Albert Grazia, M.S., N.D. 1 Chapter 24 Digestion Albert Grazia, M.S., N.D. (516) 486-8332 .

Albert Grazia, M.S., N.D. www.naturedoc.info

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Chapter 24Digestion

Albert Grazia, M.S., N.D.

(516) 486-8332

www.naturedoc.info


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Chapter 24The Digestive System

• Structure – Gross Anatomy

– Histology

• Function– Mechanical

– Chemical

• Development• Disorders


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• The digestive system consists of a tubular structure (Alimentary canal) designed for the digestion and absorption of food as well as the expulsion of unusable ingested materials.

• This system also has accessory structures that play roles in other systems of the body as well as the digestive system.


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Functions of the digestive system:

• 1. Ingest the food• 2. Break it down into small

molecules that can cross plasma membranes

• 3. Absorb these nutrient molecules

• 4. Eliminate non-digestible wastes


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Our digestive system goes through 9 basic steps:

• 1. Teeth- Digestion starts here.

• The job of the teeth is to start tearing and crushing the food down into small enough pieces so that it can fit down our throats.


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2. Saliva- The salivary gland in located underneath the back of our tongue. It creates our saliva or spit.

This helps soften the food in the mouth so that it is easier to swallow.

Saliva is also the first of several chemicals that start to break down foods into simpler forms.


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• 3. Tongue- The tongue is a muscle that works with the food and saliva to form a "ball" that can be swallowed.

• Of course, the tongue also contains taste buds that helps us tell the difference between salty, sour, sweet, and bitter foods.


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• 4. Esophagus- The esophagus is simply a transportation tube from the mouth to the stomach.

• When we swallow, what we are really doing is closing a trap door in our throat called the epiglottis.

• This sends food down the esophagus and prevents food from going down the trachea (or windpipe) and into our lungs.

• Food moves down the esophagus using muscles not gravity.


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• 5. Stomach- The first stop after the esophagus is the stomach.

• Once the food gets to the stomach the stomach uses chemicals to try to make the food tinier.

• These chemicals are called gastric juices and the include hydrochloric acid and enzymes (chemicals that break down food).

• The food is moved around in the stomach and mixed with the chemicals for about 3 or 4 hours.

• When it is done in the stomach, the food is now a cream-like liquid call chyme.

• The food is still not small enough the get into our blood stream and it has not provided the body with anything useful yet.


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• 6. Liver/Gall Bladder- At this point, our food is hit with more chemicals.

• The liver makes a chemical called bile but bile is not stored in the liver. Instead it is stored in the gall bladder.

• When the gall bladder mixes bile with our food, it does an important job: breaking down the fat (from milk, butter, cheeses) into tiny droplets.

• This fat will supply us with much energy later.


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7. Pancreas- The pancreas also adds a digestive chemical as the food leaves the stomach.

This digestive juice works on breaking down the carbohydrates (from breads, potatoes, etc.) and the proteins (from meats, cereals, peanut butter)


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8. Small Intestine- The small intestine is the real hero of the digestive system.

The small intestine is a tube that is about 22 feet long! This is where the real digestion takes place. As the food passes through, it is mixed with the new chemicals and soon our "food" is now digested small enough to be put to use by the body.

Along the walls of the intestine are thousands of tiny fingers called villi. Blood vessels (capillaries) in the villi can absorb the tiny food molecules and send them off to the rest of our body through the blood.


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• 9. Large Intestine- Whatever the body cannot put to use is sent to the large intestine.

• Many plants, for example, contain cellulose which cannot be digested.

• The big job of the large intestine is to remove water. Water has been necessary up until now but it is no longer needed and in the large intestine water is sent into the blood stream .

• Food spends about 12 hours in the large intestine where it become feces and later leaves the body through the anal opening when we go to the bathroom.


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Overview of GI tract Functions

• Mouth---bite, chew, swallow

• Pharynx and esophagus----transport

• Stomach----mechanical disruption; absorption of water & alcohol

• Small intestine--chemical & mechanical digestion & absorption

• Large intestine----absorb electrolytes & vitamins (B and K)

• Rectum and anus---defecation


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Layers of the GI Tract

1. Mucosal layer

2. Submucosal layer

3. Muscularis layer

4. Serosa layer


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Mucosa• Epithelium

– stratified squamous(in mouth,esophagus & anus) = tough

– simple columnar in the rest• secretes enzymes and absorbs nutrients

• specialized cells (goblet) secrete mucous onto cell surfaces

• enteroendocrine cells---secrete hormones controlling organ function

• Lamina propria– thin layer of loose connective tissue

– contains BV and lymphatic tissue

• Muscularis mucosae---thin layer of smooth muscle

– causes folds to form in mucosal layer

– increases local movements increasing absorption with exposure to “new” nutrients


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Submucosa

• Loose connective tissue– containing BV, glands and lymphatic tissue

• Meissner’s plexus---– parasympathetic

– innervation• vasoconstriction

• local movement by muscularis mucosa smooth muscle


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Muscularis • Skeletal muscle = voluntary control

– in mouth, pharynx , upper esophagus and anus– control over swallowing and defecation

• Smooth muscle = involuntary control– inner circular fibers & outer longitudinal fibers– mixes, crushes & propels food along by peristalsis

• Auerbach’s plexus (myenteric)--– both parasympathetic & sympathetic innervation of

circular and longitudinal smooth muscle layers


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Serosa

• An example of a serous membrane

• Covers all organs and walls of cavities not open to the outside of the body

• Secretes slippery fluid

• Consists of connective tissue covered with simple squamous epithelium


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Peritoneum

• Peritoneum– visceral layer covers

organs

– parietal layer lines the walls of body cavity

• Peritoneal cavity– potential space

containing a bit of serous fluid


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Parts of the Peritoneum

• Mesentery• Mesocolon• Lesser omentum• Greater omentum• Peritonitis =

inflammation– trauma

– rupture of GI tract

– appendicitis

– perforated ulcer


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Peritonitis

• Acute inflammation of the peritoneum

• Cause– contamination by infectious microbes during

surgery or from rupture of abdominal organs


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Greater Omentum, Mesentery & Mesocolon


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Lesser Omentum


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Mouth

• Lips and cheeks-----contains buccinator muscle that keeps food between upper & lower teeth

• Vestibule---area between cheeks and teeth• Oral cavity proper---the roof = hard, soft palate and uvula

– floor = the tongue


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Pharyngeal Arches

• Two skeletal muscles• Palatoglossal muscle

– extends from palate to tongue

– forms the first arch

– posterior limit of the mouth

• Palatopharyngeal muscle– extends from palate to

pharyngeal wall

– forms the second arch

– behind the palatine tonsil


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Salivary Glands

• Parotid below your ear and over the masseter• Submandibular is under lower edge of mandible• Sublingual is deep to the tongue in floor of mouth• All have ducts that empty into the oral cavity


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Composition and Functions of Saliva

• Wet food for easier swallowing

• Dissolves food for tasting

• Bicarbonate ions buffer acidic foods– bulemia---vomiting hurts the enamel on your teeth

• Chemical digestion of starch begins with enzyme (salivary amylase)

• Enzyme (lysozyme) ---helps destroy bacteria

• Protects mouth from infection with its rinsing action---1 to 1 and 1/2qts/day


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Salivary Gland Cellular Structure• Cells in acini (clusters)• Serous cells secrete a watery fluid• Mucous cells (pale staining) secrete a slimy, mucus

secretion


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Salivation

• Increase salivation– sight, smell, sounds, memory of food, tongue

stimulation---rock in mouth– cerebral cortex signals the salivatory nuclei in

brainstem---(CN 7 & 9)– parasympathetic nn. (CN 7 & 9)

• Stop salivation – dry mouth when you are afraid– sympathetic nerves


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Mumps• Myxovirus that attacks the parotid gland

• Symptoms– inflammation and enlargement of the parotid– fever, malaise & sour throat (especially

swallowing sour foods)– swelling on one or both sides

• Sterility rarely possible in males with testicular involvement (only one side involved)

• Vaccine available since 1967


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Structure and Function of the Tongue

• Muscle of tongue is attached to hyoid, mandible, hard palate and styloid process

• Papillae are the bumps---taste buds are protected by being on the sides of papillae


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Tooth Structure

• Crown• Neck• Roots• Pulp cavity


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Composition of Teeth• Enamel

– hardest substance in body

– calcium phosphate or carbonate

• Dentin– calcified connective

tissue

• Cementum– bone-like

– periodontal ligament penetrates it

What is the gingiva?


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Dentition

• Primary or baby teeth– 20 teeth that start erupting at 6 months– 1 new pair of teeth per month

• Permanent teeth– 32 teeth that erupt between 6 and 12 years of age– differing structures indicate function

• incisors for biting

• canines or cuspids for tearing

• premolars & molars for crushing and grinding food


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Primary and Secondary Dentition


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Digestion in the Mouth• Mechanical digestion (mastication or chewing)

• breaks into pieces

• mixes with saliva so it forms a bolus

• Chemical digestion– amylase

• begins starch digestion at pH of 6.5 or 7.0 found in mouth

• when bolus & enzyme hit the pH 2.5 gastric juices hydrolysis ceases

– lingual lipase• secreted by glands in tongue

• begins breakdown of triglycerides into fatty acids and glycerol


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Pharynx• Funnel-shaped tube extending from internal

nares to the esophagus (posteriorly) and larynx (anteriorly)

• Skeletal muscle lined by mucous membrane• Deglutition or swallowing is facilitated by saliva

and mucus– starts when bolus is pushed into the oropharynx– sensory nerves send signals to deglutition center in

brainstem– soft palate is lifted to close nasopharynx– larynx is lifted as epiglottis is bent to cover glottis


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Esophagus

• Collapsed muscular tube

• In front of vertebrae

• Posterior to trachea

• Posterior to the heart

• Pierces the diaphragm at hiatus– hiatal hernia or

diaphragmatic hernia


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Histology of the Esophagus

• Mucosa = stratified squamous• Submucosa = large mucous glands• Muscularis = upper 1/3 is skeletal, middle is mixed,

lower 1/3 is smooth– upper & lower esophageal sphincters are prominent

circular muscle

• Adventitia = connective tissue blending with surrounding connective tissue--no peritoneum


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Physiology of the Esophagus - Swallowing

• Voluntary phase---tongue pushes food to back of oral cavity

• Involuntary phase----pharyngeal stage– breathing stops & airways are closed

– soft palate & uvula are lifted to close off nasopharynx

– vocal cords close

– epiglottis is bent over airway as larynx is lifted


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Swallowing

• Upper sphincter relaxes when larynx is lifted

• Peristalsis pushes food down– circular fibers behind bolus

– longitudinal fibers in front of bolus shorten the distance of travel

• Travel time is 4-8 seconds for solids and 1 sec for liquids

• Lower sphincter relaxes as food approaches


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Gastroesophageal Reflex Disease• If lower sphincter fails to open

– distension of esophagus feels like chest pain or heart attack

• If lower esophageal sphincter fails to close– stomach acids enter esophagus & cause heartburn (GERD)

– for a weak sphincter---don't eat a large meal and lay down in front of TV

– smoking and alcohol make the sphincter relax worsening the situation

• Control the symptoms by avoiding– coffee, chocolate, tomatoes, fatty foods, onions & mint


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Anatomy of Stomach• Which side is it on? • Size when empty?

– large sausage– stretches due to rugae

• Parts of stomach– cardia– fundus---air in x-ray– body– pylorus---starts to narrow as approaches pyloric

sphincter

• Empties as small squirts of chyme leave the stomach through the pyloric valve


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The stomach has five major functions;

•Temporary food storage

•Control the rate at which food enters the duodenum

•Acid secretion and antibacterial action

•Fluidisation of stomach contents

•Preliminary digestion with pepsin, lipases etc.

http://www.le.ac.uk/pathology/teach/va/anatomy/case6/6_3a.html


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Pylorospasm and Pyloric Stenosis

• Abnormalities of the pyloric sphincter in infants

• Pylorospasm– muscle fibers of sphincter fail to relax trapping

food in the stomach – vomiting occurs to relieve pressure

• Pyloric stenosis– narrowing of sphincter indicated by projectile

vomiting– must be corrected surgically


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Histology of the Stomach


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Mucosa & Gastric Glands• Hydrochloric acid

converts pepsinogen from chief cell to pepsin

• Intrinsic factor– absorption of vitamin B12

for RBC production

• Gastrin hormone (g cell)– “get it out of here”

• release more gastric juice

• increase gastric motility

• relax pyloric sphincter

• constrict esophageal sphincter preventing entry


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Submucosa


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Muscularis

• Three layers of smooth muscle--outer longitudinal, circular & inner oblique

• Permits greater churning & mixing of food with gastric juice


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Serosa

• Simple squamous epithelium over a bit of connective tissue

• Also known as visceral peritoneum


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Mucosal layer on the surface is made up simple columnar cells and a mucosal muscularis on the deep side .

Submucosa contains fibrous connective tissue and blood vessels.

The muscularis externa is made up of a circular and a longitudinal muscle layer with a myenteric plexus in between the layers. A very thin layer of

Serosa is also present .


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Physiology--Mechanical Digestion

• Gentle mixing waves – every 15 to 25 seconds– mixes bolus with 2 quarts/day of gastric juice to

turn it into chyme (a thin liquid)

• More vigorous waves – travel from body of stomach to pyloric region

• Intense waves near the pylorus – open it and squirt out 1-2 teaspoons full with

each wave


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Physiology--Chemical Digestion• Protein digestion begins

– HCl denatures (unfolds) protein molecules– HCl transforms pepsinogen into pepsin that breaks

peptides bonds between certain amino acids

• Fat digestion continues– gastric lipase splits the triglycerides in milk fat

• most effective at pH 5 to 6 (infant stomach)

• HCl kills microbes in food• Mucous cells protect stomach walls from being

digested with 1-3mm thick layer of mucous


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Regulation of Gastric Secretion and Motility

• Cephalic phase

• Gastric phase

• Intestinal phase


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Cephalic Phase = “Stomach Getting Ready”

• Cerebral cortex =sight, smell, taste & thought – stimulate parasympathetic nervous system

• Vagus nerve– increases stomach muscle and glandular

activity


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Gastric Phase = “Stomach Working”• Nervous control keeps stomach active

– stretch receptors & chemoreceptors provide information– vigorous peristalsis and glandular secretions continue– chyme is released into the duodenum

• Endocrine influences over stomach activity– distention and presence of caffeine or protein cause G

cells secretion of gastrin into bloodstream– gastrin hormone increases stomach glandular secretion– gastrin hormone increases stomach churning and

sphincter relaxation


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Intestinal Phase = “Stomach Emptying”

• Stretch receptors in duodenum slow stomach activity & increase intestinal activity

• Distension, fatty acids or sugar signals medulla – sympathetic nerves slow stomach activity

• Hormonal influences – secretin hormone decreases stomach secretions

– cholecystokinin(CCK) decreases stomach emptying

– gastric inhibitory peptide(GIP) decreases stomach secretions, motility & emptying


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Absorption of Nutrients by the Stomach

• Water especially if it is cold• Electrolytes• Some drugs (especially aspirin) & alcohol• Fat content in the stomach slows the passage of alcohol to

the intestine where absorption is more rapid • Gastric mucosal cells contain alcohol dehydrogenase that

converts some alcohol to acetaldehyde-----more of this enzyme found in males than females

• Females have less total body fluid that same size male so end up with higher blood alcohol levels with same intake of alcohol


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Regulation of Gastric Emptying• Release of chyme is regulated by neural

and hormonal reflexes

• Distention & stomach contents increase secretion of gastrin hormone & vagal nerve impulses

– stimulate contraction of esophageal sphincter and stomach and relaxation of pyloric sphincter

• Enterogastric reflex regulates amount released into intestines

– distension of duodenum & contents of chyme

– sensory impulses sent to the medulla inhibit parasympathetic stimulation of the stomach but increase secretion of cholecystokinin and stimulate sympathetic impulses

– inhibition of gastric emptying


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Vomiting (emesis)

• Forceful expulsion of contents of stomach & duodenum through the mouth

• Cause– irritation or distension of stomach

– unpleasant sights, general anesthesia, dizziness & certain drugs

• Sensory input from medulla cause stomach contraction & complete sphincter relaxation

• Contents of stomach squeezed between abdominal muscles and diaphragm and forced through open mouth

• Serious because loss of acidic gastric juice can lead to alkalosis


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Anatomy of the Pancreas

• 5" long by 1" thick• Head close to curve in C-

shaped duodenum• Main duct joins common

bile duct from liver • Sphincter of Oddi on

major duodenal papilla• Opens 4" below pyloric

sphincter


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Histology of the Pancreas

• Acini- dark clusters – 99% of gland

– produce pancreatic juice

• Islets of Langerhans– 1% of gland

– pale staining cells

– produce hormones


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Composition and Functions of Pancreatic Juice

• 1 & 1/2 Quarts/day at pH of 7.1 to 8.2• Contains water, enzymes & sodium bicarbonate• Digestive enzymes

– pancreatic amylase, pancreatic lipase, proteases– trypsinogen---activated by enterokinase (a brush border enzyme)– chymotrypsinogen----activated by trypsin– procarboxypeptidase---activated by trypsin– proelastase---activated by trypsin– trypsin inhibitor---combines with any trypsin produced inside

pancreas

– ribonuclease----to digest nucleic acids– deoxyribonuclease


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Pancreatitis

• Pancreatitis---inflammation of the pancreas occurring with the mumps

• Acute pancreatitis---associated with heavy alcohol intake or biliary tract obstruction– result is patient secretes trypsin in the pancreas &

starts to digest himself


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Regulation of Pancreatic Secretions• Secretin

– acidity in intestine causes increased sodium bicarbonate release

• GIP– fatty acids & sugar

causes increased insulin release

• CCK– fats and proteins cause

increased digestive enzyme release


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Anatomy of the Liver and Gallbladder

• Liver– weighs 3 lbs.

– below diaphragm

– right lobe larger

– gallbladder on right lobe

– size causes right kidney to be lower than left

• Gallbladder– fundus, body & neck


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Histology of the Gallbladder

• Simple columnar epithelium• No submucosa• Three layers of smooth muscle• Serosa or visceral peritoneum


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• The gallbladder is a muscular sac located under the liver. It stores and concentrates the bile produced in the liver that is not immediately needed for digestion. Bile is released from the gallbladder into the small intestine in response to food. The pancreatic duct joins the common bile duct at the small intestine adding enzymes to aid in digestion.


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Bile Production

• One quart of bile/day is secreted by the liver– yellow-green in color & pH 7.6 to 8.6

• Components– water & cholesterol– bile salts = Na & K salts of bile acids – bile pigments (bilirubin) from hemoglobin molecule

• globin = a reuseable protein

• heme = broken down into iron and bilirubin


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Pathway of Bile Secretion

• Bile capillaries

• Hepatic ducts connect to form common hepatic duct

• Cystic duct from gallbladder & common hepatic duct join to form common bile duct

• Common bile duct & pancreatic duct empty into duodenum


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Regulation of Bile Secretion


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Histology of the Liver

• Hepatocytes arranged in lobules • Sinusoids in between hepatocytes

are blood-filled spaces • Kupffer cells phagocytize microbes

& foreign matter


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Flow of Fluids Within the Liver


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Blood Supply to the Liver• Hepatic portal vein

– nutrient rich blood from stomach, spleen & intestines

• Hepatic artery from branch off the aorta


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Liver Functions--Carbohydrate Metabolism

• Turn proteins into glucose

• Turn triglycerides into glucose

• Turn excess glucose into glycogen & store in the liver

• Turn glycogen back into glucose as needed


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Liver Functions --Lipid Metabolism

• Synthesize cholesterol

• Synthesize lipoproteins----HDL and LDL(used to transport fatty acids in bloodstream)

• Stores some fat

• Breaks down some fatty acids


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Liver Functions--Protein Metabolism

• Deamination = removes NH2 (amine group) from amino acids so can use what is left as energy source

• Converts resulting toxic ammonia (NH3) into urea for excretion by the kidney

• Synthesizes plasma proteins utilized in the clotting mechanism and immune system

• Convert one amino acid into another


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Other Liver Functions• Detoxifies the blood by removing or altering drugs &

hormones(thyroid & estrogen)• Removes the waste product--bilirubin• Releases bile salts help digestion by emulsification• Stores fat soluble vitamins-----A, B12, D, E, K• Stores iron and copper• Phagocytizes worn out blood cells & bacteria• Activates vitamin D (the skin can also do this with 1 hr

of sunlight a week)


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Liver Detoxification• One of the liver's primary functions is filtering the

blood. Almost 2 quarts of blood pass through the liver every minute for detoxification. Filtration of toxins is absolutely critical as the blood from the intestines contains high levels of bacteria, bacterial endotoxins, antigen-antibody complexes, and various other toxic substances.

• When working properly, the liver clears 99% of the bacteria and other toxins during the first pass.

• However, when the liver is damaged, such as in alcoholics, the passage of toxins increases by over a factor of 10.


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• The liver's second detoxification process involves the synthesis and secretion of bile. Each day the liver manufactures approximately 1 quart of bile, which serves as a carrier in which many toxic substances are dumped into the intestines.

• In the intestines, the bile and its toxic load are absorbed by fiber and excreted.

• However, a diet low in fiber results in inadequate binding and reabsorption of the toxins.

• This problem is magnified when bacteria in the intestine modify these toxins to more damaging forms.


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• However, when the excretion of bile is inhibited (i.e. cholestasis from gallstones in bile duct), toxins stay in the liver longer.

• Another common cause of cholestasis and impaired liver function is alcohol ingestion. In some sensitive individuals, as little as 1 ounce of alcohol can produce damage to the liver, which results in fat being deposited within the liver.

• All active alcoholics demonstrate fatty infiltration of the liver.


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• The liver utilizes a two-step enzymatic detoxification process for the neutralization of unwanted chemical compounds.

• These not only include drugs, pesticides, and toxins from the gut, but also normal body chemicals such as hormones and inflammatory chemicals (e.g. histamine) which become toxic if allowed to build up.


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Phase I Detox Pathway• Phase I enzymes directly neutralize some

chemicals, but most are converted to intermediate forms that are then processed by phase II enzymes.

• These intermediate forms are much more chemically active and therefore more toxic.

• If the phase II detoxification systems are not working adequately, these intermediates can cause substantial damage, including the initiation of carcinogenic processes.


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• Phase I detoxification of most chemical toxins involves a group of enzymes which, collectively, have been named cytochrome P450. Some 50-100 enzymes make up the cytochrome P450 system.

• Since the activity of cytochrome P450 varies from person to person, so does an individual's risk for various diseases, such as cancer.


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Phase II Detox pathway

• Phase II detoxification typically involves conjugation in which various enzymes in the liver attach small chemicals to the toxin.

• This conjugation reaction either neutralizes the toxin or makes the toxin more easily excreted through the urine or bile.

• Phase II enzymes act on some toxins directly, while others must first be activated by the phase I enzymes.


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• Another potential problem occurs because the toxins transformed into activated intermediates by phase I are substantially more reactive. Unless quickly removed from the body by phase II detoxification mechanisms, they can cause widespread problems, especially carcinogenesis.

• Therefore, the rate at which phase I produces activated intermediates must be balanced by the rate at which phase II finishes their processing.

• People with a very active phase I detoxification system coupled with slow or inactive phase II enzymes are termed pathological detoxifiers.

• These people may suffer unusually severe toxic reactions to environmental poisons.


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• An imbalance between phase I and phase II can also occur when a person is exposed to large amounts of toxins or exposed to toxins for a long period of time.

• In these situations, the critical nutrients needed for phase II detoxification are depleted, which allows the highly toxic activated intermediates to build up.


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Summary of Digestive Hormones• Gastrin

– stomach, gastric & ileocecal sphincters

• Gastric inhibitory peptide--GIP– stomach & pancreas

• Secretin– pancreas, liver & stomach

• Cholecystokinin--CCK– pancreas, gallbladder, sphincter of Oddi, &

stomach


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Anatomy of the Small Intestine

• 20 feet long----1 inch in diameter

• Large surface area for majority of absorption

• 3 parts– duodenum---10 inches– jejunum---8 feet– ileum---12 feet

• ends at ileocecal valve


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Histology of Small Intestine


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Histology of the Small Intestine• Structures that increase surface area

– plica circularis• permanent ½ inch tall folds that contain part of

submucosal layer• not found in lower ileum• can not stretch out like rugae in stomach

– villi• 1 Millimeter tall• Core is lamina propria of mucosal layer• Contains vascular capillaries and lacteals(lymphatic

capillaries)

– microvilli• cell surface feature known as brush border


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Functions of Microvilli

• Absorption and digestion

• Digestive enzymes found at cell surface on microvilli

• Digestion occurs at cell surfaces

• Significant cell division within intestinal glands produces new cells that move up

• Once out of the way---rupturing and releasing their digestive enzymes & proteins


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Cells of Intestinal Glands

• Absorptive cell• Goblet cell• Enteroendocrine

– secretin

– cholecystokinin

– gastric inhibitory peptide

• Paneth cells– secretes lysozyme


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Hormone RegulatorsThe hormones that control digestion are gastrin, secretin, and cholecystokinin (CCK):

• Gastrin causes the stomach to produce an acid for dissolving and digesting some foods. It is also necessary for the normal growth of the lining of the stomach, small intestine, and colon.

• Secretin causes the pancreas to send out a digestive juice that is rich in bicarbonate. It stimulates the stomach to produce pepsin, an enzyme that digests protein, and it also stimulates the liver to produce bile.

• CCK causes the pancreas to grow and to produce the enzymes of pancreatic juice, and it causes the gallbladder to empty.


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Nerve Regulators

• Acetylcholine causes the muscle of the digestive organs to squeeze with more force and increase the "push" of food and juice through the digestive tract. Acetylcholine also causes the stomach and pancreas to produce more digestive juice.

• Adrenaline relaxes the muscle of the stomach and intestine and decreases the flow of blood to these organs.


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Goblet Cells of GI epithelium

Unicellular glands that are part of simple columnar epithelium


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Roles of Intestinal Juice & Brush-Border Enzymes

• Submucosal layer has duodenal glands – secretes alkaline mucus

• Mucosal layer contains intestinal glands = Crypts of Lieberkuhn(deep to surface)– secretes intestinal juice

• 1-2 qt./day------ at pH 7.6

– brush border enzymes

– paneth cells secrete lysozyme kills bacteria


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Mechanical Digestion in the Small Intestine

• Weak peristalsis in comparison to the stomach---chyme remains for 3 to 5 hours

• Segmentation---local mixing of chyme with intestinal juices---sloshing back & forth


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Chemical Digestion in Small Intestine

• Chart page 853--groups enzymes by region where they are found

• Need to trace breakdown of nutrients– carbohydrates– proteins– lipids


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Digestion of Carbohydrates

• Mouth---salivary amylase

• Esophagus & stomach---nothing happens

• Duodenum----pancreatic amylase

• Brush border enzymes (maltase, sucrase & lactase) act on disaccharides– produces monosaccharides--fructose, glucose &

galactose– lactose intolerance (no enzyme; bacteria

ferment sugar)--gas & diarrhea


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Lactose Intolerance

• Mucosal cells of small intestine fail to produce lactase– essential for digestion of lactose sugar in milk– undigested lactose retains fluid in the feces– bacterial fermentation produces gases

• Symptoms– diarrhea, gas, bloating & abdominal cramps

• Dietary supplements are helpful


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Digestion of Proteins

• Stomach– HCl denatures or unfolds proteins– pepsin turns proteins into peptides

• Pancreas– digestive enzymes---split peptide bonds between

different amino acids– brush border enzymes-----aminopeptidase or

dipeptidase------split off amino acid at amino end of molecule or split dipeptide


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Digestion of Lipids

• Mouth----lingual lipase

• Small intestine– emulsification by bile– pancreatic lipase---splits into fatty acids &

monoglyceride– no enzymes in brush border


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• The gut produces chylomicrons following the absorption of fat.

• The TG component of chylomicrons is removed by lipoprotein lipase located on the vascular endothelium of muscle, myocardium, and adiposetissue.

• The resulting chylomicron remnants are cleared from the circulation by hepatic receptors that recognize apolipoprotein (Apo) E.

• The liver exports TG into the circulation in the core of VLDL particles.


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Digestion of Nucleic Acids

• Pancreatic juice contains 2 nucleases– ribonuclease which digests RNA– deoxyribonuclease which digests DNA

• Nucleotides produced are further digested by brush border enzymes (nucleosidease and phosphatase)– pentose, phosphate & nitrogenous bases

• Absorbed by active transport


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Regulation of Secretion & Motility

• Enteric reflexes that respond to presence of chyme– increase intestinal motility– VIP (vasoactive intestinal polypeptide) stimulates the

production of intestinal juice– segmentation depends on distention which sends

impulses to the enteric plexus & CNS• distention produces more vigorous peristalsis• 10 cm per second

• Sympathetic impulses decrease motility


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Absorption in Small Intestine


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Absorption of Monosaccharides

• Absorption into epithelial cell– glucose & galactose----sodium symporter(active transport)

– fructose-----facilitated diffusion

• Movement out of epithelial cell into bloodstream– by facilitated diffusion


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Absorption of Amino Acids & Dipeptides

• Absorption into epithelial cell– active transport with Na+ or H+ ions (symporters)

• Movement out of epithelial cell into blood– diffusion


118

Absorption of Lipids • Small fatty acids enter cells & then blood by simple diffusion

• Larger lipids exist only within micelles (bile salts coating)

• Lipids enter cells by simple diffusion leaving bile salts behind in gut

• Bile salts reabsorbed into blood & reformed into bile in the liver

• Fat-soluble vitamins are enter cells since were within micelles


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Absorption of Lipids (2)

• Inside epithelial cells fats are rebuilt and coated with protein to form chylomicrons

• Chylomicrons leave intestinal cells by exocytosis into a lacteal

– travel in lymphatic system to reach veins near the heart

– removed from the blood by the liver and fat tissue


120

Absorption of Electrolytes• Sources of electrolytes

– GI secretions & ingested foods and liquids

• Enter epithelial cells by diffusion & secondary active transport– sodium & potassium move = Na+/K+ pumps (active transport)

– chloride, iodide and nitrate = passively follow

– iron, magnesium & phosphate ions = active transport

• Intestinal Ca+ absorption requires vitamin D & parathyroid hormone


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Absorption of Vitamins

• Fat-soluble vitamins – travel in micelles & are absorbed by simple

diffusion

• Water-soluble vitamins– absorbed by diffusion

• B12 combines with intrinsic factor before it is transported into the cells– receptor mediated endocytosis


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Absorption of Water

• 9 liters of fluid dumped into GI tract each day

• Small intestine reabsorbs 8 liters

• Large intestine reabsorbs 90% of that last liter

• Absorption is by osmosis through cell walls into vascular capillaries inside villi


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Anatomy of Large Intestine

• 5 feet long by 2½ inches in diameter• Ascending & descending colon are retroperitoneal• Cecum & appendix • Rectum = last 8 inches of GI tract anterior to the sacrum & coccyx• Anal canal = last 1 inch of GI tract

– internal sphincter----smooth muscle & involuntary – external sphincter----skeletal muscle & voluntary control


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Appendicitis• Inflammation of the appendix due to blockage

of the lumen by chyme, foreign body, carcinoma, stenosis, or kinking

• Symptoms– high fever, elevated WBC count, neutrophil count

above 75%– referred pain, anorexia, nausea and vomiting– pain localizes in right lower quadrant

• Infection may progress to gangrene and perforation within 24 to 36 hours


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Histology of Large Intestine

• Mucosa– smooth tube -----no villi or plica– intestinal glands fill the the mucosa– simple columnar cells absorb water & goblet cells secrete mucus

• Submucosal & mucosa contain lymphatic nodules


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Histology of Large Intestine

• Muscular layer– internal circular layer is normal

– outer longitudinal muscle• taeniae coli = shorter bands

• haustra (pouches) formed

• epiploic appendages

• Serosa = visceral peritoneum

• Appendix

– contains large amounts of lymphatic tissue


127

Mechanical Digestion in Large Intestine

• Smooth muscle = mechanical digestion• Peristaltic waves (3 to 12 contractions/minute)

– haustral churning----relaxed pouches are filled from below by muscular contractions (elevator)

– gastroilial reflex = when stomach is full, gastrin hormone relaxes ileocecal sphincter so small intestine will empty and make room

– gastrocolic reflex = when stomach fills, a strong peristaltic wave moves contents of transverse colon into rectum


128

Chemical Digestion in Large Intestine

• No enzymes are secreted only mucous

• Bacteria ferment– undigested carbohydrates into carbon dioxide

& methane gas– undigested proteins into simpler substances

(indoles)----odor– turn bilirubin into simpler substances that

produce color

• Bacteria produce vitamin K and B in colon


129

Absorption & Feces Formation in the Large Intestine

• Some electrolytes---Na+ and Cl-

• After 3 to 10 hours, 90% of H2O has been removed from chyme

• Feces are semisolid by time reaches transverse colon

• Feces = dead epithelial cells, undigested food such as cellulose, bacteria (live & dead)


130

Defecation

• Gastrocolic reflex moves feces into rectum

• Stretch receptors signal sacral spinal cord

• Parasympathetic nerves contract muscles of rectum & relax internal anal sphincter

• External sphincter is voluntarily controlled


131

Defecation Problems• Diarrhea = chyme passes too quickly through

intestine – H20 not reabsorbed

• Constipation--decreased intestinal motility– too much water is reabsorbed– remedy = fiber, exercise and water


132

Dietary Fiber

• Insoluble fiber– woody parts of plants (wheat bran, vegie skins)– speeds up transit time & reduces colon cancer

• Soluble fiber– gel-like consistency = beans, oats, citrus white

parts, apples– lowers blood cholesterol by preventing

reabsorption of bile salts so liver has to use cholesterol to make more


133

Development of the Digestive System

• Endoderm forms primitive gut with help from the splanchnic mesoderm --- resulting tube is made up of epithelial, glandular, muscle & connective tissue

• Differentiates into foregut, midgut & hindgut• Endoderm grows into the mesoderm to form salivary glands, liver,

gallbladder & pancreas


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Development of the Digestive System

• Stomodeum develops into oral cavity– oral membrane

ruptures

• Proctodeum develops into anus– cloacal membrane

ruptures


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Aging and the Digestive System• Changes that occur

– decreased secretory mechanisms

– decreased motility

– loss of strength & tone of muscular tissue

– changes in neurosensory feedback

– diminished response to pain & internal stimuli

• Symptoms– sores, loss of taste, peridontal disease, difficulty swallowing,

hernia, gastritis, ulcers, malabsorption, jaundice, cirrhosis, pancreatitis, hemorrhoids and constipation

• Cancer of the colon or rectum is common


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Diseases of the GI Tract• Dental caries and periodontal disease • Peptic Ulcers• Diverticulitis• Colorectal cancer• Hepatitis• Anorexia nervosa


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• Cancer of the Liver - The most common primary malignant tumor of the liver is an hepatocellular carcinoma. Chronic carriers of hepatitis B virus, particularly those with chronic hepatitis or cirrhosis, are at substantially increased risk of developing hepatocellular carcinoma.

• Recent research also indicates that patients who have long- standing chronic hepatitis C virus infection are also at increased risk for the development of hepatocellular carcinoma.


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• Chronic Hepatitis - an ongoing injury to the cells of the liver with inflammation which lasts for longer than six months.

• Causes of chronic hepatitis are viruses, metabolic or immunologic abnormalities and medications.

• Signs and symptoms may include fatigue, mild discomfort in the upper abdomen, loss of appetite and aching joints.


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• Cirrhosis - a group of chronic liver diseases in which normal liver cells are damaged and replaced by scar tissue, decreasing the amount of normal liver tissue.

• Cirrhosis and other liver diseases take the lives of over 25,000 Americans each year and rank eighth as a cause of death in the United States.

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Albert Grazia, M.S., N.D. 1 Chapter 24 Digestion Albert Grazia, M.S., N.D. (516) 486-8332 .

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