Digestive system
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Transcript Digestive system
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%