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PowerPoint® Lecture Slides prepared by Janice Meeking, Mount Royal College

C H A P T E R

Copyright © 2010 Pearson Education, Inc.

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The Digestive System: Part C


Pancreas

• Location• Mostly retroperitoneal, deep to the greater

curvature of the stomach

• Head is encircled by the duodenum; tail abuts the spleen


Pancreas

• Endocrine function• Pancreatic islets secrete insulin and glucagon

• Exocrine function• Acini (clusters of secretory cells) secrete

pancreatic juice

• Zymogen granules of the secretory cells (Acinar) contain digestive enzymes

Copyright © 2010 Pearson Education, Inc. Figure 23.26a

SmallductAcinar cells

Basementmembrane

Zymogengranules

Roughendoplasmicreticulum

(a)


Pancreatic Juice•Watery alkaline solution (pH 8) neutralizes chyme

and allows pancreatic secreted enzymes to work

• The epithelial cells lining the small pancreatic ducts secrete the electrolytes (primarily HCO3

–)

• The bicarbonate is made in the epithelial cells – for every bicarbonate secreted a H+ is returned to the blood – thus the alkaline tide in the venous blood return from the stomach is balanced by the acidic venous blood from the pancreas


Pancreatic Juice• Acinar cells produce the enzyme rich secretion

• Enzymes

• Amylase, lipases, nucleases are secreted in active form but require ions or bile for optimal activity

• Proteases secreted in inactive form

• Protease activation in duodenum

• Trypsinogen is activated to trypsin by brush border enzyme enteropeptidase

• Procarboxypeptidase and chymotrypsinogen are activated by trypsin

Copyright © 2010 Pearson Education, Inc. Figure 23.27

Stomach

Pancreas

Epithelialcells

Trypsinogen(inactive)Chymotrypsinogen(inactive)Procarboxypeptidase(inactive)

Trypsin

Chymotrypsin

Carboxypeptidase

Membrane-boundenteropeptidase


Regulation of Bile Secretion

• Gallbladder contraction is stimulated by• Cholecystokinin (CCK) from intestinal cells

exposed to proteins and fat in chyme

• Vagal stimulation (minor stimulus)

• CKK also causes the hepatopancreatic sphincter to relax


Regulation of Bile Secretion

• Bile secretion is stimulated by• Bile salts in enterohepatic circulation – the

more bile salts in the enterohepatic circulation the more bile is secreted.

• Secretin from intestinal cells exposed to HCl and fatty chyme


Regulation of Pancreatic Secretion• Bile and pancreatic secretions are regulated

by the same factors (neural and hormonal)

• CCK induces the secretion of enzyme-rich pancreatic juice by acini

• Secretin causes secretion of bicarbonate-rich pancreatic juice by duct cells

• Vagal stimulation also causes release of pancreatic juice (minor stimulus)


Chyme enter-ing duodenum causes release of cholecystokinin (CCK) and secretin from duodenal enteroendocrine cells.

CCK (red dots) and secretin (yellow dots) enter the bloodstream.

CCK induces secretion of enzyme-rich pancreatic juice. Secretin causes secretion of HCO3

–-rich pancreatic juice.

Bile salts and, to a lesser extent, secretin transported via bloodstream stimulate liver to produce bile more rapidly.

CCK (via bloodstream) causes gallbladder to contract and hepatopancreatic sphincter to relax; bile enters duodenum.

During cephalic and gastric phases, vagal nerve stimulation causes weak contractions of gallbladder.

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Copyright © 2010 Pearson Education, Inc. Figure 23.28, step 1


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During cephalic and gastric phases, vagal nerve stimulation causes weak contractions of gallbladder.

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Digestion in the Small Intestine

• Chyme from stomach contains• Partially digested carbohydrates and proteins

• Undigested fats (minimally worked on by salivary lipase and gastric lipase)


Requirements for Digestion and Absorption in the Small Intestine

• The intestine must get slow delivery of hypertonic acidic chyme from the stomach

• 3cc or less per peristaltic wave and 3 waves per minute so 9 cc or less per minute into small intestine from stomach

• If the hypertonic chyme was delivered to the small intestine too quickly it would pull in too much water from the bloodstream

• Additionally the chyme is quite acidic – thus ulcer formation if it enters the small intestine too fast


• The small intestine only provides brush border enzymes – most digestive chemicals in the small intestine come from the liver and pancreas

• Delivery of bile, enzymes, and bicarbonate from the liver and pancreas

• Intestinal motility mixes chyme with pancreatic, bile and intestinal juices as a result of its segmentation waves

• The small intestine mainly uses segmentation waves – peristaltic waves begin after most of the materials have been absorbed


Motility of the Small Intestine• Segmentation Waves• Make the intestinal contents appear as if they are

being massaged- the chyme is moved back and forward in the lumen a few centimeters at a time by alternating contraction and relaxation of rings of smooth muscle.

• Initiated by intrinsic pacemaker cells located in circular muscles – but unlike stomach pacemaker cells which have only one rhythm – the pacemakers in the duodenum depolarize more frequently (12 -14 contractions per minute) than those in ileum ( 8 or 9 contractions per minute)


• Mixes and moves contents slowly and steadily toward the ileocecal valve – giving plenty time to complete digestion and absorption

• The intensity of the waves is altered by long and short reflexes

• The segmentation waves wane in the late intestinal (fasting) phase after most of the small intestinal contents have been absorbed

• Once the segmentation waves wane the peristaltic waves begin and a result of secretion of motilin from the duodenal mucosae


Motility of the Small Intestine• Peristalsis

• Initiated by motilin in the late intestinal phase

• As the motilin blood level rises peristaltic waves are initiated in the proximal duodenum every 90 – 120 minutes and sweep slowly along the intestines – dying out in approximately 2 feet from its initiation area.

• The next wave starts distal to the previous wave thus termed the MMC ( migrating motility complex)

• A complete trip from duodenum to ileum takes approximately two hours

• The process repeats itself thus meal remnants, bacteria, sloughed off mucosal cells and debris are moved to the large intestine


• This housekeeping function is critical for preventing the overgrowth of bacteria that migrate from the large intestine. As food enters the stomach with the next meal, peristalsis is replaced by segmentation


Motility of the Small Intestine

• The local enteric neurons coordinate intestinal motility and it depends on which neurons are activated or inhibited

• Cholinergic sensory neurons may activate the myenteric plexus• Causes contraction of the circular muscle

proximally and of longitudinal muscle distally

• Forces chyme along the tract


Motility of the Small Intestine

• Most of the time the ileocecal sphincter is closed. Two mechanisms open it

1. The stomach initiates a gastroileal reflex – a long reflex that enhances the force of segmentation in the ileum

2. Gastrin increases the motility of the ileum and relaxes the ileocecal valve

• Ileocecal valve flaps close when chyme exerts backward pressure

Copyright © 2010 Pearson Education, Inc. Figure 23.3b

(b)

Microvilli

Absorptivecell


From mouth

(a) Peristalsis: Adjacent segments of alimentary tract organs alternately contract and relax, which moves food along the tract distally.


Large Intestines (Colon)

• Approximately twice the diameter of the small intestines – 3 inches wide

• Approximately 5 feet long

• Function (absorb most of the remaining water from indigestible food residues and temporarily store the residues before elimination as feces)


Functions of the Large Intestine

• Vitamins, water, and electrolytes are reclaimed

• Major function is propulsion of feces toward the anus

• Colon is not essential for life


Large Intestine

• Unique features• Teniae coli

• Three bands of longitudinal smooth muscle in the muscularis

• Haustra

• Pocketlike sacs caused by the tone of the teniae coli

• Epiploic appendages

• Fat-filled pouches of visceral peritoneum


Large Intestine

• Regions• Cecum (pouch with attached vermiform

appendix)

• Colon

• Rectum

• Anal canal


Left colic(splenic) flexureTransversemesocolon Epiploicappendages

Descendingcolon

Teniae coli

Sigmoidcolon

Cut edge ofmesentery

External anal sphincter

Rectum

Anal canal(a)

Right colic(hepatic) flexureTransversecolon SuperiormesentericarteryHaustrumAscendingcolon IIeumIIeocecal valve

Vermiform appendixCecum


Colon

• Ascending colon and descending colon are retroperitoneal

• Transverse colon and sigmoid colon are anchored via mesocolons (mesenteries)

Copyright © 2010 Pearson Education, Inc. Figure 23.30c

Transverse colon

Greater omentum

Descending colonJejunumMesentery

Transversemesocolon

SigmoidmesocolonSigmoid colon

Ileum

(c)

Copyright © 2010 Pearson Education, Inc. Figure 23.30d

(d)

Pancreas

LiverLesser omentum

Stomach

Duodenum

Transversemesocolon

Greater omentumMesentery

Jejunum

Visceral peritoneum

Urinary bladder

Transverse colon

Ileum

Parietal peritoneum

Rectum


Rectum and Anus

• Rectum• Three rectal valves stop feces from being

passed with gas• Anal canal• The last segment of the large intestine

• Sphincters• Internal anal sphincter—smooth muscle• External anal sphincter—skeletal muscle

Copyright © 2010 Pearson Education, Inc. Figure 23.29b

(b)

Rectal valveRectum

Anal canal

Levator animuscle

AnusAnal sinuses

Anal columns

Internal analsphincter

External analsphincter

Hemorrhoidalveins

Pectinate line


Large Intestine: Microscopic Anatomy• Mucosa of simple columnar epithelium except

in the anal canal (stratified squamous)

• Abundant deep crypts with goblet cells

• Extensive mucus eases passage of feces and protects the intestinal wall from irritating acids and gases released by resident bacteria in the colon

• Low folds give anal columns and anal sinuses are between the folds – the sinuses exude mucus when defecate


• The horizontal tooth-shaped line that parallels the inferior margins of the anal sinuses is called the pectinate line. Superior to this line, the mucosa is innervated by visceral sensory fibers and is relatively insensitive to pain. The area inferior to this line is innervated by somatic sensory fibers – thus very sensitive to pain.

• Superficial venous plexuses of the anal canal form hemorrhoids if inflamed


Bacterial Flora• 10 million different types

• Enter from the small intestine or anus

• Metabolize some host products (mucin, heparin, and hyaluronic acid)

• Ferment some indigestible carbohydrates (cellulose, xylan, and others

• Release irritating acids and a mixture of gases (dimethyl sulfide, H2, N2, CH4, and CO2)

• Dimethyl sulfide is quite odorous

• About 500 ml of gas is produced each day

• Synthesize Vitamin B complex vitamins and Vitamin K


• Most bacteria exist peacefully with their host in the large intestine – but an elegant system keeps them from breaching the mucosal barrier

• The epithelial cells of the gut mucosa respond to specific bacterial components by releasing chemicals that recruit immune cells, particularly dendritic cells into the mucosa.

• The dendritic cells pry open the tight junctions between the epithelial cells and send extensions into the lumen of sample the microbial antigens

• They then migrate to the nearby lymphoid follicles (MALT) where they present antigens to T cells.

• An IgA antibody response restricted to the gut lumen is triggered that prevents the bacteria from straying into tissues deep to the mucosa.


Motility of the Large Intestine

• Haustral contractions (occur every 30 minutes or so)• Slow segmenting movements that occur

mainly in the transverse and descending colon

• Haustra sequentially contract in response to distension


Mass Movements• Long slow-moving but powerful contractions

that move over large areas of the colon- three to four times a day and force the contents towards the rectum.

• Typically, they occur during or just after eating, which indicates the presence of food in the stomach activates the gastrocolic reflex in the colon.

• Of the 500 cc of fluid entering the cecum only about 150cc becomes feces.


Motility of the Large Intestine

• Gastrocolic reflex• Initiated by presence of food in the stomach

• Activates three to four slow powerful peristaltic waves per day in the colon (mass movements)


Defecation

• Mass movements force feces into rectum• Distension initiates spinal defecation reflex• Parasympathetic signals• Stimulate contraction of the sigmoid colon and

rectum• Relax the internal anal sphincter

• Conscious control allows relaxation of external anal sphincter


Impulses fromcerebral cortex(consciouscontrol)

Voluntary motornerve to externalanal sphincter

External analsphincter(skeletal muscle)

Internal anal sphincter(smooth muscle)

Sensorynerve fibers

Involuntary motor nerve(parasympathetic division)

Stretch receptors in wall

Rectum

Sigmoidcolon

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Distension, or stretch, of therectal walls due to movement of feces into the rectum stimulates stretch receptors there. The receptors transmit signals along afferent fibers to spinal cord neurons. A spinal reflex is initiated in which parasympathetic motor (efferent) fibers stimulate contraction of the rectal walls and relaxation of the internal anal sphincter.

If it is convenient to defecate, voluntary motor neurons are inhibited, allowing the external anal sphincter to relax so that feces may pass.


Chemical Digestion

• Catabolic

• Enzymatic

• Hydrolysis


Chemical Digestion and Absorption of Carbohydrates• Digestive enzymes• Salivary amylase, pancreatic amylase, and

brush border enzymes (dextrinase, glucoamylase, lactase, maltase, and sucrase)


Bonding Carbohydrate monomers together

• Monosaccharides bond together by the removal of a water molecule (dehydration synthesis) to form a covalent bond between the two monosaccharides known as a “glycosidic bond”

•When bond two monosaccharides together termed a disaccharide, when join 3 – 10 together termed an Oligosaccharide – more than 10 together – termed a polysaccharide


Common Disaccharides

• Sucrose – table sugar (glucose alpha 1,2 to fructose)

• Maltose – in beer (glucose alpha 1,4 to glucose)

• Lactose – in milk (galactose beta 1, 4 to glucose)


AmyloseAlpha 1,4 Linkages (Linear)


AmylopectinAlpha 1,4 and Alpha 1,6 (Branching)


Alpha 1,4 and many Alpha 1,6


Humans Cannot Break the Beta Bond in Cellulose (Fiber) so it adds bulk to the diet


Chemical Digestion and Absorption of Carbohydrates• Digestive enzymes

• Salivary amylase, pancreatic amylase, and brush border enzymes (dextrinase, glucoamylase, lactase, maltase, and sucrase)

• The amylases (salivary, pancreatic and brush border type) break alpha 1, 4 linkages

• The dextrinase brush border enzyme breaks alpha 1,6 linkages

• The lactase breaks the Beta 1,4 between glucose and galactose

• Maltase breaks the alpha 1,4 between glucose and glucose

• Sucrase breaks the alpha 1,2 bond between glucose and fructose


Chemical Digestion and Absorption of Carbohydrates• Absorption• Secondary active transport (cotransport) with

Na+

• Facilitated diffusion of some monosaccharides

• Enter the capillary beds in the villi

• Transported to the liver via the hepatic portal vein

Copyright © 2010 Pearson Education, Inc. Figure 23.32 (1 of 4)

Carbohydrate digestion

• Glucose and galactose are absorbed via cotransport with sodium ions.• Fructose passes via facilitated diffusion.• All monosaccharides leave the epithelial cells via facilitated diffusion, enter the capillary blood in the villi, and are transported to the liver via the hepatic portal vein.

Starch and disaccharides

Oligosaccharidesand disaccharides

Lactose Maltose Sucrose

Glucose Fructose

Salivaryamylase

Mouth

Pancreaticamylase

Brush borderenzymes in small intestine(dextrinase, gluco-amylase, lactase, maltase, and sucrase)

Smallintestine

Smallintestine

Foodstuff

Galactose

Path of absorptionEnzyme(s)and source

Site ofaction


Protein Digestion and Absorption

• Dietary Proteins (125 grams a day)

• Enzyme Proteins in Gut (15 – 25 grams)

• Proteins in sloughed mucosal cells (15 – 25 grams)


R group still free


Chemical Digestion and Absorption of Proteins• Enzymes: pepsin in the stomach• Pancreatic proteases• Trypsin, chymotrypsin, and carboxypeptidase

• Brush border enzymes• Aminopeptidases, carboxypeptidases, and

dipeptidases• Absorption of amino acids is coupled to active

transport of Na+ and in the cases of Dipeptides and Tripeptide – coupled to H+


Peptidases (enzymes that break the peptide bond) • Exopeptidases – break the end amino acids off at the N-

terminal (Aminopeptidase) or C- Terminal (Carboxypeptidase- from pancreas and brush border)

• Endopeptidases – break peptide bonds within protein

• Pepsin – from chief cells in stomach breaks peptide bonds between tyrosine and phenylalanine

• Trypsin (from pancreas) cleaves peptide chains mainly at the carboxyl side of the amino acids lysine or arginine, except when either is followed by proline

• Chymotrypsin (from pancreas) - preferentially cleaves peptide amide bonds where the carboxyl side of amide bond (the S1 position) is a tyrosine, tryptophan, or phenylalanine

• Dipeptidase – from brush border splits dipeptides


Absorptiveepithelialcell

Apical membrane (microvilli)

Aminoacid carrier

Capillary

Lumen of intestine

Pancreaticproteases

Amino acids of protein fragmentsBrush border enzymes

Na+

Na+

1 Proteins and protein fragments are digested to amino acids by pancreatic proteases (trypsin, chymotrypsin, and carboxypeptidase), and by brush border enzymes (carboxypeptidase, aminopeptidase, and dipeptidase)of mucosal cells.

2 The amino acids are then absorbed by active transport into the absorptive cells, and move to their opposite side (transcytosis).

3 The amino acids leave the villus epithelial cell by facilitated diffusion and enter the capillary via intercellular clefts.

Active transportPassive transport


Protein digestion

• Amino acids are absorbed by cotransport with sodium ions.• Some dipeptides and tripeptides are absorbed via cotransport with H+

and hydrolyzed to amino acids within the cells.

+

• Amino acids leave the epithelial cells by facilitated diffusion, enter the capillary blood in the villi, and are transported to the liver via the hepatic portal vein.

Smallintestine

Smallintestine

Stomach

Foodstuff

Protein

Large polypeptides

Pepsin(stomach glands)in presence of HCl

Small polypeptides,small peptides

Pancreaticenzymes (trypsin, chymotrypsin,carboxypeptidase)

Amino acids(some dipeptidesand tripeptides)

Brush border enzymes(aminopeptidase,carboxypeptidase,and dipeptidase)


Site ofaction


Chemical Digestion and Absorption of Lipids• Pre-treatment—emulsification by bile salts

• Enzymes—pancreatic lipase

• Absorption of glycerol and short chain fatty acids• Absorbed into the capillary blood in villi

• Transported via the hepatic portal vein


Chemical Digestion and Absorption of Lipids• Absorption of monoglycerides and fatty acids• Cluster with bile salts and lecithin to form

micelles

• Released by micelles to diffuse into epithelial cells

• Combine with proteins to form chylomicrons

• Enter lacteals and are transported to systemic circulation


Epithelialcells ofsmallintestine

Fat dropletscoated withbile salts

Fat globule

Lacteal

Bile salts

Micelles made up of fatty acids, monoglycerides,and bile salts

1 Large fat globules are emulsified (physically broken up into smaller fat droplets) by bile salts in the duodenum.

2 Digestion of fat by the pancreatic enzyme lipase yields free fatty acids and monoglycerides. These then associate with bile salts to form micelles which “ferry” them to the intestinal mucosa.

3 Fatty acids and monoglycerides leave micelles and diffuse into epithelial cells. There they are recombined and packaged with other lipoid substances and proteins to form chylomicrons.

4 Chylomicrons are extruded from the epithelial cells by exocytosis. The chylomicrons enter lacteals. They are carried away from the intestine by lymph.


Short Chain Fatty Acids

• Passage of short chain fatty acids is quite different from what we have described. These fat breakdown products do no depend on the presence of bile salts or micelles, are not recombined to form triglycerides within the intestinal lumen cells, and simply diffuse into the portal blood for distribution.


Fat digestion

Small intestine

Small intestine

Foodstuff

Unemulsifiedfats

Emulsification by the detergent action of bile salts ductedin from the liver

Pancreatic lipases

Monoglyceridesand fatty acids

Glyceroland

fatty acids


Site ofaction

• Fatty acids and monoglycerides enter the intestinal cells via diffusion. • Fatty acids and monoglycerides are recombined to form triglycerides and then combined with other lipids and proteins within the cells, and the resulting chylomicrons are extruded by exocytosis.• The chylomicrons enter the lacteals of the villi and are transported to the systemic circulation via the lymph in the thoracic duct.• Some short-chain fatty acids are absorbed, move into the capillary blood in the villi by diffusion, and are transported to the liver via the hepatic portal vein.


Chemical Digestion and Absorption of Nucleic Acids• Enzymes• Pancreatic ribonuclease and

deoxyribonuclease – break into nucleotides.

• Intestinal brush border enzymes (nucleosidases and phosphatases)

• Absorption• Active transport

• Transported to liver via hepatic portal vein


Nitrogenous Bases


Nucleic acid digestion

• Units enter intestinal cells by active transport via membrane carriers.• Units are absorbed into capillary blood in the villi and transported to the liver via the hepatic portal vein.

Smallintestine

Smallintestine

Foodstuff

Nucleic acidsPancreatic ribo-nuclease and deoxyribonucleaseBrush borderenzymes(nucleosidasesand phosphatases)

Pentose sugars,N-containing bases,

phosphate ions


Site ofaction


Vitamin Absorption• In small intestine• Fat-soluble vitamins (A, D, E, and K) are carried

by micelles and then diffuse into absorptive cells – Thus to get maximal absorption fat soluble vitamins – need to eat some fat containing food

• Water-soluble vitamins (vitamin C and B vitamins) are absorbed by diffusion or by passive or active transporters.

• Vitamin B12 binds with intrinsic factor, and is absorbed by endocytosis in the terminal ileum


Vitamin Absorption

• In large intestine

• Vitamin K and B vitamins from bacterial metabolism are absorbed


Electrolyte Absorption

• Mostly along the length of small intestine

• Iron and calcium are absorbed mainly in duodenum • Na+ is coupled with absorption of glucose and

amino acids

• Ionic iron is stored in mucosal cells with ferritin

• K+ diffuses in response to osmotic gradients

• Ca2+ absorption is regulated by vitamin D and parathyroid hormone (PTH)


• Na+/K+ pump in basal membrane of mucosal epithelial cells sets up gradient

• Na+ helps monosaccharides get absorbed (glucose and galactose) and the amino acids

• The anions generally follow Na+

• Chloride is actively transported out of the lumen – and particularly by a HCO3- exchange transporter in the terminal intestine

• K+ follows behind water – as follows leaves the intestinal lumen it creates a high concentration gradient for K+ - so K+ is then pulled by the osmotic gradient – thus if for some reason water is not passively absorbed properly – K+ is loss from the body and some is even pulled in the lumen from the interstitial space


Iron Absorption 1• Iron is brought into the cell through an active

transport process involving the protein DMT-1 (divalent metal transporter-1), which is expressed on the apical surface of enterocytes in the initial part of the duodenum. DMT-1 is not specific to iron, and can transport other metal ions such as zinc, copper, cobalt, manganese, cadmium or lead.


Iron Absorption 2• Once inside the enterocyte, there are two

fates for iron:(1) It may leave the enterocyte and enter the body via the basolateral transporter known as ferroportin.

(2) It can be bound to ferritin, an intracellular iron-binding protein. For the most part, iron bound to ferritin in the enterocyte will remain there. This iron will be lost from the body when the enterocyte dies and is sloughed off from the tip of the villus.


Iron 3• Iron that enters the body from the basolateral

surface of the enterocyte is rapidly bound to transferrin, an iron-binding protein of the blood. Transferrin delivers iron to red blood cell precursors, that take up iron bound to transferrin via receptor-mediated endocytosis.

• Normally, the capacity of transferrin to bind iron in the plasma greatly exceeds the amount of circulating iron. The transferrin saturation (percent of transferrin occupied by iron) is measured to determine if an individual has an excessive load of iron in the body. The normal transferrin saturation is in the range of 20-45%.


Water Absorption

• 95% is absorbed in the small intestine by osmosis

• Net osmosis occurs whenever a concentration gradient is established by active transport of solutes

•Water uptake is coupled with solute uptake


Malabsorption of Nutrients

• Causes• Anything that interferes with delivery of bile or

pancreatic juice

• Damaged intestinal mucosa (e.g., bacterial infection)


Malabsorption of Nutrients

• Gluten-sensitive enteropathy (celiac disease)• Gluten damages the intestinal villi and brush

border

• Treated by eliminating gluten from the diet (all grains but rice and corn)


Developmental Aspects

• In the third week • Endoderm has folded and foregut and hindgut

have formed

• Midgut is open and continuous with the yolk sac

• Mouth and anal openings are nearly formed

• In the eighth week• Accessory organs are budding from endoderm


Stomodeum

Foregut

Site ofliverdevelopmentMidgutSpinal cord

Hindgut

Proctodeum

Endoderm

BrainOralmembrane Heart

Yolk sac

Cloacalmembrane

Bodystalk

(a)

Lung bud

Liver

Gall-bladder

Cystic ductVentral pancreatic bud

DorsalpancreaticbudDuodenum

Stomach

(b)

Bileduct



• Fetal nutrition is via the placenta, but the GI tract is stimulated to mature by amniotic fluid swallowed in utero

• The newborn’s rooting reflex helps the infant find the nipple; the sucking reflex aids in swallowing



• During old age • GI tract activity declines, absorption is less

efficient, and peristalsis is slowed

• Diverticulosis, fecal incontinence, and cancer of the GI tract


Cancer

• Stomach and colon cancers rarely have early signs or symptoms

• Metastasized colon cancers frequently cause secondary liver cancer

• Prevention• Regular dental and medical examination

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