Digestion, Absorption, and Metabolism

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Digestion, Absorption, and Metabolism
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Definitions

• Digestion
• Mechanical and chemical process in which food is
  broken down in the GI tract, releasing many
  nutrients in forms the body can use
• Absorption
• Process in which released nutrients are taken
  into the cells lining the GI tract
• Metabolism
• The sum of the body processes (catabolic and
  anabolic) involved in converting food necessary
  for energy, tissue building, and metabolic
  controls

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Digestion Basic Principles

• Principle of change
• The body cannot use food as it is eaten.
• Food must be changed into simpler substances to
  be absorbed and then used by cells to sustain
  life.
• Principle of wholeness
• The parts of the digestive process comprise a
  continuous whole.
• Food components travel through the
  gastrointestinal (GI) system until they are
  delivered to cells or excreted.

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The Gastrointestinal System

• Tube that extends from the mouth to the anus in
  which the movement of muscles and release of
  hormones and enzymes digest food (also called the
  alimentary canal or gastrointestinal tract)
• The major parts of the gastrointestinal system
• Mouth and Salivary glands
• Pharynx and Esophagus
• Stomach
• Small Intestine and Large Intestine
• Rectum
• Accessory organs liver, gallbladder, pancreas

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Functions of the GI System

• Motility
• Movement through the GI tract
• Digestion
• Mechanical and Chemical
• Secretion and Absorption
• Into the GI tract (secretion) or into blood
  (absorption)
• Storage
• Elimination

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Digestion Mechanical and Chemical Changes

• Through a balanced system of mechanical and
  chemical digestion, food is broken down into
  smaller substances and the nutrients are released
  for biologic use.
• Mechanical and chemical actions make up the
  digestive process.
• Food must undergo these changes to be delivered
  to cells.

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Mechanical Digestion

• Breaking down of food and moving it along the GI
  tract
• Mastication
• Process of chewing and biting food in the mouth
• First step in digestion
• GI motility
• Beginning in the mouth, muscles and nerves in the
  GI tract coordinate their actions to provide
  motility, an automatic response to the presence
  of food.

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Mechanical Digestion

• Layers of the GI tract provide two types of
  movement tonic contraction and peristalsis
• Tonic contraction
• Ensures continuous passage of the food mass and
  valve control along the way
• Peristalsis
• Periodic muscle contraction and relaxation which
  are rhythmic waves that mix the food mass and
  move it forward

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Mechanical Digestion

• Nerves
• The intramural nerve plexus is the network of
  nerves in the GI wall extending from the
  esophagus to the anus
• These nerves do three things
• Control muscle tone in the wall
• Regulate the rate and intensity of alternating
  muscle contractions.
• Coordinate all the various movements
• Problems with the nerves in the digestive tract
  can cause problems with the digestive process.

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Chemical Digestion

• Substances that break down food
• Digestive Enzymes specific proteins that break
  down nutrients
• Hydrochloric Acid (HCL) acid made by the stomach
  that breaks down proteins in the foods we eat and
  lowers pH and total alkalinity
• HCL activates digestive enzymes that break down
  food into small particles for absorption
• Buffer Ions produces correct pH for enzyme
  activity

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Chemical Digestion

• Substances that break down food
• Mucus A clear liquid made by the intestines.
  Lubricates and protects the GI tract tissues and
  helps mix the food mass
• Water and Electrolytes Carry and circulate the
  products of digestion through the tract and into
  the tissues. Major electrolytes in the body
  include sodium, potassium, magnesium, calcium,
  chloride, and phosphate.
• Bile A digestive fluid made by the liver and
  stored in the gallbladder which divides fat into
  smaller pieces to assist fat enzymes

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Digestion of a Meal

•  You consumed the following for lunch at
  McDonalds 
• Big Mac (542 calories, 279 fat calories and 52
  fat)
• Small French Fries (211calories, 45 fat calories
  and 47 fat calories)
• Small Chocolate Milk Shake (380 calories, 81 fat
  calories and 21 fat calories )
• Cherry pie (298 calories, 162 fat calories and
  54 fat)
• That is a whopping 1,431 calories, 567 fat
  calories and 40 fat.
• That is an artery clogging meal. I hope you
  remain awake to learn how it digests!

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Digestion in the Mouth and Esophagus

• Mechanical Digestion
• Mastication (biting and chewing) breaks down food
• Tongue pushes bolus (food mass) to pharynx
  (throat)
• Food is swallowed and passes down esophagus (long
  hollow tube from pharynx to stomach) by
  peristaltic waves controlled by nerves
• Gastroesophageal sphincter at stomach entrance
  relaxes, allowing food to enter, then constricts
  to retain food.
• If the gastroesophageal sphincter is not working
  properly, acid and mixed food seep back into
  esophagus, resulting in heartburn

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Digestion in the Mouth and Esophagus

• Chemical Digestion
• Saliva mixture of water, protein, and salts that
  makes food easy to swallow
• Salivary glands (parotid gland, sublingual gland,
  and submandibular gland) secrete material
  containing salivary amylase or ptyalin (enzyme
  that breaks down carbohydrates)
• Ebners glands at the back of the tongue secrete
  a lingual lipase (enzyme that breaks down fat)

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Digestion in the Mouth and Esophagus

• Chemical digestion
• Salivary glands also secrete a mucous material to
  lubricate and bind food particles, facilitating
  the swallowing of the food bolus.
• Secretions from the mucous glands in the
  esophagus help move food toward the stomach.

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The Stomach

• Regions of the stomach
• Fundus upper portion of the stomach
• Pyloric region (pyloric sphincter) opening or
  valve
• Duodenum first part of the small intestine
• Functions
• Store food
• Initiate digestion of proteins
• Kill bacteria with the strong acidity (low pH of
  the gastric juice)
• Make chyme

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The Stomach
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Stomach Digestion

• Mechanical Digestion
• Under sphincter control, the food enters the
  upper portion of the stomach as individual bolus
  lumps
• Stomach muscles knead, store, mix, and propel the
  food mass forward
• By the time the food mass reaches the lower
  portion of the stomach, it is a semiliquid
  acid/food mix called chyme
• Chyme is released slowly into the first section
  of the small intestine (duodenum) by the pyloric
  valve

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Stomach Digestion

• Chemical Digestion three types of gastric
  secretions
• Hydrochloric Acid Parietal cells in the stomach
  lining secrete acid to promote gastric enzyme
  activity
• Mucus Secretions protect the stomach lining from
  the erosive effect of the acid and also bind and
  mix the food mass and help move it along.
• Enzymes Pepsinogen is secreted by stomach cells
  and activated by acid to become pepsin, a
  protein-splitting enzyme

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Stomach Digestion

• Various sensations and emotions stimulate the
  nerve impulses that trigger these secretions
• Anger and stress can increase secretions
• Sadness and fear can decrease secretions and
  diminish blood flow
• Do you feel more or less hungry when you are
• Stressed, sad, or angry,

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Stomach Digestion

• High-calorie foodsespecially fatty foodsempty
  out of the stomach at a slower rate thus the
  feeling of fullness lasts longer.
• Caloric density influences the rate in which the
  stomach empties
• Problems with valves and sphincters can cause
  reflux and discomfort

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Small Intestine

• About 20 feet in length
• Small in diameter compared to large intestine
• Fills most of the abdomen
• Functions
• Peristalsis
• Protection against infection
• Hormone secretion (secretin and cholecystokinin)
• Majority of food digestion

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Small Intestine

• Three regions of the small intestine
• Duodenum upper region-connecting stomach to
  ileum
• Absorbs carbohydrates, lipids, amino acids, Ca2,
  iron
• Jejunum middle and largest region
• Absorbs carbohydrates, lipids, amino acids, Ca2,
  iron
• Ileum lower region
• Absorbs bile salts, vitamin B12, water and
  electrolytes
•  

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Small Intestine
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Small Intestine Digestion

• Mechanical Digestion
• Under nerve control, intestinal muscles produce
  several types of movements as follows
• Peristaltic waves slowly push food mass forward
• Pendular movements sweep back and forth
• Segmentation rings chop food mass into successive
  soft lumps and mix them with secretions
• Longitudinal rotation rolls food in a spiral
  motion, exposing new surfaces for absorption

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Small Intestine Digestion

• The small intestine together with accessory
  organs (pancreas, liver, and gallbladder) secrete
  enzymes
• Pancreatic enzymes
• Carbohydrate Pancreatic amylase converts starch
  to maltose and sucrose
• Protein Trypsin and chymotrypsin split large
  protein molecules into small peptide fragments
  and eventually into single amino acids
• Fat Pancreatic lipase converts fat to glycerides
  and fatty acids

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Small Intestine Digestion

• Intestinal enzymes
• Carbohydrate Disaccharides (maltose, lactose,
  and sucrose) converted into monosaccharides
  (glucose, galactose, and fructose)
• Protein Enterokinase activates trypsinogen from
  the pancreas to split the following
• polypeptides dipeptides amino
  acids
• Fat Intestinal lipase splits fat into glycerides
  and fatty acids

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Small Intestine Digestion

• Mucus is important because the contents entering
  from the stomach are highly acidic and it
  protects the mucosal lining from irritation and
  erosion
• Bile emulsifying agent produced by the liver and
  stored in the gallbladder which breaks fat into
  smaller pieces so it can be broken down easier
• Hormone secretin keeps the environment alkaline
  at a pH of 8 so pancreatic enzymes can work.
• Hormone cholecystokinin triggers release of bile
  from the gallbladder

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What Happened to the McDonalds Meal?

• Carbohydrates in bun, fries, and cherry pie
  reduced to simple sugars (glucose, fructose,
  galactose)
• Proteins in hamburger patty and milk shake become
  amino acids
• Lipids in burger, fries, milk shake and cherry
  pie become fatty acids and glycerides
• Lettuce is reduced to cellulose or fiber
• Any vitamins and minerals liberated from food
• Excess calories are stored as fat in adipose
  tissue
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Bioavailability

• Bioavailability refers to
• Amount of nutrient present in the GI tract
• Competition between nutrients for absorption
• Form in which the nutrient is present
• All nutrients present in a food are not absorbed
  because of differing bioavailability.
• This is considered when determining dietary
  intake standards

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Absorption in the Small Intestine

• The inner surface of the small intestine is 600
  times larger than its outer area, facilitating
  the delivery of nutrients into circulation
• Three absorbing structures
• Mucosal Folds surface of small intestine piles
  into folds
• Villi small, finger-like projections cover the
  mucosal folds, increasing the area of exposed
  intestinal surface
• Microvilli smaller projections cover each villi
  (look like bristles on a brush)

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Absorption in the Small Intestine

• Each villus can receive protein and carbohydrate
  material as well as receive fat materials in its
  lymph vessels
• These lymph vessels are called lacteal
• If the entire surface of the small intestine were
  spread out, it would be half the size of a
  basketball court

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Large Intestine

• 4 to 5 feet in length
• Populated by bacteria which synthesize Vitamin K
• Consists of the cecum and colon
• Primary function is to reabsorb water from the
  stool and eliminate the remaining relatively dry
  waste
• Food and drink consumption and intestinal
  secretions result in approximately 5 gallons
• (19 L) of fluid being placed in the large
  intestine daily

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Large Intestine

• Water is taken up by the large intestine
• Most water in chyme is absorbed in the first half
  of the colon
• Only about 100 mL of water is left in the large
  intestine after the digestion process to form and
  eliminate feces
• Dietary fiber is not digested
• Contributes bulk to food mass and gas
• What foods high in dietary fiber is associated
  with gas?
• Helps form feces

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Transport

• The vascular system (blood circulation) is
  responsible for supplying the body with
  nutrients, oxygen, and other vital substances
• Veins and arteries
• Transports waste, such as carbon dioxide and
  nitrogen, to lungs and kidneys for removal
• Water-soluble products of digestion can be
  absorbed into the vascular system.
• Nutrients travel to the liver by the portal
  circulation, then to cells throughout the body

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Transport

• Lymphatic system
• Route for fatty materials, which are not water
  soluble
• Fat molecules pass into lymph vessels in villi

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Metabolism
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Energy Storage Glycogenesis

• Anabolic process of converting extra glucose into
  glycogen
• Glycogen is stored in the liver and muscles for
  quick energy to be used at a later time

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Energy Storage Lipogenesis

• When glycogen reserves are full, additional
  excess energy from carbohydrates, fat, or protein
  is stored as fat in adipose tissue
• Lipogenesis The building up of triglycerides for
  storage in adipose tissue

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Energy Storage Gluconeogenesis

• Conversion of amino acids to glucose
• Excess protein is not stored as muscle but is
  further broken down
• Nitrogen unit is removed
• Remaining carbon chain can be converted to
  glucose (if needed) or to fat for storage

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Metabolic Pathways of Excess Energy
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Alcohol

• Alcohol is a food-7 kcal/gram
• Cannot be stored in the body-toxic
• Absorbed without chemical digestion
• Alcohol metabolism is first because a byproduct
  of alcohol is acetaldehyde-which destroys healthy
  tissue
• Liver detoxifies alcohol and uses remaining to
  produce fatty acids-triglycerides stored in liver
• Single drinking binge can accumulate fat in the
  liver
• Can lead to liver disease
• Blood alcohol levels peak after one drink in
  30-45 minutes 12oz of beer, 5 oz of wine, 1.5 oz
  of 80 proof alcohol
• www.health.org

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Discussion Questions

• What is the difference between mechanical and
  chemical digestion?
• What functions do the following perform
• Stomach
• Small Intestine
• Pancreas
• Gallbladder
• Liver
• Large Intestine