Download - Digestion Chabner 4
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Digestive System
alimentary canal and accessory organs
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Basic Functions
Ingestion, mastication, deglutition - eating
Movement of food along the GI - peristalsis
Digestion - chemical and physical
breakdown of food
Absorption of food breakdown products
into circulatory and lymphatic systems
Elimination - excretion of waste products
and toxins
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The Digestive System
Composed of a two parts
*A long tube - termed the alimentary canal. The alimentary canal extends from mouth to anus and is about 30 ft in length
*Accessory structures -these structures aid in digestion (salivary glands, teeth, liver, GB, pancreas)
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Digestion Begins in the Mouth
Oral or buccal cavity - moistens and aids
mechanical digestion of food (minor amount of saliva secreted here)
Tongue - helps to manipulate food, moves food to the back of the mouth for swallowing, contains taste buds, glands located here secrete lingual lipase
allows for speech
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Salivary Glands
The majority of saliva comes from
three paired structures
Parotid, submandibular, and
sublingual glands
Saliva pH is 6-7 and composed of
99.5% H2O.
1000 to 1500 ml secreted daily
under nervous system control (salivary nuclei triggered by smell, sight, thought or presence of food in mouth)
These glands secrete Salivary Amylase which breaks polysaccharides into disaccharides
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Teeth
Mastication - chewing
1st step of digestion
Two dentitions
1. Deciduous teeth
-appear in 6 months
-20 teeth
-formula 2 incisors,
1 canine, 2 molars
2. Permanent Teeth
-appear 6-12 years
-32 in all
-formula 2 incisors,
1 canine (cuspids), 2 bicuspid
(premolars) and 3 molars
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Schimmels Dental Chart
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Digestion Begins With
Mastication - chewing/teeth 20-30 bites
Moistening - saliva
Chemical -
Salivary Amylase - polysaccharides (starch) into disaccharides
Lingual Lipase - triglycerides to fatty acids and monoglycerides
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Deglutition - Swallowing
Voluntary stage - tongue moves food back into the oropharynx
Pharyngeal stage - bolus stimulates receptors in oropharynx causing a
cessation of respiration and epiglottis covers airway
Esophageal stage - dilation of upper/lower esophageal sphincters,
food moves into esophagus and down to the stomach via peristalsis
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Esophageal bolus moves behind heart!
Lower esophageal sphincter must now open
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4 layers of the GI tract
Mucosa - mucous membrane, first layer,
composed of epithelium (stratified. sqamous/simple columnar), and is in contact with food
Submucosa - composed of loose areolar connective tissue and binds mucosa to muscle layer, contains immune system of the gut
Muscularis - skeletal muscle in mouth, pharynx and esophagus. The rest of the tract is smooth muscle. This layer usually contains 2 layers (3 in stomach) - inner circular/outer longitudinal
Serosa - serous membrane, made up of connective tissue and epithelium. Below the diapham this layer is termed the visceral peritoneum.
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4 layers of the GI tract
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Peritoneum
The Peritoneum is a serous membrane that lines
the walls of the abdomen and organs
The parietal layer lines the walls of the abdomen
The visceral layer covers the the internal organs
The space between these layers is called the
peritoneal cavity and contains serous fluid
Organs behind this cavity (kidneys/pancreas) are
termed retroperitoneal
Infection in this area is called peritonitis
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Stomach
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Stomach
The mucosa of the stomach consists of gastric pits lined with 4
types of cells
1. Chief cells - pepsinogen/gastric lipase
2. Parietal cells - HCL/Intrinsic factor
3. Mucous cells - mucous barrier
4. G cells - gastrin
All secretions equal gastric juice (2000 to 3000 ml per day)
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Gastric Pits
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Mechanical digestion
Peristaltic mixing waves q 15-25 secs
Food mixed with HCL and reduced to chyme
Food is stored in the fundus then ground down in the body
and pylorus
Each wave ejects chyme through the pyloric sphincter into
the duodenum
H. pylori-bacteria associated with Ulceration
Reflux -smoking, coffee, alcohol, and vitamin deficiency ?
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Chemical Digestion in the Stomach
1. HCL denatures protein - destroys three dimensional structure of protein
2. HCL converts pepsinogen (zymogen) into pepsin (active protease)
3. Pepsin breaks large polypeptides into peptides
4. Gastric Lipase digests buttermilk in infants
Alcohol, ASA and water are absorbed in stomach
Mucus prevents auto-digestion of this layer
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Nerve/hormonal interactions
1. HCL denatures protein - destroys three
dimensional structure of protein
2. HCL converts pepsinogen (zymogen)
into pepsin (active protease)
3. Pepsin breaks large polypeptides into
peptides
4. Gastric Lipase digests buttermilk in
infants
Alcohol, ASA and water are absorbed in
stomach
Mucus prevents auto digestion of this layer
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The hormones of digestion
Acetylcholine - parasymp nervous output
stimulates parietal HCL production, peptic pepsinogen release, and mucous secretion
Histamine - amino acid derivative that stimulates HCL production via H2 parietal cell activation. This compound is required for gastrin and acetylcholine activation of parietal cells. Blocked by tagamet-cimetidine
Gastrin - secreted by G-cells in response to the presence of peptides/amino acids in the stomach. This hormone increases HCL production, contracts LES, increases stomach motility, and relaxes the pyloric sphincter leading to the SI
Alcohol, caffeine, and amino acids stimulate gastric production as well
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More hormones
Secretin - secreted by endocrine cells located in
the duodenum in response to the presence of acidic chyme. This hormone triggers the release of bicarbonate from the pancreas to neutralize acidic chyme and create the proper pH for pancreatic enzyme activity
CCK(cholecystokinin) - fat entering the small intestine triggers the release of CCK from local endocrine cells. CCK enhances the action of secretin, stimulates pancreatic enzyme secretion and causes the GB to contract releasing bile into the SI.
Both Secretin/CCK slow stomach emptying
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CCKs more interesting aspect
CCK in the brain is related to anxiety
CCK injected into humans creates feeling of
panic and intense anxiety
Drugs that Block CCK in the brain reduce
anxiety
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GIP
Glucose insulinotropic peptide is released in
response to glucose and fat in the small intestine
It slows gastric gland secretion and emptying
GIP also stimulates the release of insulin which allows for cellular uptake of glucose/glycogen storage
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The Liver, GB , Pancreatic interface
Right and Left Hepatic ducts
Common Hepatic duct
Cystic duct
Pancreatic duct and Accessory duct
Hepatopancreatic ampulla
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The Liver
Generation of bile salts
from cholesterol via HMG - CoA reductase
Lipid metabolism
Protein synthesis and
recycling
Phase1/2 detox
reactions
Hormone metabolism
Storage of glycogen,
vitamins (A,B,D,E,K) and iron
Activation of multiple vitamin prestages
Phagocytosis of worn out blood cells
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Gall Bladder
Stores and concentrates bile by reabsorbing water
Bile salts and lecithin combine with cholesterol to
form micelles that are soluble and keep cholesterol in solution preventing stone formation
Ejection of bile primed by CCK
Cholelithiasis - cholecystitis 1. Too much H2O
absorbed from bile 2. High fat high cholesterol diet 3. Too much cholesterol in bile - chenodeoxycholic bile acid therapy for the dissolution of stones
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Pancreas
Exocrine function
Digestive enzyme release
(CCK)
Bicarbonate - primed by
secretin
Endocrine function
Glucagon
Insulin
Somatostatin
What does insulin do to blood sugar?
Netters
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Pancreatic Enzymes
Pancreatic amylase -
starches to disaccharides
Trypsin, chymotrypsin and carboxypeptidase -
proteins into peptides
Pancreatic lipase
triglycerides into fatty acids and monosaccharides
Pancreatic nucleases -
RNA/DNA into nucleotides
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Small Intestine
Duodenum, Jejunum and Ileum
Consists of 4 layers with
modifications to the mucosa and submucosa to increase surface area
Villi-absorptive finger has a LP core that contains blood vessels+lymphatics (lacteal)
Microvilli-threadlike projection of the epithelial cell. Each cell has about 1,700 - this creates the brush border and secretes digestive enzymes
Plica circularis - The submucosa is thrown into large folds
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Villi and Plica Circularis
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Glandular types of the small intestine
Intestinal Glands - Crypts of leiberkuhn
these glands secrete intestinal juice (combined with the brush border secretions)
Duodenal glands - Brunners glands - secrete an alkaline mucous that combines with bicarbonate ions from the pancreas to neutralize acidic chyme from the stomach - inhibited by symp. Activity or chronic stress leading to ulcer formation
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Intestinal Juice
1 to 2 liters produced per day
Derived from intestinal glands and the surface of
enterocytes (brush border)
Brush border enzymes include;
*maltase, sucrase, lactase, alpha destrinase - disaccharides into monosaccharides
*peptidases
peptides into amino acids
*nucleosidases and phosphatases - nucleotides into N bases, pentose and P
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Chemical Digestion in the Small Intestine revisited
Pancreatic enzymes
Pancreatic amylase -
starches to disaccharides
Trypsin, chymotrypsin
and carboxypeptidase -
proteins into peptides
Pancreatic lipase
triglycerides into fatty acids and monosaccharides
Pancreatic nucleases -
RNA/DNA into
nucleotides
Brush border enzymes
Maltase, sucrase, lactase,
alpha destrinase disaccharides into monosaccharides
Peptidases
peptides into amino acids
Nucleosidases and phosphatases
nucleotides into N bases, pentose and P
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Enzymes reviewed
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Mechanical Digestion
Inner circular and outer longitudinal muscle
muscle layers allow for segmentation
Chyme is confined to one region of the small
intestine
It washes back and forth to make contact with the
mucosa
Peristalsis then moves the the chyme to the next part
of the small intestine where segmentation occurs again
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Regulation of secretion in the small intestine
The intestine has a localized nervous system called the Enteric nervous system
*it responds to the presence of chyme and performs visceral reflexes
This division is influenced heavily by both sympathetic and parasympathetic branches of the ANS.
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Absorption in the Small Intestine
To be absorbed
Carbohydrates must be converted onto
monosaccharides
Proteins must be converted to amino acids
Lipids must be converted to fatty acids
and monoglycerides
90% of absorption occurs in the small
intestine, 10% in the rest of the tract
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Absorption
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Vitamin Absorption
Fat soluble vitamins ADEK pass into
lacteals with triglycerides
Water soluble vitamins pass by diffusion
Vitamin B12 absorption requires intrinsic
factor from the parietal cells - no intrinsic factor = Pernicious anemia secondary to a lack of B12 absorption
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Gastroileal reflex
Ileocecal valve opens
Chyme passes into the cecum via peristalsis
This initiated by the hormone gastrin
Fecal material moves from one haustra to the next
(haustral churning)
Haustral churning continues until the second half
of the transverve colon where mass peristalsis moves it into the retum
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Fiber
Water Soluble Fiber -1. slows uptake of sugar
from the intestine thus preventing large spikes in blood sugar and the resulting insulin resistance 2. Serves as a food source for GI flora (FOS-fructo oligosaccharides) 3. Short chain fatty acids production as a results of bacterial mediated breakdown supplying the preferred energy supply for the cells lining the colon
Water Insoluble Fiber - 1. Bulking agent that improves bowel motility 2. Absorbs toxins in the GI tract improving excretion
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In the Colon
Water is reabsorbed
Bacteria break down whats left of the chyme
E coli; synthesis of vitamins B1, B2, B6, B5, B12,
folate, Biotin, Vitamin K
Deranged flora no longer produce these vitamins.
Instead they scavenge vitamins polyamine formation, scatole, indole
Fungal infections as an indicator
Lactobacillus acidophilus/bacterium bifidum/coli
culture preps
MALT
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Physiology of Defecation
Mass peristalsis of fecal material from
sigmoid colon into the rectum
Rectal wall stretching initiates defecation
reflex
Parasympathetic nervous system (sacral
portion)
Diarrhea
Constipation