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I.
Weighty Worries
A. Many Americans have poor eating habits resulting in being overweight, specifically too fat.
B. There are many correlations between diet and illnesses such as diabetes, anorexia, and bulimia.
II. Overview of the Digestive System
A. The digestive system (gastrointestinal tract, GI) functions to mechanically and chemically break down food into
nutrient molecules that the body can use to maintain homeostasis.
1.
Food is transported through the lumen from the mouth to the anus.
2.
Stretched out the GI tract is about 6-9 meters long.
3. Specialized regions include the mouth, pharynx, esophagus, stomach, small intestine, large intestine
rectum, and anus.
B. The digestive system performs these five functions:
1. Mechanical processing and motility includes movements that break up , mix, and transport ingested
nutrients.
2.
Secretion is the release of needed enzymes and other fluids.
3.
Digestion is the chemical breakdown of food to molecules small enough to cross the gut lining.
4.
Absorption is the passage of digested nutrients into the blood and lymph.
5.
Elimination is the expulsion of undigested and unabsorbed residues from the end of the GI tract.
C. Accessory organs (salivary glands, liver, pancreas, and gallbladder) play essential roles in digestion and
absorption.
D. Layers of the Digestive Tube
1.
The mucosa is the epithelial lining that faces the lumen of the gut; connective tissue underlies it.
2.
The submucosa contains blood and lymph vessels plus nerve networks.
3.
A smooth muscle layer (longitudinal and circular) lies outside the submucosa.
4.
The serosa forms an outer covering.
5. Circular arrays of smooth muscle make up the sphincters which close off a passageway to control
forward movement of the tract's contents.
E. Motility
1.
Peristalsis contractions move the food forward.
2.
Repeated segmentation contractions create an oscillating movement that mixes the food.
III. Chewing and Swallowing
A. Mouth, Teeth, and Salivary Glands
1.
Mechanical breakdown of food begins in the mouth (oral cavity).
2.
Teeth (32 in adults, 20 in children) chew the food.
a)
Each has an enamel coat, a dentin core, and an inner pulp.
b) Incisors bite off chunks, cuspids tear, and molars and premolars grind food.
3.
Improper dental care can lead to caries, gingivitis, and periodontal disease.
4.
Salivary glands secrete a fluid into the mouth.
a) Parotids lie just in front of the ear; submandibular glands are located in the lower jaw;
sublinguals are under the tongue.
b) Saliva contains salivary amylase to begin carbohydrate digestion, bicarbonate to neutralize
acids, and mucins to help form the food into a bolus.
B. Swallowing
1. The tongue (with the help of the palate) pushes the ball of food into the pharynx where receptors initiate
the swallowing reflex into the esophagus, which leads to the cardiac orifice of the stomach.
2.
The epiglottis closes off the trachea to prevent choking.
IV. Digestion in the Stomach and Small Intestine
A. The Stomach
1. The stomach is a muscular sac that stores and mixes food, secretes substances to dissolve and degrade
food, and controls the rate at which food enters the small intestine.
2.
Gastric fluid includes hydrochloric acid, pepsinogens, and mucus.
a) HCl dissolves bits of food to form a soupy chyme; it converts pepsinogen (inactive) to pepsin
(active), and kills microbes.
b) Pepsin begins the digestion of proteins
c) Stomach lining cells also secrete intrinsic factor which is essential for the absorption of vitamin
B12.
3. Stomach secretions begin in response to sensual perception of food and continue in response to
stretching of the stomach wall and the presence of gastrin hormone.
4. Normally, mucus and bicarbonate ions protect the stomach lining; however, a bacterium (Helicobacter
pylori) can interfere with mucus production, resulting in a peptic ulcer
5. Peristaltic contractions churn the chyme and keep the pyloric sphincter at the stomach’s exit closed, but
small amounts are released at intervals into the small intestine.
6. A large meal hastens stomach emptying while increases in acidity, fat content, fear, and depression slow
it down.
7.
Alcohol is one of the few substances absorbed by the stomach wall.
B. The Small Intestine
1.
The small intestine consists of three sections: duodenum, jejunum. and ileum.
2.
Secretions from the pancreas and liver enter via a common duct.
a)
Trypsin and chymotrypsin (from the pancreas) digest proteins to peptide fragments.
b) Carboxypeptidase (pancreas) and aminopeptidase (intestinal lining) degrade the fragments to
amino acids.
c)
Bicarbonate from the pancreas buffers from the stomach.
d) Bile (stored in the gallbladder until used) is a secretion consisting of bile salts, pigments,
cholesterol, and lecithin.
e)
Bile salts speed up fat digestion by emulsification.
f) Triglycerides tend to form large globules, but when smaller fat droplets become coated with
bile salts, the negative charges on the droplets repel and cause them to stay separated.
g) Excess cholesterol may form gallstones ,which may lodge in the ducts causing cholecystitis.
V. Absorption of Nutrients In the Small Intestine
A. Structure is the Key to Function
1.
The vast majority of nutrients are absorbed in the small intestine.
2. Absorptive surface area is increased by fingerlike projections of the intestine lining called villi which
bear even smaller microvilli, giving each cell a "brush border."
B. What Are the Absorption Mechanisms?
1. Monosaccharides (glucose), amino acids, and some nucleotides cross the gut lining by active transport
and enter the bloodstream.
2.
Absorption of fatty acids and monoglycerides is aided by bile salts.
a) Free fatty acids diffuse into the gut epithelium, aided by micelle formation (bile salts, lecithin,
cholesterol).
b) Fats combine with proteins to form chylomicrons that enter lymph vessels called chylomicrons
VI. The Liver's Role in Digestion
A. Nutrient-laden blood flows via the hepatic portal vein from the intestine to the liver, after which the blood is
returned to the general circulation via the hepatic vein.
B. The liver has several functions:
1.
It makes bile, which is useful in fat emulsification
2.
It converts glucose to glycogen --a storage form.
3.
It produces plasma proteins
4.
It detoxifies harmful substances.
5.
It forms urea.
6.
It forms and degrades red blood cells.
VII. The Large Intestine
A. The large intestine (colon) concentrates and stores feces --a mixture of undigested and unabsorbed material,
water, and bacteria.
1.
Sodium is actively transported out of the colon and water follows by osmosis.
2.
About 30 percent of the feces is bacteria which produce vitamins and intestinal gases.
B. The large intestine begins as a blind, cup-shaped pouch (cecum) at its junction with the small intestine.
1. A slender projection, the appendix, is attached here; it has no known digestive function but may
participate in immunity.
2.
Inflammation of the appendix (appendicitis) may lead to rupture and peritonitis.
3. The colon continues from the cecum and is subdivided into the ascending, transverse, and descending
regions; the last portion (sigmoid colon) connects to the rectum, which lead to an external opening (anus).
C. Diarrhea results from two little water absorption from the feces; in constipation the residues remain in the colon
too long allowing them to become dry and hard.
D. Bulk (soluble and insoluble fiber) in the diet is important in moving material in the feces through the large
intestine at the proper speed.
VIII.
Digestion Controls and Nutrient Turnover
A. Controls Over Digestion
1. Distention of the gut wall after a meal stimulates mechanoreceptors and their neurons, which respond
with muscle action and enzyme secretion.
2.
Four gastrointestinal hormones play roles.
a) Gastrin, produced by the stomach lining, stimulates secretion of acids into the stomach;
somatostatin inhibits acid production.
b) Secretin, from the intestinal lining, stimulates the pancreas to release bicarbonate.
c)
Cholecystokinin enhances the actions of secretin and stimulates gallbladder contractions.
d) GIP (glucose insulinotropic peptide), is released in the presence of glucose and fat, slows the
rate of gastric emptying, and stimulates insulin secretion .
B. Nutrient Turnover and Organic Metabolism
1.
Nutrient molecules are shuffled and reshuffled once they have been absorbed.
2. Shortly after a meal, the level of carbohydrates rises; some are converted to fat for storage in adipose,
and others are converted to glycogen in the liver and muscle tissue.
3. Between meals, glucose levels are maintained by breakdown of glycogen reserves in the liver and amino
acids are converted to glucose; fatty acids from fats can be used directly by cells for energy.
C. Lactose intolerance results from a deficiency in lactase.
D. Persons suffering from cystic fibrosis do not produce the pancreatic enzymes necessary for normal digestion and
absorption of nutrients.
a) Crohn’s disease can so severely damage the intestinal lining that portions of the tract must be
removed.
b) Food allergies also disrupt digestion and absorption.
IX. The Body's Nutritional Requirements
A. New, Improved Food Pyramids
1. Our ancient ancestors ate a diet or fresh fruits and fibrous plant material; we now prefer a diet rich in
low-fiber, high fat foods.
2. Nutritionists recommend a well-balanced diet consisting of complex carbohydrates: 55-60%; fats and
lipids: 20-25%; and proteins: 15-20%.
B. Carbohydrates
1. Complex carbohydrates are the main source of energy taken into the body; they are degraded to glucose,
the main source of energy available to individual cells.
2.
Fruits, vegetables, and grains should be at least 50 to 60 percent of daily caloric intake.
C. Lipids
1. Phospholipids and cholesterol are important components of membranes; fats are energy reserves,
provide insulation and cushioning, and store vitamins.
2. The body needs very little polyunsaturated fat to supply the essential fatty acids, those not made by the
body itself.
3.
Fats make up 40% of the American diet--they should be less than 30%.
4. Cholesterol is necessary in the synthesis of bile acids and steroid hormones as well as being a component
of membranes; however, too much can damage the circulatory system.
D. Proteins
1.
Of the twenty different amino acids in proteins, eight are essential (that is, must be supplied in the diet).
2. Because some sources of protein are "incomplete" (plants), nutritionists use the net protein utilization
(NPU) index to compare proteins from different sources.
3. Protein deficiency is very serious because it results in damage to the brain and growing tissues,
especially in the young.
X. IX. Vitamins and Minerals
A. Humans need small amounts of at least thirteen organic molecules called vitamins to assist in cellular
metabolism.
B. Inorganic substances called minerals (Ca, Mg, K, Fe, for example) are also needed.
C. A balanced diet will normally meet all requirements for these substances; excessive intake is at least wasteful and
at worst harmful.
XI. X. Food Energy and Body Weight
A. To maintain acceptable weight, caloric (measured in kilocalories) intake must balance energy output.
1.
The basal metabolic rate is the amount of energy needed to sustain the body at rest.
2. The number of kilocalories needed to maintain "acceptable" body weight can be calculated relative to
your amount of physical activity and age.
B. Assessing Body Weight
1. Obesity is an excess of fat in the body’s adipose tissue; by definition that term is applied to persons who
are 30% heavier than ideal.
2.
Clear medical evidence links obesity with conditions such as diabetes and heart disease.
C. My Genes Made Me Do It
1. In 1995, researchers identified a gene in mice called the ob gene which codes for a hormone called leptin
that influences the brain to stimulate or suppress appetite.
2.
Persons with a faulty ob gene could literally not be able to control their caloric intake.
D. Dieting and Exercise
1.
Fat storage in adipose tissue is an adaptation to survival.
2. Because fat cells refill after dieting, any effective program of weight loss must include reduced food
intake and increased physical activity.