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Digestive system
Why do we eat?
Control of Food Intake and Body Weight
 The body is in a continual state of hunger, which is
intermittently relieved by eating.
 The drive to eat is suppressed by impulses generated by
 the presence of food in the gastrointestinal tract,
 the flow of nutrients into blood and other factors.
 When these "satiety factors" have dissipated, the desire
to eat returns.
Common observations
 Most animals as adults maintain a remarkably
constant body weight.
 When it's cold, animals (including humans) eat
more than when it's hot.
 Children maintain energy balance with wildly
varying intakes of food per meal.
These observations suggest a very
complex system controlling energy
balance and body weight.
 What affects the control of eating…
 the central nervous system
 Pregastric factors
 Gastrointestinal and postabsorptive factors
 Long-term factors
CNS role
 Located in the hypothalamus.
 Lateral hypothalamus (hunger center)
 animals with lesions in this area become anorectic
and lose weight.
 Ventromedial hypothalamus (satiety center)
 animals with lesions in this area overeat and become
obese.
Pre gastric factors
 Appearance of food: we like or dislike certain
meals based on visual appearance
 Taste and/or odor of food
 Learned preferences and aversions: Almost
everyone has an aversion to one or more types of
foods.
 Psychologic factors: mental states such as fear,
depression and social interactions often affect food
intake.
GI and postabsorption factors
 The degree of gastrointestinal fill is the most
important signal from the digestive tract
 a full stomach and intestine induce satiety
 the hormone CCK induces satiety,
 the hormone ghrelin seems to stimulate appetite.
Long term control of food intake
 Adult animals tend to maintain a relatively constant
weight known as their "set weight."
 If an animal is starved for a long period of time, then allowed
access to food, it eats a far greater amount of food than a
normal animal.
 if an animal is force fed for several weeks, then allowed access
to free choice food, it will not eat very much.
 In both cases, when weight returns to "set weight," feeding
behavior normalizes.
 when food is restricted, basal metabolic rate
decreases,
 which is one reason that it is so difficult to lose weight by
dieting.
Oral cavity
 physical & chemical digestion
 chewing results increases surface area for chemical
digestion of food.
 food causes saliva to be released containing the
enzyme salivary amylase to break down starch.
 tongue forms the mixture into a bolus and starts the
swallowing process.
Pharynx
 intersection between digestive tract & the breathing
tube
 When you swallow, a flap, the epiglottis, covers the
trachea so food goes down the esophagus.
Swallowing
Two types of molecules control digestion
 Enzymes
 Catalyze the breakdown of food from polymers to monomers
 Hormones
 Signal when food is present,
 Identify what kind of food,
 stimulate release of enzymes and
 control peristalsis
stomach
 located just below the
diaphragm
 Can accommodate up to 4 liters
of food
 Store food, begin digestion of
protein
 the hormone gastrin controls
stomach secretions
 Three kinds of cells make
secretions:
 Mucus- mucus cells
 Hydrochloric acid- Parietal cells
 Pepsin- chief cells
Stomach tissue layers
SEM of stomach
Small intestine
 6 m in length
 Villi and microvilli increase surface area.
 chemical digestion completed & all absorption of
nutrients occurs here.
 three regions
 Duodenum- digestion completed
 Jejenum, ileum - absorption
Small Intestine
Digestion in small intestine
entry of chyme starts a chain reaction
1.
Intestine releases hormone secretin
pancreas releases bicarbonate (baking soda) to
neutralize the acidic chyme


intestine releases the hormone CCK to signal
2.
gall bladder releases bile salts to emulsify fats
This allows lipases to work


3.
Inactive enzymes to break down protein are
released by pancreas.
1.
4.
activated by enterokinase from the intestinal wall
Nuclease enzymes hydrolyze DNA & RNA to
nucleotides.
absorption
 villi
 projections (like shag carpet) with capillaries
surrounding a lymph vessel called a lacteal
 Amino Acids, sugars enter the capillaries
 glycerol & fatty acids enter the lacteal
 the capillaries all drain together into the hepatic portal
vein where the various molecules are used, stored, or
converted to a different form
Alcohol absorption
 Because alcohol vaporizes easily, it also passes
through cell membranes easily.
 Because food and drink can spend several hours in
the stomach, some alcohol is absorbed through the
stomach and enters the bloodstream.
 Most alcohol is absorbed in the small intestine.
 The rate at which alcohol is absorbed is affected by
several factors:
 Food
 gender
Women and alcohol
 Women absorb and metabolize alcohol differently
than men.
 They have a higher Blood Alcohol Concentration
(BAC) than men after ingesting the same amount of
alcohol.
 More susceptible to liver disease, heart muscle damage, brain
damage.
 Why?
 Women have less body water
 Fewer alcohol digesting molecules in the stomach, so more
gets into the blood.
Alcohol metabolism
 Most is metabolized by the liver.
 The rest is vaporized in breath, urine.
 Liver cannot store alcohol.
 Liver cells cannot alter their rate of metabolism.
 Liver can metabolize about 1 oz. of alcohol/hr.
 The BAC concentration peaks about 30-45 min. after
ingestion of one drink.
 A 150 lb. male would get a BAC of 0.02% from one drink.
 A 150 lb. female 0.04%