unit 8 -gastrointestinal physiology

8/10/2019 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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Oral Phase

Voluntary Phase

Hard palate compresses bolus

Tongue forces bolus into oropharynx

Soft palate elevated


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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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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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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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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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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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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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Local Response

Distention of gastric wall stimulates the release of

histamine in the lamina propria

Histamine binds to receptors on parietal cells



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Intestinal phase

Neural Responses

Chyme leaves the stomach relieving stomach

distension

Stretch and chemoreceptors 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



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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



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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



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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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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



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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



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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

Copyright 2009

Pearson Education, Inc.,

publishing as Pearson

Metabolic Regulation The liver regulates:

1. Composition of circulating blood

2. Nutrient metabolism

3. Waste product removal

4. Nutrient storage

5. Drug inactivation

Th Li


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The Liver

Copyright 2009



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

Th Li


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The Liver

Copyright 2009



Metabolic Activities of the Liver Carbohydrate metabolism

Lipid metabolism

Amino acid metabolism

Waste product removal

Vitamin storage

Mineral storage

Drug inactivation

Th Li


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The Liver

Copyright 2009



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

Th Li


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The Liver

Copyright 2009



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



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The Liver

Functions of Bile

Emulsification of fatsincrease surface area for

pancreatic lipase to work

Facilitates absorption of lipids by intestinalepithelium



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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



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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



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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

Copyright 2009



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

Copyright 2009



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

Copyright 2009



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

Copyright 2009



Figure 2424 The Mucosa and Glands of the Colon.

The Large Intestine


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The Large Intestine

Copyright 2009



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

Copyright 2009



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

Copyright 2009



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

Copyright 2009



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

Copyright 2009



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

Copyright 2009



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

unit 8 -gastrointestinal physiology

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