed.

A duct system carries bile from the liver to the duodenum and the gallbladder. Liver tissue consists of a mass of cells tunnelled with bile ducts and blood vessels.

Liver tissue is composed of thousands of lobules, and each lobule is made up of hepatic cells, the basic metabolic cells of the liver. About 60% are hepatic cells, which are highly active metabolically. A second type, Kupffer cells, play a role in blood-cell formation, antibody production, and ingestion of foreign particles and cell debris.

Bird's liver has two lobes unlike four in humans. The avian liver changes in colour and consistency during the life of the bird. The liver of the newly hatched chick is very pale and contains a large amount of fat. It changes to a more normal brownish red colour at 5-7 days of age at which time the yolk sac has been completely reabsorbed. Fat again accumulates in the liver of the female chicken when it is starting egg production. This change is physiological and under the control of estrogen. With fat infiltration the liver of a laying hen tends to be larger, paler and more friable than that of a male bird of the same age.

Functions
Liver cells or hepatocytes carry out the various functions:
Digestion   •Synthesis   •Storage   •Metabolism   •Detoxification
One of the liver's major functions is the manufacture and secretion of bile, which is stored in the gall bladder and released in the small intestine. Bile contains bile acids and salts, cholesterol, and electrolyte chemicals that keep it slightly acidic. Bile salts emulsify fats aiding in digestion by the intestinal enzymes. Some of the bile drains directly into the duodenum, and some is stored in the gallbladder.
The liver performs several roles in carbohydrate metabolism. The hepatic cells assimilate carbohydrates, fats, and proteins. They convert glucose to its stored form, glycogen, which is reconverted into glucose, as the body requires it for energy. The ability of the liver to maintain the proper.
Gluconeogenesis : The Formation of glucose from certain amino acids, lactate or glycerol
Glycogenesis : The formation of glycogen from glucose
Glycogenolysis : The formation of glucose from glycogen
The liver is responsible for the mainstay of protein metabolism The liver is also capable of synthesizing certain amino acids so-called nonessential amino acids from other amino acids in a process called transamination.
The liver also performs several roles in lipid metabolism :
Cholesterol Synthesis
The production of triglycerides
Excess carbohydrates and protein are also converted into fat by the liver
The liver produces coagulation factors.
The liver breaks down hemoglobin, creating metabolites that are added to bile as pigment.
The liver breaks down toxic substances and most medicinal products in a process called drugmetabolism. This sometimes results in toxication, when the metabolite is more toxic than its precursor.
The liver converts ammonia to uric acid.
The liver stores a multitude of substances, including glucose in the form of glycogen, vitamin B12, iron, and copper.
The liver is the main site of red blood cell production.
The liver is responsible for immunological effects- the reticuloendothelial system of the livercontains many immunologically active cells, acting as a 'sieve' for antigens carried to it via the portal system. The liver also filters harmful substances from the blood. Phagocytic cells in the liver, called Kupffer cells, remove large amounts of debris and bacteria.

Bile - Bile Acids : Secretion and Role in Avian Digestion
Bile is a complex fluid containing water, electrolytes and a battery of organic molecules including bile acids, cholesterol, phospholipids and bilirubin that flows through the biliary tract into the small intestine. There are two fundamentally important functions of bile in all species :
Bile contains bile acids, which are critical for digestion and absorption of fats and fat-soluble vitamins in the small intestine.
Many waste products, including bilirubin, are eliminated from the body by secretion into bile and elimination in feces.

Adult bird produces 30 to 70 ml of bile daily, and other animals proportionately 400 to 800 ml amounts. The secretion of bile can be considered to occur in two stages :
Initially, hepatocytes secrete bile into canaliculi, from which it flows into bile ducts. This hepatic bile contains large quantities of bile acids, cholesterol and other organic molecules.
As bile flows through the bile ducts it is modified by addition of a watery, bicarbonate-rich secretion from ductal epithelial cells.

In species with a gallbladder (bird and most domestic animals except horses and rats), further modification of bile occurs in that organ. The gall bladder stores and concentrates bile during the fasting state. Typically, bile is concentrated five-fold in the gallbladder by absorption of water and small electrolytes - virtually all of the organic molecules are retained.

Secretion into bile is a major route for eliminating cholesterol. Free cholesterol is virtually insoluble in aqueous solutions, but in bile, it is made soluble by bile acids and lipids like lecithin. Gallstones, most of which are composed predominantly of cholesterol, result from processes that allow cholesterol to precipitate from solution in bile.

Role of Bile Acids in Fat Digestion and Absorption
Bile acids are derivatives of cholesterol synthesized in the hepatocyte. Cholesterol, ingested as part of the diet or derived from hepatic synthesis is converted into the bile acids cholic and chenodeoxycholic acids, which are then conjugated to anamino acid (glycine or taurine) to yield the conjugated form that is actively secreted into canaliculi.

Bile acids are facial amphipathic, that is, they contain both hydrophobic (lipid soluble) and polar (hydrophilic) faces. The cholesterol-derived portion of a bile acid has one face that is hydrophobic (that with methyl groups) and one that is hydrophilic (that with the hydroxyl groups); the amino acid conjugate is polar and hydrophilic.



Their amphipathic nature enables bile acids to carry out two important functions :
Emulsification of lipid aggregates : Bile acids have detergent action on particles of dietary fat which causes fat globules to break down or be emulsified into minute, microscopic droplets. Emulsification is not digestion per se, but is of importace because it greatly increases the surface area of fat, making it available for digestion by lipases, which cannot access the inside of lipid droplets.
Solubilization and transport of lipids in am aqueous environment : Bile acids are lipid carriers and are able to solubilize many lipids by forming micelles - aggregates of lipids such as fatty acids, cholesterol and monoglycerides - that remain suspended in water. Bile acids are also critical for transport and absorption of the fat-soluble vitamins.

Role of Bile Acids in Cholesterol Homeostasis:
Hepatic synthesis of bile acids accounts for the majority of cholesterol breakdown in the body. In birds, roughly 40 mg of cholesterol are converted to bile acids and eliminated in bile every day. This route for elimination of excess cholesterol is probably important in all animals, but particularly in situations of massive cholesterol ingestion.

Interestingly, it has recently been demonstrated that bile acids participate in cholesterol metabolism by functioning as hormones that alter the transcription of the rate-limiting enzyme in cholesterol biosynthesis.

Enterohepatic Recirculation:
Large amounts of bile acids are secreted into the intestine every day, but only relatively small quantities are lost from the body. This is because approximately 95% of the bile acids delivered to the duodenum are absorbed back into blood within the ileum.

Venous blood from the ileum goes straight into the portal vein, and hence through the sinusoids of the liver. Hepatocytes extract bile acids very efficiently from sinusoidal blood, and little escapes the healthy liver into systemic circulation. Bile acids are then transported across the hepatocytes to be resecreted into canaliculi. The net effect of this enterohepatic recirculation is that each bile salt molecule is reused about 20 times, often two or three times during a single digestive phase.

It should be noted that liver disease can dramatically alter this pattern of recirculation for instance, sick hepatocytes have decreased ability to extract bile acids from portal blood and damage to the canalicular system can result in escape of bile acids into the systemic circulation. Assay of systemic levels of bile acids is used clinically as a sensitive indicator of hepatic disease.

Pattern and Control of Bile Secretion:
The flow of bile is lowest during fasting, and a majority of that is diverted into the gallbladder for concentration. When chyme from an ingested meal enters the small intestine, acid and partially digested fats and proteins stimulate secretion of cholecystokinin and secretin. As discussed previously, these enteric hormones have important effects on pancreatic exocrine secretion. They are both also important for secretion and flow of bile:
Cholecystokinin: The name of this hormone describes its effect on the biliary system. Cholecysto means gallbladder and kinin means movement. The most potent stimulus for release of cholecystokinin is the presence of fat in the duodenum. Once released, it stimulates contractions of the gallbladder and common bile duct, resulting in delivery of bile into the gut.
Secretin: This hormone is secreted in response to acid in the duodenum. Its effect on the biliary system. It simulates biliary duct cells to secrete bicarbonate and water, which expands the volume of bile and increases its flow out into the intestine.