12.1 Digestive Process

• Digestion


  1. Begins in the mouth

  2. Teeth break food into smaller bits

  3. Salivary glands secrete saliva

  4. Salivary amylase digests starch

  5. Food mass (bolus) is swallowed

  6. Through pharynx and esophagus to stomach

  7. Epiglottis blocks trachea
• Structures of the mouth, pharynx, and esophagus involved in the swallowing reflex

  • Nasal passages
  • Soft palate
  • Epiglottis
  • Flap of tissue at base of the tongue keeps food from going into the trachea
  • Pharynx
  • Glottis: The middle part of the larynx, the area where the vocal cords are located
  • Larynx
  • Vocal chords
  • Trachea
  • Esophagus
  • Tongue
  • Food bolus
• Motions that seal the nasal passages, mouth, and trachea during the swallowing reflex

  1. Elevation of soft palate prevents food bolus from entering nasal passages

  2. Pressure of tongue seals back of mouth and prevents bolus from backing up

  3. Larynx moves upward, pushing glottis against epiglottis to prevent bolus from entering airway
• Stomach


  1. Secretes gastric juice

  2. Hydrochloric acid and protein digesting enzyme: pepsin

  3. Chief cells secrete: pepsinogen (the precursor for pepsin)

  4. Mucus

  5. Parietal cells: secrete H+ and Cl-

  6. Stomach churns acid contents into chyme (stomach contents)

  7. Released in pulses into small intestine
• Gastric pit


  • Indentation in the stomach
  • Surface epithelial cells
  • Stomach lumen
  • Mucosa
  • Mucous cells: secrete mucus
  • Submucosa
  • Chief cells: secrete pepsinogen
  • Gastric gland
  • Parietal cells: secrete H+ and Cl-
• Gastric Juice


  1. When food enters stomach, stretch receptors in stomach wall are activated

  2. Stimulate the release of pepsinogen (from chief cells), HCl (from parietal cells), mucus (from glandular cells)

  3. Gastrin: major hormone that regulates acid secretion in the stomach

  4. Gastrin release is stimulated by stomach distension, presence of digested proteins (amino acids)
• Pepsin from Chief Cells

  1. HCl can activate pepsinogen to form pepsin

  2. Positive feedback: once pepsin is formed, it facilitates the formation of more pepsin
• Stomach


  • Acidic pH=2, optimal pH for pepsin
  • Helps unfold proteins so they can be broken down
  • Kills unwanted bacteria
  • Stomach's mucus protects stomach wall from acidity and breakdown from pepsin
• Stomach


  • Can take 1-6hrs for the stomach to empty after a meal
  • Food is not moved along faster than it can be digested
  • Fat: is digested in small intestines and is digested more slowly than other nutrients, so further emptying of the stomach is prevented until fat is processed
• Digestion in small intestine

  1. Most digestion occurs here

  2. Proteins – broken down by endopeptidases, exopeptidases

  3. Polysaccharides – hydrolyzed by amylase, then disaccharidases

  4. Triglycerides hydrolyzed by lipase

  5. Nucleic Acids hydrolyzed by nucleases (DNase, RNase)
• Absorption of nutrients begins in the small intestine

  1. Specializations to optimize absorption

  2. Intestinal villi: folds in lining of small intestine

  3. Microvilli: projections of plasma membrane of epithelial cells

  4. Villi and microvilli greatly increase the surface area of intestine
• Villi in the Small Intestine

  • Folds of small intestine
  • Section of small intestine
  • Villus
  • Brush border
  • Capillaries
  • Microvilli
  • Intestinal epithelial cell
  • Lymphatic vessel
  • Villus
• Lacteal


  The lymphatic vessels of the small intestine that absorb digested fats
• The Digestive Process- Small Intestine

  1. In the duodenum, digestive substances from the pancreas (enzymes) and liver (bile) aid the digestive process

  2. Pancreas secrete digestive enzymes and bicarbonate ions (HCO3-: neutralize acidity) into ducts that empty into the duodenum

  3. CO2+H2O -> H2CO3 -> HCO3- + H+
• Basic fluid


  High concentration of bicarbonate ions (HCO3-)
• Proteolytic Enzymes from the Pancreas

  • trypsin
  • chymotrypsin
  • carboxypeptidase
• Enzymes from the Pancreas

  • Lipase
  • Amylase
  • Nucleases
• Bicarbonate ions


  Neutralizes the pH of the chyme for optimal activity of amylase, proteases, lipases, nucleases
• Glucagon


  Released when blood glucose levels start to fall too low, causes the liver to convert stored glycogen into glucose and release it into the bloodstream, raising blood glucose levels
• Insulin


  Causes cells in the liver, muscle, and fat tissue to take up glucose from the blood, storing it as glycogen in the liver and muscle, and stopping use of fat as an energy source
• Fat digestion: small intestine

  1. Liver secretion bile (bile is a mixture of substances that act as emulsifiers)

  2. Emulsifies fats

  3. Bile salts: aid in fat digestion by forming a hydrophilic coat around fats

  4. Churning motion of small intestine emulsifies (breaks down fats) into micelles

  5. Lipase: enzyme from pancreas hydrolyzes the fats in the micelles, producing monoglycerides and free fatty acids
• Micelle


  In aqueous solution, structure that has molecules aggregated with the hydrophilic "head" regions in contact with surrounding solvent, sequestering the hydrophobic tail regions in the micelle center
• Bile


  • Made in the liver, the liver secretes bile
  • Bile is stored in the gall bladder, released when chyme enters the small intestine
  • Bile emulsifies fats
• Bile Salts


  • Bile salts form a hydrophilic coat around the fats
  • Churning motion of the small intestine emulsifies the fat: breaks the fat down into droplets called micelles
  • Lipase: hydrolyzes the fats in the micelles; producing monoglycerides and free fatty acids that can diffuse into the intestinal cell (Mucosal cell)
• Absorption


  1. Micelles (formed by the bile salts) assist in the absorption of fatty acids, monoglycerides and other products of lipid breakdown by lipase

  2. Micelles get digested and hydrophobic molecules in the micelle are released and enter the mucosal cell cytoplasm

  3. In the mucosal cell: hydrophobic molecules (fatty acids, monoglycerides etc) are packaged into chylomicrons
• Chylomicrons


  • Have a protein coat provides a hydrophilic surface that keeps the droplets suspended in the cytosol of the mucosal cell
  • Chylomicrons are secreted into the interstitial fluid of the submucosa, taken up by lymph blood circulation
  • They transport lipids absorbed from the intestine to adipose, cardiac, and skeletal muscle tissue
• Chylomicrons are one of the five major groups of lipoproteins (chylomicrons, VLDL, IDL, LDL, HDL) that enable fats and cholesterol to move within the water-based solution of the bloodstream
• Absorption of fat-soluble products of digestion by intestinal mucosa cells

  1. Lipases

  2. Micelles

  3. Monoglycerides and fatty acids associated with bile salts

  4. Bile salts released and recycled

  5. Monoglycerides and fatty acids assembled into fats and coated with proteins to form chylomicrons

  6. Coating proteins

  7. To extracellular fluid and lymph vessels
• Micelles (fats coated with bile salts) are digested to monoglycerides and fatty acids, which penetrate into cells and are assembled into fats. The fats are coated with proteins to form chylomicrons, which are released by exocytosis to extracellular fluids, where they are picked up by lymph vessels
• Absorption of fluid


  1. Much of this occurs in small intestine

  2. Active uptake of salt

  3. Passive following of water

  4. Small intestine reabsorbs much of fluid released from the stomach (about 6-8L of the 7-9 L of fluid from the stomach)

  5. Large intestine also absorbs water
• Much of the absorption of fluid occurs in the small intestine
• Pancreatic enzymes found in the lumen accomplish the crude hydrolysis, chopping proteins and starch into smaller and smaller fragments
• The final hydrolysis is driven by enzymes from the intestinal mucosa (enterocytes)
• Enterocyte enzymes tend to stay embedded in the microvilli and mucus layer
• Absorption of water-soluble products of digestion by intestinal mucosa cells

  1. Polypeptides

  2. Polysaccharides and disaccharides

  3. Nucleotides

  4. Peptidases

  5. Disaccharidases

  6. Nucleotidases, nucleosidases, phosphatases

  7. Nitrogenous bases, five-carbon sugars, and phosphates
• Fat Absorption


  1. Form chylomicrons

  2. Released by exocytosis to extracellular fluids

  3. Picked up by lymph vessels
• Absorption of fluid


  Much of this occurs in small intestine
• Absorption of fluid


  1. Active uptake of salt

  2. Passive following of water
• Small intestine


  Reabsorbs much of fluid released from the stomach (about 6-8L of the 7-9 L of fluid from the stomach)
• Large intestine


  Also absorbs water