Lesson #5: Structure and Function: The Small Intestine

Question 1

The Basics

Answer 1

The small intestine is the major organ of digestion and absorption.

The small intestine is 7m long and attached to the abdominal wall by the
mesentary (contains the blood supply)

The small intestine is divided into 3 parts:

Duodenum – the first part of the small intestine. The major site of
digestion.

Jejunum – the second part of the small intestine. The longest section,
responsible for additional digestion and absorption.

Illeum – the final section of the small intestine. Absorbs remaining
nutrients from the chyme.

Question 2

The Duodenum

Answer 2

The duodenum is the first segment of the small intestine and the major site of
digestion in the body.

The chyme gets broken down further, both chemically and mechanically.

90% of digestion happens here with the help of some “accessory” organs.

3 digestive juices are active here:

Pancreatic Juice

Bile

Intestinal Juice

Question 3

Pancreatic Juice

Answer 3

Produced by the Pancreas

When the acidic chyme enters the duodenum, the acid triggers
the hormone Prosecretin to activate into Secretin.

Secretin triggers the pancreas to start releasing pancreatic juice
into the duodenum through the Pancreatic Duct
Pancreatic Juice contains:

Bicarbonate ions – act as a buffer and neutralize the acid in the chyme
(raises pH from 2-6)

Trypsinogen – activated into Trypsin in the duodenum and breaks large
polypeptides into smaller ones.

Pancreatic Amylase – breaks starch into maltose

Pancreatic Lipase – breaks fat molecules into fatty acids and glycerol

Question 4

Bile

Answer 4

Produced by the Liver and stored in the Gall Bladder.

THIS IS NOT AN ENZYME

When fat reaches the duodenum, the hormone Cholecystokinin (CCK) is
released.

CCK triggers the gall bladder to release bile into the duodenum through
the Bile Ducts.

Bile mechanically breaks down large fat molecules through emulsification.

The smaller the fat molecules are the easier it is for the enzymes to
break them down chemically into glycerol and fatty acids.

Question 5

Intestinal Juice

Answer 5

Produced by the duodenum

Intestinal cells secrete fluid that contains the following enzymes:

Intestinal Lipase – breaks fat globules into fatty acids and
glycerol

Erepsin – breaks proteins into amino acids

Disaccharases – break disaccharides into monosaccharides.

Question 6

The Mucosa

Answer 6

The inner lining of the small
intestine is called the mucosa.

The mucosa is arranged in
ridges called villi. The villi of the
small intestine increases the
surface area in order to allow
for more efficient absorption of
nutrients.

Question 7

The Villi

Answer 7

Each villus contains many structures
that aid in the digestion and absorption
of nutrients.

Cuboidal Cells – each villus is
lined with these cells that
contain microvilli to further
increase surface area for
absorption.

Epithelial Cells – produce
mucous that protects the
walls from digestive juices.
3. Krypts of Leiberkuhn – digestive glands found at the base of
the villi of the duodenum and jejunum that release intestinal
juice.

4. Lacteal – a lymph vessel that absorbs the fatty acids and
   glycerol.
5. Capillaries – nutrients move through the cuboidal cells into
   the capillaries. The capillaries absorb monosaccharides,
   amino acids, vitamins and minerals into the blood.

Question 8

Movement of the Small Intestine

Answer 8

When food is present, the small intestine is constantly moving. This peristalsis
has 4 main functions:

Squeeze chyme through the intestine

Mix chyme with digestive enzymes

Break down food particles mechanically

Speed up absorption by bringing the chyme into contact with mucosa.