Chapter 12: Digestive System and Nutrition

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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 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.

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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 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.

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Adult mouth
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Longitudinal section of a tooth
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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.

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Swallowing
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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.

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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 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 26 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 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.

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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 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.

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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 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.

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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 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.

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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.

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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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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.

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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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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.

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Ancient versus modern diet of native Hawaiians
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Lipids

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

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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.

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• 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.

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Illnesses due to vitamin deficiency
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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.

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Minerals in the body
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• 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.

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• 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.

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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.

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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.