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Secretions of the Digestive System
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Tonye A. Ogele
This work is produced by OpenStax-CNX and licensed under the
†
Creative Commons Attribution License 3.0
Abstract
By the end of this section you will be able to: State the constituents of the secretions of the digestive
system State the functions of each of the constituents of the secretions of the digestive system State the
contributions of the secretions to the working of the digestive system
INTRODUCTION
The main secretions of the digestive system are saliva, gastric juice, bile and pancreatic juice. Aside
these there are also other secretions. Oesophageal glands in the wall of the oesophagus, for example, secrete
lubricating mucus that coats the inner surface of the oesophagus. The intestinal glands secrete the intestinal
juice which contains water, mucus and mineral salts. Mucus is also secreted to lubricate the anal canal
during defaecation.
SALIVA
Saliva is a liquid secreted into the oral cavity by the salivary glands. Saliva has a pH of 6.8 to 7. Thus
it can be said to be mildly acidic. It is made up of 97 to 99.5% water and contains the following:
• Salivary amylase which is an enzyme that digests starch
• Lingual lipase which is an enzyme that digests fat
• Mucus which moistens, binds and lubricates food to aid swallowing. It also buers the pH of the
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food, i.e. if ensures that the acidity of the food does not aect that of the oral cavity.
Lysosyme which is an enzyme that kills bacteria
Immunoglobin A (IgA) which is an antibody that inhibits bacteria growth
Electrolytes like sodium, potassium, chloride, phosphate and bicarbonate salts.
Blood-clotting factors
Aside these functions of its constituents, saliva also moistens the mouth, cleans the teeth and dissolves
molecules so that they can stimulate the taste buds. About 1.5 litres of saliva is produced daily.
GASTRIC JUICE
The gastric glands in the wall of the stomach secrete 2 to 3 litres of gastric juice daily. Gastric juice is
highly acidic (pH 0.8) and consists of:
• Water
• Mineral salts
• Mucus which acts as a barrier between the wall of the stomach and the corrosive contents of gastric
juice.
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• Hydrochloric acid (HCl) which activates pepsin and lingual lipase, liquees the food by breaking
down tissues, converts ferric ions (Fe3+ ) to ferrous ion (Fe2+ ) for absorption, and destroys microorganisms in food.
• Intrinsic factor which binds with vitamin B12 so that vitamin B12 can be absorbed in the intestine.
Without the intrinsic factor, vitamin B12 cannot be absorbed.
• Pepsinogen which is a zymogen (inactive form) of the enzyme pepsin. Hydrochloric acid (HCl)
converts pepsinogen to pepsin. Pepsin then digests proteins.
In infants the gastric juice also contains gastric lipase and chymosin (also called renin). Gastric lipase
digests some of the butter fat of milk while chymosin coagulates the proteins in milk. Chymosin is therefore
said to curdle milk.
BILE
The liver secretes 500 to 1,000 ml of bile daily. Bile is a yellow-green uid that is secreted into the left
and right hepatic ducts. It then ows into the common hepatic duct and then into the bile duct (gure 1).
When bile is not being secreted into the small intestine (duodenum), bile lls the bile duct and ows into
the gall bladder where it is stored. The gall bladder absorbs water and electrolytes from bile and therefore
concentrates it 5 to 20 times.
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Figure 1:
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The gall bladder and associated ducts.
Bile has a pH of 8 and contains water, minerals, cholesterol, mucus, neutral fats, phospholipids, bile
pigments and bile salts. Except the bile salts, all other components of bile are waste products that are to
be excreted in the faeces.
Bile salts are synthesized from cholesterol. They emulsify fat i.e. they break down fat globules into
smaller droplets that can be easily digested by enzymes. 80% of the bile salts are reabsorbed in the ileum
and returned to the liver, while the rest 20% is excreted in the faeces.
The main bile pigment is bilirubin. Bilirubin is a product of the decomposition of haemoglobin. Bilirubin
is converted to urobilinogen by bacterial in the large intestine. Urobilinogen gives faeces its brown colour
(without bile, faeces is grayish).
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PANCREATIC JUICE
The exocrine part of the pancreas secretes 1,200 to 1,500 ml of pancreatic juice daily. The pancreatic
juice is an alkaline mixture (pH 8) of
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Water
Pancreatic amylase which is an enzyme that digests starch
Pancreatic lipase which is an enzyme that digests fat
Ribonuclease which is an enzyme that digests ribonucleic acids (RNA)
Deoxyribonuclease which is an enzyme that digests deoxyribonucleic acids (DNA)
Trypsinogen which is a zymogen (inactive form) of the enzyme typsin. An enzyme on the surface
of the epithelial cells of the small intestine called enterokinase converts trypsinogen to trypsin.
• Chymotrypsinogen which is a zymogen (inactive form) of the enzyme chymotrypsin. Trypsin
converts chymotrypsinogen to chymotrypsin.
• Procarboxypeptidase which is a zymogen (inactive form) of the enzyme carboxypeptidase. Trypsin
converts procarboxypeptidase to carboxypeptidase.
• Sodium bicarbonate which neutralizes hydrochloric acid (HCl) entering the small intestine from the
stomach
• other electrolytes.
PICTURE ATTRIBUTIONS
Figure 11 Lukesanatomy (2013, January 28). Gross Gallbladder anatomy and local ducts. Retrieved
from Wikipedia Website: http://en.wikipedia.org/wiki/File:GallbladderAnatomy.png1
REFERENCES
OpenStax College (2013, May 10). Digestive systems. Retrieved from the Connexions Website: http://cnx.org/content/m4
OpenStax College (2013, April 8). Digestive system. Retrieved from the Connexions Website: http://cnx.org/content/m45
Saladin, K. S. (2007) Anatomy and physiology: The unit of form and function. (4th ed.) New York:
McGraw-Hill.
Seeley, R. R., Stephens, T. D. & Tate, P. (2002) Essentials of anatomy and physiology (4th ed.) New
York: McGraw-Hill.
Waugh, A. & Grant, A. (2006) Ross and Wilson anatomy and physiology in health and illness (10th ed.)
Philadelphia: Churchill Livingstone Elsevier.
1 http://en.wikipedia.org/wiki/File:GallbladderAnatomy.png
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