Figure 26.1

Digestive Tract

DIGESTIVE SYSTEM: the main idea/major functions

• Obtain nutrients for cells

• Nutrients are needed/used for:– Energy source (break down of nutrients can generate ATP)

– Building materials for:• Cell Growth• Cell Repair• Cell Maintenance• Production of hormones, enzymes, etc….

– Elements for normal cell function• Coenzymes, ions, etc…

Digestive System Specific Events/Processes• Ingestion: brining food into the stomach• Propulsion: movement of food through the alimentary canal

– peristalsis• Mechanical Digestion: breaking large into small without chemical

change– mastication (mixing, i.e., churning & segmentation)

• Chemical Digestion: breaking chemical bonds w/ enzymes: – large molecules smaller absorbable molecules

• Secretion: release of substances into the alimentary canal that aid digestive system function– Enzymes for chemical digestion– Mucus and other substances to neutralize acidity– Mucus for lubrication

• Absorption: movement of substances out of the digestive tract into circulation (blood or lymph)

• Elimination: digestive wastes exiting the body as feces– defecation

Macromolecules and their Components

• Carbohydrates : – Enzyme = amylase

• Polysaccharides monosaccharides• Disaccharides monosaccharides

• Proteins: – Enzyme = protease

• amino acids

• Lipids (e.g., triglycerides)– Enzyme = lipase

• Fatty Acids + Glycerol/monoglycerides

Figure 26.2 Peristalsis

• how material is moved/propelled through the digestive tract

• requires both a circularly and a longitudinally arranged layer of muscle

Digestive Tract

1. mouth

2. oral cavity

3. fauces

4. oropharynx

5. laryngopharynx

6. upper esophageal sphincter

7. esophagus

8. lower esophageal sphincter (cardiac)

9. stomach

10. pyloric sphincter

11. duodenum

12. jejunum

13. ileum

14. ileocecal valve

15. ascending colon

16. hepatic flexture

17. transverse colon

18. splenic flexture

19. descending colon

20. sigmoid colon

21. rectum

22. anal canal

salivary glands

liver/gallbladder

pancreas

Digestive Tract: epithelium

1. mouth

2. oral cavity

3. fauces

4. oropharynx

5. laryngopharynx

6. upper esophageal sphincter

7. esophagus

8. lower esophageal sphincter (cardiac)

9. stomach

10. pyloric sphincter

11. duodenum

12. jejunum

13. ileum

14. ileocecal valve

15. ascending colon

16. hepatic flexture

17. transverse colon

18. splenic flexture

19. descending colon

20. sigmoid colon

21. rectum

22. anal canal

stra

tifi

ed s

qu

amo

us

stratified squamous

simple columnar

sim

ple

co

lum

nar

Figure 26.2

= movement/propulsion = mechanical digestion

Figure 26.3 Oral Cavity

• Lined with stratified squamous

• Mechanical digestion by teeth• Chemical Digestion by

salivary and intrinsic enzymes and

– salivary amylase & lipase

• approximate pH ~ 7• Sensation: taste and texture• Propulsion (with tongue)

Figure 26.5 Teeth

• Mechanically Digest

• Know parts as per lab• Incisors: snip/clip/cut

• Canines: tear

• premolars: crush and grind

• molars: better crushers and grinders

Figure 26.4

• 3 Salivary Glands + intrinsic glands• Mucus Cells: mucinmucus

– lubricates and binds material• Serous Cells: salivary amylase &

lipase– chemical digestion – antimicrobial components

• 95% water; dissolves chemical to enhance taste sensations

• Lysozyme: antibacterial

• Dual innervation by ANS, but PD signals stimulate increases gland activity – cranial nerve VII/facial (subs)– cranial nerve

IX/glossopharyngeal (parotid)– Sight, smell, though, or taste of food

stimulates

Salivary Glands and Saliva

Digestive Tract

1. mouth

2. oral cavity

3. fauces

4. oropharynx

5. laryngopharynx

6. upper esophageal sphincter

7. esophagus

8. lower esophageal sphincter (cardiac)

9. stomach

10. pyloric sphincter

11. duodenum

12. jejunum

13. ileum

14. ileocecal valve

15. ascending colon

16. hepatic flexture

17. transverse colon

18. splenic flexture

19. descending colon

20. sigmoid colon

21. rectum

22. anal canal

stra

tifi

ed s

qu

amo

us

stratified squamous

simple columnar

sim

ple

co

lum

nar

Figure 26.3

Oropharynx & Laryngopharynx

• Function:– Propulsion

• with skeletal muscle mostly innervated by cranial nerves

• glossopharyngeal nerve (CN IX) & vagus (CNX) nerve

• Structure:– Lined with stratified squamous– Pharyngeal constrictor muscles (skeletal

muscles) along posterior wall for swallowing

Figure 26.9LAYERS OF THE DIGESTIVE TRACT WALL

Layers of digestive Tract Wall• Mucosa:

– inner-most layer in contact with ingested material– Lined with epithelial tissue (stratified squamous or simple columnar)– Lamina propria containing loose CT, capillaries, nerves, lymph capillaries

• Submucosa– Dense CT, larger vessels, – nerves/submucosal plexus that innervates/regulate mucosa and

submucosa

• Muscularis (muscularis externa)– muscle (mostly smooth) that propells– circular and longitudinal layers’– myenteric plexus (innervates/controls muscularis)

• Serosa OR adventitia– outer most layer– serosa = visceral peritoneum– adventitia=dense CT

Produces peristalsis

DIGESTIVE TRACT WALL

Figure 26.10 Esophagus• Soft, muscular tube; propells bolus to

stomach• passes through esophageal hiatus of

diaphragm• Mucosa

– stratified squamous• submucosa

– glands that produce mucous that lubricates

• Muscularis– inner circular layer– transitions from skeletal to smooth

• Adventitia• Lower Esophageal Sphincter

– poorly defined muscular thickening at end of esophagus

– reinforced by diaphragm– prevents movement of stomach

contents back into esophagus

Figure 26.12 Stomach

Processes• Mechanical digestion

– churning– involves inner oblique layer• Denaturing proteins by acids• Kills pathogens with acidity

• Chemical Digestion– proteins by enzymes

• pH ~2.5-4.5• minimal absorption

– alcohol and some lipid soluble substances

• soupy mixture/contents of stomach = chyme

Figure 26.12 StomachMucosa:• Simple Columnar

– Surface mucus cells• Secrete protective alkaline mucous

– Renew every 7 days• in body and fundus abundant

– gastric pitsgastric glands• Rugae

– allow expansionSubmucosa• unnotableMuscularis• inner oblique layer• intermediate circular• outer longitudinal layerSerosa

Unnotable

Pyloric Sphincter• well defined sphincter at junction with

duodenum• Controls emptying of stomach

Figure 26.13

Stomach is lined by surface mucus cells/simple columnar and inward projection that form gastric pits gastric glands

Figure 26.13Gastric Glands: produce gastric juice

• Parietal Cells– HCl

• activates pepsinogen• kills pathogens• denatures/breaks some bonds

– Intrinsic factor• absorption of B12

• Chief Cells– pepsinogen

• inactive enzyme

HCl + pepsinogen => pepsin (chemically digests proteins)

Stretch and ↑pH of stomach main stimulators of secretion

• Enteroendocrine Cells (gastroendocrine)– gastrin (released due to ↑pH)

• chemical regulation of digestive function• E.g., Increased parietal and chief cell

secretion• E.g., Increased gastric motility

Acid Reflux and Hiatal Hernias

• Failure of the L.E.S. to work correctly can lead to acid reflux

• Displacement of the L.E.S. relative to the diaphragm—hiatal hernia--promotes acid reflux because the diaphragm no longer reinforces the L.E.S.– Can also cause breathing difficulty in some settings

• Acid reflux is uncomfortable, diminishes quality of life, can cause histological changes in esophagus, increases likelihood of esophageal cancer, can cause airway congestions and even cause pneumonia if acids are aspirated

Figure 26.14

The 3 regions of the small intestine

Small Intestine• Main Functions:

– chemical digestion (primary site)– absorption (primary site)– propulsion– (mechanical digestion in form of segmentation)

– pH ~8 due to secretions of duodenal glands and pancreas• Structure:

– 3 regions: • doudenum (10”, retroperitoneal—not surrounded by peritoneum)• jejunum— ~ 8’, most absorption and digestion; has serosa• ileum— ~ 12’, has serosa

– bile and pancreatic secretions enter duodenum through duodenal papilla– Specializations for absorption (increase S.A. of S.I. ~600x)

1. Length2. plicea circulares/circular folds3. villi

– Contains with lacteal and capillaries to receive absorbed nutrients4. microvilli

Figure 26.15

Figure 26.15

Small Intestine

Mucosal epithelium• Simple columnar

– thin enough to pass through, large enough to be active– absorptive cells

• intrinsic enzymes for chemical digestion• absorption

– goblet cells• lubricating mucus that protects against acidic chyme

– (entero)endocrine cells• regulatory hormones

• intestinal crypts: – source of new columnar cells– Also produce alkaline mucus

• Neutralizes acidic chyme from stomach

External to Mucosa• capillaries and lacteals within mucosa accept absorbed nutrients

• Lymphatic tissue to protect again harmful microorganisms

Small Intestine Regulation of Gallbladder and Pancreas

• Acidic chyme stimulates duodenal mucosa to release secretin– Secretin stimulates production of bicarbonate by

pancreas

• Fatty acids and triglycerides stimulate duodenal mucosa to release CCK– CCK stimulates production of lipases by pancrease– CCK stimulates release of bile by gallbladder– CCK relaxes hepotopancreatic sphincter

Small Intestine Absorption

Small Intestine Trends

• Duodenum has poorly developed plicae and villi.

• Jejunum has well developed plicae and villi, but these absorptive features decline distally and eventually dissapear

• Lymphatic tissue increase in prevalence as you move from duodenum to ileum

Figure 26.16

Large Intestines• Includes:

– cecum– colon– rectum– anal canal

• Main Functions:– re-absorbs water to consolidate chyme into feces

• but NOT main site of water absorption– houses normal flora/bacteria

• produce vitamin K and some B vitamins• influence regularity, immune function (including allergies and

autoimmune), and evidence for influencing fat metabolism/deposition, neurological development, etc…

• produce flatus—farts…the gas we—all-pass– Store feces until defecation– Propulsion/defecation

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Ingestion(2 L)

1% in feces

(Water in feces = ingested + secreted – absorbed)

Absorption

Ingestion orsecretion

6 – 7%absorbed in the large intestine

92% absorbed in the small intestine

Pancreatic secretions(1.2 L)

Gastric secretions(2 L)

Salivary gland secretions(1 L)

Bile(0.7 L)

Small intestinesecretions(2 L)

Figure 26.17

Large Intestines

Structure:

• Mucosa: simple columnar

– absorptive cells– mucus cells—predominate

• to lubricate the solidifying mass of material– invaginated glands; crypts

• With many mucus cells

• muscularis – longitudinal layer concentrated into the 3 teniae coli– muscle tone (partial contraction) causes L.I. to gather and produce

haustra

• Ascending and descending colon are retroperitoneal

Rectum

• Last 6” of digestive tract• Mucosa of simple columnar that transitions to stratified

squamous by beginning of anus (the opening out of rectum)• Submucosa and lamina propria rich in blood vessels

– hemorrhoids

• thick muscular layer– For defecation– circular layer forms internal anal sphincter

• retroperitoneal

AnusPassageway out of rectum

Peritoneum and Mesentery

• Abdominal wall is lined by parietal peritoneum• Peritoneum covering surface of abdominal organs

is visceral peritoneum• double layer of peritoneum with no abdominal

organ between is mesentary• Mesentery does house blood vessels and is site of

adipose tissue deposition• there are many specific mesentaries (see next

page)• Organs which are posterior to the peritoneum and

have it covering only a single surface are retroperitoneal

Figure 26.7

Peritoneum and Mesentery

• FUNCTION:– Reduce friction– ‘suspend’ organs from the body wall by mesentary– influence movements of organs they surround

• STRUCTURE– Parietal peritoneum lines the body wall– Visceral peritoneum covers surface of many abdominal organs

• MESENTERY– Mesentery is a the visceral peritoneum folded back on itself with

no organ in between.• contain blood vessels• site for adipose tissue deposition

• Organs not surrounded by peritoneum are retroperitoneal – (will have peritoneum on one or more surface).

• duodenum pancreas, ascending Colon, descending colon, rectum, kidneys/adrenal gland and urinary bladder

MesenteriesNOTE: don’t memorize these unless study guide says to.• Lesser omentum: hold stomach and duodenum to liver and

diaphragm• Greater omentum: greater curvature to transverse colon• coronary ligament: liver to diaphragm• falciform ligament: liver to anterior body wall• mesentary proper: connectes to small intestine• mesocolon: connects the colon• mesoapendix: appendix

• Unusual mesentaries can cause torsion/obstruction/infection….– Ascending colon, gallbladder

Figure 26.8

Figure 26.20

Pancreas

PancreasDigestive Function:• Pancreatic Juice

– Created by acini cells of pancreas– Contains enzymes that break down:

• carbohydrates—pancreatic amylase• proteins—proteases• lipids—lipases• Nucleases—nucleic acids

– Enters the duodenum via duodenal papilla– pancratic ducthepatopancreatic ampullar sphincter/duodenal papilla

Regulated by hormones:• Secretin--from• Cholecytokinin (CCK)--from

endocrine function– releases insulin (reduces blood sugar and encourages cells to store away nutrients)

– releases glucagon (increases blood sugar and encourages cells to release stored energy)

Figure 26.18

Liver

Liver Functions

• Stores and Releases glucose (only organ/tissue that can release glucose)– to stabilize blood glucose

• Synthesizes and stores lipids (chlosterol, …..) also regulates circulating lipids– foie gras anyone?

• Amino Acid interconversion and breakdown (gluconeogenisis)

• Detoxify absorbed toxins• Removes old RBC’s• Produces Bile

– bile emulsifies fat (mechanically breaks it up and allows it to mix with water)

– eliminates cholesterol and bilirubin• Bile is stored in gallbladder and enters duodeum through a

system of ducts (biliary ducts)

Figure 26.19

Lobules of Liver (microscopic units)

Bile

Bile canaliculi

Hepaticsinusoid

Inferior view

Inferior vena cava

Hepatic duct

Hepatic duct

Hepatic portal veinHepatic artery

Hepatic portalveinHepatic artery

Liver lobule

Inferior venacava

Hepatic veins

Central vein

Heart

Aorta

Portaltriad

Portaltriad

Hepaticducts

Hepatic portalvein

Hepaticartery

Portaof liver

Hepaticduct branch

Hepatic portalvein branch

Hepatic artery branch

Nutrient-rich, oxygen-poor,

bloodOxygen-rich

blood

Smallintestine

Oxygen-richblood

Hepatic cords composedof hepatocytes

12

3

4

5

Liver Related Topics

• Juandice (obstructive)• Gallstones• Hepatic Portal Hypertension

Bile• Produced by liver• Stored in Gallbladder

– lined by smooth muscle– absorbs water/concentrates bile (5-10x more concentrated)

Bile Function and Movement• Bile emulsifies fat —breaks large droplets into smaller dropplets

that remain mixed with water.– Bile salts

• Bile is also used to excrete cholesterol and bilirubin (from hemoglobin breakdown—bile pigments) from body.

• Enters the duodenum via duodenal papilla– Common bile ducthepatopancreatic ampullar sphincter/duodenal

papilla

Gallbladder releases bile in response to CCK

Figure 26.21

Bile flow

• Hepatocytes

• bile canaliculi • hepatic ducts • L&R hepatic ducts • common hepatic duct

• common bile duct • duodenum

Cystic duct

Cranial Nerves and Digestive Function

• Swallowing involves use of:– hypoglossal nerve

• tongue movements

– Glossopharyngeal nerve• tongue & pharyngeal muscles

– Vagus Nerve• Pharynx & esophagus

• Vagus Nerve: is mostly PD and is widely involved with stimulation of digestive organs– esophagus, stomach, intestines

Table 26.1

Table 26.3

Table 26.4