The Digestive System

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The Digestive System

• Functions to break down food substances into
  forms that can be absorbed

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Types of Digestion

• Mechanical
• Chemical

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Digestive Processes

• Ingestion
• Digestion
• Absorption
• Elimination

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Divisions of the Digestive System

• Gastrointestinal tract
• Oral cavity, pharynx, esophagus, stomach, large
  and small intestines
• Accessory structures
• Teeth, tongue, salivary glands, liver, pancreas,
  and gallbladder

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Layers of the GI Tract

• Mucosa - innermost layer
• Epithelium - stratified squamous (pharynx and
  esophagus) and simple columnar
• Lamina propria - areolar connective tissue
• Muscularis - smooth muscle layer causing small
  folds

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Layers of GI Tract

• Submucosa - loose connective tissue containing
  blood vessels, nerves, glands, and lymphatics
• Muscularis - two layers of smooth muscle
• inner circular layer and outer longitudinal one
• some skeletal muscle

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• Serosa - outer mesothelium
• Peritoneum (parietal and visceral)
• Extensions mesentery, mesocolon, greater and
  lesser omentum, falciform ligament

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Organs and Structures
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Mouth

• Vestibule
• Tongue
• Teeth
• Salivary glands

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Mouth or Buccal Cavity

• Vestibule- area bounded by the lips and cheeks
  externally/internally by the gums and teeth
• Tongue - striated skeletal muscle that
  manipulates food, help form words, and serves as
  a sense organ
• Teeth - breaks food into smaller pieces
  increasing surface area for digestion

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Teeth

• Found in alveolar sockets
• Primary (deciduous or baby) teeth
• Usually consists of 20 teeth)
• Permanent teeth (32 teeth)
• Incisors - chisel-shaped
• Canines (Cuspids) - fang-like
• Premolars (bicuspids) - grinding and crushing
• Molars - grinding and crushing

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Tooth Structure

• Crown - exposed portion above gum
• Covered in enamel
• Dentin - fills interior of tooth
• Neck - hidden by gumline
• Root - embedded in jawbone, number of roots
  varies
• Cementum - attaches tooth to periodontal ligament

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Salivary Glands

• Parotid glands
• Submandibular glands
• Sublingual glands
• Functions
• cleanses, dissolves, moistens, digests
• 97 water
• Slightly acidic
• Contains electrolytes, salivary amylase, mucin,
  lysozymes, and immunoglobulins

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Esophagus

• Hollow muscular tube that functions to carry food
  to the stomach through an opening in the
  diaphragm called the esophageal hiatus
• Upper third composed of skeletal musle lower
  regions made of smooth muscle
• Hiatal hernia

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Digestive Processes of the mouth, pharynx,
esophagus

• Mastication - chewing
• Deglutition - swallowing
• chewed food formed into bolus
• tongue blocks mouth, soft palate blocks
  nasopharynx, epiglottis blocks trachea
• Peristalsis - rhythmic contractions of circular
  muscles causing food to move down GI tract

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Stomach

• Serves as a reservoir for food and to mix food
  with gastric juice
• Regions cardia, fundus, body, and pylorus
• Greater and lesser curvatures
• Pyloric sphincter
• Rugae
• Additional oblique smooth muscle layer

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Gastric gland cells

• Mucous goblet cells - secrete mucosal layer
• Parietal cells (oxyntic) - secrete HCl and
  intrinsic factor
• Chief cells (zymogenic) - secrete pepsinogen
• Pepsinogen converted to pepsin which breaks
  proteins into peptides
• Enteroendocrine cells (G-cells)- secrete
  hormones
• gastrin, histamine, endorphins, serotonin,
  cholecystokinin, somatostatin
• Stomach ulcers - caused by Heliobacter pylori

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Digestive Processes in the Stomach

• Mechanical chemical digestion turns food into
  chyme
• Pepsin breaks down proteins into peptides
• Rennin (in children) breaks down milk
• Intrinsic factor - required for absorption of
  vitamin B12 in the intestines

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Gastric motility emptying

• Stomach contractions produce chyme, increase
  mixing of food
• Entire stomach can hold up to 4L
• Approx. 3 ml squirts through pylorus with each
  contraction
• Stomach contracts 3X/min
• Usually empties within 4 hours but may be delayed
  depending on contents

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Pancreas

• Secretes pancreatic juice formed by acini
• Pancreatic juice
• sodium bicarbonate that serves to buffer HCl
• proteases such as trypsin, chymotrypsin, and
  carboxypeptidase
• pancreatic amylase
• pancreatic lipases
• pancreatic nucleases

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Liver and Gallbladder

• Largest internal organ in the body
• Consists of right and left lobe separated by the
  falciform ligament caudate and quadrate lobes on
  the posterior surface
• Digestive function includes the production of
  bile protein, carbohydrate lipid metabolism
  removal of drugs hormones
• Gallbladder - stores and concentrates bile

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Small Intestines

• Duodenum, Jejunum, and Ileum

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Small Intestine

• Duodenum - (10 inches) receives chyme from the
  stomach and digestive secretions from the
  pancreas and liver major digestion occurring
  here
• Secretin
• Cholecystokinin (CCK)
• Gastric inhibitory peptide (GIP)

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Jejunum and Ileum

• 8 12 20 feet long
• Region where most absorption occurs
• Plica circulares folds
• Villi lined with simple columnar epithelium
• Each villus contains an arteriole, venule,
  capillary bed and a lacteal
• Microvilli form brush border
• Peristalsis and segmentation occurs

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Enzymes of the small intestine

• Dextrinase break down
• Glucoamylase oligosaccharides
• Maltase break
• Sucrase down
• Lactase disaccharides
• Carboxypeptidase breaks
• Aminopeptidase down
• Dipeptidase proteins

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Large Intestine

• 5.5 feet long
• Functions
• reabsorption of remaining water and electrolytes
  
• absorption of vitamins B and K (produced by the
  bacterial flora)
• elimination of feces (may take 12-24 hours)

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Large Intestine

• Cecum
• Ileocecal valve
• Appendix
• Ascending, transverse, and descending colon
• Sigmoid colon
• Taeniae coli / Haustra
• Rectum / Anus
• Hemorrhoids/Diverticulitis

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Products of Digestion

• Carbohydrates
• Lipids
• Proteins

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Carbohydrates

• Monosaccharides - single sugars
• Glucose
• Fructose
• Galactose
• Deoxyribose
• Ribose

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Carbohydrates

• Disaccharides - double sugars
• Sucrose
• Lactose
• Maltose
• Polysaccharides - starches (many sugars)
• Amylose
• Cellulose
• Glycogen

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Formation of a disaccharide

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Formation of a fat
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Carbohydrate Digestion

• Carbohydrates are broken down into disaccharides
  and then into monosaccharides
• Starch (Amylose) amylase --gt Maltose
• Maltose Maltase --gt 2 Glucose molecules
• Lactose Lactase --gt Galactose Glucose
• Sucrose Sucrase --gt Glucose Fructose

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Lipid Digestion

• Lipids (fats) Lipase ---gt Glycerol Fatty
  Acids
• This process is made more efficient by the
  emulsifying action of bile.

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Protein Digestion

• Protein Pepsin --gt large polypeptides
• Protein Rennin --gt large polypeptides
• Protein Trypsin --gt small polypeptides
• Protein Chymotrypsin --gt small polypeptides
• Protein Carboxypeptidase --gt sm. Plypeptds
• Aminopeptidases - to single amino acids
• Carboxypeptidase - to single amino acids
• Dipeptidase - to single amino acids

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• Overview of Digestive Diseases
• Digestive diseases range from the occasional
  upset stomach to the more life-threatening colon
  cancer and encompass disorders of the
  gastrointestinal tract, the liver, the
  gallbladder, and the pancreas.  
• What Causes a Digestive Disease? 
• Digestive disease may develop congenitally or
  from multiple factors such as stress, fatigue,
  diet, or smoking. Abusing alcohol imposes the
  greatest risk for digestive diseases,
  particularly increasing the risk of esophageal,
  colorectal, and liver cancers.

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Who Develops a Digestive Disease? 

• Each year 62 million Americans are diagnosed with
  a digestive disorder.
• The incidence and prevalence of most digestive
  diseases increase with age.
• Women are more likely than men to report a
  digestive condition, particularly non-ulcer
  dyspepsia and irritable bowel syndrome (IBS).

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How Are Digestive Diseases Diagnosed? 

• Most digestive diseases are very complex, with
  subtle symptoms. Because of this, patients may
  undergo extensive and expensive diagnostic tests
  which may include a blood test, an upper or lower
  GI series, an ultrasound, and endoscopic
  examinations of the colon, esophagus, stomach, or
  small intestine, or more sophisticated tests such
  as a CAT (computerized axial tomography) scan or
  MRI. 

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Significant Digestive Conditions 

• Gallstones affect 20 percent of women and 10
  percent of men, or approximately 20 million adult
  Americans.
• are solid masses, primarily of cholesterol that
  develop in the gallbladder or less often in the
  bile ducts leading from the liver to the small
  intestine. 
• Symptoms may include mid- or right-upper
  abdominal pain that may lead to complications
  such as acute cholecystitis and pancreatitis.
  Gallstones are rarely associated with gallbladder
  cancer. 
• Treatment options include surgery by open
  cholecystectomy or laparoscopic cholecystectomy,
  watchful waiting, or oral bile acid therapy in
  patients who cannot tolerate surgery.

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Gastroesophageal Reflux Disease and Related
Disorders 

• Gastroesophageal reflux disease affects nearly
  one-third of the American population. GERD is the
  backward flow of the stomach's contents into the
  esophagus.  Heartburn is the most common symptom
  of GERD. Doctors also believe that diet and
  lifestyle habits, hiatal hernia, obesity, and
  pregnancy contribute to GERD. Certain
  foods,including chocolate, fried or fatty foods,
  and alcohol may weaken the LES, permitting reflux
  and heartburn. Antacids such as Tums and
  Gaviscon neutralize stomach acid for relatively
  short periods of time.  Histamine 2 (or
  H2-blockers), which suppress acid, are also
  prescribed to relieve symptoms of GERD. The
  H2-remedies currently available include
  cimetidine (Tagamet), famotidine (Pepcid),
  nizatidine (Axid), and ranitidine HCl (Zantac).
  Omeprazole (Prilosec, Losec, or Antra), are
  pump inhibitors that dramatically inhibit an
  enzyme, H(hydrogen), K(potassium)-ATPase, from
  producing stomach acid lansoprazole (Prevacid)
  has also recently been approved.  

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Inflammatory Bowel Disease 

• Inflammatory bowel disease (IBD) refers to two
  chronic intestinal disorders Crohn's disease and
  ulcerative colitis. IBD affects between 2 to 6
  percent of Americans or an estimated 300,000 to
  500,000 people. The causes of Crohn's disease and
  ulcerative colitis are not known. A leading
  theory suggests that some agent, perhaps a virus
  or bacterium, alters the body's immune response,
  triggering an inflammatory reaction in the
  intestinal wall. The onset for both diseases
  peaks during young adulthood. An individual with
  either disease may suffer persistent abdominal
  pain, bowel sores, diarrhea, fever, intestinal
  bleeding, or weight loss.  

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Crohn's Disease

• Crohn's disease primarily involves the small
  bowel and the colon. It may cause the intestinal
  wall to thicken, which may narrow the bowel
  channel and block the intestinal tract.
   Although surgery relieves chronic symptoms,
  Crohn's disease often recurs at the location
  where the healthy parts of the bowel were
  rejoined. The length of time that a Crohn's
  patient is in remission is not predictable. 

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Ulcerative Colitis

• Ulcerative colitis (UC) is an inflammatory
  disorder affecting the inner lining of the large
  intestine. The inflammation originates in the
  lower colon and spreads through the entire colon.
  Blood in the stool is the most common and
  distinct symptom of ulcerative colitis.  

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Irritable Bowel Syndrome 

• Irritable bowel syndrome (IBS) is a common
  functional disorder of the intestines estimated
  to affect 5 million Americans. The cause of IBS
  is not yet known. Doctors refer to IBS as a
  functional disorder because there is no sign of
  disease when the colon is examined. However,
  doctors believe that people with IBS experience
  abnormal patterns of colonic movement. The IBS
  colon is highly sensitive, overreacting to any
  stimuli such as gas, stress, or eating high-fat
  or fiber-rich foods. 

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Peptic Ulcer Disease 

• Peptic ulcer disease, estimated to affect 4.5
  million people in the United States, is a chronic
  inflammation of the stomach and duodenum.
   Peptic ulcers result from the breakdown of the
  lining of the stomach and duodenum caused by
  increased stomach acid and pepsin and
  Helicobacter pylori (H. pylori). One type of
  ulcer occurs in the stomach, the other in the
  duodenum, the first part of the small intestine.
  Duodenal ulcers are much more common than stomach
  ulcers, which have a greater risk of
  malignancy.