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Physiology ofDigestion
Department of Physiology
School of Medicine
University of North Sumatra
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Food
Nutrient
Non Nutrient
Assimilated
Eliminated
Digestive
system
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Motility;Movement of food through tract ,includes ingestion,
mastication (chewing food and mixing with saliva),
deglutition (swallowing) and peristalsis (rhythmic
contractions along GI tract that propel food)
muscular contraction.
Secretion;Endocrine (secretion of hormones that regulate digestiveprocess)
Exocrine (secretion of water, enzymes, acid, bicarbonate,
into GI tractenzyme & other digestive juices.
Primary Functions of Digestive SystemActivity necessary:
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Digestion;Hydrolysis reactions that break ingested polymers
(large molecules) into their smaller subunits(monomers)breakdown of substances.
proteins into amino acids
fats into glycerol and free fatty acids
complex sugars into monosaccharides
Absorption;
Transfer of monomer subunits across wall of smallintestine into blood or lymph
transport modified nutrients.
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Regulation;
There are two nerve nets (plexuses) in GI tractthat contain neurons and interneurons
sub mucosal (Meissner)
Myenteric (Auerbach)
Plexuses = brain of the gut
stimulated by stretch (bolus of food), chemicals,
and stomach content (local stimuli) Activity of plexuses can be modified by central
nervous system and the autonomic nervous
system
Neural:
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Neural regulation via autonomic nervous system
GI tract receives bothsympatheticandparasympatheticinnervations
Parasympathetic via vagus nerve and spinal nerves
in sacral region (to lower portion of large intestine) stimulates motility and secretion; favors digestion
Sympathetic
reduces motility and secretory activity andstimulates sphincter contraction
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Paracrine regulation
production of hormone-like molecules that are
produced in one cell and travel through
interstitial fluid (not bloodstream) to affect
activity of nearby cells Hormone regulation
production of hormones that are released into
the bloodstream and carried to target tissueswithin digestive system where they affect
digestive activity
Hormonal.
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Salivary glands(parotid, submaxillary, sublingual)
secrete saliva
lubricates and softens food; aids in
swallowingcontains amylase = enzyme that begins
breakdown of carbohydrates
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OropharynxTo convey food into the esophagus.
Important role in swallowing.
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Cavity at back of mouth opening toboth esophagus (digestive tract)
and trachea (windpipe)
Voluntary raising of larynx to close
(epi)glottis and prevent food entry
into windpipe
Pharynx = throat
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Esophagus.
Hollow muscular tube connecting pharynx and
stomach.
Bounded by sphincters.
Lined w/ stratified squamous epithelium.Lower esophageal (gastroesophageal) sphincter ;
transtition from low pressure ( intrathoracic )
high pressure (intra-abdominal).Dissorder o/t LES tonemajor cause esophageal
refluxheart burn.
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stomach
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Stomach:
Temporary stores ingested food.sphinctersprevent backward flow of materials intoesophagus and regulate release of stomachcontents into small intestine
Churn, mixes food with gastric juice.
Mechanical and chemical breakdown of ingestedmaterial
Produces, mucus, HCl and pepsinogen.
HCl converts pepsinogen into pepsin. Sterilization of stomach contents by acid
Pepsin digests proteins into peptide fragments.
Absorbs some water, alkohol, glucose.
Binds vit. B12allows abs. in ileum
Secretion/Digestion
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Pepsinogen
HCl
Gastrin
Histamine
Stomach:
lower region of stomach(antrum) secretes the
hormone gastrin.
Additional secretions: histamine (ECL cell)
Somatostatin
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Mucous cell, secrete mucousprotects
mucosa from acid & pepsin.
Chief cells, secrete :
- Gastric lipase.
- Pepsinogen PepsinHCl
Prietal (oxyntic) cells, secrete :
- HCl .
- Intrinsic factorbinds vit. B12
Prietal (oxyntic) cells, secrete :
- HCl .
- Intrinsic factorbinds vit. B12
Pyloric glandAlkaline mucus.
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Digestion in the stomach
CHO digestion is halted because acidic pH of thestomach inactivates salivary amylase
Little fat digestion occurs in the stomach
Protein digestion begins in the stomach
Involves mechanical breakdown of proteins by thechurning actions of the stomach
Involves the chemical digestion of proteins by acid andhormones Gastric (stomach) acid = hydrochloric acid (HCl)
pepsin
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Protein Digestion in the Stomach
Parietal cells
secrete acid (hydrochloric acid = HCL)
stimulated to produce acid by gastrin
Chief cells
secrete pepsinogen
Pepsinogen = inactive hormone Pepsinogen converted to pepsin (active hormone) by
acidic pH of the stomach
Pepsin breaks large proteins down into smaller peptides
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G cells
Secrete gastrin
Gastrin = hormone
target tissues = chief cells and parietal cells in stomachstimulates gastric juice production
HCL from parietal cells
Pepsinogen from chief cells
Increases pH of stomach Promotes conversion of pepsinogen to pepsin
Protein Digestion in the Stomach
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It also activates gastroileal reflex which moveschyme from ileum to colon
Parasympathetic stimulation releases gastrinreleasing peptide, which stimulates G cell releaseof gastrin
Mucus cells - secretes mucus to protect againstacid and digestive enzymes
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Absorption in the Stomach
Almost all products of digestion areabsorbed in the intestine
Notable exceptions
alcohol and aspirin can be absorbed directlythrough stomach wall
due to their lipid solubility
absorption of aspirin through stomach wall
associated with bleeding, may be related topeptic ulcers in people taking large dosages
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Chyme
Food in stomach is liquified
mixed with stomach juices to form pasty liquid
material = chyme
Chyme = material passed from stomach to
small intestine
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Small Intestine
Functions in digestion CHO digestion resumes and is completed here
Protein digestion continues and completes here
Fat digestion is initiated and completed here
Also functions to absorb nutrients, fluids, andelectrolytes
Divisions (@ 12 feet long total length)
duodenum = upper portion (@ 1 foot long) closest to
stomach
jejunum = middle section
ileum = lower section closest to large intestine
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Anatomy
3 segments (12 ft long, 22 ft in cadaver)
Duodenum
Jejunum Ileum
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Anatomy
Features that increase surface area Circular folds (valvulae conniventes, kerckrings
folds, plicae circularis) Project into lumen 3-10 mm Prominent in duodenum and jejunum and disappear near
mid ileum
Responsible for feathery appearance on bariumradiographs
Villi 4-5 million in entire length
0.5-1.5 mm long
Account for velvet-like appearance
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Anatomy
Microvilli
1.0 um long
Brush border
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Anatomy Structure of the villus
Lacteal
A
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Anatomy Goblet cells and absorptive cells
A t
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Anatomy Brush border enzymes
A t
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Anatomy Crypts of Lieberkuhn
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Physiology
Two primary function Digestion
Absorption of nutrients and water
Digestion
Mainly in duodenumsmall intestine andpancreatic enzymes
Bicarbonate from pancreas neutralizes acids
Mucous protects from acids
Bile emulsifies fats
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Hormones Important in Sm.
Intestine Digestive Activity
Secretin
Cholecyctokinin (CCK) Enterokinase
Pancreatic enzymes
Lipase, Amylase, Peptidases, Trypsinogen, Trypsin
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Physiology
Digestive enzymes Salivary amylase
Pepsin
Pancreatic enzymes Trypsin
Chymotrypsin
Carboxypeptidase
Nucleases
Pancreatic lipase
Pancreatic amylase
Intestinal enzymes Peptidases
Disaccharidases
Lipase
Nucleotidases
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Physiology
Absorption
Nutrients broken down into simple sugars, fattyacids and amino acids
Principle sites of absorption
Duodenum: iron, calcium, vitamins, fats, sugars, aminoacids, vitamins
Jejunum: fat, sugar, amino acid (largely complete by mid
jejunum), vitamins Ileum: vitamin B12and bile salts
Most bile salts are absorbed and recirculated to the liverimportant in maintaining bile pool
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Digestion
Most occurs by hydrolysis reactions -
reactions that add water to break chemical
bonds
Enzymes involved:
amylase - breaks complex sugars to
disaccharides
lipases - breaks down lipids
proteases - breaks down proteins
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Hormonal Control of Intestinal
Secretion
S cells secrete secretin
secretin target tissue = pancreatic ducts and liver
induces release of water and HCO3-from pancreatic ductcells and secretion of HCO3
-into bile by liver
HCO3-neutralizes acidic chyme
Stimuli for secretin secretion
acid (pH less than 4.5 stimulates secretion)
secretin = natures antacid
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I cells secrete cholecystokinin (CCK)
CCK target tissues = pancreatic acinar cells and
gall bladder induces secretion of digestive enzymes from
pancreatic acinar cells
induces contraction of gall bladder, which releases
bile into small intestine Stimuli for CCK secretion
presence of protein and/or fat in chyme
Hormonal Control of Intestinal
Secretion (continued)
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CCK and Appetite Control
CCK also thought to act on satiety center inbrain and inhibit appetite
Butabindid = drug that interfers withenzyme responsible for CCK degradation(breakdown)
keeps CCK in system longer
boosts appetite inhibition
used to treat obesity
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Pancreatic Enzymes (continued) Most pancreatic enzymes are produced as
inactive molecules = zymogens
Are transported to small intestine in zymogen
form
Protects the pancreas from self digestion
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CHO Digestion in Sm. Intestine
Pancreatic amylase secreted into duodenum in
response to secretin
Is active in the intestine because the acidic chymeis neutralized by HCO3
-also secreted from
pancreas in response to secretin
Amylase converts complex CHOs to maltriose,maltose, and short branched sugars
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Small Intestine (continued)
Inner surface (epithelial layer) extensivelyfolded and covered with smaller folds (villi)
and even smaller folds (microvilli) increase
surface area for absorption to occur
Site of absorption of carbohydrates, lipids,
amino acids, calcium and iron in duodenum
and jejunum
Bile salts, Vit B12, water, and electrolytes
mainly in ileum
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CHO Absorption in the Sm. Intestine
Enzymes in the brush border completebreakdown of maltotriose, maltose, and branched
sugars, lactose and sucrose
Lactase converts lactose to glucose and galactose
Sucrase converts sucrose to glucose and fructose
Isomaltase converts branched sugars glucose,
maltose and unbranched short oligosaccharides
Maltase converts maltose to glucose
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Fat Digestion in Sm. Intestine
Limited fat digestion occurs prior to sm. Intestine Some lipases in saliva and gastric secretions
Lipase = enzyme important in fat digestion
Secreted into sm. intestine from pancreas in response
to secretin breaks down triglycerides to free fatty acids and
monoglycerides
Activity is dependent upon the amount of surface areaon which it can work
Phospholipase A2digests phospholipids
Bile is essential for proper fat digestion
Di ti d Ab ti f Li id i
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Digestion and Absorption of Lipids in
Small Intestine
Fats stimulate I cell release of CCK
CCK triggers release of bile from gallbladder
Bile emulsifies fats breaks large fat globules into smaller globules
does not actually break bonds between glyceroland fatty acids
Increases surface area available for pancreaticlipase to act
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Liver
Largest organ in body Blood supply
hepatic artery delivers oxygenated blood
hepatic portal veinproducts absorbed into capillaries in the intestines
do not directly enter general circulation
this blood is delivered first to the liver by the hepatic
portal vein, and then passed on to the generalcirculation
liver has first crack at absorbed nutrients, except
lipids
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Non-digestive functions
circulatory functions; destroys aged or abnormal
blood cells and produces clotting factors
converts protein metabolites to urea for
elimination by kidneys
immune function (Kupffer cells)
functions as blood reservoir in regulation of blood
volume
Liver (continued)
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Bile Synthesis
This is the main digestive function of the liver;Approximately 1 liter per day is produced
bile salts are cholesterol derivatives and functionto emulsify fats
bile salts are recycled, not excreted
main bile pigment is bilirubin, derived from RBCheme
bile is synthesized in the liver and stored in thegallbladder
release is stimulated by cholecystokinin andvagus nerve
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Bile
Product of the liver cells bile contains bile pigment, bile salts, phospholipids,
cholesterol, and inorganic ions
bile pigment = bilirubin = breakdown product of hemoglobin
bile salts = derivatives of cholesterol that are combined withtaurine or glycine, form micelles = lipid aggregates with non-
polar parts in central region and polar regions toward water
Essential for absorption of fat from the digestivetract
Emulsifies fat; breaks large fat globules into smaller fatdroplets, provides greater surface area on which lipasecan act
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Micelles
Aggregates of bile salts, free fatty acids,
monglycerides, lysolecithin, and fat-soluble
vitamins
Arranged with non-polar regions to center, polar,
water-soluble portions to outside
makes lipids more water soluble in lumen of intestine
Micelle is transported to epithelial cells liningsmall intestine
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Importance of Micelle Formation
Intestinal epithelial layer is covered by an unstirred
water layer
Fats are nonpolar, and therefore insoluble in water Micelles are structures whose outer borders are polar,
but whose inner segments are nonpolar
Outer polar portion can dissolve in the unstirred water layer
and be transported to epithelial cell surfaces
Nonpolar contents can then be removed from micelle and
absorbed individually, or in some cases the micelle itself
may be absorbed
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Chylomicrons
Inside epithelial cells, triglycerides and
phospholipids are re-synthesized
Resynthesized triglycerides and phospholipids are
combined with cholesterol and protein inside the
cell to form chylomicrons
Chylomicrons are released into the lymphatic
system - NOT INTO HEPATIC PORTAL VEINliver does not get first crack at lipids
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Protein Digestion in Sm. Intestine
Proteolytic enzymes are released (in inactive,zymogen forms) from the pancreas in response tosecretin
Trypsinogen = inactive precursor that isconverted (by enterokinase) to trypsin in the brushborder of the small intestine
Trypsin converts other zymogens to their activeforms
Collectively, the proteolytic enzymes breakproteins and peptides into single amino acids, ordi- and tripeptides
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Absorption of Amino Acids in Sm.
Intestine Amino acids are absorbed by the epithelial cells of
the small intestine by active co-transport with
sodium
Di- and tripeptides are also absorbed by epithelial
cells of the small intestine and then broken down
into amino acids within the epithelial cells
Epithelial cells release amino acids into portalblood, which is transported first to the liver, then
to rest of the body
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Intestinal Contractions and Motility
2 major types ofcontractions occur in thesmall intestine:
Peristalsis: Slow movement.
Pressure at the pyloric endof small intestine is greaterthan at the distal end.
Segmentation: Major contractile activity of
the small intestine.
Contraction of circularsmooth muscle. Mix chyme.
Insert fig. 18.14
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Brush border
enzymes
reassembly
Anatomy and Physiology
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Anatomy and Physiology Anatomy
Anatomy and Physiology
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Functions (converts chyme to feces)
Absorption of water and electrolytes (mainly onright side) Absorbs 800 ml water/day
Capacity 1500-2000 ml/day (when exceeded results indiarrhea)
Sigmoid colon reservoir for dehydrated fecalmass
~200 g feces/day
Water80-90%
Food residue
Bacteria
Cells
Unabsorbed minerals
Anatomy and Physiology
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Secretes mucus (no enzymes)
Bacteria produce vitamin K and several Bs
Flatus (NH3, CO2, H2, H2S, CH4) CO2 produced when fatty acids and HCl are neutralized
by bicarbonate
Bacterial fermentation of carbohydrates produces CO2,H2, CH4
~1000 ml expelled each day Excess occurs with aerophagia and diets high in
indigestible carbohydrates
Rectum and anus sites of some of most commondisorders known to humans Constipation Hemorrhoids
Abscesses and fistulas
Colon and rectal cancer
Colon
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Colon
1,51,8 m.
Various segment.
Columnar epithelium at the surface (few, short),no villi, few folds (except in the distal rectum).
Goblet cells.
Endocrine cells.
Absortive cells.
Anatomi & Histology
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Large Intestine
Larger diameter, shorter length (@ 4 feet)
than small intestine
separated from small intestine by ileocecal
valve
Collects food residue (indigestible) material
Site of final water reabsorption - causes
chyme to become solid (forms feces)
Stores feces until eliminated from body
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Bacteria live in the colon and stimulate theproduction of vitamin K and some of the B
complex vitamins Mucus is produced but no enzymes are
secreted in the large intestine
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Divisions
cecum = section nearest small intestine
(appendix attached in this region)
colon = major portion
ascending
transverse
descendingsigmoid (leads to rectum)
Large Intestine (continued)
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Water Absorption in the Intestine
Majority ( 98%) of water that enters gut is
(re)absorbed
85% (re)absorbed in small intestine 13% in large intestine
Absorption of water is passive process
osmotic gradient created by active transport ofions
water moves according to its gradient
Large Intestine
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Large Intestine
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Large
Intestine
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Colon: Function
Absorb H2O and electrolytes
Secretion of mucus
Formation, propulsion & storage ofunabsorbed material (feces)
Some digestion by bacteria
Mass Peristaltic Movements (2-3x day)
Moves through in 12-24 hours
DIGESTIVE ACTIVITIES OF
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DIGESTIVE ACTIVITIES OFLARGE INTESTINE
STRUCTURE ACTIVITY RESULT
Mucosa Secretes
mucus
Lubricates colon &
protects mucosa
Absorbs water Maintains water
balance; solidifies
feces; absorbs
vitamins & some ions
DIGESTIVE ACTIVITIES OF
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DIGESTIVE ACTIVITIES OFLARGE INTESTINE
STRUCTURE ACTIVITY RESULT
Lumen Bacterial
activity
Breaks down
undigested
carbohydrates,protein, & amino
acids into products
that can be expelled
in feces or absorbed& detoxified by liver
Synthesizes certain B
vitamins & vitamin K
DIGESTIVE ACTIVITIES OF
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DIGESTIVE ACTIVITIES OFLARGE INTESTINE
STRUCTURE ACTIVITY RESULT
Muscularis Haustral
churning
Contractions move
contents from haustrum
to haustrum
Peristalsis Contractions of circular
& longitudinal muscles
move contents along
length of colon
DIGESTIVE ACTIVITIES OF
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DIGESTIVE ACTIVITIES OFLARGE INTESTINE
STRUCTURE ACTIVITY RESULT
Muscularis Mass
peristalsis
Forces contents into
sigmoid colon
Defecation
reflex
Eliminates feces by
contractions in sigmoid
colon & rectum
Large Intestine
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Large IntestineDigestion and Absorption
Fermentation breaks down undigested complexCHO and proteins Produce lactate and short chain fatty acids used by
the colonocytes for energy Bacteria produce absorbable vitamins like K
Gases are produced in the large intestine
Colon is responsible for absorbing most of the
water that enters it in the form of chyme Colonocytes absorb NaCl
Secretion
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Secretion
& H2Oabsorption
2000 ml150 ml = ?
Ion &
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Ion &
Vitaminabsorption
Rectum
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Rectum
The
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Figure 24.25
TheDefecation
Reflex
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Defecation process
Reflex relaxation of internal sphincter
Valsalva maneouvre raising intraabdominal
pressure Relaxation of puborectalis (anorectal
angle)
Voluntary relaxation of external sphincter
Defecation reflex
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f f> 15 mm Hg
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Continence mechanism
Rectum normally empty
Colonic movements distend rectum -
150 mls, 25 cm H2O which activatedefecation reflex
Voluntary inhibition (external sphincter)
movement of faecal material back intocolon
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