Layers of Gastrointestinal Tract
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Mucosa--epithelial cells
Submucosa
Circular muscle
Longitudinal muscle
Serosa
Proposed Causes of Ulcers
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Overproduction of acid
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Inactive until acted upon by acid or another enzyme
Breach in protection by mucus
Helicobacter pylori, a bacteria
Many protein-digesting enzymes are released as zymogens
Acid converts pepsinogen to pepsin in stomach
Enzyme enterokinase converts the pancreatic enzymes trypsinogen and
chymotripsinogen to trypsin and chymotrypsin, respectively
Neural Control of Gut
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Autonomic nervous system (vagus nerve)
Enteric nervous system
Control motility & secretions
Hormonal Control of Gut
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Gastrin-secreted by stomach
Secretin
secreted by small intestine
Cholecystokinin
Gastric Inhibitory Peptide (GIP)
Gastrin
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Stimulates gastric motility
Stimulates HCl secretion
Stimulates pepsinogen secretion
Other Gut Hormones
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Secretin antagonizes action of gastrin; also stimulates secretion of bicarbonate
by pancreas
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CCK stimulates enzyme secretion by pancrease & bile by gall bladder
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GIP inhibits gastric motility and secretion; stimulates insulin release
The Small Intestine
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Vast majority of nutrient absorption occurs here
Divided into 3 parts
– Duodenum
– Jejunum
– Ileum
Small Intestine Structure
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Mucosal layer is divided into thousands of fingerlike projections--villi
Epithelial cells of mucosa have microvilli
Net effect--increased surface area for absorption
In the Duodenum…
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Bicarbonate secreted by pancreas increases pH of chyme to over 7
Pancreatic secretions and bile enter duodenum through the common bile duct
Pancreatic Enzymes
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Proteases--trypsin, chymotrypsin, carboxypeptidase--products amino acids
and short chains
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Lipase--triglycerides to monoglycerides and fatty acids
Amylase-starch to shorter chains
Bile
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Derived from cholesterol
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Bile can be reabsorbed in ileum and recycled--enterohepatic circulation
A detergent--emulsifies fat and fat-soluble vitamins into small droplets called
micelles
Enzymes on Surface of Small Intestine
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Proteases
Sucrase
Lactase
Lactose Intolerance
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Affects over 70% of adults worldwide
Due to deficiency of enzyme lactase
Some formerly lactose-tolerant adults may develop lactose intolerance when
ill with HIV or other illnesses that affect the gut
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Most lactose-intolerant people can drink milk in small doses with a meal
How do nutrients get absorbed?
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Simple diffusion
Facilitated diffusion--requires a carrier
Active transport--against concentration gradient
Endocytosis--pinching off of cell membrane
– Antibodies in breast fed babies
Carbohydrate & Protein Absorption
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Glucose and galactose--active transport
Fructose--facilitated diffusion
Most amino acids--active transport
Di- and tripeptides, other amino acids--facilitated diffusion
Go directly into bloodstream and to liver
Fat Absorption, or
Oil & Water Don’t Mix
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Products of fat digestion diffuse across membrane into mucosal cell
Triglycerides are resynthesized and packaged into chylomicrons
Chylomicrons enter lymph system via lacteals in each villus
Lymph enters bloodstream at thoracic duct
Water Absorption
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All along gastrointestinal tract
There is no active transport of water in biological systems
Osmosis--movement of water across membrane along with nutrients
The Colon
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No mucosal villi
Mainly absorbs water and a few minerals (sodium, potassium)
Good Bacteria in the Colon
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Lactobacilli, Bifidobacter
Can digest lactose and other indigestible compounds
Bacteria in yogurt or acidophilus milk can survive upper gastrointestinal tract
and colonize the large intestine
Feces Happen
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Consist of undigested fiber, bacteria, a little water
What is regular for one person may be irregular for another
Low fiber and/or low fluid diet may be primary reasons for constipation
Diverticular Disease
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Low fiber diet is main culprit
Small pockets form in lining of colon
If a small seed gets into a pocket, inflammation can occur--diverticulitis
Prevention is via high fiber diet