unit 8 -gastrointestinal physiology
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The Digestive System
The Gastrointestinal System consists of the:
Digestive Tract Oral Cavity
Pharynx
Esophagus
Stomach Small Intestines
Large Intestines
Accessory Organs Teeth
Tongue
Salivary Glands
Liver Pancreas
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GENERAL FUNCTIONS
1. Ingestion
2. Mechanical Processing
3. Digestion
4. Secretion
5. Absorption
6. Excretion
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The Digestive Organs and the
Peritoneum
Peritoneal cavity found within the
abdominopelvic cavity, lined by a serous
membrane (mesothelium and areolar tissue)
Serous membraneSerosa (visceralperitoneum) & Parietal Peritoneum
Peritoneal fluid separates the two layers
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The Digestive Organs and the
Peritoneum
MESENTERIES
Sheets of serous membrane that suspend
portions of the digestive tract within the peritoneal
cavity Double sheets of peritoneal membrane
Provides access routes for blood vessels,
lymphatic vessels and nerves
Stabilizes position of attached organs
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Peritoneum and Mesenteries
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Mesenteries
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Mesenteries
Digestive tract &
Accessory Organs
suspended by dorsal
and ventralmesenteries in
embryonic life
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Mesenteries
Ventral Mesentery > Lesser Omentum &
Falciform Ligament
Dorsal Mesentery > Greater Omentum
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Lesser and Greater Omentum
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LAYERS OF THE WALL OF
THE DIGESTIVE TRACT
1. Mucosa
2. Submucosa
3. Muscularis Externa4. Serosa
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MucosaEpithelial Layer & Lamina Propria
Epithelial Layer
The layer is folded to facilitate a large surface
area for digestion.
The epithelium may be simple or stratified.
Stratified Squamous- Oral Cavity, Pharynx and
Esophagus
Simple Columnar- Stomach, Small Intestine andalmost all the length of the large intestine.
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Lamina Propria
Loose connective tissue
Blood vessels
Sensory nerve endings Lymphatic vessels
Smooth muscle cells
Scattered lymphoid tissues
Secretory cells of mucus glands Muscularis Mucosae (circular and longitudinal)
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Submucosa
A layer of dense connective tissues that lies
below the mucosa consisting of:
Blood vessels
Lymphatic Vessels
Exocrine glands - buffers and enzymes
Sub mucosal plexus/Plexus of Meissner Sensory neurons, parasympathetic ganglionic neurons and
sympathetic postganglionic fibers innervating mucosa and
submucosa
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Muscularis Externa
Dominated bysmooth muscle cells
Forms an inner
circular and outerlongitudinal layer
Contraction facilitatesperistalsis and
segmentation
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Muscularis Externa
Enteric Nervous System (ENS) coordinate
movementsinnervated by the ANS
Sympathetic fibers also innervate mucosa and
Myenteric Plexus
Myenteric Plexus lies between circular and
longitudinal muscle
Parasympathetic stimulation increases muscletone and activity
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Serosa/Adventitia
Found along most portions of tract except in theoral cavity, pharynx, esophagus and rectum.
The areas consist of a dense network of
collagen fibers which attaches the digestive tractto the adjacent structures.
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Control of Digestive Functions
Neural Mechanisms
CNS and Enteric Nervous System
Hormonal Mechanisms
Enteroendocrine cells
Local Mechanisms
Release of chemicals in interstitial fluid
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Digestive Systems and Their Functions
Oral Cavity
Pharynx
Esophagus
Stomach
Small Intestine
Large Intestine
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ORAL CAVITY
Functions:
Analysis of material before swallowing
Mechanical processing by teeth, tongueand palate surfaces
Lubrication with mucus and salivary
secretions
Limited digestion of carbohydrates andlipids.
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Salivary Glands
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Salivary Gland
Submandibular Glands
-buffers, mucins & salivary amylase
Sublinguals-contains mucus cells
Parotid Glands
-contains only serous cellssalivary amylase
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Saliva
Functions
Helps keep oral surfaces clean
Moistening and lubricating of mouth and food.
Aids in tasting.
Aids in Swallowing.
Helps in the metabolism of carbohydrate.
Helps to maintain the calcium phosphate matrixof the teeth.
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Four Major Components of Saliva:
1. Mucus
2. Amylase
3. Lingual Lipase
4. Alkaline Electrolyte Solution
Note:
Saliva contains thiocyanates and lysozymes that can attack and destroy the mouth bacteria.
It also contains antibodies that destroy oral bacteria in certain people.
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Control of Salivary Secretions
ANS control
PANS increases secretion
SNS produces small amounts of thick saliva
Other brain stem nuclei & higher centers
Chewing gum
Smelling or thinking about food
Irritating stimuli in esophagus, stomach &intestines
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Pharynx
Review Respiratory system
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Esophagus
Definition
A hollow muscular tube that transfers solid food
and liquids to the stomach.
Extends from cricoid cartilage, along posteriorsurface of trachea, through diaphragm to
stomach
Upper and Lower (Cardiac Sphincter)Esophageal Sphincter Muscles
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Histology of Esophagus
Mucosanonkeratinized stratified squamous
epithelium
Mucosa and submucosa thrown into folds
Submucosa contains mucus secreting glands
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Esophagus
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The Mechanism of Swallowing
Consists of three phases:
Oral Phase
Pharyngeal Phase
Esophageal Phase
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The Mechanism of Swallowing
Oral Phase
Voluntary Phase
Hard palate compresses bolus
Tongue forces bolus into oropharynx
Soft palate elevated
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The Mechanism of Swallowing
Pharyngeal Phase
Bolus enters pharynx
Tactile receptors on palatal arches and uvulastimulated by bolus
Swallowing center of medulla oblongata receivesinformation
Pharyngeal muscles stimulated to contract
Larynx elevates Respiratory centers inhibited
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The Mechanism of Swallowing
Esophageal Phase
Bolus enters esophagus
Bolus pushed to stomach by peristalsis
Cardiac sphincter muscles open Bolus enters stomach
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The Stomach
Functions:
Bulk storage of ingested food.
Mechanical breakdown of ingested food. Disruption of chemical bonds in food.
Production of intrinsic factor (for vitamin B12
absorption).
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The Stomach
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The Stomach
Histology Epitheliumsimple columnar with goblet cells
Gastric pits open onto gastric surface Gastric glands below gastric pits in fundus and
body
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Gastric Glands
Consist of:
Parietal Cells
Intrinsic factor & HCl
Chief Cells -
Pepsinogen
Vit B12
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Parietal Secretion Of HCl
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Hydrochloric Acid
Functions:
Kills microbes.
Denatures proteins and inactivates most
enzymes in food.
Breaks down plant cell walls and connective
tissues in meat.
Provides an acidic environment for activationand function of pepsin.
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Chief Cells
Secrete pepsinogen
Pepsinogen is converted to pepsin in the
gastric lumen by HCl (pH 1.52.0)
Rennin and Gastric Lipase produced in infants
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Pyloric glands
Produce:
Mucus
Hormones by Enteroendocrine cells
Gastrin by G cellsstimulates parietal and chief cellsand promotes gastric mixing
Somatostatinby D cellsinhibits gastrin release;
overridden by neural & hormonal stimuli
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Regulation of Gastric Activity
Controlled by CNS
Regulated by short reflexes of the ENS in wall
of stomach
Regulated by hormones in the digestive tract
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Regulation of Gastric Activity
There are three phases of Gastric Control:
1. Cephalic Phase2. Gastric Phase
3. Intestinal Phase
1. Cephalic Phase
-Begins before food enters the stomach, that is when we see, smell, think ortaste food.
2. Gastric Phase
-The presence of food in the stomach stimulates gastric secretions.
3. Intestinal Phase-Distension and presence of protein fragments in the duodenum trigger the
release of gastrin.
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Regulation of Gastric Activity
Cephalic Phase
Directed by CNS (Medulla)
Lasts one minute
Neural output via PANS and vagus nerves innervatesubmucosal plexus
Postganglionic parasympathetic fibers innervate
mucous cells, chief cells, parietal cells and G cells
Gastric juice production accelerates (500ml/hr)
(Phase affected by emotional states)
R l ti f G t i A ti it
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Regulation of Gastric Activity
Gastric Phase
Initiated bystomach distension, increased
pH in stomach and undigested proteins in
stomach
34 hours
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Mechanisms of Gastric Phase
Neural Response
Stretch receptors and chemoreceptors stimulated
Short reflexes triggered in submucosal andmyenteric plexuses
Postganglionic fibers leaving submucosal plexusstimulate the release of Ach which stimulateschief and parietal cells
Myenteric plexus stimulation produce mixingwaves
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Mechanisms of Gastric Phase
Hormonal Response
Neural stimulation and the presence of peptides
and amino acids stimulate Gastrin secretion
Gastrin accelerates parietal and chief cellsecretion rates
Gastrin stimulates gastric motility
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Mechanisms of Gastric Phase
Local Response
Distention of gastric wall stimulates the release of
histamine in the lamina propria
Histamine binds to receptors on parietal cells
Regulation of Gastric Activity
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Regulation of Gastric Activity
Intestinal phase
Neural Responses
Chyme leaves the stomach relieving stomach
distension
Stretch and chemo- receptors in duodenum stimulated
by the presence of chyme
Enterogastric reflexinhibits central and local
stimulation of gastrin production & gastric
contractions; also stimulating contraction of the pyloric
sphincter Local reflexes stimulate mucus production at the
duodenum
Regulation of Gastric Activity
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Regulation of Gastric Activity
Intestinal phase
Hormonal Responses
Cholecystokinin (CCK) and Gastric Inhibitory
Peptide (GIP) secreted when lipids and
carbohydrates enter the duodenum CCKinhibits acid and pepsin secretion
GIPtargets the pancreas and inhibits gastric
secretion and reduces rate and force of gastric
contractions
Regulation of Gastric Activity
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Regulation of Gastric Activity
Intestinal phase
Hormonal Responses
Secretin secreted when pH drops below 4.5
Secretin inhibits parietal and chief cell activity
Stimulates production of bicarbonate ions by thepancreas and bile by the liver
Regulation of Gastric Activity
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Regulation of Gastric Activity
Intestinal phase
Hormonal Responses
Gastrin is produced by G cells in duodenum when
partially digested proteins enter the duodenum
Accelerates acid and enzyme secretion in stomach
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High fat, high protein meals slow gastric
emptying
Large, low fat, low protein, wine and caffeine
increases gastric emptying Wine and caffeine increases gastric secretion
and motility
DIGESTION AND ABSORPTION IN
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DIGESTION AND ABSORPTION IN
THE STOMACH
Proteins break down in the presence of pepsin to formpolypeptides.
Carbohydrate and lipid digestion continue until pHthroughout the material in the stomach falls below 4.5.
The only substances absorbed by the stomach are fat-soluble (incl. alcohol and aspirin) in small amounts.
NB: The stomach lacks the absorptive surfaces contained inthe small intestine.
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Small Intestines and Accessory Organs
Extends from the pyloricsphincter to the cecum.(Approx. 2.5 cm in diameter &7m long)
Consists of three parts:
DUODENUM
JEJUNUM
ILEUM
NB
-9O% of absorption occurs inthe small intestine.
Enzymes are secreted in thesmall intestines by thepancreas.
These are:
Pancreatic Amylase Pancreatic Lipase
Nucleases
Proteolytic Enzymes
The liver and gall bladdersecretes bile into the smallintestine.
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Histology
Structural features of the Small intestines: Plicae circulares
Villi
Microvilli
Components of the villi: Lamina propria has capillaries that transports gases and
absorbed nutrients to the hepatic portal vein.
Lacteals transport substances too large to diffuse into the
bloodstream eg.Chylomicrons (protein-lipid packages). Muscularis mucosae and smooth muscle of intestinal villi move
villi for maximum absorption of nutrients and squeeze lactealstherefore moving lymph out of villi
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Histology
Structure of Duodenum, Jejunum and
ileum.
Duodenum- has few plicae, numerous villi and
mucous glands Jejunum- has plicae and villi (prominent in its
proximal portion).
Ileum- has scattered villi but lacks plicae
altogether; lymphoid tissue (Peyers Patches) atterminal portion
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Intestinal Secretion
Intestinal secretions contain
Mucus
Brush border enzymes (enterokinase,maltase,
sucrase, lactase, dipeptidases, peptidases) Intestinal secretion is controlled by
Local reflexes
Enterocrinin secretion
Parasympathetic stimulation
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Intestinal Movements
Myenteric reflexes stimulate weak peristaltic
contractions
PANS stimulation accelerates peristalsis and
segmentation Affects short segments of small intestine
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Intestinal Movements
Two reflexes coordinate activities along the
entire length of the small intestine:
Triggered by stretch receptors in stomach
Gastroenteric reflex stimulates motility and secretionalong entire small intestine
Gastroileal reflex stimulates relaxation of ileocecal
valve; enhanced by gastrin secretion
Digestion in the Small Intestine
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g
The Pancreas When acidic chyme enters the duodenum,
pancreatic juice via the pancreatic duct entersas well.
Functions of the pancreas
Endocrine cells of the pancreatic isletsproduce insulin and glucagon
Exocrine cells including acinar and
epithelial duct cells produce pancreaticjuice
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Digestion in the Small Intestine
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Digestion in the Small Intestine
The Pancreas
Pancreatic juice consists of:
Sodium bicarbonate (pH 7.5 -8.8) Buffer solution secreted when chyme enters duodenum
and secretin produced
PANCREATIC ENZYMES: CHOLECYSTOKININ stimulates their production
Digestion in the Small Intestine
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Digestion in the Small Intestine
The Pancreas
PANCREATIC ENZYMES
Pancreatic amylasecarbohydrates to tri- and di-
saccharides
Pancreatic lipaselipids to fatty acids andmonoglycerides
Nucleasesnucleic acids to simple sugars and
nitrogen bases
Digestion in the Small Intestine
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Digestion in the Small Intestine
The Pancreas
PANCREATIC ENZYMES (Proteolytic enzymes)
Trypsinogen, chymotrypsinogen,
procarboxypeptidase and proelastase
In the duodenum enterokinase convertstrypsinogen to trypsin
Trypsin activates the other enzymes to
chymotrypsin, carboxypeptidase and elastase
These enzymes produce dipeptides, tripeptides
and amino acids from proteins
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Th Li
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The Liver
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Metabolic Regulation The liver regulates:
1. Composition of circulating blood
2. Nutrient metabolism
3. Waste product removal
4. Nutrient storage
5. Drug inactivation
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The Liver
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Composition of Circulating BloodAll blood leaving absorptive surfaces of digestive tract
Enters hepatic portal system
Flows into the liver
Liver cells extract nutrients or toxins from blood
Before they reach systemic circulation through hepatic veins
Liver removes and stores excess nutrients
Corrects nutrient deficiencies by mobilizing stored reserves or
performing synthetic activities
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The Liver
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Metabolic Activities of the Liver Carbohydrate metabolism
Lipid metabolism
Amino acid metabolism
Waste product removal
Vitamin storage
Mineral storage
Drug inactivation
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The Liver
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Functions of Hematological Regulation1. Phagocytosis and antigen presentation
2. Synthesis of plasma proteins
3. Removal of circulating hormones
4. Removal of antibodies5. Removal or storage of toxins
6. Synthesis and secretion of bile
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The Liver
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The Functions of Bile Dietary lipids are not water soluble
Mechanical processing in stomach creates large drops
containing lipids
Pancreatic lipase is not lipid soluble Interacts only at surface of lipid droplet
Bile salts break droplets apart (emulsification)
Increases surface area exposed to enzymatic attack
Creates tiny emulsion droplets coated with bile salts
Digestion in the Small Intestine
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Digestion in the Small Intestine
The Liver
Functions of Bile
Emulsification of fatsincrease surface area for
pancreatic lipase to work
Facilitates absorption of lipids by intestinalepithelium
Digestion in the Small Intestine
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ges o e S a es e
The Gallbladder
Stores bile
Concentrates bile
E d f Di ti i S ll I t ti
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End of Digestion in Small Intestines
Brush border Enzymes complete digestion
Maltasemaltose to glucose
Sucrasesucrose to glucose and fructose
Lactaselactose to glucose and galactose Dipeptidases and Peptidasesdipeptides and
tripeptides to amino acids
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Absorption in the Small Intestine
Substances absorbed in the small intestines are: Monosaccharides-absorbed by the duodenum and
upper jejunum; co-transport with sodium
Amino Acids- absorbed by the end of the jejunum;co-transport with sodium and facilitated diffusion
Absorption in the Small Intestine
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Absorption in the Small Intestine
Fatty acids and Monoglycerides
absorbed in the duodenum, jejunum and ileum
diffusion in water-soluble micelles which diffuse into
epithelial cells of villi
Fatty acids and monoglycerides combine to form
triglycerides
Triglycerides combine with cholesterol, lipoprotein and
phospholipids to form chylomicrons
Chylomicrons then diffuse into lacteals
Absorption in the Small Intestine
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Absorption in the Small Intestine
Vitamins
- C and B by passive diffusion
- B12with intrinsic factor active transport
- A,D,E & K with micelles
Large Intestine
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Large Intestine
Parts of the Large Intestine
Cecum Colon - Ascending, Transverse, Descending
and Sigmoid
Rectum
Anal canal
The Large Intestine
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The Large Intestine
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Anal Sphincters
Internal anal sphincter
Circular muscle layer of muscularis externa
Has smooth muscle cells, not under voluntary control
External anal sphincter
Encircles distal portion of anal canal
A ring of skeletal muscle fibers, under voluntary control
The Large Intestine
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The Large Intestine
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Histology of the Large Intestine
Lack villi
Abundance of mucous cells
Presence of distinctive intestinal glands
Are deeper than glands of small intestine
Are dominated by mucous cells
The Large Intestine
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The Large Intestine
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Histology of the Large Intestine
Does not produce enzymes
Provides lubrication for fecal material
Large lymphoid nodules are scattered throughout
the lamina propria and submucosa
The longitudinal layer of the muscularis externa isreduced to the muscular bands of taeniae coli
The Large Intestine
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The Large Intestine
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Figure 2424 The Mucosa and Glands of the Colon.
The Large Intestine
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The Large Intestine
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Absorption in the Large Intestine
Reabsorption of water
Reabsorption of bile salts
In the cecum
Transported in blood to liver
Absorption of vitamins produced by bacteria
Absorption of organic wastes
The Large Intestine
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The Large Intestine
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VitaminsAre organic molecules
Important as cofactors or coenzymes in
metabolism
Normal bacteria in colon make three vitamins that
supplement diet
The Large Intestine
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The Large Intestine
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Three Vitamins Produced in the Large Intestine
1. Vitamin K (fat soluble):
Required by liver for synthesizing four clotting factors,
including prothrombin
2. Biotin (water soluble):
Important in glucose metabolism
3. Pantothenic acid: B5(water soluble):
Required in manufacture of steroid hormones and some
neurotransmitters
The Large Intestine
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The Large Intestine
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Organic Wastes
Bacteria convert bilirubin to urobilinogens and
stercobilinogens
Urobilinogens absorbed into bloodstream are excreted
in urine
Urobilinogens and stercobilinogens in colon convert to
urobilins andstercobilinsby exposure to oxygen
The Large Intestine
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The Large Intestine
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Organic WastesBacteria break down peptides in feces and
generate
Ammonia: as soluble ammonium ions
Indole and skatole:
nitrogen compounds responsible for odor of feces
Hydrogen sulfide:
gas that produces rotten egg odor
The Large Intestine
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The Large Intestine
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Organic WastesBacteria feed on indigestible carbohydrates
(complex polysaccharides)
Produce flatus, or intestinal gas, in large intestine
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Large Intestine
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Large Intestine
Simple columnar epithelium with goblet cells
Mucus
Protects intestinal wall and holds fecal matter
together Contains sodium bicarbonate that neutralizes
acids produced by bacterial metabolism
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Large Intestine
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Large Intestine
Produces large amounts of water andelectrolytes when irritated
Movement of fecal matter is due to peristalsis,
segmentation and contraction of longitudinalmuscle bands
top related