Chapter 24
7 – The Small Intestine
The Small Intestine
• The small intestine is a tube of the GI tract
that begins at the pyloric sphincter of the
stomach, and ends at the large intestine.
The Small Intestine
• The small intestine is divided into three
regions:
1. The duodenum
2. The jejunum
3. The ileum
The Small Intestine
1. The duodenum is the first part of the small
intestine between the stomach and the
jejunum.
The Small Intestine
2. The jejunum is the middle part of the small
intestine between duodenum and the ileum.
The Small Intestine
3. The ileum is the terminal part of the small
intestine between the jejunum and the large
intestine.
The Small Intestine
• **Recall – the four layers of the GI tract are:
the mucosa, the submucosa, the muscularis,
and the serosa.
The Small Intestine
• **Special structural features of the small
intestinal wall facilitate the process of
digestion and absorption.
The Small Intestine
• The special structural features of the small
intestinal wall include:
1. Circular folds
2. Villi
3. Microvilli
The Small Intestine
1. Circular folds are folds of the mucosa and the
submucosa that enhance absorption by
increasing surface area and causing the
chyme to spiral.
The Small Intestine
2. Villi are fingerlike projections of the mucosa
that increase the surface area available for
absorption and digestion.
The Small Intestine
3. Microvilli are microscopic fingerlike
projections of the mucosa that increase
digestive surface area and contain several
“brush-border” enzymes that have digestive
functions.
The Small Intestine
• **Digestion in the small intestine is both
mechanical and chemical.
The Small Intestine
• The two types of mechanical movements of
the small intestine are:
1. Segmentation
2. Migrating motility complex
The Small Intestine
1. Segmentation is the localized mixing and
contracting of the small intestine that mixes
chyme with digestive juices and brings the
particles of food in contact with the mucosa
for absorption.
The Small Intestine
– Sequences of segmentations cause chyme to
slosh back and forth, they do not push the
intestinal contents along the GI tract.
The Small Intestine
2. The migrating motility complex is a type of
peristalsis that occurs in the small intestine
that pushes chyme forward.
The Small Intestine
• **Recall – in the mouth, salivary amylase
begins the breakdown of starch; in the
stomach pepsin begins the break down of
proteins, & lingual and gastric lipases begin
the breakdown of triglycerides.
The Small Intestine
– Thus, chyme entering the small intestine contains
partially digested carbohydrates, proteins and
lipids.
The Small Intestine
• **Chemical digestion in the small intestine
completes the digestion of carbohydrates,
proteins, and lipids and is a collective effort of
pancreatic juice, bile and intestinal juice in
the small intestine.
The Small Intestine
• **Recall – ‘salivary amylase’ is an enzyme
secreted by the salivary glands that initiates
the breakdown of starch in the mouth.
The Small Intestine
• Pancreatic amylase secreted from pancreatic
acinar cells continues the digestion of
carbohydrates into disaccharides.
The Small Intestine
• **Carbohydrate digestion is completed in the
small intestine by three brush-border enzymes
secreted by microvilli:
1. Sucrase
2. Lactase
3. Maltase
The Small Intestine
1. Sucrase breaks down sucrose
2. Lactase breaks down lactose
3. Maltase breaks down maltose
The Small Intestine
• **Recall – ‘pepsin’ is an enzyme secreted by
the chief cells of the stomach that starts
fragmenting proteins into peptides in the
stomach.
The Small Intestine
• Trypsin and other enzymes secreted from
pancreatic acinar cells continue the digestion
of proteins into peptides in the small
intestine.
The Small Intestine
• **Protein digestion is completed in the small
intestine by two brush-border pepidases
secreted by micorvilli:
1. Aminopeptidase
2. Dipeptidase
The Small Intestine
1. Aminopeptidase is an enzyme secreted by
microvilli that cleaves off the amino acids at
the amino end of a peptide.
The Small Intestine
2. Dipeptidase is an enzyme secreted by
microvilli that splits dipetides into single
amino acids.
The Small Intestine
• **Recall – ‘lingual lipase & gastric lipase’ are
enzymes secreted by the tongue and the chief
cells of the stomach that start the digestion of
triglycerides in the stomach.
The Small Intestine
• Pancreatic lipase secreted from pancreatic
acinar cells completes the digestion of
triglycerides in the small intestine.
The Small Intestine
• **Recall – ‘bile’ is a secretion of the liver that
emulsifies lipids in the small intestine prior to
their digestion.
The Small Intestine
• **Emulsification must happen before
triglycerides are broken down by pancreatic
lipase into fatty acids and monoglycerides.
The Small Intestine
• **Recall – ‘pancreatic enzymes’ also include
“nucleases” nucleic acid digesting enzymes.
The Small Intestine
• The two nucleases contained in pancreatic
juice are:
1. Ribonuclease
2. Deoxyribonuclease
The Small Intestine
1. Ribonuclease is an enzyme secreted from
pancreatic acinar cells that digests RNA in
the small intestine.
The Small Intestine
2. Deoxyribonuclease is an enzyme secreted
from pancreatic acinar that digests DNA in
the small intestine.
The Small Intestine
• **All the chemical and mechanical phases of
digestion from the mouth through the small
intestine are directed at changing food into
forms that can pass through the absorptive
cells lining the mucosa.
The Small Intestine
• About 90% of all absorption of nutrients
occurs in the small intestine; the other 10%
occurs in the stomach and large intestine; any
unabsorbed materials left in the small
intestine passes on to the large intestine.
Homework
• Finish handout:
– “The Small Intestine”
• Study for mini-quiz