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

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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

    • chemical – hydrolysis reactions that break down macromolecules into monomers

  • ________________- uptake nutrients into blood/lymph

  • _______________- elimination of undigested material

  • 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

    Production and Action of Pepsin


    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

    Anatomy of Large Intestine


    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


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