Chapter 18LectureOutline

Functions of GI Tract

18-5

Is movement of food through GI tract by means of: Ingestion--taking food into mouth Mastication--chewing food and mixing it with saliva Deglutition--swallowing food Peristalsis--rhythmic wave-like contractions that

move food through GI tract

Motility

18-6

Includes release of exocrine and endocrine products into GI tract

Exocrine secretions include: HCl, H2O, HCO3-, bile,

lipase, pepsin, amylase, trypsin, elastase, and histamine

Endocrine includes hormones secreted into stomach and small intestine to help regulate GI system e.g. gastrin, secretin, CCK, GIP, GLP-1, guanylin,

VIP, and somatostatin

Secretion

18-7

Is passage of digested end products into blood or lymph

Absorption

18-8

Digestion

Refers to breakdown of food molecules into smaller subunits

Includes temporary storage and subsequent elimination of indigestible components of food

Storage and Elimination

18-9

Immune Barrier

Includes physical barrier formed by tight junctions between cells of small intestine

And cells of the immune system that reside in connective tissue just below epithelium

Structure of Digestive System

18-10

Digestive System continued

Organs include oral cavity, pharynx, esophagus, stomach, and small and large intestine

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

18-12

Layers of GI Tract

18-13

Is the absorptive and secretory layer lining lumen of GI tract In places is highly folded with villi to increase absorptive area Contains lymph nodules, mucus-secreting goblet cells, and thin

layer of muscle

Mucosa

18-14

Is a thick, highly vascular layer of connective tissue where absorbed molecules enter blood and lymphatic vessels

Contains glands and nerve plexuses (submucosal plexus) that carry ANS activity to muscularis mucosae of small and large int.

Submucosa

18-15

Is responsible for segmental contractions and peristaltic movement through GI tract

Has an inner circular and outer longitudinal layer of smooth muscle Activity of these layers moves food through tract

while pulverizing and mixing it Myenteric plexus between these layers is major

nerve supply to GI tract Includes fibers and ganglia from both Symp and

Parasymp systems

Muscularis

18-16

Serosa

Is outermost layer; serves to bind and protect Consists of areolar connective tissue covered with layer of

simple squamous epithelium

18-17

From Mouth to Stomach

18-19

Mastication (chewing) mixes food with saliva which contains salivary amylase An enzyme that catalyzes partial digestion of starch

From Mouth to Stomach

18-20

Deglutition (swallowing) begins as voluntary activity Oral phase is voluntary and forms a food bolus Pharyngeal and esophageal phases are involuntary

and cannot be stopped To swallow, larynx is raised so that epiglottis covers

entrance to respiratory tract A swallowing center in medulla orchestrates

complex pattern of contractions required for swallowing

From Mouth to Stomach continued

18-21

Peristalsis propels food thru GI tract = wave-like

muscular contractions

After food passes into stomach, the gastroesophageal sphincter constricts, preventing reflux

From Mouth to Stomach continued

18-23

Stomach

18-24

Is most distensible part of GI tract Empties into the duodenum Functions in: storage of food; initial digestion of

proteins; killing bacteria with high acidity; moving soupy food mixture (chyme) into intestine

Stomach

18-25

Stomach continued

Is enclosed by gastroesophageal sphincter on top and pyloric sphincter on bottom

Is divided into 3 regions: Fundus Body Antrum

18-26

Inner surface of stomach is highly folded into rugae

Contractions of stomach churn chyme, mixing it with gastric secretions Eventually these

will propel food into small intestine

Stomach continued

18-27

Stomach continued

Gastric mucosa has gastric pits in its folds

Cells that line folds deeper in the mucosa, are exocrine gastric glands

18-28

Gastric glands contain cells that secrete different products that form gastric juice Goblet cells secrete

mucus Parietal cells secrete

HCl and intrinsic factor (necessary for B12 absorption in intestine)

Chief cells secrete pepsinogen (precursor for pepsin)

Stomach continued

18-29

HCl in Stomach

Is produced by parietal cells which pump H+ into lumen via an H+/ K+ pump (pH ~1)

Cl- is secreted by facilitated diffusion

H+ comes from dissociation of H2CO3

Cl- comes from blood side of cell in exchange for HCO3

-

18-31

Is secreted in response to the hormone gastrin; and ACh from vagus These are indirect effects since both stimulate

release of histamine which causes parietal cells to secrete HCl

HCl in Stomach continued

18-32

Makes gastric juice very acidic which denatures proteins to make them more digestible

Converts pepsinogen into pepsin Pepsin is more

active at low pHs

HCl in Stomach continued

18-33

Both HCL and pepsin can damage lining and produce a peptic ulcer

1st line of defense is the adherent layer of mucus = a stable gel of mucus coating the gastric

epithelium Contains bicarbonate for neutralizing HCL Is a barrier to actions of pepsin

Gastric epithelial cells contain tight junctions to prevent HCL and pepsin from penetrating the surface

Gastric epithelial cells are replaced every 3 days

Protection of Stomach Against HCL and Pepsin

18-34

Digestion and Absorption in Stomach

Proteins are partially digested by pepsin Carbohydrate digestion by salivary amylase is soon

inactivated by acidity Alcohol and aspirin are the only commonly ingested

substances that are absorbed

18-35

Gastric and Peptic Ulcers

Peptic ulcers are erosions of mucous membranes of stomach or duodenum caused by action of HCl

In Zollinger-Ellison syndrome, duodenal ulcers result from excessive gastric acid in response to high levels of gastrin

Helicobacter pylori infection is associated with ulcers Antibiotics are useful in treating ulcers And also proton pump inhibitors such as Prilosec

Acute gastritis is an inflammation that results in acid damage due to histamine released by inflammation Is why histamine receptor blockers such as Tagamet

and Zantac can treat gastritis

18-36

Small Intestine

18-37

Is longest part of GI tract; approximately 3m long Duodenum is 1st 25cm after pyloric sphincter Jejunum is next 2/5 of length Ileum is last 3/5 of length; empties into large intestine

Small Intestine (SI)

18-38

Absorption of digested food occurs in SI Facilitated by long length and tremendous surface

area

Small Intestine (SI) continued

18-39

Surface area increased by foldings and projections

Large folds are plicae circulares

Microscopic finger-like projections are villi

Apical hair-like projections are microvilli

Small Intestine (SI) continued

18-40

Small Intestine (SI) continued

Each villus is covered with columnar epithelial cells interspersed with goblet cells

Epithelial cells at tips of villi are exfoliated and replaced by mitosis in crypts of Lieberkuhn

Inside each villus are lymphocytes, capillaries, and central lacteal

18-41

Small Intestine (SI) continued

A carpet of hair-like microvilli project from apical surface of each epithelial cell Create a brush border

18-42

Intestinal Contractions and Motility

2 major types of contractions occur in SI: Peristalsis is weak and

slow and occurs mostly because pressure at pyloric end is greater than at distal end

Segmentation is major contractile activity of SI Is contraction of circular

smooth muscle to mix chyme (shown in diagram)

18-44

Large Intestine

18-48

Has no digestive function but absorbs H2O, electrolytes, B and K vitamins, and folic acid

Internal surface has no villi or crypts and is not very elaborate

Contains large population of microflora = 1013 to 1014 commensal bacteria of 400 species

which produce folic acid and vitamin K and ferment indigestible food to produce fatty acids

And reduce ability of pathogenic bacteria to infect LI antibiotics can negatively affect commensals

Large Intestine (LI) or Colon

18-49

Extends from ileocecal valve at end of SI to anus

Outer surface bulges to form pouches (haustra)

Large Intestine (LI) or Colon continued

18-50

SI absorbs most water but LI absorbs 90% of water it receives Begins with osmotic gradient set up by Na+/K+

pumpsWater follows by osmosis

Salt and water reabsorption stimulated by aldosterone LI can also secrete H2O via active transport of NaCl

into intestinal lumen

Fluid and Electrolyte Absorption in LI

18-51

Liver

18-53

Structure of Liver Liver is the largest internal organ Hepatocytes form hepatic plates that are 1–2 cells thick

Plates are separated by sinusoids which are fenestrated and permeable even to proteins Contain phagocytic Kupffer cells

18-54

Structure of Liver continued

A damaged liver can regenerate itself from mitosis of surviving hepatocytes In some cases, such as alcohol abuse or viral

hepatitis, regeneration does not occurCan lead to liver fibrosis and ultimately cirrhosis

18-55

Food absorbed in SI is delivered 1st to liver Capillaries in digestive tract drain into the hepatic

portal vein which carries blood to liver Hepatic vein drains liver Liver also receives blood from the hepatic artery

Hepatic Portal System

18-56

Liver Lobules

Are functional units formed by hepatic plates In middle of each is central vein At edge of each lobule are branches of hepatic portal vein and

artery which open into sinusoids

18-57

Liver Lobules continued

Bile is secreted by hepatocytes in bile canaliculi Empty into bile ducts which flow into hepatic ducts that carry

bile away from liver

18-58

Enterohepatic Circulation

Is recirculation of compounds between liver and intestine

Many compounds are released in bile, reabsorbed in SI, and returned to liver to be recycled

Liver excretes drug metabolites into bile to pass out in feces

18-59

Detoxication of Blood

Liver can remove hormones, drugs, and other biologically active molecules from blood by: Excretion into bile Phagocytosis by Kupffer cells Chemical alteration of molecules

e.g. ammonia is produced by deamination of amino acids in liver Liver converts it to urea which is excreted in

urine

18-64

Gallbladder and Pancreas

18-68

Is a sac-like organ attached to inferior surface of liver

Stores and concentrates bile continuously produced by liver When SI is empty, sphincter

of Oddi in common bile duct closes and bile is forced into gallbladder Expands as it fills with bile

When food is in SI, sphincter of Oddi opens, gall bladder contracts, and bile is ejected thru cystic duct into common bile duct then to duodenum

Gallbladder

18-69

Pancreas

Is located behind stomach Has both endocrine and exocrine functions

Endocrine function performed by islets of LangerhansSecretes insulin and glucagon

Exocrine secretions include bicarbonate solution and digestive enzymesThese pass in pancreatic duct to SIExocrine secretory units are acini

18-70

18-71

Pancreatic juice and bile are secreted into the duodenum

18-72

The Pancreas is Both an Exocrine and Endocrine Gland

(a) A photomicrograph of the endocrine and exocrine portions of the pancreas

(b) An illustration depicting the exocrine acini, where acinar cells prod. Inactive enzymes stored as zymogen granules which are secreted via ducts into duodenum

Contains water, bicarbonate, and digestive enzymes Digestive enzymes include amylase for starch, trypsin

for proteins, and lipase for fats Brush border enzymes are also required for

complete digestion

Pancreatic Juice

18-73

The Activation of Pancreatic Juice Enzymes

Most pancreatic enzymes are produced in inactive form (zymogens) Trypsin is activated

by brush border enzyme, enterokinase

Trypsin in turn activates other zymogens

18-74