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 swallowing process.

and starts the

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

1.

2.

3.

4.

  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   1.

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.

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%