The small intestine is divided into three parts - the duodenum, jejunum, and ileum. The duodenum receives partially digested food from the stomach and digestive juices from the pancreas and gallbladder. The jejunum, which is around 2.5 meters long, contains villi that increase absorption of nutrients. The ileum, the final 3 meter section, absorbs vitamins and bile acids before connecting to the large intestine.

1. ANATOMY OF THE SMALL
INTESTINE
The small intestine (or small bowel) is the part of
the GI tract following the stomach and followed
by the large intestine, and is where much of the
digestion and absorption of food takes place. The
small intestine is composed of a duodenum,
jejunum, and ileum. It receives bile juice and
pancreatic juice through the pancreatic duct,
controlled by the sphincter of Oddi

2. STRUCTURE OF THE SMALL INTESTINE
• The average length of the small intestine in an
adult human male is 6.9 m (22 ft 8 in), and 7.1 m
(23 ft 4 in) in an adult female. It can vary greatly,
from as short as 4.6 m (15 ft) to as long as 9.8 m
(32 ft).[3][4] Recent studies indicate that small
intestine may be shorter, around 3.5 m (11 ft
6 in), and that the length is less affected by age
after childhood than expected[5]
• It is approximately 2.5–3 cm in diameter. The
surface area of the human small intestinal
mucosa averages 30 square meter [6]

3. THE SMALL INTESTINE IS DIVEDED
INTO THREE STRUCTUIRAL PARTS
• The small intestine is divided into three structural parts.
The duodenum is a short structure (about 20–25 cm long)
continuous with the stomach and shaped like a "C".[7] It
surrounds the head of the pancreas. It receives gastric
chyme from the stomach, together with digestive juices
from the pancreas (digestive enzymes) and the gall bladder
(bile). The digestive enzymes break down proteins and bile
and emulsify fats into micelles. The duodenum contains
Brunner's glands, which produce a mucus-rich alkaline
secretion containing bicarbonate. These secretions, in
combination with bicarbonate from the pancreas,
neutralizes the stomach acids contained in gastric chyme.

4. • The jejunum is the midsection of the small
intestine, connecting the duodenum to the
ileum. It is about 2.5 m long, and contains the
plicae circulares, and villi that increase its
surface area. Products of digestion (sugars,
amino acids, and fatty acids) are absorbed into
the bloodstream here. The suspensory muscle
of duodenum marks the division between the
duodenum and the jejunum.

5. • The ileum: The final section of the small
intestine. It is about 3 m long, and contains
villi similar to the jejunum. It absorbs mainly
vitamin B12 and bile acids, as well as any
other remaining nutrients. The ileum joins to
the cecum of the large intestine at the
ileocecal junction.

6. FUNCTION OF THE SMALL INTESTINE
• Food from the stomach is allowed into the duodenum through the
pylorus by a muscle called the pyloric sphincter.
• Digestion
• The small intestine is where most chemical digestion takes place.
Many of the digestive enzymes that act in the small intestine are
secreted by the pancreas and enter the small intestine via the
pancreatic duct. Pancreatic enzymes and bile from the gallbladder
enter the small intestine in response to the hormone
cholecystokinin, which is produced in the small intestine in
response to the presence of nutrients. Secretin, another hormone
produced in the small intestine, causes additional effects on the
pancreas, where it promotes the release of bicarbonate into the
duodenum in order to neutralize the potentially harmful acid
coming from the stomach

7. A DIAGRAM SHOWING DUODENUM,
JEJUNUM AND ILEUM