THE DIGESTIVE SYSTEM

Chapter 19

GASTROINTESTINAL (GI) TRACT

Tube that includes: mouth, Pharynx, Esophagus, Stomach, Small intestine, Large intestine

Accessory organs: teeth, tongue, salivary glands, liver, gallbladder, and pancreas

FIGURE 19.1

OVERVIEW- OPERATIONS

Ingestion: eatingSecretion: release of water,

enzymes & buffersMixing & propulsion:

movement along GI tractDigestion: mechanical and

chemical breakdown of foodsAbsorption: getting it into the

bodyDefecation: dumping waste

products = defecation

WALL LAYERS- EVERYWHERE

4 layers Mucosa- epithelium, connective layer,

glands, muscularis mucosae Submucosa- connective tissue, blood

vessels, lymphatic vessels, enteric nervous system

Muscularis- circular layer, longitudinal layer In mouth, pharynx & upper esophagus –skeletal

muscle Also in external anal sphincter

Serosa or Visceral peritoneum

FIGURE 19.2

FIGURE 19.3A

FIGURE 19.3B

MOUTH

Formed by cheeks, hard & soft palate & tongue

Soft palate at back includes a “hangy down” part = uvulaDuring swallowing uvula prevents

entry into nasal cavityTongue- muscular accessory organ

maneuvers food for chewingAdjusts shape for speech & swallowing

Lingual tonsils at base of tongue

SALIVARY GLANDS

3 pairs of salivary glandsDucts empty into oral cavity

Parotid- inferior & anterior to ears

Submandibular- in floor of mouth, medial & inferior to

mandibleSublingual

Beneath tongue and superior to submandibular

Saliva contains 99.5% water, salivary amylase, mucus and other solutesDissolves food & starts digestion of

starches

FIGURE 19.4

TEETH

Accessory organs in bony sockets of mandible & maxilla

3 external regions: Crown- above gumsRoot- 1 or more parts embedded in

socketNeck – between crown and root near gum

line3 layers of material

Enamel- covers crownDentin- majority of interior of toothPulp cavity - nerve, blood vessel &

lymphatics

FIGURE 19.5

DIGESTION IN THE MOUTH

Mechanical breakdown- chewing

Mixed with saliva by tongueSalivary amylase chemically breaks down polysaccharides (starch)maltose and larger fragmentsContinues in the stomach until

acidifiedRounds up food into a soft bolus for swallowing

PHARYNX & ESOPHAGUS

On swallowing:Bolus of food oropharynxLaryngopharynx esophagus

Muscular contractions in pharynx help

Upper esophageal sphincter (UES)Skeletal muscle –controls entry to

esophagusLower esophageal sphincter (LES)

Smooth muscle- regulates entry to stomach

FIGURE 19.6A,B

SWALLOWING

Voluntary: bolus forced into oropharynx

Triggers oropharyngeal stageInvoluntary & breathing interruptedSoft palate move up-close

nasopharynxEpiglottis seals off larynxBolus moves into esophagus through

UESEsophageal stage peristalsis

moves it toward stomach

FIGURE 19.6C

STOMACH

J- shaped enlargement of tractServes as mixing chamber and

holding reservoirVery elastic & muscular4 regions

Cardia- surrounds upper openingFundus- superior & to left of cardiaBody – large central portionPylorus- lower part leading to pyloric

sphincter & duodenum

FIGURE 19.7

STOMACH WALL

Mucosa:Folds called rugaeEpithelium- simple columnar mucousForm gastric glands lining gastric pits

Secretory cells: mucous neck cellsChief cells inactive enzyme

pepsinogeParietal cells HCl & intrinsic factorCollectively = gastric juice

Muscularis- 3 Layers: longitudinal, circular & oblique

FIGURE 19.8

FIGURE 19.9

DIGESTION & ABSORPTION

Food entry stretch & rise in pHNerve impulses secretion & mixing

waves Food mixed with juice ChymeSmall amount pushed through pyloric

sphincter= gastric emptying- Carb. foods fastest,

lipids next & proteins slowestEntry in duodenum feedback inhibition

of stomach activityPepsin digests protein peptidesLittle absorption- water, ions & some

drugs

PANCREAS

Behind stomach- Produces pancreatic juice in acinar cellsto duodenum via pancreatic duct

NaHCO3 solution (pH 7.1-8.2)– 1000ml/dayNeutralize stomach acid and dilutes chyme

Panceas- digestive enzymesProteases: chymotrypsinogen, trypsinogen,

et. al.Activated by entreokinase from intestineStarch digesting- pancreatic amylasePancreatic lipaseNucleotidases – RNAase & DNAase

LIVER & GALL BLADDER

Largest organ after the skinOn right below diaphragmFunctional unit is lobule-

Hepatocytes around central veinOpen capillaries = sinusoids

Bile canaliculi ducts hepatic duct

Gall bladder =Pear-shaped organ on front (stores bile)

cystic duct common bile duct

BILE

Bicarbonate, bile salts & waste. – 1000 ml/day

Important for emulsifying fats Increases surface area for digestion

Pigment is bilirubin- from broken-down heme during RBC recycling

Digested to strecobilin- brown colorBile salts reabsorbed at end of small

intestine- ileumrecycle to liver in portal circulation

FIGURE 19.10

FIGURE 19.11A

FIGURE 19.11B

LIVER FUNCTION

Maintains blood glucoseStores as glycogen Uses absorbed sugars & Converts amino

acids glucoseLipid metabolism

Produces cholesterol & triglycerides, makes bile

Makes lipoproteins for lipid transportExcretion of bilirubinProcesses drugs and other chemicalsStore fat soluble vitaminsMake active vitamin D

SMALL INTESTINE

3 parts: duodenum, jejunum, ileum Where most of the digestion occurs Essentially all of the nutrient absorption Ends in ileocecal sphincter

FIGURE 19.12A

FIGURE 19.12B

WALL STRUCTURE

Same 4 layersEpithelial- simple columnar

Absorptive cells with microvilliGoblet cells- secrete mucus

Intestinal glands- intestinal juice & hormonesSecretin, cholecystokinin (CCK),

Glucose-dependent-insulinotrophic peptide (GIP)

Lymphatic tissue- defense

WALL STRUCTURE (CONT.)

Duodenal glands- alkaline mucus Helps neutralize stomach acid

Circular folds- increase surface area Villi- finger like projections of mucosa

Increase surface area for absorption Include lacteals for lipid absorption

FIGURE 19.13

MOTILITY & SECRETIONS Secretions: alkaline, some enzymes

Peptidases-breaks small peptides Disaccharidases attached to wall Water and salt to balance osmolality ~2000 ml/day

Segmentation activity- for mixing Peristalsis for movement after most

absorption completed- slow waves

DIGESTION & ABSORPTION

Chyme enters with partially digested carbohydrates & proteins

Bile + pancreatic juice + intestinal juice completes the job

Absorption is of monosaccharides; amino acids; phosphate sugar & bases of DNA & RNA; fatty acids & monoglycerides

CARBOHYDRATE DIGESTION

Amylases: Starch & dextrin maltose

Disaccharidases at surface: Maltose: maltose glucose Sucrase: sucrose glucose & fructose Lactase: lactose glucose & galactose

PROTEIN & FAT DIGESTION Trypsin, chymotrypsin, elastase,

carboxypeptidase & pepsinProteins small peptides

Peptidases at surface:Peptides amino acids & di- & tri-

peptides Lipase:

glycerides fatty acids & monoglycerides

ABSORPTION

By diffusion, facilitated diffusion, osmosis & active transport

Carbohydrates monosaccharidesVia portal system to liver

Proteins (jejunum & ileum) amino acidsVia portal system to liver

Lipids reformed to triglyceridesPackaged in chlyomicrons with proteinVia lacteals lymphatics

ABSORPTION (CONT.)

Water & salt Primarily osmotic movement along with other

nutrients Vitamins:

Fat soluble absorbed with fat Water soluble with simple diffusion B12 combines with intrinsic factor & absorbed by

active transport in ileum

FIGURE 19.14A

FIGURE 19.14B

LARGE INTESTINE

Cecum, colon, rectum, anal canalIleocecal canal large intestine

Below is cecum with appendixColon- ascending, transverse,

descending & sigmoid rectum anal canalStandard 4 layers with mucus

secretionFew folds , little specialization for

absorptionMuscularis: circular + bands of

longitudinal muscle

FIGURE 19.15A

FIGURE 19.15B

FIGURE 19.16

DIGESTION & ABSORPTION

Slow emptying of ileumSlow peristalsisMass peristalsis with food in stomachMoves from middle of colon

rectumBacterial digestion

Produce some B-vitamins & Vit. KProduce gases= flatusColon absorbs salt & water

DEFECATION REFLEX

Stretch of rectum wall neural reflex contraction of longitudinal muscleCombined pressure + parasympathetic

activity relaxing of internal anal sphincter

External anal sphincter is voluntaryContraction of diaphragm & abdominal

wall muscles aid defecation

CONTROL

Rule: activate forward and inhibit behindthree phases: Cephalic, gastric, intestinalCephalic- smell, sight, thought of food

Neural signals stimulates salivary glands & gastric glands

Gastric- stretching, pH of stomachGastrin activates stomach & LES & relaxes

pyloric sphincterNeural signals + gastrin signal satiety

(fullness)

CONTROL (CONT.)

Intestinal- responses to food entering duodenumneural & endocrine

CCK stimulated by AA & fatsPancreatic enzyme releaseGall bladder contractionContraction of pyloric sphincter

Acid stimulates secretinStimulates HCO3

- ions in pancreatic juiceInhibits gastrin action in stomach

AGING

Decreased secretion, motility, strength of responses

loss of taste, periodontal disease, hiatal hernia, gastritis & peptic ulcer disease

Increased incidence of gall bladder problems, cirrhosis of liver, pancreatitis, constipation, hemorrhoids & diverticulitis