1. Parts of the digestive tract produce digestive enzymes.
The human digestive tract is a tube with specialized regions and organs between the mouth and the anus.
Food is ingested, mechanically processed, and chemically digested to small molecules that are absorbed; indigestible remains are eliminated.
Parts of the digestive tract produce digestive enzymes.
Three pairs of salivary glands send saliva (containing salivary amylase for digestion of starch to maltose) into the mouth.
During chemical digestion, many different enzymes break down macromolecules to small organic molecules that can be absorbed. Each enzyme has a particular job to do.

2. Digestive system
Trace the path of food from mouth to the anus. The large intestine consists of the cecum, colon (consisting of the ascending, transverse, descending, and signoid colon), and the rectum and anal canal. Note also the location of the accessory organs of digestion: the pancreas, the liver, and the gallbladder.

3. The Mouth Lips and cheeks enclose the mouth.
Taste buds on the tongue provide the sense of taste; skeletal muscle in the tongue allows it to move.
The roof of the mouth is formed by the hard and soft palates that separate it from the nasal cavities.
The soft palate ends in a finger-shaped projection called the uvula.
The red portion of the lips is poorly keratinized, and this allows blood to show through.
The hard palate of the roof of the mouth contains several bones, but the soft palate is composed entirely of muscle.

4. The Teeth
Twenty deciduous (baby) teeth are replaced by 32 adult teeth.
Each tooth has a crown and a root.
The crown has a layer of enamel, dentin, and an inner pulp with nerves and blood vessels that extend into the root.
The tongue mixes the chewed food with saliva and then forms the mixture into a mass called a bolus in preparation for swallowing.
Fluoride treatments make tooth enamel stronger and reduce dental caries or cavities. With aging, inflammation of the gums and periodontal membrane are more serious.

5. Adult mouth
The chisel-shaped incisors bite; the pointed canines tear; the flat premolars grind; and the flattened molars crush food. The last molar, called the wisdom tooth, mail fail to erupt, or if it does, it is sometimes crooked and useless. Often dentists recommend the extraction of the wisdom teeth.

6. Longitudinal section of a tooth
The crown is the portion that projects above the gum line and can be replaced by a dentist if damaged. When a “root canal” is done, the nerves are removed. When the periodontal membrane is inflamed, the teeth can loosen.

7. The Pharynx
The air passage and food passage cross in the pharynx because the trachea is ventral to the esophagus.
Swallowing occurs in the pharynx and is a reflex action.
During swallowing, the air passage is usually blocked off by the soft palate and uvula, and the trachea moves under the epiglottis to cover the glottis opening to the windpipe.
Breathing and swallowing cannot occur at the same time.

8. Swallowing
When food is swallowed, the soft palate closes off the nasopharynx, and the epiglottis covers the glottis, forcing the bolus to pass down the esophagus. Therefore, a person does not breathe during swallowing.

9. The Esophagus
The esophagus is a muscular tube that conducts food through the thoracic cavity and diaphragm into the stomach.
Peristalsis begins in the esophagus; this collapsed tube moves the bolus of food downward after swallowing occurs.
Heartburn is a burning pain when acidic stomach contents enter the esophagus.
Peristalsis is a rhythmic contraction that pushes food through the digestive tube. When peristalsis occurs with no food present, it produces a sensation of a lump in the throat.
When vomiting occurs, a contraction of the abdominal muscles and diaphragm propels the contents of the stomach upward through the esophagus.

10. No chemical digestion occurs in the esophagus.
The entrance of the esophagus to the stomach is marked by a constriction, called a sphincter; the sphincter must relax in order for food to enter the stomach.
The sphincter prevents food from backing up into the esophagus.
The sphincter between the esophagus and the stomach is not as developed as a true sphincter, but is still referred to as a sphincter.

11. The Wall of the Digestive Tract
The digestive tract wall has four layers:
Mucosa (mucous membrane – secretes digestive enzymes and mucus),
Submucosa (loose connective tissue – houses blood and lymph vessels),
Muscularis (two layers of smooth muscle - for peristalsis), and
Serosa (serous membrane – secretes serous fluid to prevent sticking).
The serosa secretes a serous fluid that keeps the outer surface of the intestines moist so that the organs of the abdominal cavity slide against one another. The esophagus has an outer layer of adventitia composed only of loose connective tissue.

12. Wall of the digestive tract
Several different types of tissues are found in the wall of the digestive tract. Note the placement of circular muscle inside longitudinal muscle. On the right is a micrograph of the wall of the esophagus.

13. The Stomach The stomach expands to store food.
Food in the stomach is churned, mixing the food with gastric juices containing hydrochloric acid and pepsin for the digestion of protein to peptides.
Alcohol, but not food, is absorbed here.
In 2–6 hours, the soupy chyme leaves the stomach.
Ulcers are usually caused by a bacterial infection.
The wall of the stomach is protected by a thick layer of mucus secreted by goblet cells in its lining. If, by chance, HCl penetrates this mucus, the wall can begin to break down, and an ulcer results. An ulcer is an open sore in the wall caused by the gradual disintegration of tissue. It now appears that most ulcers are due to a bacterial infection (Heliobacter pylori) that impairs the ability of goblet cells to produce protective mucus.

14. Anatomy and histology of the stomach
The stomach has a thick wall with folds that allow it to expand and fill with food. The mucosa contains gastric glands, which secrete mucus and a gastric juice active in protein digestion.

15. Ulcer
Shown here is a bleeding ulcer viewed through an endoscope (a tubular instrument bearing a tiny lens and a light source) inserted into the abdominal cavity.

16. The Small Intestine
The small intestine, averaging about 6 meters in length, is small in diameter.
The first 25 cm is the duodenum that receives bile from the gallbladder and pancreatic juice containing pancreatic lipase and trypsin for digestion of protein to peptides, as well as lipase for digestion of fat to glycerol and fatty acids.
Pancreatic juice contains NaHCO3 that is basic and neutralizes the acidic chyme.
Bile emulsifies fats – emulsification causes fat droplets to disperse in water.
NaHCO3 is sodium bicarbonate.

17. The small lymphatic capillary in a villus is called a lacteal.
Enzymes that finish the process of digestion are produced by the intestinal wall.
Walls of the small intestine have finger-like projections called villi where nutrient molecules are absorbed into the cardiovascular and lymphatic systems.
Villi have microvilli that increase the surface area available for absorption.
The small lymphatic capillary in a villus is called a lacteal.
The large surface area of the small intestine facilitates absorption of nutrients into the cardiovascular system (glucose and amino acids) and the lymphatic system (fats).

18. Anatomy of the small intestine
The wall of the small intestine has folds that bear fingerlike projections called villi. The products of digestion are absorbed into the blood capillaries and the lacteals of the villi.

19. Regulation of Gastric Secretions
Both the nervous system and chemicals called hormones regulate digestive juice secretion.
In response to eating protein foods, the hormone gastrin is produced by the lower part of the stomach and flows through the bloodstream to stimulate the stomach to produce digestive juice.
A hormone is a substance produced by one set of cells that affects a different set of cells, the so-called target cells. Hormones are usually transported by the bloodstream.

20. The duodenal wall produces gastric inhibitory peptide (GIP) to inhibit gastric gland secretion.
The hormones secretin and cholecystokinin (CCK) are produced by the duodenal wall and stimulate the pancreas to secrete digestive juice and the gallbladder to release bile.
Acidic chyme stimulates the secretion of secretin, while fatty chyme with protein triggers CCK release.

21. Hormonal control of digestive gland secretions
Gastrin (blue), produced by the lower part of the stomach, enters the bloodstream and thereafter stimulates the upper part of the stomach to produce more digestive juice. Secretin (green) and CCK (purple), produced by the duodenal wall, stimulate the pancreas to secrete its digestive juice and the gallbladder to release bile.

22. The Large Intestine
The large intestine consists of the cecum, colon, rectum and anal canal.
The large intestine does not produce digestive enzymes but does absorb water, salts, and some vitamins.
The colon includes the ascending colon, the transverse colon, the descending colon, and the sigmoid colon.

23. The appendix is an extension of the cecum.
Indigestible material is stored in the rectum until the anus allows defecation.
Anaerobic bacteria in the feces break down indigestible material and produce some vitamins.
Water tests that show the presence of the bacterium Escherichia coli indicate water is contaminated.
In humans, the appendix may play a role in fighting infections.
Bacterial action on indigestible materials causes the odor of feces and also accounts for the presence of gas. A breakdown product of bilirubin and the presence of oxidized iron causes the brown color of feces.

24. Junction of the small intestine and the large intestine
The cecum is the blind end of the ascending colon. The appendix is attached to the cecum.

25. Three Accessory Organs
The pancreas, liver, and gallbladder are accessory organs of digestion; their secretions assist digestion.
Accessory organs are not part of the digestive tube but produce enzymes and other substances that assist digestion.
These three accessory organs send secretions to the duodenum via ducts.

26. The Pancreas
The pancreas produces pancreatic juice, which contains digestive enzymes for carbohydrate (pancreatic amylase), protein (trypsin), and fat (lipase), along with sodium bicarbonate (NaHCO3) to neutralize acid in chyme.
The pancreas is also an endocrine gland that secretes insulin and glucagon, hormones that keep blood glucose within normal limits.
In cystic fibrosis, a thick mucus blocks the pancreatic duct, and the patient must take supplemental pancreatic enzymes by mouth for proper digestion to occur.

27. The Liver The liver produces bile, which is stored in the gallbladder.
Bile emulsifies fats; it is a yellowish-green substance containing bilirubin from hemoglobin breakdown and bile salts derived from cholesterol.
The liver acts as gatekeeper to the blood and receives blood from the small intestine by way of the hepatic portal vein.
Bilirubin is derived from the breakdown of hemoglobin, the red pigment of red blood cells.

28. Hepatic lobules
The liver contains over 100,000 lobules. Each lobule contains many cells that perform the various functions of the liver. They remove from and/or add materials to blood and deposit bile in bile ducts.

29. The functions of the liver are many: detoxifies blood,
stores iron and vitamins,
makes plasma proteins,
stores glucose as glycogen,
produces urea from amino acids,
removes bilirubin after dismantling blood cells, and
regulates blood cholesterol level when producing bile salts.
If the supply of oxygen is depleted, the liver converts glycerol (from fats) and amino acids to glucose molecules. The conversion of amino acids to glucose necessitates deamination, the removal of amino groups. The liver then combines ammonia with carbon dioxide to form urea. Urea is then excreted by the kidneys.

30. Hepatic portal system
The hepatic portal vein takes the products of digestion from the digestive system to the liver, where they are processed before entering a hepatic vein.

31. Liver Disorders
When a person has a liver disorder, jaundice may occur.
Jaundice is a yellowish tint to eyes and skin, indicating abnormal levels of blood bilirubin.
Hepatitis is inflammation of the liver; different strains of virus cause hepatitis A, B, etc.
Cirrhosis is scar tissue that can form when the liver is diseased or killed by exposure to alcohol.
In hemolytic jaundice, red blood cells have been broken down in abnormally large amounts; in obstructive jaundice, bile ducts are blocked or liver cells are damaged.
The liver has amazing regenerative powers and can recover if the rate of regeneration exceeds the rate of damage. During liver failure, however, there may not be enough time left to let the liver heal itself.

32. The Gallbladder
The gallbladder is a pear-shaped muscular organ that stores bile until it is sent to the duodenum.
Water is reabsorbed in the gallbladder making the bile thick and mucus-like.
Bile enters the duodenum via the common bile duct.
Gallstones are crystals of cholesterol.
The cholesterol content of bile can come out of solution and form crystals. If the crystals grow in size, they form gallstones. The passage of the stones from the gallbladder may block the common bile duct and cause obstructive jaundice. Then the gallbladder must be removed.

33. Digestive Enzymes
Digestive enzymes are present in digestive juices and introduce water at specific bonds to break down food into sugars, amino acids, fatty acids, and glycerol.
Starches are broken down into simpler sugars by salivary amylase and pancreatic amylase.
Pepsin in the stomach, and trypsin from the pancreas break proteins into peptides.

34. Peptidases and maltase, produced by the small intestine, complete the digestion of proteins and starches, respectively.
Glucose and amino acids are absorbed into the blood capillaries of the villi.
Fatty acids and glycerol rejoin in the villi to produce lipoprotein droplets which enter the lacteals.
Digestive enzymes speed specific reactions and function best at a warm body temperature and optimum pH.
Individuals with lactose (milk sugar) intolerance do not produce lactase, the enzyme that breaks down lactose.

35. Conditions for Digestion
For digestion to occur the correct enzyme, optimum pH, optimum temperature, and the correct substrate must be present.
Exact conditions can be determined during laboratory experiments.
Most digestive enzymes, aside from pepsin, require a basic pH.

36. Digestion experiment
This experiment is based on the optimum conditions for digestion by pepsin in the stomach. Knowing that the correct enzyme, optimum pH, optimum temperature, and correct substrate must be present for digestion to occur, explain the result of this experiment. Colors indicate pH of test tubes (blue, basic; red, acidic).

37. Nutrition
The U.S. Department of Agriculture recommends a diet high in carbohydrates (whole grains), at least five servings of fruits and vegetables, and limited but adequate protein as illustrated in a “food pyramid.”
Fats and sweets should be used sparingly.

38. Food guide pyramid: A guide to daily food choice
The U.S. Department of Agriculture uses a pyramid to show the ideal diet because it emphasizes the importance of eating grains, fruits, and vegetables. Meats and dairy products are needed in limited amounts; fats, oils, and sweets should be used sparingly.

39. Carbohydrates
Complex carbohydrates from foods like breads and pasta can be converted to glucose and used rapidly.
Body cells can utilize fatty acids as an energy source, but brain cells require glucose, thus carbohydrates are an essential part of the diet.
Complex, rather than simple, carbohydrates should make up the bulk of the diet.

40. Simple carbohydrates like table sugar (sucrose) contribute to energy needs and weight gain without supplying other nutrients.
Insoluble fiber helps regularity and may help prevent cancer by limiting the time substances are in contact with the intestinal wall.
Soluble fiber combines with bile acids and cholesterol in the intestine and prevents them from being absorbed.

41. Complex carbohydrates
To meet our energy needs, dietitians recommend consuming foods rich in complex carbohydrates, such as those shown here, rather than foods consisting of simple carbohydrates, such as candy and ice cream. Simple carbohydrates provide monosaccharides but few other types of nutrients.

42. Proteins
Meat, milk or eggs are complete proteins; they provide all 20 essential amino acids.
Because individual vegetables do not provide all essential amino acids, vegetarians must be careful to consume a combination of legumes, grains, vegetables, seeds and nuts to secure complementary proteins.
The amino acid pool relies on continual uptake; amino acids are not stored.
Table 12.4 (page 228 of the text) gives a list of complementary proteins in legumes, seeds and nuts, grains, and vegetables. Complementary proteins together supply the 20 essential amino acids; individually, these foods are lacking one or more amino acid.
Nutritionists do not recommend taking protein supplements.

43. Ancient versus modern diet of native Hawaiians
Among those Hawaiians who have switched back to the native diet, the incidence of cardiovascular disease, cancer, and diabetes has dropped.

44. Vitamins
Vitamins are organic compounds that the body cannot produce but needs for metabolic purposes; some are portions of coenzymes.
Vitamins A, E, and C are antioxidants that protect cell contents from damage due to free radicals.
Free radicals donate an electron to DNA, proteins, enzymes, membranes, etc. and can damage cell structures or cause cancer.
Table 12.6 lists the fat-soluble vitamins. Table 12.7 lists the water-soluble vitamins. Free radicals are unstable molecules produced by cell metabolism. Nutritionists suggest that it is better to get needed vitamins from food rather than from taking supplements.

45. Vitamin D
A precursor molecule in skin is converted to vitamin D after exposure to ultraviolet (UV) light.
Vitamin D is modified first in the kidneys and then the liver until it becomes calcitriol, which is needed for calcium absorption in intestines.
In the U.S., milk is often fortified by vitamin D.
Vitamin D is a fat-soluble vitamin.

46. Illnesses due to vitamin deficiency
Bowing of bones (rickets) due to vitamin D deficiency is shown at the left. In the middle photograph is an individual with dermatitis (pellagra) of areas exposed to light due to niacin (vitamin B3) deficiency. On the right, the bleeding of gums (scurvy) due to vitamin C deficiency is apparent.

47. Minerals
The body contains more than 5 grams of each major minerals and less than 5 grams of each trace minerals.
Calcium and phosphorus are in bones and teeth.
Potassium and sodium are involved in nerve conduction.
Trace minerals are critical in various enzymes and hormones.
Figure graphs both types of minerals and Table 12.8 lists their functions.

48. Minerals in the body
These charts show the usual amount of certain minerals in a 60-kilogram (135 lb) person. The functions of minerals are given in Table The major minerals are present in amounts larger than 5 grams (about a teaspoon). Trace minerals are present in lesser amounts.

49. Calcium
Calcium is needed to have strong bones.
Older women in particular are at risk for osteoporosis, a degenerative bone disease due to insufficient intake of calcium because bone cells are constantly building and eroding bone tissue.
Calcium supplement with vitamin D (and also estrogen for women) can help prevent this bone loss.

50. Most Americans have too much salt in their diet.
Sodium
Most Americans have too much salt in their diet.
High sodium intake is linked to hypertension in some persons.
About one-third of the sodium we consume occurs naturally in foods; another one-third is added during commercial processing; and the final one-third is added during cooking or at the table in the form of table salt.
Table 12.9 (page 233 of the text) gives suggestions of how to reduce dietary sodium.

51. Eating Disorders
Obesity is defined as a body weight of more than 20% above the ideal weight for that person.
Obesity can have hormonal, metabolic, and social causes.
For many, a commitment to a sensible diet and exercise program can prevent obesity or a harmful cycle of weight gain-and-loss.

52. Recognizing obesity
Persons with obesity have: weight 20% or more above appropriate weight for their height; body fat in excess of that consistent with optimal health, probably due to a diet rich in fats; and low levels of exercise.

53. Bulimia nervosa is characterized by a restrictive diet, binging, and purging.
Psychotherapy and antidepressants may help.
Anorexia nervosa is characterized by a distorted body image and feeling fat even when emaciated.
It can be life-threatening and carries the same risks as starvation.

54. Recognizing bulimia nervosa
Persons with bulimia nervosa have: recurrent episodes of binge eating characterized by consuming an amount of food much higher than normal for one sitting and a sense of lack of control over eating during the episode; an obsession about their body shape and weight, but often without exercising; and an increase in fine body air, halitosis, and gingivitis. Body weigh in these individuals is regulated by: a restrictive diet, excessive exercise; and/or purging (self-induced vomiting or misuse of laxatives).

55. Recognizing anorexia nervosa
Persons with anorexia nervosa have: a morbid fear of gaining weight and body weight is no more than 85% of normal; a distorted body image so that person feels fat even when emaciated; and, in females, an absence of a menstrual cycle for at least three months.
Body weight is kept too low by either/or: a restrictive diet, often with excessive exercise; and binge eating/purging (person engages in binge eating and then self-induces vomiting or misuses laxatives).

56. Chapter Summary
The human digestive tract is a tube with specialized regions and organs between the mouth and the anus.
Food is ingested, mechanically processed, and chemically digested to small molecules that are absorbed; indigestible remains are eliminated.

57. The mouth, pharynx, esophagus, stomach, small and large intestines have distinct functions and hormones control digestive gland secretions.
The pancreas, liver, and gallbladder are accessory organs of digestion; their secretions assist digestion.
The products of digestion are small molecules, such as amino acids and glucose, that can cross plasma membranes.

58. Digestive enzymes are specific and have an optimum temperature and pH.
Proper nutrition supplies the body with energy and nutrients, including essential amino acids and fatty acids, and all vitamins and minerals.
Anorexia nervosa, bulimia, and obesity are primary eating disorders in the United States.