The small intestine or small bowel is an organ in the gastrointestinal tract where most of the absorption of nutrients from food takes place. It lies between the stomach and large intestine, and receives bile and pancreatic juice through the pancreatic duct to aid in digestion. The small intestine is about 5.5 metres (18 feet) long and folds many times to fit in the abdomen. Although it is longer than the large intestine, it is called the small intestine because it is narrower in diameter. The small intestine has three distinct regions – the duodenum, jejunum, and ileum. The duodenum, the shortest, is where preparation for absorption through small finger-like protrusions called villi begins.[2] The jejunum is specialized for the absorption through its lining by enterocytes: small nutrient particles which have been previously digested by enzymes in the duodenum. The main function of the ileum is to absorb vitamin B12, bile salts, and whatever products of digestion that were not absorbed by the jejunum.

2. Small Intestine
Dr Mathew Joseph MBBS, MD(AIIMS), BCC(Palliative Medicine)
Assistant Professor
Department of Anatomy
Amala Institute of Medical Sciences, Thrissur

3. Small Intestine

6. SMALL INTESTINE
The small intestine extends from the pylorus to the ileocaecal junction. It is
about 6 metres long.
It is divided into:
1. An upper, fixed part, called the duodenum, which measures about 25
cm in length; and
2. A lower, mobile part, forming many convolutions
The upper two-fifths of the mobile intestine are known as the jejunum,
and the lower three-fifths are known as the ileum.

7. RELEVANT FEATURES
Large Surface Area
a. The great length of the intestine.
b. The presence of circular folds of mucous membrane, villi and microvilli.
Intestinal Glands
The duodenal glands or Brunner’s glands lie in the submucosa.
Lymphatic Follicles
Solitary lymphatic follicles
Aggregated lymphatic follicles or Peyer’s patches.

8. Arterial Supply
The arterial supply to jejunum and ileum is derived from the jejunal and ileal
branches of the superior mesenteric artery.
Lymphatics
The lymphatics (lacteals) have a circular course in the walls of the intestine.
Tubercular ulcers and subsequent strictures are due to involvement of these
lymphatics.
Nerve Supply
The nerve supply of the small intestine is sympathetic (T9 to T11) as well as
parasympathetic (vagus), both of which pass through the coeliac and superior
mesenteric plexuses.

9. Duodenum
1. First or superior part, 5 cm or 2 inches long.
2. Second or descending part, 7.5 cm or 3 inches long.
3. Third or horizontal part, 10 cm or 4 inches long.
4. Fourth or ascending part, 2.5 cm or 1 inch long.

10. Parts of the duodenum

11. Relations of First Part
Relations of the first part of the duodenum: (a) Sagittal section
viewed from the left side, and (b) posterior relations

12. Relations of Second Part
Relations of the second part of the duodenum: (a) Anterior
relations, and (b) posterior relations

13. • The interior of the second part of the duodenum shows the following
special features.
• The major duodenal papilla is an elevation present posteromedially, 8 to
10 cm distal to the pylorus.
• The minor duodenal papilla is proximal to major duodenal papilla
Relations of Third Part
Relations of Fourth Part
SUSPENSORY MUSCLE OF DUODENUM OR LIGAMENT OF TREITZ
Composed of smooth muscle fibres skeletal fibres and elastic fibres.
Arterial Supply of Duodenum (See the Figure)

14. Relations of the third part of the duodenum: (a) Anterior
relations, and (b) posterior relations

15. Posterior relations of the fourth part of the duodenum

16. Suspensory muscle of the duodenum

17. Arterial supply of the duodenum

18. JEJUNUM AND ILEUM
Features
The jejunum and ileum are suspended from the posterior abdominal wall by
the mesentery.
The differences between the jejunum and the ileum are given in Table 20.1.

20. MECKEL’S DIVERTICULUM (Diverticulum Ilei)
1. It occurs in 2% subjects.
2. Usually it is 2 inches or 5 cm long.
3. It is situated about 2 feet or 60 cm proximal to the ileocaecal valve,
attached to antimesenteric border of the ileum.
4. Its calibre is equal to that of the ileum.

26. Basic Histological Layers
1. Mucosa:
a. Epithelium
b. Lamina Propria
c. Muscularis Mucosae
2. Submucosa:
Submucosal plexus
“Plexus of Meissner”
3. Muscularis:
Myenteric plexus
“Plexus of Auerbach”
4. Serosa

27. Small Intestine
It is divided into duodenum, jejunum and
ileum.
• Mucosa: characteristic features-
Plicae circularis (valves of Kerkring)
Villi & Microvilli
Goblet cells (few)
Crypts of Lieberkuhn (intestinal glands)
Glands are lined by columnar cells, goblet
cells, Paneth cells & enteroendocrine cells

30. Types of Cells

31. Layers of Small Intestine
•Mucosa: E,LP,MM
•Submucosa: contains
blood vessels, lymphatics
and Meissner’s plexus.
•Muscularis externa: Outer
longitudinal and inner
circular layers of smooth
muscle.
•Serosa/Adventitia

32. Duodenum
Functions
• Villi have absorptive function
• Microvilli increase the surface area
of absorption
• Muscularis externa helps in churning
food particles i.e peristalsis
• Serosa is supportive and protective
in function
• Brunner’s glands secrete alkaline
fluid rich in HCO3‾
1. Brunner’s glands present in
submucosa
2. Numerous leaf-shaped villi
3. Goblet cells
4. Crypts of Lieberkuhn present

33. Duodenum
Presence of Brunner’s glands in
submucosa

37. Jejunum
Functions
• Villi have absorptive function
• Microvilli increase the surface area
of absorption
• Muscularis externa helps in churning
food particles i.e peristalsis
• Serosa is supportive and protective
in function
• Tongue shaped villi lined by simple
columnar epithelium.
• Absence of brunner’s glands in
submucosa.
• Crypts of Lieberkuhn present

41. Ilieum
Functions
• Villi have absorptive function
• Microvilli increase the surface area
of absorption
• Muscularis externa helps in churning
food particles i.e peristalsis
• Serosa is supportive and protective
in function
• Peyer's patches have an
immunologic function
• Finger-like villi lined by simple
columnar epithelium.
• Villi rich in goblet cells.
• Crypts of lieberkuhn present.
• Mucosa containing Peyer’s patches

45. Histology of the Mucosa
Organ Epithelium
Mouth Nonkeratinized Stratified Squamous
Pharynx Nonkeratinized Stratified Squamous
Esophagus Nonkeratinized Stratified Squamous
Stomach Simple Columnar
Small Intestine Simple Columnar
Large Intestine Simple Columnar
Anus Nonkeratinized Stratified Squamous

46. Histology of the Mucosa
Organ Folds of the epithelium
Esophagus none
Stomach Rugae, Gastric pits
Small Intestine Plicae circulares, Villi, Crypts of
Lieberkuhn, microvilli
Large Intestine Haustra , Intestinal glands

47. Histology of the Submucosa
Organ Specialized structures
Esophagus Submucosal mucous glands
Stomach None
Duodenum Brunner’s glands
Ileum Peyer’s Patches
Large Intestine None

48. Histology of the Muscularis
Organ Smooth muscle layers
Esophagus 2, circular and longitudinal
Stomach 3, oblique, circular, and longitudinal
Small Intestine 2, circular and longitudinal
Large Intestine 2, circular and longitudinal

49. Histology of the Serosa
Organ Serosa
Esophagus Adventitia due to the fact that the
esophagus is not in a cavity
Stomach Visceral Peritoneum
Small Intestine Visceral Peritoneum
Large Intestine Visceral Peritoneum
Anus Adventitia

50. Thank You