Unit 6
Digestive System
Session 21
Accessory Digestive Organs
Session Outline
Introduction
21.1 Salivary glands
21.2 Liver and gall bladder
21.3 Biliary system
21.4 Pancreas
Summary
Learning Outcomes
Introduction
The process of digestion is aided by the secretion of mucus, many enzymes, and enzyme
precursors. The final digested nutrients are absorbed into the blood stream and are taken up by the
liver
SALIVARY GLANDS (Fig.12.18)
21.1 Salivary glands
Salivary glands pour their secretions into the mouth. There are three pairs: the parotid glands, the
submandibular glands and the sublingual glands.
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Fig. 21.1 Salivary glands
Parotid glands
These glands are located one on each side of the face just below the external acoustic meatus.
Each gland has a parotid duct opening into the mouth at the level of the second upper molar
tooth.
Submandibular glands
These lie one on each side of the face under the angle of the jaw. The two submandibular ducts
open on the floor of the mouth, one on each side of the lower mucosal attachment.
Sublingual glands
These glands lie under the mucous membrane of the floor of the mouth in front of the
submandibular glands.
They have numerous small ducts that open into the floor of the mouth.
Structure of the salivary glands
The glands are all surrounded by a fibrous capsule. They consist of a number of lobules made up
of small acini lined with secretory cells. The secretions are poured into ductules which join up to
form larger ducts leading into the mouth.
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Nerve supply
The glands are supplied by parasympathetic and sympathetic nerve fibres. Parasympathetic
stimulation increases secretion, whereas sympathetic stimulation decreases it.
Blood supply
Arterial supply is by various branches from the external carotid arteries and venous drainage is
into the external jugular veins.
Composition of saliva
Saliva is the combined secretions from the salivary glands and the small mucus-secreting glands
of the lining of the oral cavity. About 1.5 litres of saliva is produced daily and it consists of:
Fig.21.2 Organs of the gastrointestinal system
21.2 Liver and Gallbladder
Structure of liver and biliary system
Liver is both secretory and excretory organ. It is the largest Gland in the body. It weights about
1.5 Kg in man . It is located in the upper and right side of the abdominal cavity immediately
beneath diaphragm.
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Liver
Liver is made up of liver cells called heapatocytes and a system of blood vessles. Liver consists
of many lobes. Each lob consists of large number is lobules. Each lobule is a honey comb like
structure. The hepatocytes are arranged in different plates. Each plate is one cell thick with a
central vein. In between the cells are bill canaliculi. Each lobule is surounded by portal triads.
Each portal triad consists of a branch of hepatic artery a branch of portal vein and a tributary of
bile duct .In between the plates, the sinusoid receives blood from a branch triad. Sinusoids are
lined by endothelial cells. Few between cells called Kupffer’s cells are also found in between the
endothelial cells.
BLOOD SUPPLY TO LIVER
Liver receives blood from two sources namely, the hepatic artery and portal vein.
Functions of liver

Secretion of bile by the hepatocytes into the billiary canaliculi.

Storage of glycogen and vitamin B12, A ad D. It also stores iron.

Synthesis of almost all the plasma proteins except gamma globulin.

Excretion of cholesterol and bile pigments.

Haemopoetic function in fetal life.

Detoxification of certain hormones and drugs

Reticuloendothelial functions by the kupffer’s cells.
Liver and bile
The bile secreted by the hepatocytes in the liver enters the gall bladder through the cystic duct
and gets stored. During the storage time there will be certain alteration in the volume and
composition of the bile in the gall bladder. The contraction of the gall bladder will empty the bile
into the intestine through the bile duct.
Secretion of bile is continuous one whereas the emptying of bile into the intestine is intermittent.
Bile secreted from the hepatocytes reach the gall bladder and get stored. In the gall bladder there
is reabsorption of water and hence the volume of bile is reduced from the initial 2000 ml/day to
about 500 ml/day that is reaching the intestine. While it is stored in the gall bladder bile becomes
a little more acidic due to the absorption of HCO3 ions and walls of the gall bladder also secrete
mucus. All these aspects constitute the functions of the gall bladder.
21.3 Biliary system
The Biliary system is formed by gallbladder and the ducts called extrahepatic bile ducts. The bile
secreted in the hepatic cells is poured into a thin canaliculus called bile canaliculus. Few
canaliculi unite to form small ducts, which finally form left and right hepatic ducts. The hepatic
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ducts join to form common hepatic duct which subsequently binds to cystic duct from gallbladder
to form common bile duct.
Right and left hepatic duct, cystic duct and common bile duct are structurally similar. The
common bile duct unites with pancreatic duct forming the common hepatopancreatic duct, which
is otherwise known as Ampulla of Vater. Ampulla of Vater opens into the duodenum.
There some smooth muscle fibres at the lower part of common bile duct before it joins the
pancreatic duct .These muscle fibres form the sphincter of Oddi, Which plays an important role in
the storage of bile in gall bladder. Because of the closure of this sphincter , the bile from liver is
filled in gallbladder.
Functions of the gall bladder are;
1.
2.
3.
4.
Storage of bile.
Concentration of bile
Expulsion of bile.
Acidification of bile
PROPERTIES AND COMPOASITION OF BILE
Bile is a golden yellow or greenish fluid. It is poured into the digestive tract along with pancreatic
juice through the common opening called ampula of Vater.
PROPERTIES OF BILE
Volume
:800 to 200 ml/day
Reaction
: Alkaline
PH
: 8 to 8.6
Specific gravity
: 1010 to 1011
COMPOSITION OF BILE
Bile contains of 97.6% of water and 2.4% of solids. Solids are organic and inorganic substances.
Functions of bile salts
 Emulsification of fats is brought about by decreasing the surface tension.
 Hydrotropic action; Micelle formation allows fat to be made water – soluble and hence help
for absorption of fat – soluble vitamins through the through the entreocytes into the lacteals.
 Keeps the cholesterol in solution; this prevents the formation of gall stones.
 Bactericidal action; Bile salts do have some amount of bactericidal action and kill be the
bacteria in the intestinal regions.
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 Bile, secreted by the liver, is unable to enter the duodenum when the hepatopancreatic sphincter
is closed; therefore it passes from the hepatic duct along the cystic duct to the gall bladder where
it is stored. Bile has a pH of 8. About 500 - 1000 ml of bile is secreted daily. It consists of
water, mineral salts, mucus, bile salts , bile pigments, mainly bilirubin and cholester
21.4 Pancreas
The pancreas is a pale grey gland weighing about 60 grams. It is situated in the epigastric and left
hypochondriac regions of the abdominal cavity. It consists of a broad head, a body and a narrow
tail. The head lies in the curve of the duodenum, the body behind the stomach and the tail lies in
front of the left kidney and just reaches the spleen. The abdominal aorta and the inferior vena
cava lie behind the gland. The pancreas is both an exocrine and endocrine gland.
Fig 21.3 Pancreas and related structure
The exocrine pancreas
This consists of a large number of lobules made up of small alveoli, the walls of which consist of
secretory cells. Each lobule is drained by a tiny duct and these unite eventually to form the
pancreatic duct, which extends the whole length of the gland and opens into the duodenum. Just
before entering the duodenum the pancreatic duct joins the common bile duct to form the
hepatopancreatic ampulla. The duodenal opening of the ampulla is controlled by the
hepatopancreatic sphincter (of Oddi). The function of the exocrine pancreas is to produce
pancreatic juice containing enzymes that digest carbohydrates.
Pancreatic juice
Pancreatic juice enters the duodenum at the hepatopancreatic ampulla and it consists of
mainly water, mineral salts, enzymes (amylase and lipase), inactive enzymes & its
precursors ( trypsinogen, chymotrypsinogen, procarboxypeptidase). Pancreatic juice
contains bicarbonate and is alkaline (pH 8).
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When acidic chyme enters the duodenum from the stomach, it is mixed with pancreatic
juice and bile and the pH rises to alkaline levels. At this pH the pancreatic enzymes,
amylase and lipase, act most effectively.
Self-Assessment Questions
Name the main salivary glands?
Describe the basic structure of the liver lobule?
List the basic functions of the liver?
List the basic functions of the gallbladder?
Describe the exocrine functions of the pancreas?
Summary
In this session you learnt about the organs of the digestive tract and their main structural features.
You learnt about the features of the their functions and importance of them in digestion. Also you
learnt about how these functions are coordinated.
Learning Outcomes
After studying this section, you should be able to describe the structure and the function of the
principal salivary glands explains the role of saliva in digestion.
List the main organs of the alimentary tract.
List the accessory organs of digestion
Discuss the digestive functions of the small intestine and its secretions
Explain how nutrients are absorbed in the small intestine
Identify the different sections of the large intestine
Describe the structure and functions of the large intestine, the rectum and the anal canal.
After studying this section, you should be able to:
Describe the location of the liver in the abdominal cavity
Describe the structure of a liver lobule
List the functions of the liver.
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