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Chapter 12: Digestive System and Nutrition


Chapter 12: Digestive System and Nutrition The Digestive Tract The human digestive tract is a tube with specialized regions and organs between the mouth and the anus. – PowerPoint PPT presentation

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Title: Chapter 12: Digestive System and Nutrition

Chapter 12 Digestive System and Nutrition
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

Digestive system
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
  • The roof of the mouth is formed by the hard and
    soft palates that separate it from the nasal
  • The soft palate ends in a finger-shaped
    projection called the uvula.

  • 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
  • Three pairs of salivary glands send saliva
    (containing salivary amylase for digestion of
    starch to maltose) into the mouth.

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.

Adult mouth
Longitudinal section of a tooth
The Pharynx
  • The air passage and food passage cross in the
    pharynx because the trachea is ventral to the
  • Swallowing occurs in the pharynx and is a reflex
  • 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.

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.

  • 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 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).

Wall of the digestive tract
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
  • Alcohol, but not food, is absorbed here.
  • In 26 hours, the soupy chyme leaves the stomach.
  • Ulcers are usually caused by a bacterial

Anatomy and histology of the stomach
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
  • Pancreatic juice contains NaHCO3 that is basic
    and neutralizes the acidic chyme.

  • 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.

Anatomy of the small intestine
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.

  • 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.

Hormonal control of digestive gland secretions
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
  • The colon includes the ascending colon, the
    transverse colon, the descending colon, and the
    sigmoid colon.

  • 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

Junction of the small intestine and the large
  • Polyps are small growths arising from the
    epithelial lining that may be benign or
  • 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.

  • Water and fiber in the diet can prevent
    constipation where the feces become too dry and
  • 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.

Defecation reflex
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.

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.

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

Hepatic lobules
  • 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,
  • regulates blood cholesterol level when producing
    bile salts.

Hepatic portal system
Liver Disorders
  • When a person has a liver disorder, jaundice may
  • 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

The Gallbladder
  • The gallbladder is a pear-shaped muscular organ
    that stores bile until it is sent to the
  • Water is reabsorbed in the gallbladder making
    the bile thick and mucus-like.
  • Bile enters the duodenum via the common bile
  • Gallstones are crystals of cholesterol.

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.

  • 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
  • Digestive enzymes speed specific reactions and
    function best at a warm body temperature and
    optimum pH.

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.

Digestion experiment
  • 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.

Food guide pyramid A guide to daily food choice
  • Complex carbohydrates from foods like breads and
    pasta can be converted to glucose and used
  • 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.

  • 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.

Complex carbohydrates
  • 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.

Ancient versus modern diet of native Hawaiians
  • Fat and cholesterol are lipids.
  • Lipids, found in fats and oils, should be used
  • 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.

  • 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.

Fake Fat
  • Olestra 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
  • Those who consume olestra have reduced
    carotenoids in their blood.

  • 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
  • Free radicals donate an electron to DNA,
    proteins, enzymes, membranes, etc. and can damage
    cell structures or cause cancer.

  • Vitamin D
  • A precursor molecule in skin is converted to
    vitamin D after exposure to ultraviolet (UV)
  • 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
  • Vitamin D is a fat-soluble vitamin.

Illnesses due to vitamin deficiency
  • The body contains more than 5 grams of each major
    minerals and less than 5 grams of each trace
  • Calcium and phosphorus are in bones and teeth.
  • Potassium and sodium are involved in nerve
  • Trace minerals are critical in various enzymes
    and hormones.

Minerals in the body
  • 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

  • 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.

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
  • For many, a commitment to a sensible diet and
    exercise program can prevent obesity or a harmful
    cycle of weight gain-and-loss.

Recognizing obesity
  • 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.

Recognizing bulimia nervosa
Recognizing anorexia nervosa
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.

  • 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.

  • 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.
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