Transcript Small and Large Intestines

Small and Large Intestines
J. Gilbert
4th March 2004
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The Wall of the Small Intestine
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Small Intestine
Functions:
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Digestion: neutralise acid from stomach, add digestive
enzymes and bile, break proteins, carbohydrates and
lipids to absorbable materials
Absorption: 95% of food absorbed here
Structure
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Regions: duodenum, jejunum, ileum
Mucosa adaptations: villi containing blood and lacteal
capillaries
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Major enzymes of digestion
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Accessory Organs: Aid Digestion
and Absorption
Pancreas: exocrine functions
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Secretes digestive enzymes and sodium bicarbonate
Liver
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Produces bile (acts as emulsifier – begins fat
breakdown
Hepatic portal system: drains blood from digestive tract
Metabolic functions: storage, synthesis, chemical
processing
Gallbladder: stores bile
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From the duodenum…
The intestinal epithelium secretes sodium
and chloride ions into the lumen
Results in a water potential gradient
Water potential in the first part of the lumen of
the SI is lower than epithelial cells
 Water moves into lumen by osmosis

Na+ Cl- water
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Sodium Ions
All cells are required to maintain a low
intracellular concentration of sodium.
This is maintained in the intestinal epithelial cells
by sodium pumps embedded in the membrane of
the microvilli
In rats, there are about 150,000 sodium pumps per
SI epithelial cell, this means that 4.5 billion
sodium ions move out of each cell per minute.
This flow and accumulation of sodium is
ultimately responsible for absorption of water,
amino acids and glucose.
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Absorption of Proteins and
Carbohydrates
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Absorption of Glucose
Glucose is absorbed from the lumen by
active transport
This requires specific carrier molecules
(sodium dependent glucose transporters) in
the plasma membrane of epithelial.
 These proteins transport both sodium and
glucose into the cell
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Absorption of Glucose
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Absorption of Glucose
Once inside the SI epithelium, glucose and sodium
must be exported from the cell into the blood.
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Glucose diffuses down the concentration gradient
(facilitated diffusion) into the capillary blood in the
villus
Na+ diffuses into the blood via sodium-potassium
pumps (this requires ATP)
Other monosaccharides such as fructose are not
absorbed by active transport
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This only requires facilitated diffusion
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Absorption of Amino Acids
Similar to that of glucose (active transport)
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However sodium-dependent amino acid
transporters are required
Absorption of amino acids is dependent on
the gradient of the sodium ions
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So far…
Absorption of glucose and amino acids is
dependent on the sodium ion gradient.
Active transport from lumen into epithelial cell
Sodium is pumped out of cell into blood to help
maintain concentration gradient
Facilitated diffusion of glucose and amino acids
from epithelium into the blood
Blood travels from the gut to the liver via the
hepatic portal vein
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Absorption of Fats
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Emulsification of Fats
Bile salts play a critical role in lipid assimilation
by promoting emulsification
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Bile salts have hydrophobic domain that intercalate into
the lipid.
They also have a hydrophilic domain that remains at the
surface
This aids in the breakdown
of large aggregates or droplets
into smaller and smaller droplets
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Hydrolysis of Fats
Hydrolysis of triglyceride into monoglyceride and
free fatty acids is accomplished predominantly by
pancreatic lipase.
Lipase is a water-soluble enzyme, and with a little
imagination, it's easy to understand why
emulsification is a necessary prelude to its
efficient activity.
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Formation of Micelles
As monoglycerides and fatty acids are liberated
through the action of lipase, they retain their
association with bile salts and complex with other
lipids to form structures called micelles.
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Micelles are essentially small aggregates of mixed
lipids and bile salts suspended within the ingesta.
As the ingesta is mixed, micelles bump into the brush
border and the lipids, including monoglyceride and
fatty acids, are absorbed.
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Absorption of hydrolysed lipids
Lipids are absorbed by a mechanism distinctly
different from what we've seen for
monosaccharides and amino acids.
The major products of lipid digestion - fatty acids
and monoglycerides - enter the SI epithelium by
simple diffusion across the plasma membrane
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Fatty acids and monoglyceride are transported into the
endoplasmic reticulum, where they are used to
synthesise triglyeride!
The vesicles formed are then transported (via
exocytosis) to the lacteal in the villus, which ends up
being transported via the lymphatic vessel
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Absorption of Nutrients
Proteins and carbohydrates: active transport
Lipids: broken down and reassembled
Water: osmosis
Vitamins and minerals: assorted means
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Large Intestine
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Large Intestine: Structure and
Function
Functions: absorbs nutrients and water, and
eliminates waste
Structure:
Cecum (blind pouch), appendix
 Colon: ascending, transverse, descending,
sigmoid
 Rectum, anus
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Large Intestine
Water and electrolytes (i.e. sodium and chloride)
needs to be recovered by absorption
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Na+ transported from lumen by sodium pumps.
Water follows by osmosis
Bicarbonate is secreted to neutralise acids generated
from microbes. This also aids the absorption of
chloride ions
Goblet cells secrete mucus
Normal faeces are roughly 75% water and 25% solids
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Endocrine and Nervous Systems
Regulation of Digestion
Regulation dependent on volume and
content of food
Nervous system: stretch receptors in stomach
 Hormones:
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Gastrin: stimulates release of gastric juice
 Secretin: stimulates pancreas to secrete water and
bicarbonate
 Cholecystokinin (CCK): signals pancreas to secrete
digestive enzymes
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Nutrients:
Utilised or Stored Until Needed
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