GI Anatomy and Physiology Presented by Derrick Anderson 1.
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Transcript of GI Anatomy and Physiology Presented by Derrick Anderson 1.
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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