UNIT 8 The Digestive System Part 2 of 2 (Chapter 23)

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UNIT 8 The Digestive System Part 2 of 2 (Chapter
23)

• Functional Anatomy - The Liver and Gallbladder
• Functional Anatomy - The Pancreas
• Functional Anatomy - The Large Intestine
• Digestive Physiology Motility, Secretion,
  Digestion, and Absorption
• Digestive Pathologies and Disorders

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Functional Anatomy - The Liver and Gallbladder

• Functions of the Liver
• largest gland in the body
• performs over 500 functions, including many
  metabolic functions
• converts glucose into glycogen
• detoxifies many poisons and drugs (e.g. alcohol)
• hepatocytes (liver cells) produce bile, an
  emulsifier of fats - it turns large droplets of
  fat into many smaller droplets, thus increasing
  surface area and promoting more efficient fat
  digestion (more surface area for enzymes to act
  upon) remember that small objects have more
  relative surface area (higher surface area to
  volume ratios)
• Gross Anatomy of the Liver (fig. 23.23)
• 4 lobes right lobe, left lobe, caudate lobe,
  quadrate lobe
• ligamentum venosum (remnant of fetal circulation
  in the fetus it served as a liver bypass),
  ligamentum teres (remnant of the fetal umbilical
  vein), and falciform ligament (a mesentery)
• hepatic portal vein carries blood from digestive
  tract to the liver
• bile is stored and concentrated in the
  gallbladder (note the gallbladder does not make
  bile that is the job of the liver)

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Functional Anatomy - The Liver and Gallbladder

• Microscopic Anatomy of the Liver (fig. 23.24)
• liver lobules containing hepatocytes (liver
  cells) radiate out from a central vein
• portal triad portal arteriole portal venule
  a bile duct
• Kupffer cells - destroy bacteria and other
  foreign particles
• Bile Flow (fig. 23.20)
• when the hepatopancreatic sphincter is relaxed
  bile flows from the right and left hepatic ducts
  of the liver into the common hepatic duct, then
  into the (common) bile duct, out through the
  hepatopancreatic ampulla, and finally into the
  duodenum where it is needed
• when the hepatopancreatic sphincter is
  contracted bile backs up into the (common) bile
  duct and cystic duct, and finally the
  gallbladder, where it is stored
• Thus, the hepatopancreatic sphincter controls
  whether bile is secreted or stored when no
  digestion is occurring the sphincter remains
  closed

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Functional Anatomy - The Pancreas

• Functions
• both an exocrine gland (enzymes) and endocrine
  gland (hormones)
• makes, stores, and secretes enzymes for digestion
• produces the hormones glucagon and insulin to
  regulate blood glucose
• Anatomy of the Pancreas (fig. 23.20)
• head and tail the head touches the duodenum and
  the tail connects to the spleen
• main pancreatic duct and accessory pancreatic
  duct
• hepatopancreatic ampulla and sphincter control
  release of pancreatic secretions
• Microscopic Anatomy - pancreatic islets (islets
  of Langerhans) contain clusters of hormone
  secreting cells (secrete insulin and glucagon)

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Functional Anatomy - The Large Intestine

• Functions
• small amount of digestion by bacteria
• absorption of water and electrolytes to form
  feces
• Gross Anatomy of the Large Intestine (fig. 23.29)
• cecum - first portion of the large intestine
  connects to the ileum at the ileocecal valve
• vermiform appendix - worm-shaped has some
  lymphatic function
• colon - ascending, transverse, descending, and
  sigmoid segments
• rectum - located in the pelvic region
• anal canal (fig. 23.29b)- internal anal sphincter
  (smooth muscle) and external anal sphincter
  (skeletal muscle) remember that you have
  voluntary control over skeletal muscle but not
  smooth muscle thus, you can voluntarily contract
  the external anal sphincter, but not the internal
  anal sphincter
• teniae coli - maintain constant muscle tone to
  constrict the large intestine and form pouches
  called haustrae

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Functional Anatomy - The Large Intestine

• Microscopic Anatomy of the Large Intestine
• unlike the small intestine, there is an absence
  of villi and microvilli because digestion and
  absorption are not the primary functions of the
  large intestine, it does not need the added
  surface area provided by villi and microvilli
• like the small intestine, the lining is composed
  of simple columnar epithelium

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Digestive Physiology MOTILITY

• two purposes
• moving food from mouth to anus
• mechanically mixing food to break it down into
  smaller pieces, to increase surface area for
  exposure to digestive enzymes
• nerves (e.g. vagus nerve), hormones, and
  paracrines can alter motility parasympathetic
  input from the vagus nerve increases motility
  (remember that we call the parasympathetic
  nervous system the rest and digest part)
• smooth muscle of the GI tract is connected by gap
  junctions to create contracting segments
• like cardiac muscle, autorhythmic cells
  spontaneously depolarize to cause contraction in
  this case, contraction of smooth muscle in the
  wall of the GI tract

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Digestive Physiology MOTILITY

• peristaltic contractions (fig. 23.3)
  peristalsis progressive waves of contraction
  move from one section of the GI tract to another
  the ANS, hormones, and paracrines influence
  peristalsis in all regions of the GI tract
• circular muscle contract just behind a bolus
  (mass of food)
• this pushes the bolus forward into a receiving
  segment, where the circular muscles are relaxed
• the receiving segment then pushes the mass
  forward, continuing the forward movement
• segmental contractions (fig. 23.3) - segments of
  the small intestine contract and relax
  alternating segmental contractions churn
  intestinal contents back and forth, mixing them
  and keeping them in contact with the absorptive
  epithelium

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Digestive Physiology SECRETION

• typically, about 9 liters of fluid pass through
  an adults GI tract in one day most is
  reabsorbed, and thus not lost to the external
  environment
• fluid input about 2.0 L from food and drink
  7.0 L from digestive secretions (e.g. saliva,
  bile, mucus, and various digestive enzymes)
• fluid removal from GI tract about 7.5 L
  absorbed from small intestine and 1.4 L absorbed
  from large intestine 0.1 L excreted in the feces
• some enzymes are secreted in an inactive form
  for example, specialized cells in the stomach
  produce pepsinogen (inactive) the inactive
  pepsinogen does not damage the cells that produce
  it later, upon encountering a lower pH
  environment, the pepsinogen is converted into the
  active form, pepsin
• mucus is a viscous secretion - it forms a
  protective coating over the mucosa of the GI
  tract, and helps to lubricate the contents of the
  gut
• mucous cells - secrete mucus in the stomach
• goblet cells - part of the simple columnar
  epithelium of the intestine secrete mucus in the
  small and large intestines
• salivary glands - secrete mucus in the saliva of
  the mouth

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Digestive Physiology DIGESTION and ABSORPTION

• digestion of macromolecules (carbohydrates,
  proteins, and fats) is accomplished by a
  combination of mechanical and chemical processes
• chewing and churning (mechanical) create smaller
  pieces of food, which expose more surface area to
  digestive enzymes segmentation (mechanical)
  results in food mixing
• bile from the liver creates droplets of lipids
  (fats) with greater surface area bile is an
  emulsifier of fats
• the optimum pH for various digestive enzymes
  reflects the location where they are most active
  enzymes in the stomach have an acidic optimum pH,
  whereas enzymes in the small intestine work best
  at alkaline (basic) pH
• most nutrient absorption takes place in the small
  intestine some additional absorption of water
  and ions also occurs in the large intestine

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Digestive Physiology DIGESTION and ABSORPTION

• carbohydrate digestion (fig. 23.33)
• most carbohydrates are ingested in the form of
  starch (a polysaccharide) and disaccharides, such
  as sucrose (table sugar), lactose (milk sugar),
  and maltose other dietary carbohydrates include
  monosaccharides (simple sugars) such as glucose
  and fructose, and the polysaccharides glycogen
  and cellulose (fiber)
• we are unable to digest cellulose, also known as
  fiber, because we lack the necessary enzymes
• the enzyme amylase breaks down starch into
  smaller glucose chains and into the disaccharide
  maltose
• other enzymes (e.g. maltase, lactase, and
  sucrase) break down their corresponding
  disaccharides into monosaccharides maltase
  breaks maltose down into 2 glucose molecules,
  lactase breaks lactose down into glucose and
  galactose, and sucrase breaks sucrose down into
  glucose and fructose the disaccharides must be
  broken down into monosaccharides before they can
  be absorbed across the epithelium of the GI tract

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Digestive Physiology DIGESTION and ABSORPTION

• protein digestion (fig. 23.33)
• plant protein is the least digestible, whereas
  animal protein is the most digestible (egg
  protein is the best, with 85-90 in a form that
  can be digested and absorbed)
• two broad groups of protein enzymes
  endopeptidases and exopeptidases endopeptidases
  attack peptide bonds in the interior of the amino
  acid chain, whereas exopeptidases chop off single
  amino acids from the ends of the amino acid
  chains

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Digestive Physiology DIGESTION and ABSORPTION

• lipid digestion (fig. 23.33)
• on average, about 90 of our ingested fat comes
  from triglycerides, because they are the primary
  form found in both plants and animals
• other lipid molecules in our diet include
  cholesterol, phospholipids, and fat-soluble
  vitamins
• most lipids are not water-soluble therefore,
  they appear as clumps in the chyme solution of
  the digestive tract thus, they must be first
  broken down into smaller particles (via bile)
  before digestion can proceed
• enzymatic fat digestion is carried out by
  lipases these enzymes remove 2 fatty acids from
  triglycerides, resulting in a glycerol, a
  monoglyceride, and two free fatty acids
  phospholipids are digested by pancreatic
  phospholipase free cholesterol does not need to
  be broken down before it is absorbed

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Digestive Pathologies and Disorders

• Gastroesophageal Reflux Disease (GERD) - abnormal
  relaxation or weakness of the narrowed area at
  the esophageal-stomach junction (at the cardia)
  symptoms include heartburn, regurgitation of
  stomach contents, and belching persistent
  exposure to stomach acid can lead to an
  esophageal ulcer
• Peptic Ulcer - a craterlike erosion of the mucosa
  of any part of the alimentary canal that is
  exposed to stomach acid usually occur in the
  pyloric region (pylorus) of the stomach or the
  duodenum
• Gallstones - either too much cholesterol or too
  little bile salts can lead to the crystallization
  of cholesterol in the gallbladder producing
  gallstones the gallstones can block the cystic
  duct, and thus require surgery to remove the
  gallbladder
• Cirrhosis (of the Liver) - a progressive
  inflammation of the liver that usually results
  from chronic alcoholism resulting scar tissue
  can impede the flow of blood through the liver
• Diarrhea - pathological state in which intestinal
  secretion of fluid is not balanced by absorption,
  resulting in watery stools in extreme
  circumstances (e.g. cholera) can cause severe
  dehydration and even death if not treated
• Constipation - caused by consciously ignoring the
  defecation reflex or through decreased motility
  continued water absorption creates hard, dry
  feces that are difficult to expel
• Vomiting (emesis) - forceful expulsion of
  gastric (stomach) and duodenal contents from the
  mouth protective reflex designed to remove toxic
  material from the GI tract before it can be
  absorbed

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