Accessory organs of digestive systems

Accessory Organs of Digestive System

By the end of this session, students are expected to be able to:

• Describe the Accessory Organs of Digestive System
• Describe the Structure and Functions of the Liver
• Describe the Structure and Functions of Gallbladder
• Explain Organization of the Biliary System
• Explain Bile Production, Circulation and Functions

Identify Structure of Pancreas

Overview of Accessory Organs of Digestive System

What are the accessory organs of the digestive system?

Accessory organs of digestive systems

Accessory organs of gastrointestinal tract: The organs which are not directly part of gastrointestinal tract but their products are essential for the digestive system to accomplish its primary function These organs include:

• The liver
• Gallbladder and bile duct

Pancreas

These are essential for the digestive system as they either produce or store secretions essential the digestion to take place

Structure and Functions of the Liver

• Liver is the largest gland in the body, weighing between 1 and 2.3Kg

In adults the liver weighs 2% of body mass Occupying the greater part the right hypochondriac, part of the epigastric and frequently extending into the left hypochondriac regions as far as the left lateral line The liver is divided into right and left lobes by fossae for the gallbladder and the inferior vena cava The right lobe of liver is the largest lobe, whereas the left lobe of liver is smaller wedge shaped The quadrate and caudate lobes are described as arising from the right lobe of liver, but functionally are distinct these are seen on the posterior side of the liver making it to have four lobes:

The Quadrate lobe

It is visible on the upper part of the visceral surface of the liver It is bounded on the left by the fissure for ligamentum teres and on the right by the fossa for the gallbladder Functionally it is related to the left lobe of the liver The caudate lobe

It is visible on the lower part of the visceral surface of the liver It is bounded on the left by the fissure for the ligamentum venosum and on the right by the groove for the inferior vena cava Functionally, it is separate from the right and the left lobes of the liver

Structures related to Liver

• The right anterior part of the stomach
• The superior part of the duodenum
• The lesser omentum
• The gallbladder
• The right colic flexure
• The right transverse colon
• The right kidney

The right suprarenal gland

Accessory organs of digestive systems

The porta hepatis serves as the point of entry into the liver for the hepatic arteries and the portal vein, and the exit point for the hepatic ducts The liver is attached to the anterior abdominal wall by the falciform ligament and, except for a small area of the liver against the diaphragm (the bare area), the liver is almost completely surrounded by visceral peritoneum Additional folds of peritoneum connect the liver to the stomach (hepatogastric ligament), the duodenum (hepatoduodenal ligament), and the diaphragm (right and left triangular ligaments and anterior and posterior coronary ligaments)

The connective tissue septa divide the liver into hexagon-shaped lobules with a portal triad at each corner

*The triad are so named because three vessels – the hepatic portal vein, hepatic artery and hepatic duct are commonly located in them Hepatic nerves and lymphatic vessels often too small to be easily seen in light microscope are also located in these areas Liver lobules are hexagonal in outline and are formed by cubical-shaped cells; the hepatocytes arranged in pair, between them are columns of cells called sinusoids Hepatic macrophages (Kupffer cells) are also present which function in destroy and ingest worn out cells and foreign particles in the liver A central vein is in the centre of each lobule

Accessory organs of digestive systems

Central veins unite to form hepatic veins, which exit the liver on its posterior and superior surface and empty into the inferior vena cava Hepatic cord radiate out from the central vein of each lobule like the spokes of a wheel The hepatic cords are composed of hepatocytes, the functional cells of the liver The spaces between the hepatic cords are blood channels called hepatic sinusoids The sinusoids are lined with a very thin, irregular squamous epithelium consisting of two cell populations; endothelial cells and Hepatic phagocytic cells (kupffer cells) A cleft-like lumen, the bile canaliculus lie between the cells within each cord

Functions of hepatocytes

What are the function of hepatocyte

Hepatocytes have six major functions, which are:

• o Bile production
• o Storage
• o Interconversion of nutrients
• o Detoxication
• o Phagocytosis (kuppfer cells)

o Synthesis of blood components

Main Functions of the Liver

• Nutrient and vitamin metabolism and storage

o Glucose and other sugars

o Amino acids (Deamination of amino acids),the end product is urea which is then excreted in the urine) o Lipids

• o Fatty acids
• o Cholesterol
• o Lipoproteins
• o Fat-soluble vitamins

o Water-soluble vitamins

Breakdown of erythrocytes and defence against microbes

• Detoxification of drugs and noxious substances
• Inactivation of hormones such as Steroids
• Production of heat
• Formation and Secretion of biles
• Synthesis of plasma proteins:
• -Acute-phase proteins
• Albumin
• -Clotting factor e.g fibrinogen, prothrombin, factor V,VII,IX,X,XII,
• Steroid-binding and other hormone-binding proteins

Immunity;

Vascular Supply and Lymphatic Drainage

The vessels connected with the liver are the portal vein, hepatic artery and hepatic veins The portal vein and hepatic artery ascend in the lesser omentum to the porta hepatis, where each bifurcates The hepatic bile duct and lymphatic vessels descend from the porta hepatis in the same omentum The hepatic veins leave the liver via the posterior surface and run directly into the inferior vena cava

Structure and functions of the Gallbladder

The gallbladder is a pear-shaped sac lying on the visceral surface of the right lobe of the liver in a fossa between the right and quadrate lobes It is about 8 cm long and 4 cm wide

The gallbladder has the following structures o A rounded end (fundus of gallbladder), which may project from the inferior border of the liver o A major part in the fossa (body of gallbladder), which may be against the transverse colon and the superior part of the duodenum o A narrow part (neck of gallbladder) with mucosal folds forming the spiral fold

Hepatobilliary system

Organisation of the Biliary System

Biliary system

Is the duct system for the passage of bile extends from the liver, connects with the gallbladder, and empties into the descending part of the duodenum The coalescence of ducts begins in the liver parenchyma and continues until the right and left hepatic ducts are formed These drain the respective lobes of the liver The two hepatic ducts combine to form the common hepatic duct, which runs, near the liver, with the hepatic artery proper and portal vein in the free margin of the lesser omentum As the common hepatic duct continues to descend, it is joined by the cystic duct from the gallbladder

This completes the formation of the bile duct

At this point, the bile duct lies to the right of the hepatic artery proper and usually to the right of, and anterior to, the portal vein in the free margin of the lesser omentum The omental foramen is posterior to these structures at this point The bile duct continues to descend, passing posteriorly to the superior part of the duodenum before joining with the pancreatic duct to enter the descending part of the duodenum at the major duodenal papilla Therefore the bile either empties directly into the duodenum or is diverted for minutes up to several hours through the cystic duct into the gallbladder !The clinical importance of this system is if there is a tumour of the head of pancrease will cause obstruction to the bile duct and cause a condition known as obstructive jaundice

Bile Production, Circulation and Functions

• The liver produces and secretes about 600-1000ml of bile each day
• Continuous bile formation is an important function of the liver

Bile is used as a vehicle for the secretion of bile acids Bile salts are quantitatively the major constituents of bile and are circulated in the enterohepatic circulation between the liver and the small intestine with high efficiency Synthesis of bile acids is a major route of cholesterol metabolism Bile is synthesised through a series of chemical reactions in hepatocytes then secreted into minute bile canaliculi that originate between the hepatic cells

Bile circulation.

• Bile is produced by hepatocytes in the liver

Bile draining through the many bile ducts that penetrate the liver During this process, the epithelial cells add a watery solution that is rich in bicarbonates that dilutes and increases alkalinity of the solution Bile then flows into the common hepatic duct, which joins with the cystic duct from the gallbladder to form the common bile duct The common bile duct in turn joins with the pancreatic duct to empty into the duodenum If the sphincter of Oddi is closed, bile is prevented from draining into the intestine and instead flows into the gallbladder, where it is stored and concentrated to up to five times its original potency between meals This concentration occurs through the absorption of water and small electrolytes, while retaining all the original organic molecules Cholesterol is also released with the bile, dissolved in the acids and fats found in the concentrated solution.

Accessory organs of digestive systems

Bile plays an important role in fat digestion and absorption due to the presence of bile acids by do the following;

o They help to emulsify the large fat particles of the food into many minute particles, the surface of which can then be attacked by lipase enzymes secreted in pancreatic juice.

o They aid in absorption of the digested fat end products through the intestinal mucosal membrane.

Bile serves as a means for excretion of several important waste products from the blood.

These include especially bilirubin, an end product of haemoglobin destruction, and excesses of cholesterol

Composition of Bile

• Water constitute 97.0%, in the gall bladder the amount is reduced 5 folds
• Bile pigments 0.2%

Cholesterol0.06%

• Inorganic salts 0.7% such as Na+, K+& Ca2+ salts Cl- HCO3 & Phosphorus
• Fatty acids 0.15%

Lecithin 0.1%

Fat 0.1%

• Alkaline phosphatase

proteins

Structure and Functions of Pancreas

• The pancreas lies mostly posterior to the stomach

It is elongated,tapered flat structure (flat-fish like) It extends across the posterior abdominal wall from the duodenum, on the right, to the spleen, on the left It is 12 to 15 cm long; weighs approximately 60gm Composed of endocrine and exocrine glandular tissue o Exocrine portion makes up majority of pancreas; has a compound acinar arrangement and tiny ducts which unite to form the main pancreatic duct, which empties into the duodenum It secretes pancreatic juice which consists of pancreatic enzymes o Endocrine portion – embedded between exocrine units; called pancreatic islets; constitute only 2% of the total mass of the pancreas; made up of alpha cells and beta cells which produce glucagon and insulin respectively and pass its secretions into capillaries

In the lower part of the head of pancreas, the pancreatic duct joins the bile duct

The joining of these two structures forms the hepatopancreatic ampulla (Ampulla of Vater), which enters the descending part of the duodenum at the major duodenal papilla Surrounding the ampulla is the sphincter of ampulla (sphincter of Oddi), which is a collection of smooth muscle The accessory pancreatic duct empties into the duodenum just above the major duodenal papilla at the minor duodenal papilla If the accessory duct is followed from the minor papilla into the head of the pancreas, a branch point is discovered o One branch continues to the left, through the head of the pancreas, and may connect with the pancreatic duct at the point where it turns inferiorly o A second branch descends into the lower part of the head of pancreas, anterior to the pancreatic duct, and ends in the uncinate process

The main and accessory pancreatic ducts usually communicate with each other

The presence of these two ducts reflects the embryologic origin of the pancreas from dorsal and ventral processes The blood supply; the arterial supply is through pancreatic arteries the branches of splenic artery, gastro-duodenal and superior mesenteric arteries Venous drainage follows arterial supply Splenic vein drains the body and tail Innervation of Pancreas is from sympathetic and parasympathetic nerves of coelic and superior mesenteric ganglion of thoracic plexus and vagus nerve Note: Digestive enzymes produced by pancreas will be discussed in Session: 24 of this module.

Key points

Accessory organs of the gastrointestinal tract are the liver, gallbladder, bile duct and pancreas The liver is divided into right and left lobes by fossae for the gallbladder and the inferior vena cava Bile contains water, bile pigments, cholesterol ,inorganic salts such as Na+, K+& Ca2+ salts Cl- HCO3 & Phosphorus ,fatty acids, lecithin, fat ,Alkaline phosphatase, proteins Pancreas composed of endocrine and exocrine glandular tissue

evaluation

• What are the accessory organs of digestive system?
• What are the components of bile?
• What are the functions of the pancreas?

What are the functions of gallbladder?