Ch 25 The Digestive System Jen Tynes SELU. The Digestive System General anatomy and digestive processes General anatomy and digestive processes Mouth.

Ch 25 The Digestive System Jen Tynes SELU

The Digestive System General anatomy and digestive processes General anatomy and digestive processes Mouth through esophagus Mouth through esophagus Stomach Stomach Liver, gallbladder and pancreas Liver, gallbladder and pancreas Small intestine Small intestine Chemical digestion and absorption Chemical digestion and absorption Large intestine Large intestine

Digestive Functions Ingestion Ingestion intake of food intake of food Digestion Digestion breakdown of molecules breakdown of molecules Absorption Absorption uptake nutrients into blood/lymph uptake nutrients into blood/lymph Defecation Defecation elimination of undigested material elimination of undigested material

Stages of Digestion Mechanical digestion Mechanical digestion physical breakdown of food into smaller particles physical breakdown of food into smaller particles teeth and churning action of stomach and intestines teeth and churning action of stomach and intestines Chemical digestion Chemical digestion series of hydrolysis reactions that break macromolecules into their monomers series of hydrolysis reactions that break macromolecules into their monomers enzymes from saliva, stomach, pancreas and intestines enzymes from saliva, stomach, pancreas and intestines results results polysaccharides into monosaccharides polysaccharides into monosaccharides proteins into amino acids proteins into amino acids fats into glycerol and fatty acids fats into glycerol and fatty acids

Digestive Processes Motility Motility muscular contractions that break up food, mix it with enzymes and move it along muscular contractions that break up food, mix it with enzymes and move it along Secretion Secretion digestive enzymes and hormones digestive enzymes and hormones Membrane transport Membrane transport absorption of nutrients absorption of nutrients

Subdivisions of Digestive System Digestive tract (GI tract) Digestive tract (GI tract) 30 foot long tube extending from mouth to anus 30 foot long tube extending from mouth to anus Accessory organs Accessory organs teeth, tongue, liver, gallbladder, pancreas, salivary glands teeth, tongue, liver, gallbladder, pancreas, salivary glands

General Anatomy Digestive tract is open to the environment at both ends Digestive tract is open to the environment at both ends Most material in it has not entered the body tissues Most material in it has not entered the body tissues Is considered to be external to the body until it is absorbed by the epithelial cells of the alimentary canal Is considered to be external to the body until it is absorbed by the epithelial cells of the alimentary canal In a strict sense, defecated food residue was never in the body In a strict sense, defecated food residue was never in the body 25-7

Tissue Layers of GI Tract Mucosa Mucosa epithelium epithelium lamina propria lamina propria muscularis mucosae muscularis mucosae Submucosa Submucosa Muscularis externa Muscularis externa inner circular layer inner circular layer outer longitudinal layer outer longitudinal layer Adventitia or Serosa Adventitia or Serosa areolar tissue or mesothelium areolar tissue or mesothelium

25-9 Variations in Layers of GI Tract mucosa (mucous membrane) – lines the lumen and consists of: mucosa (mucous membrane) – lines the lumen and consists of: inner epithelium inner epithelium simple columnar in most of digestive tract simple columnar in most of digestive tract stratified squamous from mouth through esophagus, and in lower anal canal stratified squamous from mouth through esophagus, and in lower anal canal lamina propria – loose connective tissue layer lamina propria – loose connective tissue layer muscularis mucosa – thin layer of smooth muscle muscularis mucosa – thin layer of smooth muscle tenses mucosa creating grooves and ridges that enhance surface area and contact with food tenses mucosa creating grooves and ridges that enhance surface area and contact with food improves efficiency of digestion and nutrient absorption improves efficiency of digestion and nutrient absorption mucosa-associated lymphatic tissue (MALT) – the mucosa exhibits an abundance of lymphocytes and lymphatic nodules mucosa-associated lymphatic tissue (MALT) – the mucosa exhibits an abundance of lymphocytes and lymphatic nodules submucosa – thicker layer of loose connective tissue submucosa – thicker layer of loose connective tissue contains blood vessels, lymphatic vessels, a nerve plexus, and in some places mucus secreting glands that dump lubricating mucus into the lumen contains blood vessels, lymphatic vessels, a nerve plexus, and in some places mucus secreting glands that dump lubricating mucus into the lumen MALT extends into the submucosa in some parts of the GI tract MALT extends into the submucosa in some parts of the GI tract

25-10 Variations in Layers of GI Tract muscularis externa – consists of usually two layers of muscle near the outer surface muscularis externa – consists of usually two layers of muscle near the outer surface inner circular layer inner circular layer in some places, this layer thickens to form valves (sphincters) that regulate the passage of material through the tract in some places, this layer thickens to form valves (sphincters) that regulate the passage of material through the tract outer longitudinal layer outer longitudinal layer responsible for the motility that propels food and residue through the tract responsible for the motility that propels food and residue through the tract serosa – composed of a thin layer of areolar tissue topped by simple squamous mesothelium serosa – composed of a thin layer of areolar tissue topped by simple squamous mesothelium begins in the lower 3 to 4 cm of the esophagus begins in the lower 3 to 4 cm of the esophagus ends just before the rectum ends just before the rectum adventitia – a fibrous connective tissue layer that binds and blends the pharynx, most of the esophagus, and the rectum into the adjacent connective tissue of other organs adventitia – a fibrous connective tissue layer that binds and blends the pharynx, most of the esophagus, and the rectum into the adjacent connective tissue of other organs

Tissue Layers of GI Tract

Enteric Nervous Control Able to function independently of CNS Able to function independently of CNS Composed of two nerve networks Composed of two nerve networks submucosal plexus submucosal plexus controls glandular secretion of mucosa controls glandular secretion of mucosa contractions of muscularis mucosae contractions of muscularis mucosae myenteric plexus myenteric plexus controls peristalsis controls peristalsis contractions of muscularis externa contractions of muscularis externa

Relationship to Peritoneum Only duodenum, pancreas and parts of large intestine are retroperitoneal Only duodenum, pancreas and parts of large intestine are retroperitoneal Dorsal mesentery suspends GI tract and forms serosa (visceral peritoneum) of stomach and intestines Dorsal mesentery suspends GI tract and forms serosa (visceral peritoneum) of stomach and intestines Ventral mesentery forms lesser and greater omentum Ventral mesentery forms lesser and greater omentum lacy layer of connective tissue that contains lymph nodes, lymphatic vessels, blood vessels lacy layer of connective tissue that contains lymph nodes, lymphatic vessels, blood vessels

Lesser and Greater Omentum Lesser - attaches stomach to liver Greater - covers small intestines like an apron

Mesentery and Mesocolon Mesentery of small intestines holds many blood vessels Mesocolon anchors colon to posterior body wall

Regulation of Digestive Tract Neural control Neural control short myenteric reflexes (swallowing) short myenteric reflexes (swallowing) long vagovagal reflexes (parasympathetic stimulation of digestive motility and secretion) long vagovagal reflexes (parasympathetic stimulation of digestive motility and secretion) Hormones Hormones messengers diffuse into bloodstream, distant targets messengers diffuse into bloodstream, distant targets Paracrine secretions Paracrine secretions messengers diffuse to nearby target cells messengers diffuse to nearby target cells

Mouth or Oral Cavity

25-18 The Mouth the mouth is known as the oral, or buccal cavity the mouth is known as the oral, or buccal cavity functions include: functions include: ingestion (food intake) ingestion (food intake) other sensory responses to food – chewing and chemical digestion other sensory responses to food – chewing and chemical digestion swallowing, speech, and respiration swallowing, speech, and respiration mouth enclosed by cheeks, lips, palate, and tongue mouth enclosed by cheeks, lips, palate, and tongue oral fissure – anterior opening between lips oral fissure – anterior opening between lips fauces – posterior opening to the throat fauces – posterior opening to the throat stratified squamous epithelium lines mouth stratified squamous epithelium lines mouth keratinized in areas subject to food abrasion – gums and hard palate keratinized in areas subject to food abrasion – gums and hard palate nonkeratinized in other areas – floor of mouth, soft palate, and inside of cheek and lips nonkeratinized in other areas – floor of mouth, soft palate, and inside of cheek and lips

Features of Oral Cavity Cheeks and lips Cheeks and lips keep food between teeth for chewing; essential for speech and suckling in infants keep food between teeth for chewing; essential for speech and suckling in infants vestibule - space between teeth and cheeks vestibule - space between teeth and cheeks lips: cutaneous area versus red area (vermilion) lips: cutaneous area versus red area (vermilion) Tongue is sensitive, muscular manipulator of food Tongue is sensitive, muscular manipulator of food papillae and taste buds on dorsal surface papillae and taste buds on dorsal surface lingual glands secrete saliva, tonsils in root lingual glands secrete saliva, tonsils in root Hard and soft palate Hard and soft palate allow breathing and chewing at same time allow breathing and chewing at same time palatoglossal and palatopharyngeal arches palatoglossal and palatopharyngeal arches

25-20 The Tongue tongue – muscular, bulky, but remarkably agile and sensitive organ tongue – muscular, bulky, but remarkably agile and sensitive organ manipulates food between teeth while it avoids being bitten manipulates food between teeth while it avoids being bitten can extract food particles from the teeth after a meal can extract food particles from the teeth after a meal sensitive enough to feel a stray hair in a bite of food sensitive enough to feel a stray hair in a bite of food nonkeratinized stratified squamous epithelium covers its surface nonkeratinized stratified squamous epithelium covers its surface lingual papillae – bumps and projections on the tongue that are the sites of the taste buds lingual papillae – bumps and projections on the tongue that are the sites of the taste buds body – anterior two-thirds of the tongue occupies oral cavity body – anterior two-thirds of the tongue occupies oral cavity root – posterior one-third of the tongue occupies the oropharynx root – posterior one-third of the tongue occupies the oropharynx

25-21 The Tongue vallate papillae – a V-shaped row of papillae that mark the boundary between the body and root of the tongue vallate papillae – a V-shaped row of papillae that mark the boundary between the body and root of the tongue terminal sulcus – groove behind the V-shaped vallate papillae terminal sulcus – groove behind the V-shaped vallate papillae lingual frenulum – median fold that attaches the body to the floor of the mouth lingual frenulum – median fold that attaches the body to the floor of the mouth intrinsic muscles are contained entirely within the tongue intrinsic muscles are contained entirely within the tongue produce the subtle tongue movements of speech produce the subtle tongue movements of speech extrinsic muscles – with origins elsewhere and insertions in the tongue extrinsic muscles – with origins elsewhere and insertions in the tongue produce stronger movements of food manipulation produce stronger movements of food manipulation genioglossus, hyoglossus, palatoglossus, and styloglossus genioglossus, hyoglossus, palatoglossus, and styloglossus lingual glands – serous and mucous glands amid the extrinsic muscles lingual glands – serous and mucous glands amid the extrinsic muscles secrete a portion of the saliva secrete a portion of the saliva lingual tonsils – contained in the root lingual tonsils – contained in the root

25-22 Tongue Figure 25.5 a-b Epiglottis Root Body Hyoid bone Mylohyoid m. Sublingual gland Submandibular gland Mandible Genioglossus m. Hyoglossus m. Styloglossus m. 1st molar Buccinator m. Intrinsic muscles of the tongue (a) Superior view (b) Frontal section, anterior view Lingual tonsils Palatine tonsil Terminal sulcus Vallate papillae Foliate papillae Fungiform papillae Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

The Palate palate – separates the oral cavity from the nasal cavity palate – separates the oral cavity from the nasal cavity makes it possible to breathe while chewing food makes it possible to breathe while chewing food hard (bony) palate – anterior portion that is supported by the palatine processes of the maxillae and the palatine bones hard (bony) palate – anterior portion that is supported by the palatine processes of the maxillae and the palatine bones palatine rugae – transverse ridges that help the tongue hold and manipulate food palatine rugae – transverse ridges that help the tongue hold and manipulate food soft palate – posterior with a more spongy texture soft palate – posterior with a more spongy texture composed of skeletal muscle and glandular tissue composed of skeletal muscle and glandular tissue no bone no bone uvula – conical medial projection visible at the rear of the mouth uvula – conical medial projection visible at the rear of the mouth helps retain food in the mouth until one is ready to swallow helps retain food in the mouth until one is ready to swallow pair of muscular arches on each side of the oral cavity pair of muscular arches on each side of the oral cavity palatoglossal arch – anterior arch palatoglossal arch – anterior arch palatopharyngeal arch – posterior arch palatopharyngeal arch – posterior arch palatine tonsils are located on the wall between the arches palatine tonsils are located on the wall between the arches 25-23

Dentition Dentition Baby teeth (20) by 2 years; Adult (32) between 6 and 25 Occlusal surfaces and cusp numbers differ

25-25 The Teeth dentition – the teeth dentition – the teeth masticate food into smaller pieces masticate food into smaller pieces makes food easier to swallow makes food easier to swallow exposes more surface area for action of digestive enzymes speeding chemical digestion exposes more surface area for action of digestive enzymes speeding chemical digestion 32 adult teeth – 20 deciduous (baby) teeth 32 adult teeth – 20 deciduous (baby) teeth 16 in mandible 16 in mandible 16 in maxilla 16 in maxilla from midline to the rear of each jaw from midline to the rear of each jaw 2 incisors – chisel-like cutting teeth used to bite off a piece of food 2 incisors – chisel-like cutting teeth used to bite off a piece of food 1 canine – pointed and act to puncture and shred food 1 canine – pointed and act to puncture and shred food 2 premolars – broad surface for crushing and grinding 2 premolars – broad surface for crushing and grinding 3 molars – even broader surface for crushing and grinding 3 molars – even broader surface for crushing and grinding

25-26 The Teeth alveolus – tooth socket in bone alveolus – tooth socket in bone gomphosis joint formed between tooth and bone gomphosis joint formed between tooth and bone periodontal ligament – modified periosteum whose collagen fibers penetrate into the bone on one side and into the tooth on the other periodontal ligament – modified periosteum whose collagen fibers penetrate into the bone on one side and into the tooth on the other anchors tooth firmly in alveolus anchors tooth firmly in alveolus allows slight movement under pressure of chewing allows slight movement under pressure of chewing gingiva (gum) – covers the alveolar bone gingiva (gum) – covers the alveolar bone regions of a tooth regions of a tooth crown – portion above the gum crown – portion above the gum root – the portion below the gum, embedded in alveolar bone root – the portion below the gum, embedded in alveolar bone neck – the point where crown, root, and gum meet neck – the point where crown, root, and gum meet gingival sulcus – space between the tooth and the gum gingival sulcus – space between the tooth and the gum hygiene in the sulcus in important to dental health hygiene in the sulcus in important to dental health Figure 25.6 Names of teeth 2nd molar 1st molar Canine Lateral incisor Central incisor 6–9 16–20 12–16 20–26 Names of teeth (a) Deciduous (baby) teeth (b) Permanent teeth Central incisor Lateral incisor 1st premolar 2nd premolar 1st molar Canine 2nd molar 6–8 7–9 9–12 10–12 6–7 11–13 17–25 Age at eruption (months) 7– 11 Age at eruption (years) 3rd molar (wisdom tooth) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Periodontal ligament is modified periosteum Periodontal ligament is modified periosteum anchors into alveolus anchors into alveolus Cementum and dentin are living tissue Cementum and dentin are living tissue Enamel is noncellular secretion formed during development Enamel is noncellular secretion formed during development Root canal leads into pulp cavity Root canal leads into pulp cavity nerves and blood vessels nerves and blood vessels Gingiva or gums Gingiva or gums Tooth Structure

Permanent and Deciduous Teeth

Deciduous Teeth 20 deciduous teeth (milk teeth or baby teeth) 20 deciduous teeth (milk teeth or baby teeth) teeth develop beneath the gums and erupt in a predictable order teeth develop beneath the gums and erupt in a predictable order erupt from 6 to 30 months erupt from 6 to 30 months beginning with incisors beginning with incisors between 6 and 32 years of age, are replaced by 32 permanent teeth between 6 and 32 years of age, are replaced by 32 permanent teeth third molars (wisdom teeth) erupt from 17 – 25 third molars (wisdom teeth) erupt from 17 – 25 may be impacted – crowded against neighboring teeth and bone so the cannot erupt may be impacted – crowded against neighboring teeth and bone so the cannot erupt 25-29

25-30 Tooth and Gum Disease the human mouth is home to more than 700 species of microorganisms, especially bacteria the human mouth is home to more than 700 species of microorganisms, especially bacteria plaque – sticky residue on the teeth made up of bacteria and sugars plaque – sticky residue on the teeth made up of bacteria and sugars calculus – calcified plaque calculus – calcified plaque bacteria metabolize sugars and release acids that dissolve the minerals of enamel and dentin to form dental caries (cavities) bacteria metabolize sugars and release acids that dissolve the minerals of enamel and dentin to form dental caries (cavities) root canal therapy is necessary if cavity reaches pulp root canal therapy is necessary if cavity reaches pulp calculus in the gingival sulcus wedges the tooth and gum apart calculus in the gingival sulcus wedges the tooth and gum apart allows bacterial invasion of the sulcus allows bacterial invasion of the sulcus gingivitis – inflammation of the gums gingivitis – inflammation of the gums periodontal disease – destruction of the supporting bone around the teeth which may result in tooth loss periodontal disease – destruction of the supporting bone around the teeth which may result in tooth loss

Mastication or Chewing Breaks food into smaller pieces to be swallowed Breaks food into smaller pieces to be swallowed  surface area exposed to digestive enzymes  surface area exposed to digestive enzymes Contact of food with sensory receptors triggers chewing reflex Contact of food with sensory receptors triggers chewing reflex tongue, buccinator and orbicularis oris manipulate food tongue, buccinator and orbicularis oris manipulate food masseter and temporalis elevate the teeth to crush food masseter and temporalis elevate the teeth to crush food medial and lateral pterygoids swing teeth in side-to- side grinding action of molars medial and lateral pterygoids swing teeth in side-to- side grinding action of molars

Saliva Functions of saliva Functions of saliva moisten, begin starch and fat digestion, cleanse teeth, inhibit bacteria, bind food together into bolus moisten, begin starch and fat digestion, cleanse teeth, inhibit bacteria, bind food together into bolus Hypotonic solution of 99.5% water and solutes Hypotonic solution of 99.5% water and solutes salivary amylase, begins starch digestion salivary amylase, begins starch digestion lingual lipase, digests fat activated by stomach acid lingual lipase, digests fat activated by stomach acid mucus, aids in swallowing mucus, aids in swallowing lysozyme, enzyme kills bacteria lysozyme, enzyme kills bacteria immunoglobulin A, inhibits bacterial growth immunoglobulin A, inhibits bacterial growth electrolytes = Na +, K +, Cl -, phosphate and bicarbonate electrolytes = Na +, K +, Cl -, phosphate and bicarbonate pH of 6.8 to 7.0 pH of 6.8 to 7.0

Salivary Glands Small intrinsic glands found under mucous membrane of mouth, lips, cheeks and tongue - secrete at constant rate 3 pairs extrinsic glands connected to oral cavity by ducts parotid, submandibular and sublingual

25-34 Salivary Glands intrinsic salivary glands – small glands dispersed amid other oral tissues lingual glands – in the tongue – produce lingual lipase labial glands – inside of the lips buccal glands – inside of the cheek all secrete saliva at a fairly constant rate extrinsic salivary glands – three pair connected to oral cavity by ducts parotid – located beneath the skin anterior to the earlobe mumps is an inflammation and swelling of the parotid gland caused by a virus submandibular gland – located halfway along the body of the mandible its duct empties at the side of the lingual frenulum, near the lower central incisors sublingual glands – located in the floor of the mouth has multiple ducts that empty posterior to the papilla of the submandibular duct

Histology of Salivary Glands Compound tubuloacinar glands Mucous cells secrete mucus Serous cells secrete thin fluid rich in amylase Mixed acinus has both

Salivation Total of 1 to 1.5 L of saliva per day Total of 1 to 1.5 L of saliva per day Cells filter water from blood and add other substances Cells filter water from blood and add other substances Food stimulates receptors that signal salivatory nuclei in medulla and pons Food stimulates receptors that signal salivatory nuclei in medulla and pons parasympathetic stimulation  salivary glands produce thin saliva, rich in enzymes parasympathetic stimulation  salivary glands produce thin saliva, rich in enzymes sympathetic stimulation  produce less abundant, thicker saliva, with more mucus sympathetic stimulation  produce less abundant, thicker saliva, with more mucus Higher brain centers stimulate salivatory nuclei so sight, smell and thought of food cause salivation Higher brain centers stimulate salivatory nuclei so sight, smell and thought of food cause salivation

Pharynx Skeletal muscle Skeletal muscle deep layer – longitudinal orientation deep layer – longitudinal orientation superficial layer – circular orientation superficial layer – circular orientation superior, middle and inferior pharyngeal constrictors superior, middle and inferior pharyngeal constrictors

Esophagus Straight muscular tube 25-30 cm long Straight muscular tube 25-30 cm long nonkeratinized stratified squamous epithelium nonkeratinized stratified squamous epithelium esophageal glands in submucosa esophageal glands in submucosa skeletal muscle in upper part and smooth in bottom skeletal muscle in upper part and smooth in bottom Extends from pharynx to cardiac stomach passing through esophageal hiatus in diaphragm Extends from pharynx to cardiac stomach passing through esophageal hiatus in diaphragm inferior pharyngeal constrictor excludes air from it inferior pharyngeal constrictor excludes air from it Lower esophageal sphincter closes orifice to reflux Lower esophageal sphincter closes orifice to reflux

Swallowing

Swallowing (Deglutition) Series of muscular contractions coordinated by center in medulla and pons Series of muscular contractions coordinated by center in medulla and pons motor signals from cranial nerves V, VII, IX and XII motor signals from cranial nerves V, VII, IX and XII Buccal phase Buccal phase tongue collects food and pushes it back into oropharynx tongue collects food and pushes it back into oropharynx Pharyngeal-esophageal phase Pharyngeal-esophageal phase soft palate rises and blocks nasopharynx soft palate rises and blocks nasopharynx infrahyoid muscles lift larynx; epiglottis folded back infrahyoid muscles lift larynx; epiglottis folded back pharyngeal constrictors push bolus down esophagus pharyngeal constrictors push bolus down esophagus liquids in 2 seconds -- food bolus may take 8 seconds liquids in 2 seconds -- food bolus may take 8 seconds lower esophageal sphincter relaxes lower esophageal sphincter relaxes

X-ray: Swallowing in Esophagus

Stomach Mechanically breaks up food, liquifies food and begins chemical digestion of protein and fat Mechanically breaks up food, liquifies food and begins chemical digestion of protein and fat resulting soupy mixture is called chyme resulting soupy mixture is called chyme Does not absorb significant amount of nutrients Does not absorb significant amount of nutrients absorbs aspirin and some lipid-soluble drugs absorbs aspirin and some lipid-soluble drugs

Gross Anatomy of Stomach Muscular sac (internal volume from 50ml to 4L) Muscular sac (internal volume from 50ml to 4L) J - shaped organ with lesser and greater curvatures J - shaped organ with lesser and greater curvatures regional differences regional differences cardiac region just inside cardiac orifice cardiac region just inside cardiac orifice fundus - domed portion superior to esophageal opening fundus - domed portion superior to esophageal opening body - main portion of organ body - main portion of organ pyloric region - narrow inferior end pyloric region - narrow inferior end antrum and pyloric canal antrum and pyloric canal Pylorus - opening to duodenum Pylorus - opening to duodenum thick ring of smooth muscle forms a sphincter thick ring of smooth muscle forms a sphincter

Innervation and Circulation Innervation by Innervation by parasympathetic fibers from vagus parasympathetic fibers from vagus sympathetic fibers from celiac plexus sympathetic fibers from celiac plexus All blood from stomach enters hepatic portal circulation and is filtered through liver before returning to heart All blood from stomach enters hepatic portal circulation and is filtered through liver before returning to heart

Gross Anatomy of Stomach Notice: bulge of fundus, narrowing of pyloric region, thickness of pyloric sphincter and greater and lesser curvatures

Gross Anatomy of Stomach

25-47 Microscopic Anatomy simple columnar epithelium covers mucosa apical regions of its surface cells are filled with mucin swells with water and becomes mucus after it is secreted mucosa and submucosa flat when stomach is full, but form longitudinal wrinkles called gastric rugae when empty muscularis externa has three layers instead of two outer longitudinal, middle circular and inner oblique layers gastric pits – depressions in gastric mucosa lined with simple columnar epithelium two or three tubular glands open into the bottom of each gastric pit cardiac glands in cardiac region pyloric glands in pyloric regions gastric glands in the rest of the stomach

Unique Features of Stomach Wall Mucosa simple columnar glandular epithelium lamina propria is filled with tubular glands (gastric pits) Muscularis externa has 3 layers outer longitudinal, middle circular and inner oblique layers

Gastric Gland

Cells of Gastric Glands Mucous cells secrete mucus Mucous cells secrete mucus Regenerative cells Regenerative cells divide rapidly to produce new cells that migrate to surface divide rapidly to produce new cells that migrate to surface Parietal cells Parietal cells secrete HCl acid and intrinsic factor secrete HCl acid and intrinsic factor Chief cells Chief cells secrete pepsinogen secrete pepsinogen chymosin and lipase in infancy chymosin and lipase in infancy Enteroendocrine cells Enteroendocrine cells secrete hormones and paracrine messengers secrete hormones and paracrine messengers

Opening of Gastric Pit

Gastric Secretions 2 to 3 L of gastric juice/day (H 2 O, HCl and pepsin) Parietal cells contain carbonic anhydrase (CAH) CAH CO 2 + H 2 O  H 2 CO 3  HCO 3 - + H + H + is pumped into stomach lumen by H + K + ATPase antiporter uses ATP to pump H + out and K + in HCO 3 - exchanged for Cl - (chloride shift) Cl - pumped out to join H + forming HCl  HCO 3 - in blood causes alkaline tide (blood pH  )

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Functions of Hydrochloric Acid Activates pepsin and lingual lipase Activates pepsin and lingual lipase Breaks up connective tissues and plant cell walls Breaks up connective tissues and plant cell walls liquefies food to form chyme liquefies food to form chyme Converts ingested ferric ions (Fe 3+ ) to ferrous ions (Fe 2+ ) Converts ingested ferric ions (Fe 3+ ) to ferrous ions (Fe 2+ ) absorbed and used for hemoglobin synthesis absorbed and used for hemoglobin synthesis Destroys ingested bacteria and pathogens Destroys ingested bacteria and pathogens

25-55 Functions of Hydrochloric Acid activates pepsin and lingual lipase activates pepsin and lingual lipase breaks up connective tissues and plant cell walls breaks up connective tissues and plant cell walls helps liquefy food to form chyme helps liquefy food to form chyme converts ingested ferric ions (Fe 3+ ) to ferrous ions (Fe 2+ ) converts ingested ferric ions (Fe 3+ ) to ferrous ions (Fe 2+ ) Fe 2+ absorbed and used for hemoglobin synthesis Fe 2+ absorbed and used for hemoglobin synthesis contributes to nonspecific disease resistance by destroying most ingested pathogens contributes to nonspecific disease resistance by destroying most ingested pathogens

Gastric Enzymes and Intrinsic Factor Intrinsic factor Intrinsic factor essential for B 12 absorption by small intestine essential for B 12 absorption by small intestine RBC production (lack causes pernicious anemia) RBC production (lack causes pernicious anemia) Pepsin - protein digestion Pepsin - protein digestion secreted as pepsinogen (inactive) secreted as pepsinogen (inactive) HCl converts it to pepsin (active) HCl converts it to pepsin (active) Gastric lipase and chymosin Gastric lipase and chymosin lipase digests butterfat of milk in infant lipase digests butterfat of milk in infant chymosin curdles milk by coagulating proteins chymosin curdles milk by coagulating proteins

25-57Pepsin zymogens – digestive enzymes secreted as inactive proteins zymogens – digestive enzymes secreted as inactive proteins converted to active enzymes by removing some of their amino acids converted to active enzymes by removing some of their amino acids pepsinogen – zymogen secreted by the chief cells pepsinogen – zymogen secreted by the chief cells hydrochloric acid removes some of its amino acids and forms pepsin that digests proteins hydrochloric acid removes some of its amino acids and forms pepsin that digests proteins autocatalytic effect – as some pepsin is formed, it converts more pepsinogen into more pepsin autocatalytic effect – as some pepsin is formed, it converts more pepsinogen into more pepsin pepsin digests dietary proteins into shorter peptide chains pepsin digests dietary proteins into shorter peptide chains protein digestion is completed in the small intestine protein digestion is completed in the small intestine

Production and Action of Pepsin

Gastric Lipase gastric lipase – produced by chief cells gastric lipase – produced by chief cells gastric lipase and lingual lipase play a minor role in digesting dietary fats gastric lipase and lingual lipase play a minor role in digesting dietary fats digests 10% - 15% of dietary fats in the stomach digests 10% - 15% of dietary fats in the stomach rest digested in the small intestine rest digested in the small intestine 25-59

25-60 Intrinsic Factor intrinsic factor – a glycoprotein secreted by parietal cells intrinsic factor – a glycoprotein secreted by parietal cells essential to absorption of vitamin B 12 by the small intestine essential to absorption of vitamin B 12 by the small intestine binds vitamin B 12 and intestinal cells absorb this complex by receptor-mediated endocytosis binds vitamin B 12 and intestinal cells absorb this complex by receptor-mediated endocytosis vitamin B 12 is needed to synthesize hemoglobin vitamin B 12 is needed to synthesize hemoglobin prevents pernicious anemia prevents pernicious anemia secretion of intrinsic factor is the only indispensable function of the stomach secretion of intrinsic factor is the only indispensable function of the stomach digestion can continue if stomach is removed (gastrectomy), but B 12 supplements will be needed digestion can continue if stomach is removed (gastrectomy), but B 12 supplements will be needed

Chemical Messengers Many produced by enteroendocrine cells Many produced by enteroendocrine cells hormones enter blood  distant cells hormones enter blood  distant cells paracrine secretions  neighboring cells paracrine secretions  neighboring cells Gut-brain peptides Gut-brain peptides signaling molecules produced in digestive tract and CNS signaling molecules produced in digestive tract and CNS

Gastric Motility Swallowing center signals stomach to relax Swallowing center signals stomach to relax Food stretches stomach activating a receptive-relaxation response Food stretches stomach activating a receptive-relaxation response resists stretching briefly, but relaxes to hold more food resists stretching briefly, but relaxes to hold more food Rhythm of peristalsis controlled by pacemaker cells in longitudinal muscle layer Rhythm of peristalsis controlled by pacemaker cells in longitudinal muscle layer gentle ripple of contraction every 20 seconds churns and mixes food with gastric juice gentle ripple of contraction every 20 seconds churns and mixes food with gastric juice stronger contraction at pyloric region; ejects 3 ml stronger contraction at pyloric region; ejects 3 ml typical meal emptied from stomach in 4 hours typical meal emptied from stomach in 4 hours

25-63 Gastric Motility antrum holds about 30 mL of chyme antrum holds about 30 mL of chyme as a parastaltic wave passes down the antrum, it squirts about 3 mL of chyme into the duodenum at a time as a parastaltic wave passes down the antrum, it squirts about 3 mL of chyme into the duodenum at a time allowing only a small amount into the duodenum enables the duodenum to: allowing only a small amount into the duodenum enables the duodenum to: neutralize the stomach acid neutralize the stomach acid digest nutrients little by little digest nutrients little by little if duodenum is overfilled it inhibits gastric motility if duodenum is overfilled it inhibits gastric motility typical meal emptied from stomach in 4 hours typical meal emptied from stomach in 4 hours less time if the meal is more liquid less time if the meal is more liquid as long as 6 hours for a high fat meal as long as 6 hours for a high fat meal

Vomiting Induced by Induced by excessive stretching of stomach, psychological stimuli or chemical irritants (bacterial toxins) excessive stretching of stomach, psychological stimuli or chemical irritants (bacterial toxins) Emetic center in medulla causes Emetic center in medulla causes retching retching lower esophageal sphincter to relax lower esophageal sphincter to relax stomach and duodenum to contract spasmodically stomach and duodenum to contract spasmodically vomiting vomiting when abdominal contraction forces upper esophageal sphincter to open when abdominal contraction forces upper esophageal sphincter to open

25-65 Vomiting vomiting – occurs when abdominal contractions and rising thoracic pressure force the upper esophageal sphincter to open vomiting – occurs when abdominal contractions and rising thoracic pressure force the upper esophageal sphincter to open esophagus and body of the stomach relax esophagus and body of the stomach relax chyme is driven out of the stomach and mouth by strong abdominal contractions combined with reverse peristalsis of gastric antrum and duodenum chyme is driven out of the stomach and mouth by strong abdominal contractions combined with reverse peristalsis of gastric antrum and duodenum projectile vomiting – sudden vomiting with no prior nausea or retching projectile vomiting – sudden vomiting with no prior nausea or retching common in infants after feeding common in infants after feeding chronic vomiting causes: chronic vomiting causes: dangerous fluid, electrolyte, and acid-base imbalances dangerous fluid, electrolyte, and acid-base imbalances bulimia – eating disorder in which the tooth enamel becomes eroded by the hydrochloric acid in the chyme bulimia – eating disorder in which the tooth enamel becomes eroded by the hydrochloric acid in the chyme aspiration (inhalation) of acid is very destructive to the respiratory tract aspiration (inhalation) of acid is very destructive to the respiratory tract surgical anesthesia may induce nausea and must be preceded by fasting until the stomach and small intestine are empty surgical anesthesia may induce nausea and must be preceded by fasting until the stomach and small intestine are empty

Digestion and Absorption salivary and gastric enzymes partially digest protein and lesser amounts of starch and fat in the stomach salivary and gastric enzymes partially digest protein and lesser amounts of starch and fat in the stomach most digestion and nearly all absorption occur after the chyme has passed into the small intestine most digestion and nearly all absorption occur after the chyme has passed into the small intestine stomach does not absorb any significant amount of nutrients stomach does not absorb any significant amount of nutrients aspirin aspirin some lipid-soluble drugs some lipid-soluble drugs alcohol is absorbed mainly by small intestine alcohol is absorbed mainly by small intestine intoxicating effects depends partly on how rapidly the stomach is emptied intoxicating effects depends partly on how rapidly the stomach is emptied 25-66

25-67 Protection of the Stomach living stomach is protected in three ways from the harsh acidic and enzymatic environment it creates living stomach is protected in three ways from the harsh acidic and enzymatic environment it creates mucous coat – thick, highly alkaline mucus resists action of acid and enzymes mucous coat – thick, highly alkaline mucus resists action of acid and enzymes tight junctions - between epithelial cells prevent gastric juice from seeping between them and digesting the connective tissue of the lamina propria and beyond tight junctions - between epithelial cells prevent gastric juice from seeping between them and digesting the connective tissue of the lamina propria and beyond epithelial cell replacement – stomach epithelial cells live only 3 to 6 days epithelial cell replacement – stomach epithelial cells live only 3 to 6 days sloughed off into the chyme and digested with the food sloughed off into the chyme and digested with the food replaced rapidly by cell division in the gastric pits replaced rapidly by cell division in the gastric pits breakdown of these protective measures can result in inflammation and peptic ulcer breakdown of these protective measures can result in inflammation and peptic ulcer

Healthy Mucosa and Peptic Ulcer

Regulation of Gastric Secretion

Regulation of Gastric Function (Phases 1-2) Cephalic phase Cephalic phase sight, smell, taste or thought of food; vagus nerve stimulates gastric secretion and motility sight, smell, taste or thought of food; vagus nerve stimulates gastric secretion and motility Gastric phase Gastric phase activated by presence of food or semidigested protein activated by presence of food or semidigested protein by stretch or  in pH by stretch or  in pH secretion stimulated by secretion stimulated by ACh (from parasympathetic fibers), histamine (from gastric enteroendocrine cells) and gastrin (from pyloric G cells) ACh (from parasympathetic fibers), histamine (from gastric enteroendocrine cells) and gastrin (from pyloric G cells) receptors on parietal and chief cells receptors on parietal and chief cells

Regulation of Gastric Function (Phase 3) Intestinal phase - duodenum regulates gastric activity through hormones and nervous reflexes Intestinal phase - duodenum regulates gastric activity through hormones and nervous reflexes at first gastric activity increases (if duodenum is stretched or amino acids in chyme cause gastrin release) at first gastric activity increases (if duodenum is stretched or amino acids in chyme cause gastrin release) enterogastric reflex - duodenum inhibits stomach enterogastric reflex - duodenum inhibits stomach caused by acid and semi-digested fats in duodenum caused by acid and semi-digested fats in duodenum chyme stimulates duodenal cells to release secretin, cholecystokinin (CCK) and gastric inhibitory peptide chyme stimulates duodenal cells to release secretin, cholecystokinin (CCK) and gastric inhibitory peptide all 3 suppress gastric secretion and motility all 3 suppress gastric secretion and motility

25-73 Regulation of Gastric Function enterogastric reflex – duodenum sends inhibitory signals to the stomach by way of the enteric nervous system and signals to the medulla oblongata - triggered by acid and semi-digested fats in the duodenum enterogastric reflex – duodenum sends inhibitory signals to the stomach by way of the enteric nervous system and signals to the medulla oblongata - triggered by acid and semi-digested fats in the duodenum inhibits vagal nuclei – reducing vagal stimulation of the stomach inhibits vagal nuclei – reducing vagal stimulation of the stomach stimulate sympathetic neurons – send inhibitory signals to the stomach stimulate sympathetic neurons – send inhibitory signals to the stomach chyme also stimulates duodenal enteroendocrine cells to release secretin and cholecystokinin chyme also stimulates duodenal enteroendocrine cells to release secretin and cholecystokinin they stimulate the pancreas and gall bladder they stimulate the pancreas and gall bladder also suppress gastric secretion also suppress gastric secretion pyloric sphincter contracts tightly to limit chyme entering duodenum pyloric sphincter contracts tightly to limit chyme entering duodenum gi ves duodenum time to work on chyme gi ves duodenum time to work on chyme enteroendocrine cells also secrete glucose-dependent insulinotropic peptide (GIP) originally called gastrin-inhibiting peptide enteroendocrine cells also secrete glucose-dependent insulinotropic peptide (GIP) originally called gastrin-inhibiting peptide stimulates insulin secretion in preparation for processing nutrients about to be absorbed by the small intestine stimulates insulin secretion in preparation for processing nutrients about to be absorbed by the small intestine

Positive Feedback Control- Gastric Secretion

Liver, Gallbladder and Pancreas All release important secretions into small intestine to continue digestion All release important secretions into small intestine to continue digestion Small intestine receives chyme from stomach Small intestine receives chyme from stomach Also secretions from liver and pancreas Also secretions from liver and pancreas Enter digestive tract near the junction of stomach and small intestine Enter digestive tract near the junction of stomach and small intestine Secretions are so important to the digestive process of the small intestine Secretions are so important to the digestive process of the small intestine

Gross Anatomy of Liver 3 lb. organ located inferior to the diaphragm 3 lb. organ located inferior to the diaphragm 4 lobes - right, left, quadrate and caudate 4 lobes - right, left, quadrate and caudate falciform ligament separates left and right falciform ligament separates left and right round ligament, remnant of umbilical vein round ligament, remnant of umbilical vein Gallbladder adheres to ventral surface between right and quadrate lobes Gallbladder adheres to ventral surface between right and quadrate lobes

25-77 Gross Anatomy of Liver four lobes - right, left, quadrate, and caudate four lobes - right, left, quadrate, and caudate falciform ligament separates left and right lobes falciform ligament separates left and right lobes sheet of mesentery that suspends the liver from the diaphragm sheet of mesentery that suspends the liver from the diaphragm round ligament (ligamentum teres) – fibrous remnant of umbilical vein round ligament (ligamentum teres) – fibrous remnant of umbilical vein carries blood from umbilical cord to the liver of the fetus carries blood from umbilical cord to the liver of the fetus from inferior view, squarish quadrate lobe next to the gall bladder and a tail-like caudate lobe posterior to that from inferior view, squarish quadrate lobe next to the gall bladder and a tail-like caudate lobe posterior to that porta hepatis – irregular opening between these lobes porta hepatis – irregular opening between these lobes point of entry for the hepatic portal vein and proper hepatic artery point of entry for the hepatic portal vein and proper hepatic artery point of exit for the bile passages point of exit for the bile passages all travel in lesser omentum all travel in lesser omentum gall bladder – adheres to a depression on the inferior surface of the liver, between right and quadrate lobes gall bladder – adheres to a depression on the inferior surface of the liver, between right and quadrate lobes bare area on superior surface where it is attach to diaphragm bare area on superior surface where it is attach to diaphragm

Inferior Surface of Liver

Microscopic Anatomy of Liver Tiny cylinders called hepatic lobules (2mm by 1mm) Central vein surrounded by sheets of hepatocyte cells separated by sinusoids lined with fenestrated epithelium Blood filtered by hepatocytes on way to central vein

Microscopic Anatomy of Liver hepatic lobules – tiny innumerable cylinders that fill the interior of the liver hepatic lobules – tiny innumerable cylinders that fill the interior of the liver about 2 mm long and 1 mm in diameter about 2 mm long and 1 mm in diameter consists of: consists of: central vein – passing down the core central vein – passing down the core hepatocytes – cuboidal cells surrounding central vein in radiating sheets or plates hepatocytes – cuboidal cells surrounding central vein in radiating sheets or plates each plate of hepatocytes is an epithelium one or two cells thick each plate of hepatocytes is an epithelium one or two cells thick hepatic sinusoids – blood-filled channels that fill spaces between the plates hepatic sinusoids – blood-filled channels that fill spaces between the plates lined by a fenestrated endothelium that separates hepatocytes from blood cells lined by a fenestrated endothelium that separates hepatocytes from blood cells allows plasma into the space between the hepatocytes and endothelium allows plasma into the space between the hepatocytes and endothelium hepatocytes have brush border of microvilli that project into this space hepatocytes have brush border of microvilli that project into this space blood filtered through the sinusoids comes directly from the stomach and intestines blood filtered through the sinusoids comes directly from the stomach and intestines hepatic macrophages (Kupffer cells) – phagocytic cells in the sinusoids that remove bacteria and debris from the blood hepatic macrophages (Kupffer cells) – phagocytic cells in the sinusoids that remove bacteria and debris from the blood 25-80

Microscopic Anatomy of Liver bile canaliculi – narrow channels into which the liver secretes bile bile canaliculi – narrow channels into which the liver secretes bile bile passes into bile ductules of the triads bile passes into bile ductules of the triads ultimately into the right and left hepatic ducts ultimately into the right and left hepatic ducts common hepatic duct – formed from convergence of right and left hepatic ducts on inferior side of the liver common hepatic duct – formed from convergence of right and left hepatic ducts on inferior side of the liver cystic duct coming from gall bladder joins common hepatic duct cystic duct coming from gall bladder joins common hepatic duct bile duct - formed from union of cystic and common hepatic ducts bile duct - formed from union of cystic and common hepatic ducts descends through lesser omentum toward the duodenum descends through lesser omentum toward the duodenum near duodenum, bile duct joins the duct of the pancreas near duodenum, bile duct joins the duct of the pancreas forms expanded chamber – hepatopancreatic ampulla forms expanded chamber – hepatopancreatic ampulla terminates in a fold of tissue – major duodenal papilla on duodenal wall terminates in a fold of tissue – major duodenal papilla on duodenal wall major duodenal papilla contains muscular hepatopancreatic sphincter (sphincter of Oddi) major duodenal papilla contains muscular hepatopancreatic sphincter (sphincter of Oddi) regulates passage of bile and pancreatic juice into duodenum regulates passage of bile and pancreatic juice into duodenum between meals, sphincter closes and prevents release of bile into the intestines between meals, sphincter closes and prevents release of bile into the intestines 25-81

25-82 Functions of Hepatocytes after a meal, the hepatocytes absorb from the blood after a meal, the hepatocytes absorb from the blood glucose, amino acids, iron, vitamins, and other nutrients for metabolism or storage glucose, amino acids, iron, vitamins, and other nutrients for metabolism or storage removes and degrades removes and degrades hormones, toxins, bile pigments, and drugs hormones, toxins, bile pigments, and drugs secretes into the blood: secretes into the blood: albumin, lipoproteins, clotting factors, angiotensinogen, and other products albumin, lipoproteins, clotting factors, angiotensinogen, and other products between meals, hepatocytes breaks down stored glycogen and releases glucose into the blood between meals, hepatocytes breaks down stored glycogen and releases glucose into the blood

Histology of Liver - Hepatic Triad 3 structures found in corner between lobules hepatic portal vein and hepatic artery bring blood to liver bile duct collects bile from bile canaliculi between sheets of hepatocytes to be secreted from liver in hepatic ducts

Ducts of Gallbladder, Liver & Pancreas

Ducts of Gallbladder, Liver, Pancreas Bile passes from bile canaliculi between cells to bile ductules to right and left hepatic ducts Bile passes from bile canaliculi between cells to bile ductules to right and left hepatic ducts Right and left ducts join outside liver to form common hepatic duct Right and left ducts join outside liver to form common hepatic duct Cystic duct from gallbladder joins common hepatic duct to form bile duct Cystic duct from gallbladder joins common hepatic duct to form bile duct Duct of pancreas and bile duct combine to form hepatopancreatic ampulla emptying into duodenum at major duodenal papilla Duct of pancreas and bile duct combine to form hepatopancreatic ampulla emptying into duodenum at major duodenal papilla sphincter of Oddi (hepatopancreatic sphincter) regulates release of bile and pancreatic juice sphincter of Oddi (hepatopancreatic sphincter) regulates release of bile and pancreatic juice

Gallbladder and Bile Sac on underside of liver -- 10 cm long Sac on underside of liver -- 10 cm long 500 to 1000 mL bile are secreted daily from liver 500 to 1000 mL bile are secreted daily from liver Gallbladder stores and concentrates bile Gallbladder stores and concentrates bile bile backs up into gallbladder from a filled bile duct bile backs up into gallbladder from a filled bile duct between meals, bile is concentrated by factor of 20 between meals, bile is concentrated by factor of 20 Yellow-green fluid containing minerals, bile acids, cholesterol, bile pigments and phospholipids Yellow-green fluid containing minerals, bile acids, cholesterol, bile pigments and phospholipids bilirubin pigment from hemoglobin breakdown bilirubin pigment from hemoglobin breakdown intestinal bacteria convert to urobilinogen = brown color intestinal bacteria convert to urobilinogen = brown color bile acid (salts) emulsify fats and aid in their digestion bile acid (salts) emulsify fats and aid in their digestion enterohepatic circulation - recycling of bile acids from ileum enterohepatic circulation - recycling of bile acids from ileum

25-87Gallstones gallstones (biliary calculi) - hard masses in either the gallbladder or bile ducts gallstones (biliary calculi) - hard masses in either the gallbladder or bile ducts composed of cholesterol, calcium carbonate, and bilirubin composed of cholesterol, calcium carbonate, and bilirubin cholelithiasis - formation of gallstones cholelithiasis - formation of gallstones most common in obese women over 40 due to excess cholesterol most common in obese women over 40 due to excess cholesterol obstruction of ducts obstruction of ducts painful painful cause jaundice - yellowing of skin due to bile pigment accumulation, poor fat digestion, and impaired absorption of fat-soluble vitamins cause jaundice - yellowing of skin due to bile pigment accumulation, poor fat digestion, and impaired absorption of fat-soluble vitamins lithotripsy - use of ultrasonic vibration to pulverize stones without surgery lithotripsy - use of ultrasonic vibration to pulverize stones without surgery

Gross Anatomy of Pancreas Retroperitoneal gland posterior to stomach Retroperitoneal gland posterior to stomach head, body and tail head, body and tail Endocrine and exocrine gland Endocrine and exocrine gland secretes insulin and glucagon into the blood secretes insulin and glucagon into the blood secretes 1500 mL pancreatic juice into duodenum secretes 1500 mL pancreatic juice into duodenum water, enzymes, zymogens, and sodium bicarbonate water, enzymes, zymogens, and sodium bicarbonate other pancreatic enzymes are activated by exposure to bile and ions in the intestine other pancreatic enzymes are activated by exposure to bile and ions in the intestine Pancreatic duct runs length of gland to open at sphincter of Oddi Pancreatic duct runs length of gland to open at sphincter of Oddi accessory duct opens independently on duodenum accessory duct opens independently on duodenum

The Pancreas pancreas – spongy retroperitoneal gland posterior to the greater curvature of the stomach pancreas – spongy retroperitoneal gland posterior to the greater curvature of the stomach measure 12 to 15 cm long, and 2.5 cm thick measure 12 to 15 cm long, and 2.5 cm thick has head encircled by duodenum, body, midportion, and a tail on the left has head encircled by duodenum, body, midportion, and a tail on the left both an endocrine and exocrine gland both an endocrine and exocrine gland endocrine portion – pancreatic islets that secrete insulin and glucagon endocrine portion – pancreatic islets that secrete insulin and glucagon exocrine portion – 99% of pancreas that secretes 1200 to 1500 mL of pancreatic juice per day exocrine portion – 99% of pancreas that secretes 1200 to 1500 mL of pancreatic juice per day secretory acini release their secretion into small ducts that converge on the main pancreatic duct secretory acini release their secretion into small ducts that converge on the main pancreatic duct 25-89

The Pancreas pancreatic duct runs lengthwise through the middle of the gland pancreatic duct runs lengthwise through the middle of the gland joins the bile duct at the hepatopancreatic ampulla joins the bile duct at the hepatopancreatic ampulla hepatopancreatic sphincter controls release of both bile and pancreatic juice into the duodenum hepatopancreatic sphincter controls release of both bile and pancreatic juice into the duodenum accessory pancreatic duct – smaller duct that branches from the main pancreatic duct accessory pancreatic duct – smaller duct that branches from the main pancreatic duct opens independently into the duodenum opens independently into the duodenum bypasses the sphincter and allows pancreatic juice to be released into the duodenum even when bile is not bypasses the sphincter and allows pancreatic juice to be released into the duodenum even when bile is not pancreatic juice – alkaline mixture of water, enzymes, zymogens, sodium bicarbonate, and other electrolytes pancreatic juice – alkaline mixture of water, enzymes, zymogens, sodium bicarbonate, and other electrolytes acini secrete the enzymes and zymogens acini secrete the enzymes and zymogens ducts secrete bicarbonate ducts secrete bicarbonate bicarbonate buffers HCl arriving from the stomach bicarbonate buffers HCl arriving from the stomach 25-90

Pancreatic Acinar Cells Zymogens = proteases trypsinogen chymotrypsinogen procarboxypeptidase Other enzymes amylase lipase ribonuclease and deoxyribonuclease

Activation of Zymogens Trypsinogen converted to trypsin by intestinal epithelium Trypsin converts other 2 (also digests dietary protein)

25-93 Regulation of Secretion three stimuli are chiefly responsible for the release of pancreatic juice and bile three stimuli are chiefly responsible for the release of pancreatic juice and bile acetylcholine (ACh) - from vagus and enteric nerves acetylcholine (ACh) - from vagus and enteric nerves stimulates acini to secrete their enzymes during the cephalic phase of gastric control even before food is swallowed stimulates acini to secrete their enzymes during the cephalic phase of gastric control even before food is swallowed enzymes remain in acini and ducts until chyme enters the duodenum enzymes remain in acini and ducts until chyme enters the duodenum cholecystokinin (CCK) - secreted by mucosa of duodenum in response to arrival of fats in small intestine cholecystokinin (CCK) - secreted by mucosa of duodenum in response to arrival of fats in small intestine stimulate pancreatic acini to secrete enzymes stimulate pancreatic acini to secrete enzymes strongly stimulates gall bladder strongly stimulates gall bladder induces contractions of the gallbladder and relaxation of hepatopancreatic sphincter causing discharge of bile into the duodenum induces contractions of the gallbladder and relaxation of hepatopancreatic sphincter causing discharge of bile into the duodenum secretin - released from duodenum in response to acidic chyme arriving from the stomach secretin - released from duodenum in response to acidic chyme arriving from the stomach stimulates ducts of both liver and pancreas to secrete more sodium bicarbonate stimulates ducts of both liver and pancreas to secrete more sodium bicarbonate raising pH to level pancreatic and intestinal digestive enzymes require raising pH to level pancreatic and intestinal digestive enzymes require

Hormonal Control of Secretion Cholecystokinin released from duodenum in response to arrival of acid and fat Cholecystokinin released from duodenum in response to arrival of acid and fat causes contraction of gallbladder, secretion of pancreatic enzymes, relaxation of hepatopancreatic sphincter causes contraction of gallbladder, secretion of pancreatic enzymes, relaxation of hepatopancreatic sphincter Secretin released from duodenum in response to acidic chyme Secretin released from duodenum in response to acidic chyme stimulates all ducts to secrete more bicarbonate stimulates all ducts to secrete more bicarbonate Gastrin from stomach and duodenum weakly stimulates gallbladder contraction and pancreatic enzyme secretion Gastrin from stomach and duodenum weakly stimulates gallbladder contraction and pancreatic enzyme secretion

Small Intestine Nearly all chemical digestion and nutrient absorption occurs in small intestine

Small Intestine Duodenum curves around head of pancreas (10 in.) Duodenum curves around head of pancreas (10 in.) retroperitoneal along with pancreas retroperitoneal along with pancreas receives stomach contents, pancreatic juice and bile receives stomach contents, pancreatic juice and bile neutralizes stomach acids, emulsifies fats, pepsin inactivated by pH increase, pancreatic enzymes neutralizes stomach acids, emulsifies fats, pepsin inactivated by pH increase, pancreatic enzymes Jejunum - next 8 ft. (in upper abdomen) Jejunum - next 8 ft. (in upper abdomen) has large tall circular folds; walls are thick, muscular has large tall circular folds; walls are thick, muscular most digestion and nutrient absorption occur here most digestion and nutrient absorption occur here Ileum - last 12 ft. (in lower abdomen) Ileum - last 12 ft. (in lower abdomen) has peyer’s patches – clusters of lymphatic nodules has peyer’s patches – clusters of lymphatic nodules ends at ileocecal junction with large intestine ends at ileocecal junction with large intestine

25-97 Gross Anatomy ileocecal junction – the end of the small intestine ileocecal junction – the end of the small intestine where the ileum joins the cecum of the large intestine where the ileum joins the cecum of the large intestine ileocecal valve – a sphincter formed by the thickened muscularis of the ileum ileocecal valve – a sphincter formed by the thickened muscularis of the ileum protrudes into the cecum protrudes into the cecum regulates passage of food residue into the large intestine regulates passage of food residue into the large intestine both jejunum and ileum are intraperitoneal and covered with serosa both jejunum and ileum are intraperitoneal and covered with serosa

Circular folds (plicae circularis) up to 10 mm tall Circular folds (plicae circularis) up to 10 mm tall involve only mucosa and submucosa involve only mucosa and submucosa chyme flows in spiral path causing more contact chyme flows in spiral path causing more contact Small Intestine - Surface Area Villi are fingerlike projections 1 mm tall contain blood vessels and lymphatics (lacteal) nutrient absorption Microvilli 1 micron tall; cover surface brush border on cells brush border enzymes for final stages of digestion

Microscopic Anatomy tissue layers have modifications for nutrient digestion and absorption tissue layers have modifications for nutrient digestion and absorption lumen lined with simple columnar epithelium lumen lined with simple columnar epithelium muscularis externa is notable for a thick inner circular layer and a thinner outer longitudinal layer muscularis externa is notable for a thick inner circular layer and a thinner outer longitudinal layer large internal surface area for effective digestion and absorption large internal surface area for effective digestion and absorption great length and three types of internal folds or projections great length and three types of internal folds or projections circular folds (plicae circulares) - increase surface area by a factor of 2 to 3 circular folds (plicae circulares) - increase surface area by a factor of 2 to 3 villi – increase surface area by a factor of 10 villi – increase surface area by a factor of 10 microvilli – increase the surface area by a factor of 20 microvilli – increase the surface area by a factor of 20 25-99

Pores opening between villi lead to intestinal crypts absorptive cells, goblet cells and at base, rapidly dividing cells life span of 3-6 days as migrate up to surface and get sloughed off and digested paneth cells – antibacterial secretions Brunner’s glands in submucosa secrete bicarbonate mucus Peyer patches are populations of lymphocytes to fight pathogens Secrete 1-2 L of intestinal juice/day water and mucus, pH 7.4-7.8 Intestinal Crypts

Intestinal Villi Villi of Jejunum Histology of duodenum

Intestinal Secretion intestinal crypts secrete 1 to 2 L of intestinal juice per day intestinal crypts secrete 1 to 2 L of intestinal juice per day in response to acid, hypertonic chyme, and distension of the intestines in response to acid, hypertonic chyme, and distension of the intestines pH of 7.4 to 7.8 pH of 7.4 to 7.8 contains water, mucus, and little enzyme contains water, mucus, and little enzyme most enzymes that function in the small intestine are found in the brush border and pancreatic juice most enzymes that function in the small intestine are found in the brush border and pancreatic juice 25-102

Intestinal Motility 1. Mixes chyme with intestinal juice, bile and pancreatic juice 2. Churns chyme to increase contact with mucosa for absorption and digestion 3. Moves residue towards large intestine segmentation segmentation random ringlike constrictions mix and churn contents random ringlike constrictions mix and churn contents 12 times per minute in duodenum 12 times per minute in duodenum peristaltic waves begin in duodenum but each one moves further down peristaltic waves begin in duodenum but each one moves further down push chyme along for 2 hours push chyme along for 2 hours suppressed by refilling of stomach suppressed by refilling of stomach Food in stomach causes gastroileal reflex (relaxing of valve and filling of cecum) Food in stomach causes gastroileal reflex (relaxing of valve and filling of cecum)

Segmentation in Small Intestine Purpose of segmentation is to mix and churn not to move material along as in peristalsis

Peristalsis Gradual movement of contents towards colon Begins after absorption occurs Migrating motor complex controls waves of contraction second wave begins distal to where first wave began

Carbohydrate Digestion - Small Intestine Salivary amylase stops working in stomach (pH < 4.5) 50% of dietary starch digested before it reaches small intestine Pancreatic amylase completes first step in 10 minutes Brush border enzymes act upon oligosaccharides, maltose, sucrose, lactose and fructose lactose indigestible after age 4 in most humans (lactase declines)

Carbohydrate Absorption Sodium-glucose transport proteins (SGLT) in membrane help absorb glucose and galactose Fructose absorbed by facilitated diffusion then converted to glucose inside the cell

25-108 Lactose Intolerance lactose passes undigested into large intestine lactose passes undigested into large intestine increases osmolarity of intestinal contents increases osmolarity of intestinal contents causes water retention in the colon and diarrhea causes water retention in the colon and diarrhea gas production by bacterial fermentation of the lactose gas production by bacterial fermentation of the lactose occurs in many parts of the population occurs in many parts of the population 15% American whites, 90% of American blacks, 70% of Mediterranean's; and nearly all of Asian descent 15% American whites, 90% of American blacks, 70% of Mediterranean's; and nearly all of Asian descent can consume yogurt and cheese since bacteria have broken down lactose can consume yogurt and cheese since bacteria have broken down lactose

Protein Digestion and Absorption Pepsin has optimal pH of 1.5 to 3.5 -- inactivated when passes into duodenum and mixes with alkaline pancreatic juice (pH 8)

25-110Proteins amino acids absorbed by the small intestine come from three sources: amino acids absorbed by the small intestine come from three sources: dietary proteins dietary proteins digestive enzymes digested by each other digestive enzymes digested by each other sloughed epithelial cells digested by enzymes sloughed epithelial cells digested by enzymes endogenous amino acids from last two sources total about 30g/day endogenous amino acids from last two sources total about 30g/day exogenous amino acids from our diet total about 44 to 60 g/day exogenous amino acids from our diet total about 44 to 60 g/day proteases (peptidases) - enzymes that digest proteins proteases (peptidases) - enzymes that digest proteins begin their work in the stomach in optimum pH of 1.5 to 3.5 begin their work in the stomach in optimum pH of 1.5 to 3.5 pepsin hydrolyzes any peptide bond between tyrosine and phenylalanine pepsin hydrolyzes any peptide bond between tyrosine and phenylalanine pepsin digests 10-15% of dietary protein into shorter peptides and some free amino acids pepsin digests 10-15% of dietary protein into shorter peptides and some free amino acids

25-111Proteins continue to digest proteins in the small intestine continue to digest proteins in the small intestine pepsin inactivated when it passes into the duodenum and mixes with the alkaline pancreatic juice (pH 8) pepsin inactivated when it passes into the duodenum and mixes with the alkaline pancreatic juice (pH 8) pancreatic enzymes trypsin and chymotrypsin take over the process pancreatic enzymes trypsin and chymotrypsin take over the process hydrolyzing polypeptides into even shorter oligopeptides hydrolyzing polypeptides into even shorter oligopeptides oligopeptides taken apart one amino acid at a time by three more enzymes oligopeptides taken apart one amino acid at a time by three more enzymes carboxypeptidase – removes amino acids from –COOH end of the chain carboxypeptidase – removes amino acids from –COOH end of the chain aminopeptidase – removes them from the –NH 2 end aminopeptidase – removes them from the –NH 2 end dipeptidase – split dipeptides in the middle and release two free amino acids dipeptidase – split dipeptides in the middle and release two free amino acids carboxypeptidase is a pancreatic secretion carboxypeptidase is a pancreatic secretion aminopeptidase and dipeptidase are brush border enzymes aminopeptidase and dipeptidase are brush border enzymes

25-112 Figure 25.29 H N H O C HO H N H O C No chemical digestion occurs. O C HO H N H O C H N H O C OH H H N H H N O C O C HO H N H O C H N H O C OH H H N H H N O C O C HO N H N H C OOH H H N O C H H N O C O C H H N H N H C O O C H H N O C H H N O C H H N O C H H N O C H H N O C H H N O C H H N O C H H N O C H H N Mouth Stomach CarboxypeptidaseAminopeptidaseDipeptidase Protein Polypeptides Oligopeptides Pepsin ( ) hydrolyzes certain peptide bonds, breaking protein down into smaller polypeptides. Trypsin ( ) and chymotrypsin ( ) hydrolyze other peptide bonds, breaking polypeptides down into smaller oligopeptides. Carboxypeptidase ( ) removes one amino acid at a time from the carboxyl (–COOH) end of an oligopeptide. Carboxypeptidase ( ) of the brush border continues to remove amino acids from the carboxyl (–C OOH) end. Aminopeptidase ( ) of the brush border removes one amino acid at a time from the amino (–N H ) end. Dipeptidase ( ) splits dipeptides ( ) into separate amino acids ( ). Small intestine Actions of brush border enzymes (contact digestion) Blood capillary of intestinal villus Small intestine Actions of pancreatic enzymes Protein Digestion and Absorption Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Protein Digestion and Absorption Pancreatic enzymes take over protein digestion by hydrolyzing polypeptides into shorter oligopeptides

Protein Digestion and Absorption Brush border enzymes finish task, producing amino acids that are absorbed into intestinal epithelial cells amino acid cotransporters move into epithelial cells and facilitated diffusion moves amino acids out into blood stream Infants absorb proteins by pinocytosis (maternal IgA)

25-115Lipids hydrophobic quality of lipids makes their digestion and absorption more complicated that carbohydrates and proteins hydrophobic quality of lipids makes their digestion and absorption more complicated that carbohydrates and proteins lipases – fat digesting enzymes lipases – fat digesting enzymes lingual lipase secreted by the intrinsic salivary glands of the tongue lingual lipase secreted by the intrinsic salivary glands of the tongue active in mouth, but more active in stomach along with gastric lipase active in mouth, but more active in stomach along with gastric lipase 10-15% of lipids digested before reaches duodenum 10-15% of lipids digested before reaches duodenum pancreatic lipase – in the small intestine digest most of the fats pancreatic lipase – in the small intestine digest most of the fats fat enters duodenum as large globules exposed to lipase only at their surface fat enters duodenum as large globules exposed to lipase only at their surface globules broken up into smaller emulsification droplets by certain components of bile globules broken up into smaller emulsification droplets by certain components of bile lecithin and bile acids lecithin and bile acids

25-116 Lipids agitation by segmentation breaks up the fats into droplets as small as 1 μm in diameter agitation by segmentation breaks up the fats into droplets as small as 1 μm in diameter the coating of lecithin and bile acids keep it broken up, exposing far more of its surface to enzymatic action the coating of lecithin and bile acids keep it broken up, exposing far more of its surface to enzymatic action there is enough pancreatic lipase in the small intestine after a meal to digest the average daily fat intake in as little as 1 to 2 minutes there is enough pancreatic lipase in the small intestine after a meal to digest the average daily fat intake in as little as 1 to 2 minutes lipase acts on triglycerides lipase acts on triglycerides removes the first and third fatty acids from glycerol backbone removes the first and third fatty acids from glycerol backbone leaves the middle one leaves the middle one the product of lipase action are two free fatty acids (FFAs) and a monoglyceride the product of lipase action are two free fatty acids (FFAs) and a monoglyceride

Fat Digestion and Absorption

25-118 Lipid Absorption absorption of free fatty acids, monoglycerides, and other lipids depends on minute droplets in the bile called micelles absorption of free fatty acids, monoglycerides, and other lipids depends on minute droplets in the bile called micelles made in the liver made in the liver consist of 20 to 40 bile acid molecules aggregated with their hydrophilic side groups facing outward and their hydrophobic steroid rings facing inward consist of 20 to 40 bile acid molecules aggregated with their hydrophilic side groups facing outward and their hydrophobic steroid rings facing inward bile phospholipids and cholesterol diffuse into the center of the micelle to form its core bile phospholipids and cholesterol diffuse into the center of the micelle to form its core micelles pass down the bile duct into the duodenum micelles pass down the bile duct into the duodenum where they absorb fat soluble vitamins, more cholesterol, and the FFAs and monoglycerides produced by fat digestion where they absorb fat soluble vitamins, more cholesterol, and the FFAs and monoglycerides produced by fat digestion they transport lipids to the surface of the intestinal absorptive cells they transport lipids to the surface of the intestinal absorptive cells lipids leave the micelles and diffuse through the plasma membrane into the cells lipids leave the micelles and diffuse through the plasma membrane into the cells micelles are reused, picking up another cargo of lipid, transporting them to the absorptive cells micelles are reused, picking up another cargo of lipid, transporting them to the absorptive cells

Fat Digestion and Absorption

Nucleic Acids, Vitamins, and Minerals Nucleases hydrolyze DNA and RNA to nucleotides Nucleases hydrolyze DNA and RNA to nucleotides nucleosidases and phosphatases of brush border split them into phosphate ions, ribose or deoxyribose sugar and nitrogenous bases nucleosidases and phosphatases of brush border split them into phosphate ions, ribose or deoxyribose sugar and nitrogenous bases Vitamins are absorbed unchanged Vitamins are absorbed unchanged A, D, E and K with other lipids -- B complex and C by simple diffusion and B 12 if bound to intrinsic factor A, D, E and K with other lipids -- B complex and C by simple diffusion and B 12 if bound to intrinsic factor Minerals are absorbed all along small intestine Minerals are absorbed all along small intestine Na + cotransported with sugars and amino acids Na + cotransported with sugars and amino acids Cl - exchanged for bicarbonate reversing stomach Cl - exchanged for bicarbonate reversing stomach Iron and calcium absorbed as needed Iron and calcium absorbed as needed

25-122 Calcium Absorption calcium is absorbed throughout the intestine by different mechanisms calcium is absorbed throughout the intestine by different mechanisms active transport in the duodenum active transport in the duodenum enters through calcium channels in apical cell membrane enters through calcium channels in apical cell membrane binds to calbindin protein so concentration gradient will continue to favor calcium influx binds to calbindin protein so concentration gradient will continue to favor calcium influx actively transported out of base of cell into bloodstream by calcium- ATPase and Na + -Ca 2+ antiport actively transported out of base of cell into bloodstream by calcium- ATPase and Na + -Ca 2+ antiport diffusion between epithelial cells in jejunum and ileum diffusion between epithelial cells in jejunum and ileum parathyroid hormone – secreted in response to a drop in blood calcium levels parathyroid hormone – secreted in response to a drop in blood calcium levels stimulates kidney to synthesize vitamin D from precursors made by epidermis and liver stimulates kidney to synthesize vitamin D from precursors made by epidermis and liver vitamin D affects the absorptive cells of the duodenum in three ways: vitamin D affects the absorptive cells of the duodenum in three ways: increases number of calcium channels in apical membrane increases number of calcium channels in apical membrane increases the amount of calbindin in the cytoplasm increases the amount of calbindin in the cytoplasm increase the number of calcium-ATPase pumps at basal membrane increase the number of calcium-ATPase pumps at basal membrane parathyroid hormone increases the level of calcium in the blood parathyroid hormone increases the level of calcium in the blood

Water Balance Digestive tract receives about 9 L of water/day Digestive tract receives about 9 L of water/day.7 L in food, 1.6 L in drink, 6.7 L in secretions.7 L in food, 1.6 L in drink, 6.7 L in secretions 8 L is absorbed by small intestine and 0.8 L by large intestine 8 L is absorbed by small intestine and 0.8 L by large intestine Water is absorbed by osmosis following the absorption of salts and organic nutrients Water is absorbed by osmosis following the absorption of salts and organic nutrients Diarrhea occurs when too little water is absorbed Diarrhea occurs when too little water is absorbed feces pass through too quickly if irritated feces pass through too quickly if irritated feces contains high concentrations of a solute (lactose) feces contains high concentrations of a solute (lactose)

Anatomy of Large Intestine

Gross Anatomy of Large Intestine 5 feet long and 2.5 inches in diameter in cadaver 5 feet long and 2.5 inches in diameter in cadaver Begins as cecum and appendix in lower right corner Begins as cecum and appendix in lower right corner Ascending, transverse and descending colon frame the small intestine Ascending, transverse and descending colon frame the small intestine Sigmoid colon is S-shaped portion leading down into pelvis Sigmoid colon is S-shaped portion leading down into pelvis Rectum - straight portion ending at anal canal Rectum - straight portion ending at anal canal

Microscopic Anatomy Mucosa - simple columnar epithelium Mucosa - simple columnar epithelium anal canal has stratified squamous epithelium anal canal has stratified squamous epithelium No circular folds or villi to increase surface area No circular folds or villi to increase surface area Intestinal crypts (glands sunken into lamina propria) produce mucus only Intestinal crypts (glands sunken into lamina propria) produce mucus only Muscularis externa Muscularis externa muscle tone in longitudinal muscle fibers (concentrated in taeniae coli) form pouches (haustra) muscle tone in longitudinal muscle fibers (concentrated in taeniae coli) form pouches (haustra) Transverse and sigmoid have a serosa, rest retroperitoneal Transverse and sigmoid have a serosa, rest retroperitoneal epiploic appendages are suspended fatty sacs epiploic appendages are suspended fatty sacs

Bacterial Flora and Intestinal Gas Bacterial flora populate large intestine Bacterial flora populate large intestine ferment cellulose and other undigested carbohydrates; we absorb resulting sugars ferment cellulose and other undigested carbohydrates; we absorb resulting sugars synthesize vitamins B and K synthesize vitamins B and K Flatus (gas) Flatus (gas) average person produces 500 mL per day average person produces 500 mL per day most is swallowed air but hydrogen sulfide, indole and skatole produce odor most is swallowed air but hydrogen sulfide, indole and skatole produce odor

Absorption and Motility Transit time is 12 to 24 hours Transit time is 12 to 24 hours reabsorbs water and electrolytes reabsorbs water and electrolytes Feces consist of water and solids (bacteria, mucus, undigested fiber, fat and sloughed epithelial cells Feces consist of water and solids (bacteria, mucus, undigested fiber, fat and sloughed epithelial cells Haustral contractions occur every 30 minutes Haustral contractions occur every 30 minutes distension of a haustrum stimulates it to contract distension of a haustrum stimulates it to contract Mass movements occur 1 to 3 times a day Mass movements occur 1 to 3 times a day triggered by gastrocolic and duodenocolic reflexes triggered by gastrocolic and duodenocolic reflexes filling of the stomach and duodenum stimulates motility filling of the stomach and duodenum stimulates motility moves residue for several centimeters with each contraction moves residue for several centimeters with each contraction

Anatomy of Anal Canal Anal canal is 3 cm total length Anal columns are longitudinal ridges separated by mucus secreting anal sinuses Hemorrhoids are permanently distended veins

Defecation Stretching of the rectum stimulates defecation Stretching of the rectum stimulates defecation intrinsic defecation reflex via the myenteric plexus intrinsic defecation reflex via the myenteric plexus causes muscularis to contract and internal sphincter to relax causes muscularis to contract and internal sphincter to relax relatively weak contractions relatively weak contractions defecation occurs only if external anal sphincter is voluntarily relaxed defecation occurs only if external anal sphincter is voluntarily relaxed parasympathetic defecation reflex involves spinal cord parasympathetic defecation reflex involves spinal cord stretching of rectum sends sensory signals to spinal cord stretching of rectum sends sensory signals to spinal cord splanchnic nerves return signals intensifying peristalsis splanchnic nerves return signals intensifying peristalsis Abdominal contractions increase abdominal pressure as levator ani lifts anal canal upwards Abdominal contractions increase abdominal pressure as levator ani lifts anal canal upwards feces will fall away feces will fall away

Neural Control of Defecation 1. Filling of the rectum 2. Reflex contraction of rectum and relaxation of internal anal sphincter 3. Voluntary relaxation of external sphincter

25-133 Man with a Hole in His Stomach Canadian shot accidentally by shotgun in 1822 Canadian shot accidentally by shotgun in 1822 a hole “large enough to receive forefinger” covered by a flap of tissue remained after wound healed a hole “large enough to receive forefinger” covered by a flap of tissue remained after wound healed local physician experimented on him local physician experimented on him removed samples and made observations removed samples and made observations proved digestion required HCl proved digestion required HCl published book in 1833 that laid foundation for modern gastric physiology published book in 1833 that laid foundation for modern gastric physiology

Ch 25 The Digestive System Jen Tynes SELU. The Digestive System General anatomy and digestive processes General anatomy and digestive processes Mouth.

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Ch 25 The Digestive System Jen Tynes SELU. The Digestive System General anatomy and digestive processes General anatomy and digestive processes Mouth.

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