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 bythe 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 observationsMost 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 roleLocated 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 foodLearned 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 satietythe 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 digestionchewing 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.

Pharynxintersection 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

EnzymesCatalyze the breakdown of food from polymers to monomers

HormonesSignal 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 foodStore food, begin digestion of

protein the hormone gastrin controls

stomach secretions Three kinds of cells make

secretions:Mucus- mucus cellsHydrochloric acid- Parietal cellsPepsin- chief cells

Stomach tissue layers

SEM of stomach

Small intestine

6 m in lengthVilli and microvilli increase surface area.

chemical digestion completed & all absorption of nutrients occurs here.

three regionsDuodenum- digestion completed Jejenum, ileum - absorption

Small Intestine

Digestion in small intestine

1. entry of chyme starts a chain reaction Intestine releases hormone secretin pancreas releases bicarbonate (baking soda) to

neutralize the acidic chyme2. intestine releases the hormone CCK to signal

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. activated by enterokinase from the intestinal wall4. Nuclease enzymes hydrolyze DNA & RNA to

nucleotides.

absorption

villiprojections (like shag carpet) with capillaries

surrounding a lymph vessel called a lactealAmino Acids, sugars enter the capillariesglycerol & 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 absorptionBecause 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:Foodgender

Women and alcoholWomen 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 waterFewer alcohol digesting molecules in the stomach, so more

gets into the blood.

Alcohol metabolismMost 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%