Chapter 25 the digestive system
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Chapter 25: The Digestive system. BSC 1086C Fall 2007. 4 basic phases of digestion. _______________________- intake of food _______________- breakdown of molecules mechanical – teeth and churning action of stomach and intestines

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Chapter 25 the digestive system
Chapter 25: The Digestive system

BSC 1086C Fall 2007

4 basic phases of digestion

  • _______________________- intake of food

  • _______________- breakdown of molecules

    • mechanical – teeth and churning action of stomach

      and intestines

    • chemical – hydrolysis reactions that break down macromolecules into monomers

  • ________________- uptake nutrients into blood/lymph

  • _______________- elimination of undigested material

  • Subdivisions of digestive system
    Subdivisions of Digestive System

    • Digestive tract (GI tract)

      • 30 foot long tube extending from mouth to anus

    • Accessory organs

      • teeth, tongue, liver, gallbladder, pancreas, salivary glands


    Ingestion via Mouth (Oral Cavity)

    • Food enters the GI tract (ingestion) via the mouth

    • Mechanical digestion starts as the teeth physically break down the food

      (increased surface area exposed to digestive enzymes!)

    • Chemical digestion starts due to enzymes in saliva


    Salivary Glands

    • Small __________________ found under mucous membrane of mouth, lips, cheeks and tongue - secrete at constant rate

    • 3 pairs _________________ connected to oral cavity by ducts

      • parotid, submandibular and sublingual

    Secrete Saliva!



    • Functions of saliva

      • moisten, begin starch and fat digestion, cleanse teeth, inhibit bacteria, bind food together into bolus

    • Hypotonic solution of 99.5% water and solutes

      • salivary amylase, begins starch digestion

      • lingual lipase, digests fat activated by stomach acid

      • mucus, aids in swallowing

      • lysozyme, enzyme kills bacteria

      • immunoglobulin A, inhibits bacterial growth

    • pH of 6.8 to 7.0 (relatively neutral)

    • Produce 1-1.5 L per day

    • Saliva is released following signals from the CNS

    2: Pharynx

    • Skeletal muscle

      • deep layer – longitudinal orientation

      • superficial layer – circular orientation

        • superior, middle and inferior pharyngeal constrictors

        • Drive food downwards (towards esophagus) during swallowing

    3: Esophagus

    • Straight muscular tube 25-30 cm long

      • esophageal glands in submucosa (lubrication)

      • skeletal muscle in upper part and smooth in bottom

    • Extends from pharynx to cardiac stomach passing through esophageal

      hiatus in diaphragm

    • Lower esophageal sphincter

      closes orifice to reflux (thus

      preventing stomach acid from

      entering the lower end of the

      esophagus causing heartburn)

    4: Stomach

    • Mechanically breaks up food, liquifies food and begins chemical digestion of protein and fat

      • resulting soupy mixture is called ____________

    • Does not absorb significant amount of nutrients

      • absorbs aspirin and some lipid-soluble drugs (alcohol)

    Gross Anatomy of Stomach


    • Notice: bulge of fundus, narrowing of pyloric region, thickness of pyloric sphincter and greater and lesser curvatures

    Unique Features of Stomach Wall


    • Mucosa

      • simple columnar glandular epithelium

      • lamina propria is filled with tubular glands (gastric pits)

    • Muscularis externa has 3 layers

      • outer longitudinal, middle circular and inner oblique layers

    Cells of Gastric Glands

    • _____________ cells secrete mucus

    • __________________ cells

      • divide rapidly to produce new cells that migrate to surface

    • __________________ cells

      • secrete HCl acid and intrinsic factor

    • __________________ cells

      • secrete pepsinogen

      • chymosin and lipase in infancy

    • __________________ cells

      • secrete hormones and paracrine messengers



    • Parietal cells contain __________________ (CAH)


      • CO2 + H2O  H2CO3  HCO3- + H+

      • H+ is pumped into stomach lumen by H+-K+ ATPase

        • antiporter uses ATP to pump H+ out and K+ in

      • HCO3- exchanged for Cl- (chloride shift)

        • Cl- pumped out to join H+ forming HCl


    • 2 to 3 L of gastric juice/day (H2O, HCl and pepsin)

    Functions of Hydrochloric Acid

    • Activates _________ and _______________

    • Breaks up connective tissues and plant cell walls

      • liquefies food to form chyme

    • Converts ingested ferric ions (Fe3+) to ferrous ions (Fe2+)

      • absorbed and used for hemoglobin synthesis

    • Destroys ingested bacteria and pathogens

    Intrinsic Factor

    • Intrinsic factor

      • A glycoprotein released by parietal cells

      • essential for vitamin B12 absorption by small intestine

      • necessary for hemoglobin production (lack causes pernicious anemia)

    First: the liver

    Accessory digestive organs: Liver, Gallbladder and Pancreas

    • All release important secretions into small intestine to continue digestion

    Gross Anatomy of Liver

    • 3 lb. organ located inferior to the diaphragm

    • 4 lobes - right, left, quadrate and caudate

      • falciform ligament separates left and right

      • round ligament, remnant of umbilical vein

    • Gallbladder adheres to ventral surface between right and quadrate lobes


    Microscopic Anatomy of Liver


    • Tiny cylinders called hepatic lobules (2mm by 1mm)

    • Central vein surrounded by sheets of hepatocyte cells separated by sinusoids lined with fenestrated_____________

    • Blood filtered by hepatocytes on way to central vein

    Gallbladder and Bile

    • Sac on underside of liver

      -- 10 cm long

    • 500 to 1000 mL bile are secreted daily from liver

    • Gallbladder stores and concentrates bile

      • bile backs up into gallbladder from a filled bile duct

      • between meals, bile is concentrated by factor of 20

    • Yellow-green fluid containing minerals, bile acids, cholesterol, bile pigments and phospholipids

      • bilirubin pigment from hemoglobin breakdown

      • bile acid (salts) emulsify fats and aid in their digestion

    Ducts of Gallbladder,Liver, Pancreas

    • Bile passes from _______________between cells to

      ______________________to right and


    • Right and left ducts join outside liver to form_________________________

    • __________________from gallbladder joins common hepatic duct to form bile duct

    • Duct of pancreas and bile duct combine to form ________________________emptying into duodenum at major duodenal papilla


    Gross Anatomy of Pancreas

    • Retroperitoneal gland

      posterior to stomach

      • head, body and tail

    • Endocrine and

      exocrine gland

      • secretes insulin and glucagon into the blood

      • secretes 1500 mL pancreatic juice into duodenum

        • water, enzymes, zymogens, and sodium bicarbonate

    • Pancreatic duct runs length of gland to open at sphincter of Oddi

      • accessory duct opens independently on duodenum


    Activation of Zymogens


    • _______________converted to _____________ by intestinal epithelium

    • Trypsin converts other 2 (also digests dietary protein)

    Small Intestine


    • Nearly all chemical digestion and nutrient absorption occurs in small intestine

    Small Intestine

    • ____________curves around head of pancreas (10 in.)

      • retroperitoneal along with pancreas

      • receives stomach contents, pancreatic juice and bile

      • neutralizes stomach acids, emulsifies fats, pepsin inactivated by pH increase, pancreatic enzymes

    • ______________- next 8 ft. (in upper abdomen)

      • has large tall circular folds; walls are thick, muscular

      • most digestion and nutrient absorption occur here

    • ______________ - last 12 ft. (in lower abdomen)

      • has peyer’s patches – clusters of lymphatic nodules

      • ends at ileocecal junction with large intestine

    Small Intestine - Surface Area

    • _______________(plicae circularis) up to 10 mm tall

      • involve only mucosa and submucosa

      • chyme flows in spiral path causing

        more contact


    • ____________ are fingerlike projections 1 mm tall

      • contain blood vessels and lymphatics (lacteal)

        • nutrient absorption

    • ____________:1 micron tall;

      cover surface

      • brush border on cells

      • brush border enzymes for final stages of digestion

    Intestinal Crypts

    • Pores opening between villi lead to intestinal crypts

      • absorptive cells, goblet cells and at base, rapidly dividing cells

      • paneth cells – antibacterial secretions

    • Brunner’s glands in submucosa secrete bicarbonate mucus

    • Peyer patches are populations of lymphocytes to fight pathogens

    • Secrete 1-2 L of intestinal juice/day

      • water and mucus, pH 7.4-7.8


    Carbohydrate Digestion - Small Intestine


    • Salivary amylase stops working in stomach (pH < 4.5)

      • 50% of dietary starch digested before it reaches small intestine

    • Pancreatic amylase completes first step in 10 minutes

    • Brush border enzymes act upon oligosaccharides, maltose, sucrose, lactose and fructose

      • lactose indigestible after age 4 in most humans (lactase declines)

    Carbohydrate Absorption


    • Sodium-glucose transport proteins (SGLT) in membrane help absorb glucose and galactose

    • Fructose absorbed by facilitated diffusion then converted to glucose inside the cell

    Protein Digestion and Absorption


    • Pepsin has optimal pH of 1.5 to 3.5 -- inactivated when passes into duodenum and mixes with alkaline pancreatic juice (pH 8)

    Protein Digestion and Absorption


    • Pancreatic enzymes take over protein digestion by hydrolyzing polypeptides into shorter oligopeptides

      • Trypsin and chymotrypsin: break peptides via hydrolysis

      • Carboxypeptidase: removes 1 amino acid at a time from the

        end of the oligopeptide

    Protein Digestion and Absorption


    • _____________________________finish task, producing amino acids that are absorbed into intestinal epithelial cells

      • ________________________, ____________________, and _________________________

      • amino acid cotransporters move into epithelial cells and facilitated diffusion moves amino acids out into blood stream

    Fat Digestion and Absorption


    Step 1: emulsification of large fat globules by

    bile acids and lecithin (a phosholipid

    component in bile)

    Fat Digestion and Absorption

    Step 2: _____________in the fat droplet are

    hydrolyzed by pancreatic lipase

    to form _______________________and


    Fat Digestion and Absorption


    Step 3: micelles produced in the bile pick up

    lipids from the lumen of the small


    Fat Digestion and Absorption


    Step 4: micelles transport lipids to brush border

    where they are absorbed by the intestinal

    cells. Complex lipids are resynthesized,

    packaged (chylomicron), exocytotically

    released and finally enter lacteal

    Function of Large Intestine


    • Reabsorbs water and electrolytes (NaCl)

    • Reduces undigested food to feces

      • feces = 75% water, 25% solids

      • solids = 30% bacteria, 30% fiber, 10-20% fat,

        small amounts of protein, dead epithelial cells, etc…

    Anatomy of Anal Canal


    • Anal canal is 3 cm total length

    • Anal columns are longitudinal ridges separated by mucus secreting anal sinuses

    Neural Controlof Defecation

    1. Filling of the rectum

    - stretches walls to

    stimulate receptors

    2. Reflex contraction of rectum and relaxation of internal anal sphincter

    3. Voluntary relaxation of external sphincter