The Digestive System

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The Digestive System Ingestion Digestion Secretio
n Absorption Excretion
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• What happens in digestion?
• Ingestion
• Mechanical (chewing)
• Chemical (enzymes)

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Esophagus ca. 10 inches long peristalsis indepe
ndent of gravity False sphincter at junction
of esophagus and stomach
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Empty stomach capacity is 50 ml Can expand to
hold several liters stretch receptors signal
HYPOTHALAMUS when stomach is full Hypothalamus-
a major control center in the brain hunger (and
satiety) thirst body temperature emotions memo
ry
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Dr. William Beaumont, 1820s had a
unique opportunity to study the
stomach Contains acid Acid breaks down
protein Acid is produced when you eat Chewing
affects rate of digestion
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Protein digestion starts in stomach (acid and
enzymes) Rennin and lipase help digest
milk Mucus protects stomach also epithelial
cells divide very rapidly Very little
absorption- aspirin and alcohol Taks 3-5 hours
to move to intestine
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Heartburn- gastric juice (acid) backs up
into esophagus Bicarbonate- neutralize
acid Tagamet, Prilosec, Zantac- inhibit acid
formation Propulsid- acts on sphincter Peptic
ulcers- erosion of stomach lining bacterial
infection- H. pylori Product of stomach
digestion is called chyme- moves into small
intestine
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Small intestine Digestion completed Absorption
Digestive enzymes come mostly from pancreas and
liver (recruited by small intestine) Proteases,
lipases, nucleases Bile
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Gastrin promotes digestion of protein Cells in
small intestine secrete hormones CCK and
secretin These hormones cause pancreas to
secrete various digestive enzymes gallbladder
to secrete bile
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Pancreatic juice contains several enzymes that
digest several kinds of molecules (see p.
414) also contains sodium bicarbonate to
neutralize acid Bile- helps with fat
digestion produced in liver, stored in
gallbladder
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Large intestine Cecum, colon, anus Absorption
of water and a few other molecules Whats
left? undigested (indigestible)
food bacteria Bacteria actually produce
vitamins (B and K)
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Gas production by bacteria (from
undigested food) Lactose intolerance- lack of
LACTASE (enzyme that breaks down lactose, a
milk sugar) Some bacteria make this enzyme
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Diarrhea- too little water absorbed bacterial
toxins nervous stimulation can lead to
dehydration, electrolyte imbalance Constipation
not enough water intake behavioral dietary
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After absorption into bloodstream- then
what? Goes to liver
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Liver cleans up blood Stores iron, vitamins,
glycogen Makes important blood
proteins Controls blood cholesterol levels
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Nutrition What do we need from
food? calories carbohydrates fats proteins vi
tamins minerals Food pyramid- out of date?
(USDA, 1992)
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Carbohydrates Simple Complex other
nutrients fiber aids in digestion may help
reduce blood cholesterol
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Carbohydrates and lipids can be stored in
the body. Proteins are not. Some amino acids
can be synthesized in the body eight cannot.
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Sources of complete protein Animal
proteins Combinations of grains legumes nuts
and seeds vegetables
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Lipids (fat and cholesterol) Saturated fats
raise blood cholesterol (LDL) some unsaturated
fats lower it Vitamins- coenzymes fat-soluble
(A, D, E, K) water-soluble (B vitamins, C)
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Minerals many different types Too much or too
little can be harmful
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Why do we need calories? How many do we need?