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Chapter 12: Digestive System and Nutrition
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The Digestive Tract
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.
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Digestive system
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The MouthLips 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.
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Tonsils at the back sides of the mouth protect against infections.
Tonsillitis results when the tonsils become inflamed; the infection can spread to the middle ears.
Three pairs of salivary glands send saliva (containing salivary amylase for digestion of starch to maltose) into the mouth.
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The TeethTwenty 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.
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Adult mouth
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Longitudinal section of a tooth
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The PharynxThe 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.
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Swallowing
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The EsophagusThe 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.
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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.
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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).
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Wall of the digestive tract
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The StomachThe 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.
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Anatomy and histology of the stomach
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Ulcer
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The Small IntestineThe 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.
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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.
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Anatomy of the small intestine
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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.
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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.
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Hormonal control of digestive gland secretions
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The Large IntestineThe 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.
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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.
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Junction of the small intestine and the large intestine
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Polyps are small growths arising from the epithelial lining that may be benign or cancerous.
Diarrhea and constipation are two common complaints of the large intestine.
Causes of diarrhea include infection of the lower tract and nervous stimulation, both moving feces more rapidly than normal, but also causing dehydration if prolonged.
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Water and fiber in the diet can prevent constipation where the feces become too dry and hard.
Hemorrhoids are enlarged and inflamed blood vessels at the anus; this condition is associated with chronic constipation.
Regular elimination reduces the time the colon wall is exposed to cancer-promoting agents in the feces and may help prevent cancer.
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Defecation reflex
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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.
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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.
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The LiverThe 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.
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Hepatic lobules
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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.
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Hepatic portal system
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Liver DisordersWhen 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.
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The GallbladderThe 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.
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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.
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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.
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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.
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Digestion experiment
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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.
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Food guide pyramid: A guide to daily food choice
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CarbohydratesComplex 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.
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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.
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Complex carbohydrates
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ProteinsMeat, 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.
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Ancient versus modern diet of native Hawaiians
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LipidsFat and cholesterol are lipids.
Lipids, found in fats and oils, should be used sparingly.
Current guidelines suggest that fat should account for 30% or less of daily calories.
High-density lipoproteins (HDL) carry cholesterol to the liver and is considered to be “good”.
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Low density lipoprotein (LDL) takes cholesterol to the cells and may contribute to the development of plaque on blood vessels walls; it is considered to be “bad”.
Saturated fatty acids lack double bonds and raise LDL cholesterol levels.
Linoleic acid and linolenic acid are two essential fatty acids the body cannot make; polyunsaturated fats supply these.
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Fake FatOlestra looks, tastes, and acts like real
fat but the digestive system cannot digest it; therefore, it is called “fake fat”.
However, fat-soluble vitamins are taken up by olestra and pass through the digestive system unabsorbed.
Those who consume olestra have reduced carotenoids in their blood.
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VitaminsVitamins 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.
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Vitamin DA 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.
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Illnesses due to vitamin deficiency
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MineralsThe 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.
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Minerals in the body
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CalciumCalcium 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.
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SodiumMost 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.
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Eating DisordersObesity 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.
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Recognizing obesity
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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.
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Recognizing bulimia nervosa
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Recognizing anorexia nervosa
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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.
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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.
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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.
