Digestive system

1
Digestive system
2
Why do we eat?
3
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.

4
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.

5
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

6
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.

7
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.

8
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.

9
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.

10
(No Transcript)
11
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.

12
Pharynx

• intersection between digestive tract the
  breathing tube
• When you swallow, a flap, the epiglottis, covers
  the trachea so food goes down the esophagus.

13
Swallowing
14
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

15
(No Transcript)
16
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

17
(No Transcript)
18
Stomach tissue layers
19
SEM of stomach
20
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

21
Small Intestine
22
(No Transcript)
23
(No Transcript)
24
(No Transcript)
25
Digestion in small intestine

• entry of chyme starts a chain reaction
• Intestine releases hormone secretin
• pancreas releases bicarbonate (baking soda) to
  neutralize the acidic chyme
• intestine releases the hormone CCK to signal
• gall bladder releases bile salts to emulsify fats
  
• This allows lipases to work
• Inactive enzymes to break down protein are
  released by pancreas.
• activated by enterokinase from the intestinal
  wall
• Nuclease enzymes hydrolyze DNA RNA to
  nucleotides.

26
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

27
(No Transcript)
28
(No Transcript)
29
(No Transcript)
30
(No Transcript)
31
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

32
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.

33
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