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10/15/2018

Digestive System Alimentary Tract and Accessory Organs

Function of Digestive System

1. Take in food and – ingestion or intake 2. Break food down into molecules 3. Absorb molecules into the bloodstream 4. Eliminate any indigestible remains – elimination or 5. Dispose of some metabolic wastes

Overview of Digestive System

• Alimentary canal (gastrointestinal or GI tract or gut) • Continuous muscular tube that runs from the to • Breaks down food into smaller fragments: • Mechanical: movement • Chemical: • Absorbs nutrient molecules through lining into blood • Organs: mouth, , , , , , anus • Accessory digestive organs • Oral cavity: • Teeth • Tongue • Salivary glands: lubrication • • • • Digestive glands: produce secretions that help break down foodstuffs

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Digestive Processes

Processing of food involves six essential activities: 1. Ingestion 2. Propulsion • • 3. Mechanical breakdown • Chewing • Churning, mixing • Segmentation: local constriction of intestine that mixes food with digestive juices 4. Chemical Breakdown • series of catabolic steps supported by groups of enzymes 5. Absorption • blood or 6. Defecation - Elimination • indigestible substances • includes wastes produced as liver processes blood

Peristalsis Segmentation

Adjacent segments of the Nonadjacent segments of the alimentary canal organs alternately alimentary canal organs contract contract and relax. and relax. • Food is moved distally • Food is moved forward, along the tract then backward • Primarily propulsive; some • Primarily mixes food and breaks mixing may occur it down mechanically; some From propulsion may occur mouth

Histology of the Alimentary Canal 1. Mucosa • Functions • Secretes , digestive enzymes, and • Absorbs end products of • Protects against infectious disease • , lymphoid follicles, localized movement, glands • Made up of three sublayers • – stratified squamous or simple columnar, mucus producing • – areolar: ample space for follicles, capillaries, and mucosal glands • 2. • Consists of areolar connective tissue • abundant amount of elastic tissue • blood and lymphatic vessels • lymphoid follicles • submucosal plexus 3. Muscularis externa • Circular and longitudinal muscle layer responsible for segmentation, peristalsis, and 4. Serosa ( in some areas) • Aka the visceral

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Figure 23.5 Basic structure of the alimentary canal.

Intrinsic nerve plexuses • Myenteric nerve plexus • Submucosal nerve plexus

Glands in submucosa

Mucosa • Epithelium • Lamina propria • Muscularis mucosae

Submucosa

Muscularis externa • Circular layer • Longitudinal layer

Serosa • Connective tissue • Epithelium (mesothelium) Nerve Gland in mucosa Mucosa-associated Duct of lymphoid tissue gland outside alimentary canal

Nerve and Blood Supply to Walls

• Blood Supply: • Splanchnic circulation includes • that branch off aorta to serve digestive organs • Hepatic, splenic, and left gastric arteries • Inferior and superior mesenteric arteries • Hepatic portal circulation • Drains nutrient-rich blood from digestive organs • Delivers blood to liver for processing • Enteric • Submucosal nerve plexus • Myenteric nerve plexus • Semiautonomous • Controls patterns of segmentation and peristalsis • Linked to CNS: visceral sensory fibers (afferent), autonomic motor fibers synapse with internal plexi (called long reflex arcs) • 100 million neurons in plexi

Neural reflex pathways initiated by stimuli inside or outside the .

External stimuli (sight, smell, taste, Central nervous system thought of food) Long reflexes Visceral afferents Extrinsic visceral (autonomic) efferents

Internal Chemoreceptors, Local (intrinsic) Effectors: (GI tract) osmoreceptors, or nerve plexus stimuli mechanoreceptors (“gut brain”) or glands

Short reflexes

Gastrointestinal Response: wall (site of short Change in reflexes) contractile or secretory activity Lumen of the alimentary canal

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Path of Alimentary Tract Begins at Oral Cavity and Pharynx . Ingestion . Propulsion . Posterior nasal . Oral mucosa aperture

Nasopharynx • Lubricants • Pharyngeal tonsil • Opening of • Antimicrobial pharyngotympanic proteins tube • Early Oropharynx Hard palate introduction • Palatine tonsil Soft palate • • Isthmus of the Particle size fauces Tongue reduction Lingual tonsil • Mechanical Laryngopharynx Hyoid bone Larynx mixing • Epiglottis • Vestibular fold • Thyroid cartilage o Taste cell Esophagus • Vocal fold • Cricoid cartilage activation o Olfactory Thyroid gland membrane (b) Structures of the pharynx and larynx involvement

The salivary glands and tongue papillae

Tongue

Teeth Ducts of Parotid sublingual gland gland Parotid duct Frenulum of tongue Masseter muscle Sublingual Body of mandible gland (cut)

Mylohyoid Posterior belly of muscle (cut) digastric muscle • Intrinsic glands Anterior belly of Submandibular • Extrinsic glands duct digastric muscle • Continuous production and more upon activation Submandibular • Average of 1500 ml/day gland • Mostly under parasympathetic control

Deglutition (swallowing) Slide 6 Bolus of food Nasopharynx

Tongue Uvula

Oro- Bolus pharynx Epiglottis

Upper Trachea Upper esophageal esophageal Bolus Esophagus sphincter

1 Buccal phase: 2 Pharyngeal-esophageal phase begins: 3 Pharyngeal-esophageal phase • The upper esophageal sphincter is • The tongue blocks the mouth. continues (steps 3  5 ): contracted (closed). • The soft palate and its uv ula rise, closing • The constrictor muscles of the • The tongue presses against the hard off the nasopharynx. pharynx contract, forcing food into the esophagus inferiorly. palate, forcing the food bolus into • The larynx rises so that the epiglottis the oropharynx. blocks the trachea. • The upper esophageal sphincter contracts after food enters. • The upper esophageal sphincter relaxes; used here food enters the esophagus.

Relaxed muscles Relaxed Circular muscles muscles contract All 4 tunics present Bolus of food

Longitudinal muscles Circular muscles contract All smooth contract muscle here

Gastroesophageal Gastroesophageal sphincter opens sphincter closed 5 The gastroesophageal sphincter surrounding the 4 Stomach Peristalsis moves cardial orifice opens. After food through the food enters the stomach, esophagus to the the sphincter closes, stomach. preventing regurgitation.

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Stomach Organization

• Storage • Expands from 50 ml to 4 l • Gastric juice • Food becomes • Region of the pyloris squishes chyme and allows (‘filters’) small amount of chyme to pass to ; remainder pushed back into fundus – prepares food for SI • Enteric pacemaker cells – 3 peristaltic waves per minute • Lesser and • Stretch receptors

• Environment best for protein digestion

Slide 4 Peristaltic waves in the stomach.

Pyloric Pyloric Pyloric valve valve valve closed slightly closed opened

1 Propulsion: Peristaltic 2 Grinding: The most 3 Retropulsion: The peristaltic waves move from the vigorous peristalsis and wave closes the pyloric valve, fundus toward the . mixing action occur close forcing most of the contents of to the pylorus. The pyloric the pylorus backward into the end of the stomach acts stomach. as a pump that delivers small amounts of chyme into the duodenum.

Microscopic of the stomach.

Gastric juice: • Mucus producing cells • Parietal cells: HCl + (for SI absorption of B12 & RBC production) • Chief cells: pepsinogen,

lipase Surface • Enteroendocrine cells: epithelium paracrine and endocrine substances influencing blood flow and stomach secretions Mucosa • Up to 3 l/day Lamina propria

Muscularis mucosae Submucosa Mucosal barrier: (contains Stomach : submucosal Oblique • Tissue regeneration plexus) layer • Kills many • Tight junctions join Muscularis Circular • Chemically degrades externa layer (contains Longitudinal epithelial cells myenteric layer food • -rich, plexus) • Activates enzymes Stomach wall thick mucus Serosa supplementing chemical Layers of the stomach wall breakdown

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Neural and hormonal mechanisms that regulate release of gastric juice. Stimulatory events Inhibitory events

Cephalic 1a Sight and thought Cerebral cortex Para- Cerebral 1c Loss of phase of food sympathetic cortex appetite, activity depression 1b Stimulation of Hypothalamus Vagus taste and smell and medulla nerve receptors oblongata

2a Stomach Long reflexes (via medulla G cells 2c Excessive distension and ) release to acidity Gastric activates blood (pH < 2) stretch phase in stomach receptors Short Sympathetic 2d Emotional reflexes nervous stress system activation 2b Food chemicals G cells Gastrin (overrides (especially peptides and release parasympathetic caffeine) and rising pH to blood controls) activate chemoreceptors Stomach secretory activity

Enterogastric reflex 3b Distension Intestinal (involves both short of duodenum; phase and long reflexes) presence of 3a Presence of Intestinal fatty, acidic, or (enteric) partially digested hypertonic gastrin foods in duodenum chyme or distension of the release Brief duodenum when to blood effect Release of stomach begins to enterogastrones empty (, )

Stimulate Inhibit

Presence of fatty, hypertonic, acidic chyme in duodenum

Controlling stomach Duodenal entero- Chemoreceptors and emptying via receptors at endocrine cells stretch receptors duodenum

• Liquids move quickly Secrete Trigger • Solids must liquify

Enterogastrones Enterogastric reflex • Carbs get to (secretin, cholecystokinin) duodenum first • Fats remain in Short Long reflex via stomach longest reflex CNS centers

via  sympathetic • Maybe >6 hrs. enteric activity; neurons  parasympathetic activity

 Contractile force of stomach  Rate of stomach emptying Initial stimulus Physiological response Stimulate Result Inhibit

The Small Intestine Gross Anatomy • Small intestine is the major of digestion and absorption • 2–4 m long (7–13 ft) from pyloric sphincter to , point at which it joins large intestine • Small diameter of 2.5–4 cm (1.0–1.6 inches) • Subdivisions • Duodenum: mostly retroperitoneal; ~25.0 cm (10.0 in) long; curves around head of pancreas • Has most features • : ~2.5 m (8 ft) long Intraperitoneal, suspended by • : ~3.6 m (12 ft) long • Blood supply: • Superior mesenteric artery brings blood supply • (carrying nutrient-rich blood) drain into superior mesenteric veins, then into hepatic portal vein, and finally into liver • Nerve supply • Parasympathetic innervation via vagus nerve, and sympathetic innervation from thoracic splanchnic

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Mesenteries of the abdominal digestive organs.

Greater omentum

Transverse colon

Transverse mesocolon

Descending colon

Jejunum Mesentery

Sigmoid mesocolon

Ileum

Microscopic Anatomy Vein carrying blood to • Modifications of small intestine hepatic portal for absorption vessel • huge surface area for nutrient absorption • Surface area is increased 600 to ~200 m2 (size of a tennis court) • Modifications include: Muscle • Length layers Lumen • • Permanent folds (~1 cm deep) Circular • Villi folds • Fingerlike projections of mucosa Villi (~1 mm high) • dense bed • Lacteals • Absorptive • Microvilli • brush border • brush border enzymes, used for final and protein digestion Modifications decline in distal SI

Loss of cells here and regeneration from crypt base Microvilli Enterocytes (brush border) (absorptive cells)

Lacteal

Goblet cell Villus

Blood Entero- capillaries endocrine Mucosa- cells associated lymphoid tissue Paneth 1-2 l Intestinal juice/day Intestinal cells • Water crypt Venule • Mucus Muscularis • Lysozyme & defensins – mucosae Lymphatic vessel paneth cells Duodenal Submucosa gland Hormones – enteroendocrine cells Alkaline mucus

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Gross anatomy of the human liver – anterior view

Sternum Nipple Bare area Liver

Falciform ligament

Left lobe Right lobe of liver of liver

Round Gallbladder ligament (ligamentum teres)

Lobes are artificial designations not structural or functional designations

Gross anatomy of the human liver – posterior view

Hepatic veins Caudate lobe Inferior vena cava

Left lobe Bare area

Porta hepatis allows passage of • Hepatic portal vein • Hepatic artery proper •

Falciform ligament Gallbladder

Round ligament Right lobe

Quadrate lobe

Microscopic anatomy of the liver.

Liver Lobules: • Hepatocytes lining sinusoidal capillaries • Stellate macrophages Lobule • Central vein • Bile ducts

Portal triads ‘at the corners’

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Microscopic anatomy of the liver – marvel of natural selection Interlobular veins (to hepatic vein) Central vein

Sinusoids Bile Plates of canaliculi hepatocytes

Bile duct (receives bile from bile canaliculi)

Fenestrated lining (endothelial cells) of sinusoids

Bile duct Portal venule Portal triad Portal arteriole Stellate macrophages in sinusoid walls

Portal vein

Hepatocyte Function

• Hepatocytes have increased rough and smooth ER, Golgi apparatus, peroxisomes, and mitochondria • Produce ~900 ml bile per day • Process bloodborne • Example: store glucose as glycogen and make plasma proteins • Store fat-soluble • Perform detoxification • Example: converting ammonia to urea

• Regenerate readily – complete regeneration in 6-12 months after 80% loss

Bile

• Yellow-green, alkaline solution containing: • Bile : cholesterol derivatives that function in fat emulsification and absorption • : pigment formed from heme • Bacteria break down in intestine to stercobilin that gives brown color of • Cholesterol, triglycerides, phospholipids, and • Enterohepatic circulation • Recycling mechanism that conserves bile salts • Bile salts are: 1. Reabsorbed into blood by ileum (the last part of small intestine) 2. Returned to liver via hepatic portal blood 3. Resecreted in newly formed bile • About 95% of secreted bile salts are recycled, so only 5% is newly synthesized each time

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The Gallbladder

• Gallbladder is a thin-walled muscular sac on ventral surface of liver • Functions to store and concentrate bile by absorbing water and ions • Contains many honeycomb folds that allow it to expand as it fills • Muscular contractions release bile via cystic duct, which flows into bile duct

The Pancreas

• Location: mostly retroperitoneal, deep to greater curvature of stomach • Exocrine function: produce pancreatic juice • Acini: zymogen granules containing proenzymes • Ducts: secrete to duodenum via main ; smaller duct cells produce water and bicarbonate • 1200-1500 ml/day watery, alkaline solution (pH 8) − • Electrolytes, primarily HCO3 • Digestive enzymes • Proteases (for proteins): secreted in inactive form to prevent self-digestion – activated by intestinal enzymes • (for ) • Lipases (for lipids) • Nucleases (for nucleic )

• Endocrine function: secretion of insulin and glucagon by pancreatic islet cells – one control over blood sugar concentration

Structure of the enzyme-producing tissue of the pancreas.

Small duct Acinar cell (secretes enzymes) • Zymogen granules • Rough Basement membrane One acinus

Duct cell (secretes  HCO3 and H2O)

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Activation of pancreatic proteases in the small intestine. Stomach

Pancreas

Epithelial cells

Membrane-bound enteropeptidase Trypsin (inactive) Chymotrypsinogen (inactive) Procarboxypeptidase Carboxypeptidase (inactive)

What Are Enzymes?

• Enzyme: “A protein molecule produced by living organisms able to catalyze, or facilitate, a specific chemical reaction involving other substances without itself being destroyed or changed in any way.” • “…speeding the rate at which a biochemical reaction proceeds but not altering the direction or nature of the reaction.”

Relationship of the Accessory organs (liver, gallbladder and pancreas) to the duodenum

Liver • Right and left hepatic ducts • Common hepatic duct • Bile duct and sphincter

Accessory pancreatic duct Pancreas • Tail of pancreas • Main pancreatic duct and sphincter Gallbladder • Head of pancreas • Mucosa with folds • Cystic duct Jejunum Duodenum Hepatopancreatic • Major duodenal ampulla and sphincter papilla

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Bile and Pancreatic Secretion into the Small Intestine • Bile duct and pancreatic duct unite in wall of duodenum • Fuse together in bulblike structure called hepatopancreatic ampulla • Ampulla opens into duodenum via volcano-shaped • Hepatopancreatic sphincter controls entry of bile and pancreatic juice into duodenum • Accessory pancreatic duct: smaller duct that empties directly into duodenum • Regulation of bile and pancreatic secretions • Bile and pancreatic juice secretions are both stimulated by neural and hormonal controls • Hormonal controls include: • Cholecystokinin (CCK) • Secretin • Bile secretion is increased when: • Enterohepatic circulation returns large amounts of bile salts • Secretin, from intestinal cells exposed to HCl and fatty chyme, stimulates gallbladder to release bile • Hepatopancreatic sphincter is closed, unless digestion is active • Bile is stored in gallbladder and released to small intestine only with contraction

Mechanisms promoting secretion and release 3 Bile secretion by liver: of bile and pancreatic juice. • Bile salts returning from enterohepatic circulation are the most powerful 1 CCK and secretin are stimulus for bile secretion. secreted by duodenal • Secretin is a minor enteroendocrine cells. stimulus. • Cholecystokinin (CCK) release is stimulated by proteins and fats in chyme. 4 Gallbladder contraction: • Secretin release is • CCK causes gallbladder stimulated by acidic chyme. contraction. • CCK and secretin enter the • Vagus nerve stimulates circulation and cause the weak gallbladder following four events contraction during cephalic ( 2 – 5 ). and gastric phases.

5 Hepatopancreatic sphincter relaxation: • CCK causes 2 Pancreatic secretion: hepatopancreatic • CCK induces secretion by sphincter to relax. Bile acinar cells of enzyme- and pancreatic juice enter rich pancreatic juice. duodenum. • Secretin causes secretion  by duct cells of HCO3 . rich pancreatic juice. • Vagus nerve weakly CCK stimulates during cephalic and gastric Secretin phases.

Gross anatomy of the large intestine. Left colic (splenic) flexure

Right colic Transverse mesocolon (hepatic) flexure Epiploic appendages Superior mesenteric artery

Ascending colon IIeum Cut edge of mesentery

IIeocecal valve Tenia coli

Sigmoid colon

Appendix

Anal canal

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Large Intestine Subdivisions of large intestine 1. Cecum: first part of large intestine 2. : masses of lymphoid tissue • Part of MALT • Bacterial storehouse capable of Rectal valve recolonizing gut when necessary Rectum 3. Colon: has several regions, most which are retroperitoneal (except Hemorrhoidal transverse and veins sigmoid) muscle • ends at right colic (hepatic) flexure • Transverse colon ends left colic (splenic) flexure External anal • Descending colon sphincter • Sigmoid colon: S-shaped portion that Internal anal travels through sphincter 4. Rectum: three rectal valves 5. Anal canal: last segment of large intestine • Has two sphincters • : smooth muscle Anus • External anal sphincter: skeletal muscle

Reflexes and the Colon and Rectum

Impulses from cerebral cortex (conscious control) Defecation reflex.

Sensory nerve fibers

1 Feces move into Voluntary motor and distend the rectum, nerve to external stimulating stretch receptors anal sphincter there. The receptors transmit signals along afferent fibers Sigmoid colon to neurons.

Stretch receptors in wall 2 A spinal reflex is initiated in which parasympathetic motor (efferent) fibers stimulate contraction of the rectum and sigmoid colon, and relaxation of the Rectum internal anal sphincter. External anal Involuntary motor nerve sphincter (parasympathetic division) (skeletal muscle) Internal anal sphincter (smooth muscle)

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

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Slide 5

Lumen of intestine 1 Pancreatic amylase breaks Pancreatic down starch and glycogen amylase into and disaccharides. Na+ Disaccharides 2 Brush border enzymes break oligosaccharides and Carbohydrate disaccharides into digestion and monosaccharides. Brush Apical absorption in border membrane the small Na+ enzymes (microvilli) intestine. Monosac- charides

3 Monosaccharides (glucose and galactose) are cotransported across the apical membrane of the absorptive epithelial cell. This active transport uses the + Basolateral Na concentration gradient membrane established by the Na+ -K+ ATPase (pump) in the Na+ ATP basolateral membrane.

Na+ -K+ Monosaccharide ATPase carrier Na+ K+ 4 Monosaccharides exit across the basolateral membrane by facilitated diffusion and enter the capillary via intercellular Capillary clefts.

Slide 5 Lumen Amino acids of intestine of protein fragments 1 Pancreatic Pancreatic proteases break Protein proteases down proteins and protein Brush fragments into smaller pieces and digestion and border some individual amino acids. + enzymes absorption in Na 2 Brush border enzymes break protein fragments the small into amino acids. intestine. Na+ Apical membrane (microvilli)

3 Amino acids are cotransported across the apical membrane of the absorptive epithelial cell. This active transport uses the Na+ concentration Amino Basolateral gradient established by the + + acid membrane Na -K ATPase (pump) in carrier the basolateral membrane. Na+ ATP

Na + -K+ ATPase

Na+ K+

4 Amino acids exit across the basolateral membrane via facilitated diffusion and enter the capillary via intercellular clefts. Capillary

Fat globule Emulsification, digestion, and

Bile salts absorption of 1 Emulsification. Bile salts in the duodenum break large fat globules into smaller fat fats. droplets, increasing the surface area available to lipase enzymes.

2 Digestion. Pancreatic lipases hydrolyze triglycerides, yielding monoglycerides and Fat droplets free fatty acids. coated with bile salts 3 Micelle formation. Micelles (consisting of fatty acids, monoglycerides, and bile salts) ferry their contents to epithelial cells.

4 Diffusion. Fatty acids and monoglycerides diffuse from micelles into epithelial cells.

5 Chylomicron formation. Fatty acids and monoglycerides are recombined and packaged with other fatty substances and proteins to form chylomicrons.

Epithelial 6 Chylomicron transport. cells of Chylomicrons are extruded small from the epithelial cells by intestine Lacteal exocytosis, enter lacteals, and are carried away from the intestine in lymph.

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Nucleic Acid Digestion

Foodstuff Enzyme(s) and source Site of action Path of absorption

Nucleic acids • Units enter intestinal cells by active Pancreatic ribo- Small transport via membrane carriers. nuclease and intestine • Units are absorbed into capillary deoxyribonuclease blood in the villi and transported to Nucleic acid the liver via the hepatic portal vein. digestion Brush border Small enzymes intestine Pentose sugars, (nucleosidases N-containing bases, and phosphatases) phosphate ions

Absorption of Vitamins, Electrolytes, and Water • absorption • In small intestine • Fat-soluble vitamins (A, D, E, and K) are carried by micelles; diffuse into absorptive cells • Water-soluble vitamins (C and B) are absorbed by diffusion or by passive or active transporters • (large, charged molecule) binds with intrinsic factor and is absorbed by endocytosis • In large intestine: and from bacterial metabolism are absorbed • Absorption of electrolytes • Most ions are transported actively along length of small intestine • Iron and calcium are absorbed in duodenum • Na+ absorption is coupled with active absorption of glucose and amino acids • Cl− is transported actively • K+ diffuses in response to osmotic gradients; lost if water absorption is poor • Usually amount in intestine is amount absorbed • Iron and calcium absorption is related to need • Ionic iron is stored in mucosal cells with ferritin • When needed, transported in blood by transferrin • Ca2+ absorption is regulated by vitamin D and parathyroid (PTH) • Absorption of water • 9 L water, most from GI tract secretions, enter small intestine • 95% is absorbed in the small intestine by • Most of rest is absorbed in large intestine • Net osmosis occurs if concentration gradient is established by active transport of solutes • Water uptake is coupled with solute uptake

Table 23.2-1 Overview of the Functions of the Gastrointestinal Organs

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Table 23.2-2 Overview of the Functions of the Gastrointestinal Organs (continued)

Parotid gland Mouth (oral cavity) Sublingual gland Salivary Tongue* Submandibular glands* gland

Esophagus Pharynx

Stomach Pancreas* () Liver*

Gallbladder* Transverse colon Duodenum Descending colon Small Jejunum Ascending intestine Ileum colon Cecum Large intestine Sigmoid colon Rectum Appendix Anus Anal canal

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