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Digestion
Chapter 8
Section 8.4 - 1
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Small Intestine
• Small Diameter – 2.5cm
• 4X longer than Large Intestine
• Physical Digestion = Segmentation
• Segments = bands of circular muscle that
briefly contract, chopping food into
segments.
• Peristalsis helps food move along.
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Main Function of Small Intestine
• Fully digest macromolecules and
absorb its subunits for use in the body.
• Macromolecules are digested by
hydrolysis (adding water).
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Regions/Structures
• Small intestine can be divided into 3
regions:
1. Duodenum
2. Jejunum
3. Ileum
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Duodenum
• The first 25 – 30 cm of small intestine.
• U – Shaped.
• Shortest and widest part.
• Channels from LIVER and PANCREAS
converge into ONE CHANNEL that
enters here.
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Duodenum Con’t
• Its Function: To increase surface area for
digestion.
• To increase surface area it has many hills and
ridges.
• ON TOP OF RIDGES are VILLI (s. villus)
– Finger-like projections that extend into small intestine to
further increase surface area.
• MICROVILLI on top of villi
– Microscopic villi (looks fuzzy) that also help to increase
surface area.
Ridges of Duodenum
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Duodenum Con’t.
• Each villus has a capillary network and lymph
vessels called lacteals.
• They conduct absorbed substances from small
intestine into the bloodstream and lymphatic
system.
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Jejunum
• 2.5m long
• More folds and secretory
glands than duodenum.
• Continues to break down
food.
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Ileum
• 3m long
• Contains fewer and
smaller villi.
• Absorbs nutrients.
• Pushes undigested food
into large intestine.
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Accessory Organs
• There are 3 accessory organs to the small intestine:
PANCREAS, LIVER and GALL BLADDER.
• Their purpose is to secrete (produce) substances that aid
in the digestion of food.
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Pancreas
• Secretes ~1L of fluid to duodenum
everyday.
• Pancreatic fluid contains many
enzymes:
– Proteases
– Carbohydrases
– Lipase
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Pancreatic Enzymes
• Proteases
–
–
–
–
Digests proteins
2 proteases: Trypsin and Erepsin
Trypsin is activated by the enzyme Enterokinase
Enterokinase changes Trypsinogen  Trypsin
• Carbohydrases
– Digests carbohydrates
– 1 carbohydrase: Pancreatic Amylase
• Lipase
– Digests lipids or fat
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Pancreatic Enzymes
• They are secreted in their inactive form.
• They are activated by other enzymes
secreted by the duodenal lining.
• Pancreas also secretes a bicarbonate
(HCO3-)
– Neutralizes the HCl and chyme from the
stomach that enters the small intestine.
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Liver
• Largest organ in body.
• Main secretion: BILE
– Bile is the liver’s waste product that will
be excreted in feces eventually.
– It assists lipases in small intestine by
breaking down fats.
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How does bile help lipase?
• Bile disperses large fat droplets into smaller
droplets.
• This creates a greater surface area for
lipase to act on.
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Gall Bladder
• Stores bile sent from liver in
between meals.
• Fat-containing chyme (thick liquid
from stomach) in duodenum
stimulates gall bladder to
CONTRACT.
• This causes bile to be injected into
the duodenum.
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