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GI Anatomy and Physiology

Presented by Derrick Anderson

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Definitions: Digestive Process

Ingestion Taking food in Propulsion moving food through alimentary canal

Peristalsis Mechanical digestion

Chewing, churning by the stomach, segmentation Chemical digestion: Breaking down molecules Absorption: Taking in molecules Defecation eliminating indigestible substances

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Where does the GI system start???

Mouth

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Gastrointestinal Tract

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Peritoneum

The serous membrane of the abdominopelvic cavity Peritonitis inflammation of the peritoneum Two Parts

Visceral Peritoneum Covers external surface of most digestive organs Continuous with the parietal peritoneum

Parietal Peritoneum Lines the body wall Continuous with the visceral peritoneum

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Peritoneal Cavity

Potential space containing slippery fluid secreted by the serous membranes

Filled with serous fluid lubrication Following diagram depicts the “potential space” of

the peritoneal cavity

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Mesentery

Double layer of peritoneum extending to the digestive organs from the body wall

Provide routes for blood vessels, lymphatics and nerves to reach digestive viscera

Hold organs in place Stores fat Can be dorsal or ventral

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Alimentary canal – continuous, muscular canal aka gastrointestinal Mouth, pharynx, esophagus, stomach, small

intestine, large intestine Accessory digestive organs

Teeth, tongue, gallbladder, salivary glands, liver and pancreas

Continuous Pathway

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Histology of the Alimentary Canal*

Four basic layers (“tunics”) from the esophagus to the anal canal Mucosa Submucosa Muscularis externa Serosa

* All layers are presented starting from the innermost layer

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Alimentary Canal

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Mucosa

Aka mucus membrane Innermost layer Epithelial membrane Lines the lumen of the alimentary canal

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Functions of the Mucosa

Secrete mucus, digestive enzymes and hormones Absorb end products of digestion into the blood Protect against infections

Depending on the region of the mucosa one or all of these functions may be occur

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Mucosa Sublayers

Epithelieum Lamina propria Muscularis mucosae

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Mucosal Epithelium

Simple columnar epithelium Contains mucus-secreting cells

Protects the GI tract from the digestive enzymes Eases passage of substances

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Lamina Propria

Loose areolar connective tissue Capillaries nourish epithelium and absorb

nutrients Contains lymphoid follicles

MALT mucosa-associated lymphatic tissue Defend against pathogens

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Muscularis Mucosa

Smooth muscle cells Local movements of the mucosa Exist as many folds in the small intestine

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Alimentary Canal

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Submucosa

Areolar connective tissue Rich supply of blood and lymphatic vessels,

lymphoid follicles and nerve fibers Contains elastic fibers

Allows stomach to return to normal shape after being stretched from storing food

Extensive vascular network supplies surrounding tissue

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Alimentary Canal

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Muscularis Externa

Aka muscularis Responsible for peristalsis and segmentation Inner circular layer of smooth muscle

Thickens in some areas to form sphincters which prevent backflow

Contraction = smaller lumen Outer longitudinal layer of smooth muscle

Contraction = shorter tube

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Alimentary Canal

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Serosa

Protective outermost layer Same as Visceral peritoneum Areolar connective tissue covered with mesothelium

Mesothelium = single layer of squamous epithelial cells Replaced by adventitia in the esophagus

Fibrous connective tissue binding the esophagus to the surrounding structures

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Alimentary Canal

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Enteric Nervous System

Nerve supply for the GI Regulate digestive system activity Semiautonomous 2 major intrinsic nerve plexuses

Submucosal Myenteric

Also subject to control by the CNS and ANS

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Submucosal Nerve Plexus

In the submucosa Sensory and motor neurons Regulates activity of glands and smooth muscle in

the mucosa

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Myenteric Nerve Plexus

Between the circular and longitudinal smooth muscle of the muscularis

Provide major nerve supply to GI tract wall and control GI motility

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Mouth and Associated Organs

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Mouth

Lined with stratified squamous epithelium Withstand friction

Epithelium of the gums, hard palate and dorsal tongue are slightly keratinized

Contains antimicrobial peptides called defensins

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Salivary glands

Paired tubuloalveolar glands Saliva

Cleanses mouth Dissolves food chemicals so that they can be tasted Moistens food and aids in forming a bolus Contains enzymes that begin chemical breakdown of

starchy food

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Salivary Glands

Parotid gland Largest Anterior to ear Mumps

Submandibular gland Sublingual gland

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Salivary Glands

2 types of secretory cells Serous

Produce watery secretion containing enzymes ions and mucin Parotid gland only has these

Mucus Produce mucus Sublingual contains mostly these

Submandibular contains equal amounts of both cell types

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Esophagus

Joins stomach at cardiac orifice Cardiac orifice surrounded by gastroesophageal or

cardiac sphincter Physiological sphincter slight thickening Not a true sphincter emesis; heartburn

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Esophagus

Nonkeratinized stratified squamous mucosa Changes to simple columnar epithelium at the esophagus-stomach

junction Specialized for secretion

Submucosa contains mucus-secreting esophageal glands “greases” the lumen of the esophagus to ease passage of the bolus

Muscularis externa (Muscle type transition) Upper (skeletal)Middle(skeletal and smooth)Lower (smooth) muscle

Adventitia (serosa layer) Fiborus Connective tissue

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Stomach Tissue

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Digestive Process: Mouth to Stomach

Mastication = chewing Mechanical breakdown Voluntary and reflexive

Deglutination = swallowing 2 phases

Buccal phase In the mouth; voluntary Tongue forces bolus down

Pharyngeal-esophageal phase Triggered by bolus reaching receptors in the pharynx Involuntary; controlled by swallowing center located in the brain

stem Respiration temporarily inhibited

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Stomach

Upper left quadrant of the peritoneal cavity “Storage tank” Size

Empty = 50 ml Can expand to a volume of 4 L (about 1 gallon)

Chemical breakdown of proteins begins Food converted to creamy paste called chyme

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Stomach

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Stomach

Contains folds called rugae Regions

Cardiac region (near the heart) or cardia Surrounds cardiac orifice

Fundus Body Pyloric region

Pyloric antrum wider and more superior Pyloric canal narrower

Pylorus Pyloric sphincter a true sphincter

Controls stomach emptying

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Stomach

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Stomach

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Omenta mesenteries extending from the greater and lesser curvatures Tether stomach to abdominal wall Lesser omentum Greater omentum extends down to cover the small

intestine Contains a lot of fatty tissue and lymph nodes

Arterial supply comes form celiac trunk Subject to para control

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Abdominal Omentum

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Microscopic Anatomy

Modified muscularis and mucosa Muscularis has an additional innermost layer of

smooth muscle fibers running obliquely Allows stomach to mix, churn move and pummel food

Mucosa is simple columnar epithelium comprised entirely of mucus cells

Mucus cells produce protective two-layer coat of alkaline

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Stomach Mucosa

Deep gastric pits Mucous cells form the walls of the gastric pits

Pits lead into gastric glands produce gastric juice Mucus neck cells upper region of gastric glands

Produce thin soluble mucus different from mucus from the mucus cells

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Parietal And Chief Cells

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Parietal Cells middle region of glands Secrete hydrochloric acid (HCl) and intrinsic factor Have microvilli increased surface are for more HCl

secretion HCl creates extremely acidic environment of the stomach

which is required for optimal activity of pepsin (enzyme) Intrinsic factor is a glycoprotein required for vitamin B12

absorption in the small intestine

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Chief Cells mostly basal region of gastric glands Produce pepsinogen (inactive form of pepsin) Pepsin digests protein At first, pepsinogen is converted to pepsin due to

presence of HCl However, once pepsin is present, pepsin itself catalyzes

activation of pepsinogen Positive feedback

Also release lipase

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Parietal and Chief Cells

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Enteroendocrine cells deep in gastric glands Release variety of chemical messengers directly into

the lamina propria Histamine, serotonin, somatastatin and gastrin

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Vulnerable Stomach?

No! Mucosal Barrier

Thick coating of bicarbonate rich mucus on stomach wall

Tight junctions between epithelial cells prevent leaking of gastric juices into underlying tissue

Damaged epithelial cells are shed quickly and quickly replaced by undifferentiated stem cells

Stomach mucus cells renewed every 3-6 days

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H. pylori and Gastric Ulcers

Acid resistant bacteria that burrow through mucus and destroy protective mucosal layer

Release… ammonia to neutralize the environment Cytotoxin that damages stomach epithelium

Detected by a breath test Treated with 2 week long antibiotic course

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Gastric ulcers

For ulcers not caused by H. pylori usually caused by long-term use of NSAIDs (eg. Aspirin, ibuprofen)

Treated with H2 (histamine) receptor blockers Cimetidine (Tagamet) Ranitidine (Zantac)

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Digestive Process in the Stomach

Protein digestion is the primary digestion that occurs in the stomach HCl Pepsin

In infants, stomach also secretes enzyme called rennin Acts on milk protein casein

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Alcohol and Aspirin

Lipid soluble substance Absorbed directly from the stomach into the blood

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Intrinsic Factor

Secreted by parietal cells Required for intestinal absorption of vitamin B12 B12 important for production of mature

erythrocytes Deficiency can cause anemia

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Regulation of Gastric Secretions

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Both neural and hormonal control Neural control

Long nerve reflexes (vagus nerve) Vagus nerve stimulation = increased activity of nearly all

glands Sympathetic nerve stimulation = opposite

Short nerve reflexes (local enteric) Hormonal control is mostly mediated with gastrin

(stomach) and hormones from the small intestine Stimulates secretion of enzymes and HCl

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Phases of gastric secretion

Cephalic phase Gastric phase Intestinal phase

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Cephalic phase

Occurs before food enters the stomach Few minutes long Stimulated by smell, taste, sight or thought of

food and gets stomach ready

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Cephalic Phase- Extrinsic

Activated olfactory receptors and taste buds send signals to the hypothalamus

Hypothalamus stimulates vagal nuclei of the medulla oblongata

Vagus nerves transmits motor impulses to parasympathetic enteric ganglia

Enteric ganglionic neurons stimulate the stomach glands

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Gastric Phase

Occurs once food enters stomach 3-4 hours Provides 2/3rd of gastric juices released Most important stimuli:

Distension activates stretch receptors and local (myenteric) and long (vagovagal) reflexes Both reflexes cause acetylcholine (ACh) to be released which

stimulates gastric juice secretion Peptides Low acidity

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Gastrin

Chemicals from partially digested proteins, caffeine, and rising pH directly activate enterendocrine cells called G cells G cells release gastrin

Gastrin stimulates HCl secretion Highly acidic environment inhibits gastrin secretion

G cells also stimulated by neural reflexes G cells inhibited by sympathetic nervous system

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HCl and Parietal Cells

HCl secretion stimulated by 3 chemicals all of which work via 2nd messengers ACh from parasympathetic fibers increase intracellular Ca2+

levels Gastrin from G cells increase intracellular Ca2+ levels Histamine from enterochromaffin-like cells acts through cAMP

When all 3 chemicals bind to parietal cells much HCl secretion When only one binds not much HCl

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HCl formation

H+ actively pumped into stomach lumen by H+ K+ ATPase for exchange of K+ ions into the cell

Cl- ions follow H+ ions into the lumen in order to maintain an electrical balance

Cl- comes from blood plasma H+ comes from carbonic acid within the parietal cells

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As H+ is pumped into the stomach, bicarbonate ion (HCO3

-) accumulates in cell and moves into the blood

HCO3- moves into blood via the HCO3

- Cl- antiporter

As HCO3- moves into blood, Cl- moves in opposite

direction into cell

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Intestinal Phase

Has an excitatory and inhibitory component Excitatory component

As food enters the duodenum intestinal mucosal cells are stimulated to release intestinal (enteric) gastrin, a hormone that encourages gastric glands to continue activity

As intestine distends, inhibitory component takes over because intestines can’t handle harsh acidic chyme

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Enterogastric Reflex

Begins the inhibitory component of the intestinal phase

Inhibits vagal nuclei in medulla Inhibits local reflexes Activates sympathetic fibers to cause pyloric

sphincter to tighten and prevent more food entering the small intestine

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Causes release of intestinal hormones called enterogastrones Secretin Cholescystokinin (CCK) Vasoactive intestinal peptide (VIP) All inhibit gastric secretion when stomach is very

active

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Small Intestine

Convoluted tube extending from pyloric sphincter to ileocecal valve where it joins the large intestine

Longest part of the alimentary canal 3 subdivisions: duodenum, jejunum, ileum

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Duodenum

Shortest Bile duct and pancreatic duct unit at wall of the

duodenum hepatopancreatic ampulla Hepatopancreatic sphincter controls entry of bile

and pancreatic juices

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Liver

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Jejunum and Ileum

Jejunum is in between duodenum and ileum Ileum is longest Ileum joins long intestine at the ileocecal valve Jejunum and ileum are suspended by mesentery

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Nerve Supply

Parasympathetic from the vagus nerve Sympathetic from the thoracic splanchnic nerves Both relayed through the superior mesenteric

plexus

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Arterial Supply

Mostly from the superior mesenteric artery Veins parallel the arteries and drain into the

superior mesenteric vein Superior mesenteric vein then drains into the

hepatic portal vein which carries nutrient-rich blood to the liver

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Structural Modifications of the Small Intestine

Circular folds Villi Microvilli All increase surface area

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Circular Folds

Aka plicae circulares Deep permanent folds of the mucosa and

submucosa Slow chyme movement to allow time for full

nutrient absorption

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Small Intestine

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Villi

Fingerlike projections of the mucosa Epithelial cells of villi are mostly columnar cells for

absorption Lacteal lymph capillary + dense capillary bed at

the core of each villus Nutrients are absorbed into the blood and the

lymph

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Microvilli

Densely packed “brush border” of mucosa Contain brush border enzymes which complete

digestion of carbohydrates and proteins

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Histology

Mucosa is mostly made up of simple columnar cells for absorption

Epithelium also contain mucus-secreting goblet cells

Pits between villi that lead into tubular glands called intestinal crypts

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Intestinal Crypts

Secrete intestinal juices (watery with mucus) Carrier fluid for nutrients

Enteroendocrine cells scattered through crypts Source of enterogastrones (secretin and CCK) Intraepithelial lymphocytes

Type of T-cell Immediately kill infected cells

Paneth Cells Secretory cells that release defensins and and lysozyme

(antimicrobial enzyme) Secretions kill some bacteria and keep others

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Intestinal Submucosa

Contains typical areolar connective tissue and erpeyer’s patches

Peyer’s patches Aggregated lymphoid follicles Increase in number along the length of the small intestine (more in

the ileum) Duodenal glands

Mucus secreting Only in submucosa of duodenum Produce alkaline mucus to neutralize acidic chyme

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Peyer’s patch component of Gut Associated lymphoid tissue (GALT)

SEM of surface of ileumH&E Ileum

Domed areas lacking villi

Lymphoid follicles with germinal centers

Follicles lined by specialized M cells, short microfolds (the name M Cell)

Basolateral pockets of B cells

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Accessory Organs of the Small Intestine

Liver Gallbladder

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Liver

Largest gland in the body Has 4 primary lobes Ventral mesentery anchors liver to lesser curvature of

the stomach Digestive role = produce bile

Bile is a fat emulsifier Also has metabolic functions

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Liver Anatomy

Hepatic artery and hepatic portal vein enter liver at porta hepatis

Bile leaves liver through several bile ducts which fuse to form the common hepatic duct

Common hepatic duct travels towards duodenum and fuses with the cystic duct (drains gallbladder) to form the bile duct

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Liver lobules as structural and functional units Hexagonal shaped Consist of hepatocytes “stacked” upon each other Hepatocytes radiate from a central vein

Portal triad exist at each of the six corners to the lobules Hepatic arteryHepatic portal vein Bile duct

Liver sinusoids leaky capillaries found between hepatocyte cells Blood from hepatic artery and hepatic portal vein goes through liver sinusoids to

the central vein Contains Kupffer cells which remove debris (bacteria and old blood cells)

Secreted bile flows through bile canaliculi between hepatocytes towards the bile duct of the portal triads

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Bile

Yellow-green alkaline solution Contains bile salts, bile pigments, cholesterol,

triglycerides, phospholipids and electrolytes Only bile salts and phospholipids help in the digestive

process Major mechanism of cholesterol elimination from the

body Too much cholesterol or low levels of bile salts cholesterol

crystallization = gallstones

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Bile salts

Cholesterol derivatives Emulsify fats Facilitate fat and cholesterol absorption Help solubilize cholesterol Not excreted but rater recycled via enterohepatic circulation

process Bile salts reabsorbed by the ileum Returned to liver via hepatic portal blood Resecreted in newly formed bile

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Bilirubin

Primary bile pigment Waste product of the heme of hemeglobin during

breakdown of old erythrocytes Metabolized in small intestine by resident bacteria One of its breakdown products is responsible for the brown

color of feces Absence of bile = grey-white color of feces

Not fats are being digested or absorbed

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Gallbladder

Thin-walled, green muscular sac Stores bile Concentrates bile (by absorbing water and some

of it ions) Contraction expels bile into the cystic duct which

then flows into bile duct

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Pancreas

Gland Accessory digestive organ Produces enzymes that breakdown all categories of

foodstuffs Pancreatic juice drains pancreas via main pancreatic duct

which fuses with the bile duct (hepatopancreatic ampulla) as it empties into the duodenum

Smaller accessory pancreatic duct empties directly into duodenum

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Acini clusters of secretory cells surrounding ducts Full of rough endoplasmic reticulum

Islets of Langerhans Scattered among the acini cells Endocrine glands Release insulin and glucagon

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Pancreas Cells

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Pancreas

Highly vascularized clusters of endocrine cells called Islets of Langerhans (endocrine pancreas) surrounded by more

abundant acinar cells (exocrine pancreas)

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Immunocytochemistry Identifies Which Islet Cell Secretes a Given Hormone

Beta Cells Alpha Cells

Delta Cells F Cells

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Pancreatic Juice

Mostly water, enzymes and electrolytes Acinar cells produce enzyme-rich component Epithelial cells lining pancreatic duct release

bicarbonate ions to make it alkaline Amount of HCl produced in the stomach is exactly

matched to the amount of bicarbonate secreted by the pancreas

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Pancreatic proteases are released in inactive form and activated in the duodenum Prevents pancreas from digesting itself

Trypsinogen trypsin Endopeptidase is an intestinal brush border protease Trypsin activates more trypsinogen

Trypsin also activates procarboxypeptidase and chymotrypsinogen to carboxypeptidase and chymotrypsin, respectively

Amylases, lipases and nucleases released in active form

endopeptidase

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Small Intestine

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Regulation of Bile and Pancreatic secretions

Bile salts are major stimulus for bile secretion As more bile salts are recycled, more bile is secreted

Secretin released from intestine also stimulates liver cells to secrete bile

Live continuously makes bile, but it’s not released until gallbladder contracts CCK (intestinal enzyme) is major stimulus for gallbladder

contraction CCK released into blood when acidic fatty chyme enters duodenum

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CCK

Stimulates gallbladder to contract Stimulates secretion of pancreatic juice Relaxes hepatopancreatic sphincter so that bile

and pancreatic juice enters duodenum Stimulates acini to release enzyme-rich pancreatic

juice Potentiates effects of secretin

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Secretin

Released in response to HCl in intestine Targets pancreatic duct cells for a bicarbonate-rich

pancreatic juice

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Roles of gastrin, secretin and cholecystokinin in digestion

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Large Intestine

Frames small intestine Extends from ileocecal valve to the anus Major digestive function is to absorb remaining water Stores food residues temporarily Eliminates remaining food residues in the form of feces Not essential for life (nor major digestion occurs here)

Removal (which may occur in cases of colon cancer) does not inhibit life

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Anatomical modifications of the Large Intestine

Teniae coli The three bands which represent the reduction of the longitudinal

layer of the muscularis Haustra

Pocket-like sacs Tone of teniae colie causes haustra

Epiploic appendages Small fat-filled pouches of visceral peritoneum that hang from the

surface of the large intestine Significance is unknown

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Large Intestine

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Subdivisions

Cecum Appendix Colon Rectum Anal canal

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Appendix

Contains masses of lymphoid tissue (part of MALT) Small and twisted infection likely Appendicitis inflammation of the appendix

Blockage that traps bacteria in the appendix Appendix swells cuts off blood supply appendix

decays If rupture peritonitis (infection of peritoneum)

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Colon

Ascending colon Right colic/hepatic flexure

bend between ascending and transvers colon

Transverse colon Left colic/splenic flexure

Bend between transverse and descending colon

Descending colon Sigmoid colon

S-shaped In the pelvis

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Rectum

Located in the pelvis Three lateral curves Internally, these curves present as three

transverse folds called rectal valves Rectal valves separate feces from flatus (gas)

Prevent feces from being passed along with gas

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Anal Canal

Located in the perineum 2 sphincters

Internal anal sphincter Involuntary Smooth muscle

External anal sphincter Voluntary Skeletal muscle

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Microscopic Anatomy

Mucosa is simple columnar epithelium except in the anal canal

No villi No cells that secrete digestive enzymes Thicker mucosa Deeper crypts

Large number of goblet cells in crypts

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Mucosa of anal canal is stratified squamous epithelium (increased abrasion)

Anal canal hangs in longitudinal folds called anal columns

Anal sinuses are between the anal columns Secrete mucus to ease feces movement

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Motility

Fecal matter stays in large intestine for 12-24 hours Haustral contractions slow segmenting movements

More frequent (every 30 minutes) Mass movements long, slow-moving, powerful

contractile waves over large areas of the colon Less frequent (3-4 times a day) Usually occur during or just after eating

Fiber increases strength of contractions and softens stool

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Defecation

When mass movements force feces into rectum, rectal wall stretches and initiates defecation reflex Sigmoid colon and rectum contract Internal anal sphincter relaxes If defecation is delayed reflex contractions end

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Diarrhea and Constipation

Diarrhea Watery stools Results from any condition that rushes food through the large

intestine (reduced time to absorb water) Dehydration and electrolyte imbalance

Constipation Hard stools When feces remains in the large intestine too long Lack of fiber, improper bowel habits, lack of exercise, emotional

upset or laxative abuse

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Chemical digestion is a metabolic process in which large food molecules are broken down to monomers

Hydrolysis enzymatic breakdown of food molecule Involves addition of water molecule to each bond

being broken

Chemical Digestion and Absorption

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Carbohydrates

Broken down to monosaccharaides Includes starch Indigestible carbohydrates (cellulose) act as bulk and help

move foodstuffs along the GI tract Chemical digestion begins in the mouth with salivary amylase Amylase works best in slightly acidic to neutral environment Inactivated by stomach acid

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Digestion continues in small intestine with aid of pancreatic amylase

Amylases convert starch to oligosaccharides Intestinal brush border enzymes convert

oligosaccharides to monosaccharaides Dextrinase, glucoamylase, maltase, sucrase, lactase

Digestion of carbohydrates ends with the small intestine (no digestion in large intestine)

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Proteins

Digested to its amino acid monomers Digestion begins in the stomach Pepsinogen from chief cells is activated to pepsin which

digests proteins Optimal functioning in acidic environment (low pH) Inactivated by high pH in duodenum Activity restricted to stomach

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Trypsin and chymotrypsin from the pancreas continue to cleave protein bonds into smaller peptide bonds in the intestine

Carboxypeptidase (brush border enzyme) cleaves amino acids one-by-one at the carboxyl end

Aminopeptidase does the same from the amino end

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Lipids

Digestion only occurs in the small intestine Pancreas provides lipase Triglycerides are insoluble in water, therefore “pre-treated” with bile

salts Bile salts emulsify and increase surface area of the triglycerides to aid in

fat digestion Bile salts have both polar and nonpolar ends Polar and faces aqueous environment. Nonpolar end faces fat molecules

Digestion occurs with LIPASE. BILE SALTS DO NOT DIGEST Breakdown into fatty acids and monoglycerides

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Nucleic Acids

DNA and RNA in the nuclei of cells of food hydrolyzed to their nucleotide monomers

Pancreatic nucleases responsible Nucleotides later broken down by brush border

enzymes nucleosidases and phosphatases

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Absorption

Most absorption complete by the time the chyme reaches the ileum

Ileum mostly responsible for reclaiming bile salts Most nutrients absorbed via active transport Tight junctions exist between mucosal epithelial

cells, therefore nutrients must move through the cell

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Carbohydrate Absorption

Glucose and galactose use secondary active transport (with Na+) to move into epithelial cells

Move out of epithelial cells and into blood via facilitated diffusion

Fructose moves entirely by facilitated diffusion

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Protein Absorption

Several types of protein transporters move different amino acids

Most transporters are coupled to the active transport with Na+

Short-chained amino acids are absorbed with H+ cotransport into the epithelial cell Broken down to single amino acids before moving

into circulation

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Lipid Absorption

Monoglycerides and fatty acids associate with bile salts and lecithin (phospholipid) to form micelles

Micelles collections of fatty elements clustered together with bile salts

Micelles easily diffuse between microvilli Fat absorption complete in the ileum

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Once inside the epithelial cells, they are resynthesized into triglycerides by the smooth ER

Triglycerides coated with a “skin” of proteins to form chylomicrons (water-soluble lipoprotein droplet)

Chylomicrons leave via exocytosis and enter lacteals to join the lymph (not blood)

Later emptied into venous blood in the neck region

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In the blood, chylomicrons liberate triglycerides Triglycerides hydrolyzed to fatty acids and glycerol

by lipoprotein lipase

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Nucleic Acid Absorption

Pentose sugars, nitrogenous bases and phosphate ions from nucleotide breakdown are actively transported across epithelium by special carriers

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Vitamin Absorption

Small intestine absorbs dietary vitamins Large intestine absorbs K and some B vitamins Fat soluble vitamins (A, D, E and K) dissolve in dietary

fats and are absorbed in micelles Water soluble vitamins (B and C) absorbed via diffusion

or specific active or passive transporters Exception: Vitamin B12 is very large. Requires intrinsic

factor (stomach) Intrinsic factor binds to B12. Intrinsic factor binds to its receptor

in the ileum allowing for endocytosis of B12

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Water Absorption

Mostly absorbed in the small intestine by osmosis Osmosis in the large intestine Water moves freely across intestinal mucosa in

both directions but net osmosis occurs when there is a concentration gradient established by active transport of solutes into mucosal cells

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General Organization and Structure of the Digestive/Alimentary tube

From Kierszenbaum,

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Basic mucosal forms:

Protective-esophagus Secretory-only in stomach

Absorptive- entire small intestine Absorptive/Protective- Large intestine