Lecture Outline - Chapter 12

12.1. Digestive System (p. 202)

1. Digestion occurs within the digestive tract which begins at the mouth and ends with the anus. (Fig. 12.1, Table 12.1)
2. Digestive System Functions:
   • a. to ingest food.
   • b. digest food to small molecules that can cross plasma membranes.
   • c. absorb nutrient molecules.
   • d. eliminate undigestible wastes.
3. Mouth
   • a. Functions to receive (ingest) food, break food into smaller pieces with aid of teeth, combine food with saliva, taste food, and form food into a bolus for swallowing.
   • b. Tongue has taste buds; sends taste sensation to brain via cranial nerves.
   • c. Taste is also heavily determined by olfactory (smell) receptors in nose.
   • d. Teeth
     • i. 20 deciduous teeth are replaced by 32 adult teeth.
     • ii. Structure consists of crown and root; hard outer enamel of crown, middle layer of bonelike dentin, and inner pulp containing nerves and blood vessels.
     • iii. Periodontal membrane, between root and gum, lines tooth socket.
     • iv. Diseases include:
       - dental caries or tooth decay and cavities, caused by mouth bacteria that metabolize sugar into acids that erode the teeth.
       - gingivitis or inflammation of the gums which can lead to...
       - periodontitis or infection of the periodontal membrane and loosening of the teeth.
   • e. Roof of the Mouth
     • i. Separates the nasal chamber from the mouth cavity.
     • ii. Consists of the anterior hard palate with several hard bones.
     • iii. Also consists of the posterior soft palate mostly of muscle.
     • iv. Soft palate ends in the uvula.
   • f. Tonsils lie at base of tongue, sides of oral cavity, and in nose (where they are called adenoids); provide minor protection from disease organisms.
   • g. Salivary Glands (Fig. 12.1)
     • i. Three pairs:
       - parotid glands are at side of face below and in front of ears.
       - sublingual glands are beneath the tongue.
       - and submandibular beneath the floor of the mouth (lower jaw).

       ii. Glands release saliva through ducts into mouth.

       iii. An enzyme in saliva begins starch digestion.

       iv. Chewed food is mixed with saliva to form a bolus for swallowing.
4. Pharynx (Fig. 12.3) (p. 204)
   • a. The region between mouth and esophagus.
   • b. Swallowing is a reflex action.
     • i. Nasopharyngeal opening closes when soft palate moves back.
     • ii. The opening to trachea (glottis) closes when the epiglottis folds over it as Adam's apple moves up; usually prevents food from going down air passage.
5. Esophagus (Fig. 12.4) (p. 205)
   • a. Transports bolus through the thoracic cavity to the stomach.
   • b. Layers of esophagus, and gut in general, include:
     • i. the mucosa that lines the lumen or space within the tube.
     • ii. the submucosa that contains connective tissue, nerves, blood vessels.
     • iii. the muscularis, a smooth muscle layer with both longitudinal and circular muscles.
     • iv. outer serosa that secretes a fluid that lubricates outer intestines.
   • c. Rhythmical contractions called peristalsis push food along.
   • d. The cardiac sphincter, or lower esophageal sphincter, usually prevents acidic stomach contents from backing up esophagus (sphincters are circular muscles that act as valves to regulate flow).
   • e. Heartburn occurs when stomach acid backs up into esophagus through the lower esophageal sphincter muscle.
   • f. Vomiting occurs from contraction of abdominal muscles and diaphragm.
6. Stomach (Fig. 12.5) (p. 206)
   • a. Thick, J-shaped organ connecting esophagus to duodenum of small intestine.
   • b. Structure has deep folds that disappear as the stomach fills.
   • c. Functions to store food, mix food with gastric juice; gastric refers to stomach.
   • d. Columnar epithelial lining contains millions of gastric pits leading to gastric glands.
   • e. Gastric secretions:
     • i. pepsin, enzyme to digest protein.
     • ii. hydrochloric acid is strong acid that kills bacteria, breaks down connective tissue in meat and activates pepsin.
     • iii. mucus secreted by goblet cells protect lining of stomach.
   • f. Ulcers, once considered to be caused by stress and oversecretion of gastric juice, are now considered to also be caused by the bacterium Helicobacter pylori that impairs ability of epithelial cells to produce mucus.
   • g. Alcohol, a small molecule, is absorbed through the stomach, but not other food substances.
   • h. Stomach empties in 2- 6 hours.
   • i. Chyme leaves the stomach by pyloric sphincter to enter duodenum.
7. Small Intestine (p. 207)
   • a. Three meter (20 foot) long tube has a small diameter relative to large intestine.
   • b. First segment is called the duodenum (Fig. 12.6); it receives ducts from gallbladder and pancreas; sometimes gastric juice in chyme forms duodenal ulcers.
   • c. Sodium bicarbonate (NaHCO₃) in pancreatic juice neutralizes acidity of chyme from stomach.
   • d. Functions chemical digestion of food; absorbs nutrients, and transports undigested material to large intestine.
   • e. Structural unit of the small intestine is the villus. (Fig. 12.6)
     • i. Provides inner intestine with velvet appearance.
     • ii. Lacteal is a small vessel that returns lymph fluid back to bloodstream.
     • iii. Layer of columnar epithelial cells contain microvilli that increase surface area for final digestion and absorption of nutrients; microvilli give cells a fuzzy brush border which produces intestinal enzymes.
     • iv. Blood vessels absorb the sugars and amino acids across villus border.
8. Regulation of Digestive Secretions
   • a. Beginning in late 1860s, Ivan Pavlov demonstrated dog would salivate at ringing of a bell associated with being fed; the thought of food caused the nervous system to order secretion of digestive juices.
   • b. Later investigators discovered that hormones secreted by one organ into the bloodstream triggered activity in other tissues.
     • i. In response to protein, stomach produces gastrin that increase gastric secretions.
     • ii. Duodenal wall cells produce secretin in response to acidic chyme and CCK (cholecystokinin) in response to protein and fat; both hormones increase pancreatic juice output and trigger gallbladder to release bile.
     • iii. Duodenal wall produces GIP (gastric inhibitory peptide) which works opposite to gastrin.
9. Large Intestine (p. 208)
   • a. Has larger diameter than small intestine.
   • b. Functions to absorb water, salts, and some vitamins; stores nondigestible material until defecation.
   • c. Parts of the large intestine include:
     • i. Cecum (Fig. 12.8)
       - lies below entrance of the small intestine.
       - contains the vermiform appendix which may have a role in immunity; when inflamed, it causes appendicitis.
       - Peritonitis is a serious infection of lining of abdominal cavity that can result from a burst appendix.

       ii. Colon

       - consists of the ascending, transverse, and descending colon.

       - feces is three-quarters water and one-quarter solids. (Fig. 12.9)

       - brown color of feces is due to bacterial breakdown of bilirubin.

       - odor of feces is due to bacterial products, including gases.

       - bacteria including Escherichia coli and anaerobic bacteria breakdown undigestible materials, produce some vitamins.

       - presence of E. coli in water sample is indication of fecal contamination and increased risk of disease-causing bacteria.

       - colon may develop polyps that may be benign or cancerous. Risk is decreased if one eats a low-fat, high-fiber diet.

       iii. Rectum

       - last 20 cm of the large intestine.

       - storage of feces.
10. Anal canal: last portion of rectum for defecation of feces. Feces contains undigested residues, bile pigments, heavy metals, and the bacterium E. coli.
11. Diarrhea and Constipation
    • a. Major causes of diarrhea:
      • i. Infection of the lower tract, for instance from food poisoning, increases peristalsis and less water is absorbed; diarrhea rapidly rids body of infectious agent.
      • ii. Nervous stimulation also increases peristalsis by triggering contractions.
    • b. Consequences of diarrhea:
      • i. Less water is absorbed; this may lead to dehydration and salt imbalance.
      • ii. Salt imbalance may disturb heart contraction.
    • c. Causes of constipation:
      • i. May result from social customs inhibiting defecation.
      • ii. Can be prevented by taking in more water and more fiber bulk in diet.

1. Pancreas
   • a. Elongated organ located in abdominal cavity on posterior wall.
   • b. Functions as exocrine gland.
     • i. Produces digestive enzymes and fluid that enters duodenum by way of the pancreatic duct.
     • ii. Pancreatic cells produce enzymes that digest carbohydrate, protein and fat.
     • iii. In cystic fibrosis, thick mucus blocks the pancreatic duct and patient must take enzymes by mouth.
   • c. Also is endocrine gland secreting insulin for glucose balance.
2. Liver (Fig. 12.10) (p. 210)
   • a. Largest gland in the body.
   • b. Located in abdominal cavity under diaphragm.
   • c. Has two main lobes with larger lobe to right and smaller lobe to left.
   • d. Contains about 100,000 lobules that are functional units.
   • e. Hepatic artery provides oxygenated blood; hepatic portal vein shunts nutrients from intestines; bile duct takes bile away from liver, and hepatic vein carries blood away from liver.
   • f. Liver lies between hepatic portal vein and hepatic vein.
   • g. Functions
     • i. Detoxifies the blood by removing poisonous substances from blood coming from hepatic portal vein from the intestine.
     • ii. Removes and stores iron and fat soluble vitamins A, D, E, and K.
     • iii. Maintains blood glucose level at 100 mg/100 ml or at 0.1% even when person eats intermittently; also converts amino acids to glucose.
     • iv. Deamination is removal of amino groups from amino acids; results in ammonia and carbon dioxide converting to urea, a waste excreted by kidneys.
     • v. Old red blood cells are destroyed; hemoglobin is converted into bile pigments bilirubin and biliverdin.
     • vi. Bile salts are produced to emulsify fat, in duodenum, into smaller droplets for lipase digestion; bile is stored in the gallbladder and enters duodenum via bile duct.
     • vii. Makes plasma proteins from amino acids for various functions.
   • h. Liver Disorders (p. 211)
     • i. Jaundice
       - Skin has yellowish tint due to build up of bilirubin in the blood.
       - Hemolytic jaundice results when too many RBCs broken down.
       - Obstructive jaundice occurs when bile duct is blocked due to gallstones.

       ii. Hepatitis

       - Inflammation of liver that can also cause jaundicing.

       - Viral hepatitis A is usually acquired from sewage-contaminated water.

       - Viral hepatitis B is usually spread by sexual contact, blood transfusions or contaminated needles; more contagious than AIDS virus.

       - Hepatitis C, also spread by infected blood, has no vaccine.

       iii. Cirrhosis

       - Common among alcoholics.

       - Liver becomes fatty; tissue is replaced by inactive fibrous scar tissue (cirrhosis).

       iv. Liver failure is treated with:

       - whole organ transplant.

       - cloned liver cells that service patient while damaged liver has chance to heal.
3. Gallbladder
   • a. Pear-shaped muscular sac attached to ventral surface of liver. (Fig. 12.1)
   • b. Stores the 1,000 ml of bile produced each day.
   • c. Water is reabsorbed to thicken bile to mucus-like material.
   • d. Bile exits gallbladder by common bile duct.

1. Digestive enzymes are hydrolytic enzymes that catalyze degradation of a molecule by addition of water at specific bonds.
2. Digestive enzymes are proteins that are specific to a substrate and have a preferred pH.
3. Specific enzymes (Table 12.2)
   • a. Salivary amylase plus water in saliva converts starch to maltose, additional digestive action in the small intestine must convert maltose to glucose.
   • b. Precursor pepsinogen in stomach converts to enzyme pepsin when exposed to HCl; pepsin converts protein to peptides.
   • c. Pancreatic amylase converts starch plus water to maltose.
   • d. Pancreatic trypsin digests protein plus water to peptides.
   • e. Pancreatic lipase converts emulsified fat droplets to glycerol plus fatty acids that rejoin inside the cells of the villi; later they enter the lacteals.
   • f. Small intestine peptidases convert peptides (plus water) to amino acids, which enter intestinal cells and pass to capillaries in the villus.
   • g. Small intestine maltase digests maltose (plus water) to glucose, which also enters and passes to capillaries in the villus.
   • h. Small intestine lactase converts lactose in milk to glucose and galactose; seventy-five percent of African Americans lack lactase and cannot digest milk (termed lactose intolerance); this leads to gas, cramps, etc.; lactose-free dairy products utilize synthetic enzymes or bacteria.
4. Best Conditions for Digestion (p. 213)
   • a. Laboratory experiments demonstrate digestion of egg protein requiring enzyme pepsin and HCl to be present. (Fig. 12.12)
   • b. Tube 1 is control; no digestion has occurred.
   • c. Tube 2 lacks HCl; no digestion has occurred.
   • d. Tube 3 lacks enzyme; no digestion has occurred.
   • e. Tube 4 contains both HCl and enzyme; digestion has occurred.

1. Macronutrients (carbohydrate, protein, and fat) supply both energy and building blocks for synthesizing cellular contents.
2. Micronutrients (mostly vitamins and minerals) are necessary for cellular metabolism.
3. Recommendations in the food pyramid suggest a diet that may provide some protection against heart disease, cancer, and other serious illnesses. (Fig. 12.13)
   • a. The bulk of the recommended diet is bread, cereal, rice, and pasta; unmilled grains provide both vitamins and minerals, and roughage.
   • b. Vegetables and fruits also provide vitamins, minerals, and roughage.
   • c. Women have high requirements for calcium; this is covered by milk in the diet.
   • d. Meat, fats and sweets are needed in lower quantities; saturated fats increase risk of cardiovascular disease.
4. Carbohydrates (p. 215)
   • a. Are the quickest, most readily available source of energy.
   • b. Blood glucose level is maintained at about 0.1% by liver that stores excess as glycogen.
   • c. Simple carbohydrates such as sugars are labeled as "empty calories" since they contribute to energy needs of body without supplying other nutritional requirements.
   • d. It is possible to cut down on consumption of dietary sugar. (Table 12.3)
   • e. Complex carbohydrates, along with fiber, are considered to be beneficial to health, in particular reducing the risk of colon cancer.
5. Proteins (p. 217)
   • a. Found in red meat, fish, poultry, dairy products, legumes, nuts, and cereals.
   • b. Mostly incorporated into structural proteins in muscles, skin, hair, and nails; some synthesize proteins such as hemoglobin, enzymes, hormones, etc.
   • c. Twenty amino acids are necessary to synthesize our proteins; eleven can be produced by our body leaving nine the body cannot itself produce, termed essential amino acids.
   • d. Complete protein sources such as meat contain all of the amino acids but some vegetables and grains are incomplete.
6. Lipids (p. 218)
   • a. Fats are present in butter, oils, and in high-protein foods.
   • b. We can alter ingested fat except our body cannot produce the essential fatty acid linoleic acid; it is therefore the essential fatty acid.
   • c. Dietary fat appears to increase the risk of cancers of the colon, pancreas, and liver, and cardiovascular disease.
7. Cholesterol (p. 218)
   • a. Saturated fatty acids along with cholesterol contribute to formation of plaque, fatty deposits on artery walls that leads to high blood pressure and heart disease.
   • b. Eggs, liver, and kidneys are foods high in cholesterol.
   • c. Cholesterol is carried from the liver to cells by low-density lipoproteins (LDL) and carried away from the cells to the liver by high-density lipoprotein (HDL). LDL contributes to plaque in the arteries while HDL is protective against clogged arteries.
8. Vitamins (p. 218)
   • a. Are organic compounds that the body is unable to produce but needs for metabolic purposes; many are portions of coenzymes.
   • b. If lacking in the diet, various symptoms develop (Fig. 12.16).
   • c. There are thirteen vitamins divided into fat soluble vitamins (Table 12.5) and water-soluble vitamins (Table 12.6).
   • d. Free radicals are unstable molecules that are generated by cellular metabolism and which can damage some cellular molecules; anti-oxidants defend against the effects of free radicals and include vitamins C, E, and A.
9. Minerals (p. 220)
   • a. Various minerals are required by the body (Table 12.7); they are divided into macrominerals (recommended amount of more than 100 mg/day) and microminerals or trace elements (less than 20 mg per day).
   • b. Iron is needed for production of hemoglobin; adult females need more per day due to blood loss during menstruation.
   • c. Calcium is a constituent of cells and body fluids, including bone; osteoporosis is loss of bone tissue in older age, especially among women; best defense against osteoporosis is exercise, dietary calcium, and estrogen replacement after menopause if needed.
10. Sodium
    • a. Average American diet contains eight to nine times the recommended intake.
    • b. High sodium intake has been linked to high blood pressure in some people.
    • c. One-third of sodium is naturally in foods, one-third is added in processing and one-third is from table salt.
    • d. Recommendations are to cut down sodium intake. (Table 12.8)
11. Eating Disorders (p. 221)
    • a. Obesity
      • i. Defined as a body weight of more than 20% above the ideal weight for sex and age of the person.
      • ii. Causes include a combination of factors: genetic, endocrine, metabolic, and social.
      • iii. Dieting combined with exercise is usually recommended.
    • b. Bulimia
      • i. Individuals eat in excess and then purge themselves by vomiting or laxatives.
      • ii. Usually occurs in depressed young women.
    • c. Anorexia Nervosa
      • i. Individual is extremely thin but still claims to "feel fat" and continues to diet.
      • ii. Mostly afflicts adolescent girls.
      • iii. May have psychological problems, including a desire to suppress their sexuality.