The small intestine has four layers and is divided into three regions: the duodenum, jejunum, and ileum. Its interior walls contain circular folds, villi, and microvilli that greatly increase its surface area for nutrient absorption. The small intestine secretes enzymes like enterokinase in the duodenum as well as hormones like gastrin, cholecystokinin, and secretin from endocrine cells in response to food constituents. These secretions aid in digestion and regulate secretions from other organs to maximize nutrient absorption in the small intestine.

1. Secretions of Small
Intestine
BY
SYEDA BATOOL ABBAS

2. RECAP
Layers of Small intestine (4)
1)Mucosa ( Innermost )
Absorb nutrients from chyme
2)Submucosa
Provides blood vessels ,lymphatic vessels and
nerves to support mucosa on the surface.
3)Muscularis layer
Contracts and moves the small intestine
4)Serosa (outermost )
Continuous throughout and surrounds the
intestine.

3. RECAP
3 major regions of Small intestine:
1:Duodenum: ( 10 inches)
 Shortest region of small intestine
 Chyme and bile mix completing digestion
2:Jejunum: (3 feet)
 Middle section
 Serves as primary site of nutrient absorption
3:Ileum: (6 feet)
 Final section of small intestine
 Empties into large intestine
 Completes nutrient absorption

4.  The interior walls of the small intestine are tightly wrinkled into projections called
circular folds that greatly increase their surface area.
 Microscopic examination of the mucosa reveals that the mucosal cells are
organized into finger-like projections known as villi, which further increase the
surface area. Many villi are present on the surface of intestine.
 The cells on the surface of the mucosa also contain finger-like projections of their
cell membranes known as microvilli, which further increase the surface area of the
small intestine.
 Epithelial cells have a ‘brush like’ border
 Purpose:
All of these wrinkles and projections help to greatly increase the amount of contact
between the cells of the mucosa and chyme to maximize the absorption of vital
nutrients.
Each square inch of mucosa contains around 20,000 villi.
It is estimated that there are around 130 billion microvilli per square inch in the mucosa of the small intestine.
Circular folds
Villi
Microvilli
Crypts / glands
of Lieberkuhan
between villi.

5. Secretions from the intestine – enzymes
and hormones
Digestive enzymes: Small intestine does not secrete any digestive
enzymes ( comes from pancreas or found on enterocytes )
Except Enterokinase secreted from Duodenal mucosa
Hormones: secreted from endocrine cells in mucosa
Stimulated by activation of chemoreceptors in response to
constituents of food and act to stimulate production of digestive
secretions from other organs
Gastrin stomach ,duodenum , pancreas (G cells)
Cholecystokinin pancreas
Secretin pancreas

6. SECRETIONS OF SMALL INTESTINE
( focus: channels and cells )
Secretions are from cells
within the crypts of
Lieburkuhn and fall into two
groups:
 Secretions into the lumen
( from enterocytes)
Secretions into the blood
( from endocrine cells)

8. Secretion of Bicarbonate

9. Nature of Secretion:
 Mucus / alkali secretions : mucosal protection
 Aqueous secretions :
• Under local nervous control
• Some minor hormonal control
a) Secretin is a peptide hormone that regulates water homeostasis throughout
the body, and influences the environment of the duodenum by regulating
secretions in the stomach and pancreas. Secretin is produced in the S cells of the
duodenum, which are located in the intestinal glands.
b) CCK: Cholecystokinin is a peptide hormone of the gastrointestinal system
responsible for stimulating the digestion of fat and protein.

12. Summary – Points to remember !
 Mucus and fluid :
Involved in protection
and absorption.
 Hormones :
Control of pancreatic and
bile secretions.

13. HELPFUL REFERENCES !!!
 Video links:
1) Focuses on Villi and digestion of fats, proteins and carbs inside
http://playit.pk/watch?v=P1sDOJM65Bc
2) Focuses on Layers , major parts and nature of secretion in the intestine
http://playit.pk/watch?v=xu5jDCX2cHM
3) Intestinal crypt
http://playit.pk/watch?v=qq5k1sWqLO0
 Article links:
1) Article that has everything related to the small intestine
http://www.innerbody.com/image_digeov/dige10-new3.html