Gastro Digestive system pg 565-616
Wednesday, November 28, 2018
1:18 AM
- Autotrophs vs. Heterotrophs
○ Autotrophs --> phtosynthesis and make own food
○ Heterotrophs --> derive energy from food
§ Organic molecules (in food) + O2 --> Energy + CO2 + H2O (for use by cells of
the body)
- Overview of the Digestive Organs:
○ Mouth and Salivary Glands
○ Pharynx and Esophogus
○ Stomach
○ Small Intestine
§ Duodenum
§ Jejunum
§ Ileum
○ Large intestine
§ cecum
§ Appendix
§ Colon
§ Rectum
- General structure of the digestive tract wall
-
-
1. Serosa
i. Secretes serous fluid-lubricates
ii. Continuous with mesentery throughout much of the tract
- Supports digestive organs in proper place while allowing them freedom
for mixing and propulsive movements
iii.
2. Muscularis externa
i. Major smooth muscle coat of digestive tube
ii. Usually two layers
1) Inner circular layer: contraction decreases diameter of lumen
2) Outer longitudinal layer: contraction shortens the tube
iii. Contractile activity produces propulsive and mixing movements
iv. Myenteric plexus: part of the enteric nervous system (in between muscle
layers)
1) Distributed in between muscle layers of the tract, the own brain of
digestive system
3. Submucosa
i. Thick layer of connective tissue
ii. For distensibility and elasticity
iii. Contains submucosal plexus nerve network part of the enteric nervous
system
4. Mucosa
i. Lines lumen: highly folded surface increases absorptive area
1) Epithelial layer (mucous membrane)
a) Cells for secretion and absorption
b) Contains exocrine gland cells--> secrete digestive juices, mucus,
enzymes into lumen (exocrine because considering lumen as
outside of body)
c) Contains endocrine gland cells--> secrete gastrointestinal
hormones into capillaries
2) Lamina propria- loose connective tissue
a) Small blood vessels, lymphatics, and enteric neurons
mucus, enzymes into lumen (exocrine because
considering lumen as outside of body)
c) Contains endocrine gland cells--> secrete
gastrointestinal hormones into capillaries
2) Lamina propria- loose connective tissue
a) Small blood vessels, lymphatics, and enteric neurons
b) Contains gut-associated lymphoid tissue (GALT)
3) Muscularis mucosa
a) Spares layer of smooth muscle
- The lumen of the gastrointestinal tract is continuous with the external
environment, important because:
○ pH in the stomach can fall as low as 2. Inside the body the range of pH
that is compatible with life is 6.8 - 8.0 (homeostatic setpoint is 7.2)
○ Harsh enzymes that hydrolyze food could destroy the body's own tissues.
Therefore enzymes are synthesized in an inactive form and are activated
when they reach the lumen
○ Millions of microorganisms inhabit the GI-tract, and these could be lethal
if they entered the body
- 4 Basic digestive processes: (Motility, Secretion, digestion, absorption)
• Ingestion, chewing, swallowing, defecation- transfers food into the
digestive tract via the mouth (skeletal muscle--> Voluntary)
1. Motility: Muscular contractions that mix and move forward the contents
within the tract, facilitating later steps in the digestive process (smooth
muscle --> involuntary), There are 2 types of movements:
I. Propulsive movements (peristalsis): move the contents forward
through the digestive tract
II. Mixing movements (segmentation, non-linear): serve 3 purposes:
1) Aid digestion by mixing food with digestive juices
2) Facilitate absorption by exposing food to absorbing surfaces
3) Forward movement (slow and non-linear)
II. Mixing movements (segmentation, non-linear): serve 3 purposes:
1) Aid digestion by mixing food with digestive juices
2) Facilitate absorption by exposing food to absorbing surfaces
3) Forward movement (slow and non-linear)
2. Secretion (Exocrine and endocrine)
I. Exocrine: digestive juices are secreted into the lumen by exocrine
glands upon appropriate neuronal or hormonal stimulation
1) Secretion when food is present
2) Secretion contains enzymes, acids, buffers, electrolytes, and
water that promote digestion, adjust tonicity and provide
lubrication for better movement through the tract
I. Endocrine: gut hormones are secreted into the blood by endocrine
glands in endocrine tissue upon appropriate neuronal or nutritional
1) Secretion when food is present
2) Secretion contains enzymes, acids, buffers, electrolytes, and
water that promote digestion, adjust tonicity and provide
lubrication for better movement through the tract
I. Endocrine: gut hormones are secreted into the blood by endocrine
glands in endocrine tissue upon appropriate neuronal or nutritional
stimulation
1) Released to blood through exocytosis
2) Gut hormones are chemical messengers released into
circulation and act on receptors in distal locations to regulate
motility, pancreatic secretions, and other digestive tract (and
non-digestive tract) functions
3. Digestion (= chemical): accomplishes the breakdown of structurally
complex foodstuffs into smaller, and eventually absorbable units
I. Chemical Digestion: enzymatic hydrolysis of carbohydrates,
proteins, and fats into absorbable units (carbs, proteins, fats)
1) Carbohydrates: absorbable units are glucose, fructose,
galactose
a) Polysaccharides: Breakdown of starch and glycogen
through amylase into maltose and from maltose to
Glucose
b) Disaccharides:
i) Sucrose into glucose, fructose using sucrase
ii) Lactose into glucose and galactose using lactase
2. Proteins: protein --> peptide fragments --> amino acids ,
absorbable units are peptide fragments & amino acids,
a) Pepsin, trypsin, chymotrypsin, carboxypeptidase (to
break proteins)
b) Aminopeptidase to break peptide fragments
3. Fats:
a) Triglycerides --> Monoglyceride, free fatty acids (using
Lipase)
4. Absorption : the transfer of small absorbable units along with water,
vitamins, and electrolytes from the lumen into the blood or lymph
a) Triglycerides --> Monoglyceride, free fatty acids (using
Lipase)
4. Absorption : the transfer of small absorbable units along with water,
vitamins, and electrolytes from the lumen into the blood or lymph
I. Digestive system, takes in as much as possible
II. Transport of nutrients: from lumen--> basal lateral side of lumen -->
capillary by diffusion
III. Into blood: carb absorbable units (glucose, galactose, fructose),
protein absorbable units (amino acids, peptide fragments)
IV. Into lymphatic capillary: fat absorbable units (monoglycerides and
free fatty acids, converted to chylomicron and into lymphatic
capillary)
- Regulation of Digestive System Functions (Intrinsic and Extrinsic factors)
-
○ Intrinsic factors:
§ Autonomous smooth muscle cells are connected by gap junctions,
thereby forming a functional syncytium. Single-unit smooth muscle
□ Allow electrical signals to travel fast in muscle sheets, big web
of cells that has their own nucleus, multinuclei
§ Interstitial cells of Cahal (ICC) acts as pacesetter cells and generate
slow-wave potentials (Basic Electrical Rhythm; BER). If the
threshold is reached and action potentials are triggered, then the
whole muscle sheet contracts as a unit. (frequency and where the
peristalsis happens)
□ Muscle contraction
□ Distributed throughout the length of the digestive tract
□ Regulation of gastric motility when there is no food in lumen:
green line: threshold potential , spikes: action potential,
peristalsis (stomach growling)
□
1. The membrane potential of pacemaker cells (Interstitial
Cells of Cahal, or ICC) oscillates at 3-5 times per sec (3-5
Hz): this is the Basic Electrical Rhythm (BER) in the
stomach
2. ICCs in the small intestine depolarize more frequently:
8-11 Hz: the BER in the small intestine
3. These depolarizations spread thru gap junctions to
smooth muscle cells, then signal propagated through
the tract by the enteric nervous system
4. However, these depolarizations exceed spike threshold
only 10-15 times per day = the migrating motility
complex, which triggers contractions that are
frequently enough to "sweep" residual contents from
smooth muscle cells, then signal propagated through
the tract by the enteric nervous system
4. However, these depolarizations exceed spike threshold
only 10-15 times per day = the migrating motility
complex, which triggers contractions that are
frequently enough to "sweep" residual contents from
the stomach and small intestine to the large intestine
(triggered by motili = extrinsic regulation).
i) Contractions even if we have no food in the
stomach
□ Regulation of gastric motility when there is food in lumen:
reach threshold much frequently
□
1. Stretch and gastrin (hormone induced by protein in the
stomach) activate neural circuits that increase the
amplitude and frequency of the BER depolarizations
2. When these depolarizations exceed spike threshold
(approx -35 mV), the smooth muscles spike and
therefore contract
3. Stretch and gastrin thereby increase digestive tract
motility
§ Enteric nervous system (myenteric +submucosal nerve plexuses) an
interconnecting network of nerve cells localized within the digestive
tract wall; coordinates local activity within the digestive tract
○ Extrinsic Factors:
§ Extrinsic nerves (originate from outside the digestive system) from
both the sympathetic and parasympathetic branch influence
motility and secretion by:
□ Modifying activity of the enteric nervous system
□ Altering gastric hormone secretion
□ Acting directly on smooth muscle and glands
§ Gastrointestinal hormones (can be triggered by intrinsic factors
but hormones are considered extrinsic factors)
□ Long-range chemical messengers secreted into blood and act
on receptors in distal locations to regulate digestive tract (and
non-digestive tract) functions
□ Acting directly on smooth muscle and glands
§ Gastrointestinal hormones (can be triggered by intrinsic factors
but hormones are considered extrinsic factors)
□ Long-range chemical messengers secreted into blood and act
on receptors in distal locations to regulate digestive tract (and
non-digestive tract) functions
- The Mouth and Salivary Glands
-
○ Lips and tongue - contain food in moth, guide food during chewing and
swallowing
○ Teeth - begin mechanical breakdown by chewing of food
○ Palate - roof of the oral cavity
§ Separates oral cavity from nasal passage
§ Allows chewing and breathing to occur simultaneously
○ Uvula: soft tissue that hangs from the rear of the mouth and seals off
nasal passage during swallowing
○ Salivary glands:
§ Sublingual
§ Submandibular
§ Parotid
§ Secrete saliva in response to autonomic stimulation
□ Contains:
® Mucus to moisten food and lubricate
® Lysozyme to lyse bacteria
® Bicarbonate buffers which neutralize acids
® Amylase, which begins chemical digestion of
carbohydrates by cleaving polysaccharides into maltose
□ Contains:
® Mucus to moisten food and lubricate
® Lysozyme to lyse bacteria
® Bicarbonate buffers which neutralize acids
® Amylase, which begins chemical digestion of
carbohydrates by cleaving polysaccharides into maltose
(carb--> maltose--? Absorbable units)
- Pharynx and esophagus
○ Swallowing- refers to the entire process of moving food from the mouth,
through pharynx and esophagus, to the stomach
§ Is a sequentially programmed all-or-none reflex, initiated when
bolus is voluntarily forced by tongue to rear of mouth into pharynx
§ Can be initiated voluntarily but cannot be stopped once it has
begun
§ 2 stages: Oropharyngeal stage, Esophageal stage
□ Oropharyngeal stage: moved bolus through pharynx and into
esophagus. During this stage, food must be prevented from:
® Re-entering the mouth: position of tongue
® Entering the nasal passages: elevation of uvula
® Entering the trachea: epiglottis is presses down over
closed glottis as auxiliary mechanism to prevent food
from entering airways
®
□ Esophageal stage: peristaltic (propulsive) waves move bolus
□ Esophageal stage: peristaltic (propulsive) waves move bolus
to stomach
□ * at end of oropharyngeal stage, the pharyngoesophageal
sphincter closes and breathing resumes
-
- Stomach
○ The stomach is a J-shaped chamber located between the esophagus and
the small intestine
○ Divided into 3 sections:
○
1. Fundus: locates above the gastroesophageal sphincter
2. Body: the middle portion (storing food)
3. Antrum: bottom portion, where mixing happens
1. Thick layer of smooth muscle
2. Connected to small intestine by the pyloric sphincter
○ The stomach has 3 major functions:
I) Store ingested food until it can be emptied into the small intestine.
2. Body: the middle portion (storing food)
3. Antrum: bottom portion, where mixing happens
1. Thick layer of smooth muscle
2. Connected to small intestine by the pyloric sphincter
○ The stomach has 3 major functions:
I) Store ingested food until it can be emptied into the small intestine.
This occurs in the Body of the stomach
□ Gastric filling: gastric volume can expand ~20-fold during a
meal, by expansion/flattening of deep folds
® receptive relaxation: a vagally-mediated process of the
expansion of gastric volume
II) Created gastric secretion: including HCl and enzymes that begin
chemical digestion of protein
□ Two distinct areas of secretory gastric mucosa
1. Oxyntic mucosa (body and fundus)
◊ In oxyntic mucosa, 3 types of EXOCRINE secretory
cells, associated with gastric pits, exocrine
secretion make up digestive juice
} Mucus cells secrete thin, watery mucus
} Chief cells secrete enzyme precursor,
pepsinogen
– Pepsinogen activated by HCl to
pepsin to digest protein
} Parietal (oxyntic) cells secrete HCl and
intrinsic factor (important for Vitamin B12
absorption: essential for normal function of
RBC)
– HCl :
w activates pepsinogen in the
lumen, protecting stomach
from itself
w Denatures protein
w Along with salivary lysozymes,
kills most of the
microorganisms ingested with
food
2. Pyloric gland area (Antrum, PGA)
i) Endocrine secretory cells: secrete the hormone
gastrin into bloodstream
} G cells secrete Gastrin stimulates parietal,
chief and ECL cells
2. Pyloric gland area (Antrum, PGA)
i) Endocrine secretory cells: secrete the hormone
gastrin into bloodstream
} G cells secrete Gastrin stimulates parietal,
chief and ECL cells
– Gastrin increases gastric motility and
promotes movement of leftover,
undigested/unabsorbed material out
of ileum into large intestine
} D cells secrete hormone somatostatin into
bloodstream to inhibit parietal and ECL
cells
} No acid secreted here
•
III. Gastric motility converts pulverized food to chyme - a thick liquid
mixture of pulverized food ad gastric secretions
□ Gastric mixing and gastric emptying- strong peristaltic
contractions occur in the antrum that:
® Mix food with gastric secretions to produce chyme
® Propel chyme towards pyloric sphincter, where a small
amount is pushed into duodenum
® In response to chyme, sphincter closes and remaining
chyme is tumbled back into the lumen
-
§ Control of gastric mixing and gastric emptying(pyloric function):
□ Factors In stomach
a) Volume of the chyme - distention directly stimulates
stretch receptors on the smooth muscle, stimulates
enteric and parasympathetic nervous system as well as
the stomach hormone gastrin to increase motility
b) Fluidity of the chyme - liquids do not require extensive
mixing and churning; contents must be rendered fluid
before they are evacuated
□ Factors in the duodenum (4 factors) P.581
a) Fat is only digested and absorbed within the small
intestine. When fat is present in the small intestine
further emptying is inhibited
◊ Fat is only digested by lipase secreted from the
pancreas, slows down gastric emptying by CCK
b) Acid - highly acidic chyme from the stomach is
neutralized by sodium bicarbonate (secreted from
pancreas) in the duodenum. Un-neutralized acid in the
duodenum inhibits gastric emptying
◊ Slows gastric emptying by secretin, unneutralized chyme is neutralized by Sodium
Bicarbonate released from the pancreas
c) Hypertonicity - increased osmolarity in the duodenum
indicates a back-up of nutrients and delays gastric
emptying
◊ Adding nutrients faster than we absorb so inhibits
gastric emptying
d) Distention - too much chyme in the duodenum inhibits
gastric emptying
□ These factors regulate gastric motility by triggering both
neural and hormonal responses:
◊ Enterogastric Reflex (Neural) : neural responses
are mediated through both intrinsic nerves (short
reflex) and autonomic nerves (long reflex)
□ These factors regulate gastric motility by triggering both
neural and hormonal responses:
◊ Enterogastric Reflex (Neural) : neural responses
are mediated through both intrinsic nerves (short
reflex) and autonomic nerves (long reflex)
◊ Hormonal responses involves release of
hormones from duodenal mucosa
(Enterogastrones)
} Cholecystokinin (CCK), stimulated by fat in
the duodenum. CCK inhibits antral
contractions and induces contraction of the
pyloric sphincter
– This is how fat in duodenum inhibits
gastric emptying
– Triggered by fat and amino acids
released in the duodenum
– Causes contraction of gallbladder and
release bile (open a sphincter)
– Decreases gastric emptying,
duodenum need more time for
emulsification to absorb and digest
the lipids
} Secretin, stimulated by un-neutralized acid
in the duodenum. Secretin is released by S
cells and slows gastric emptying
□ Control of gastric secretion has 3 phases
1. Cephalic Phase(Stimulation for secretion)
i) Stimuli in the head: smelling, seeing, tasting,
chewing, swallowing food stimulates vagus
nerve --> stimulates intrinsic nerves --> release
Ach --> stimulates Chief and parietal cells -->
increase gastric secretion and Histamine
ii) Stimuli in the head: smelling, seeing, tasting,
chewing, swallowing food stimulates vagus
nerve --> stimulates G cells --> secretion of
gastrin --> stimulates ECL cells --> high Histamine
2. Gastric Phase(stimulation for secretion)
i) Stimuli in the stomach: protein, (peptide
fragments), distension, caffeine, alcohol
stimulates Vagus Nerve --> stimulates intrinsic
nerves --> Ach --> stimulates Chief and parietal
gastrin --> stimulates ECL cells --> high Histamine
2. Gastric Phase(stimulation for secretion)
i) Stimuli in the stomach: protein, (peptide
fragments), distension, caffeine, alcohol
stimulates Vagus Nerve --> stimulates intrinsic
nerves --> Ach --> stimulates Chief and parietal
cells --> increase gastric secretion
ii) Stimuli in the head: smelling, seeing, tasting,
chewing, swallowing food stimulates vagus
nerve --> stimulates G cells --> secretion of
gastrin --> stimulates ECL cells --> high Histamine
3. Intestinal Phase(inhibitory, Duodenum )
i) Stimuli: Fat, Acid, Hypertonicity, Distension -->
stimulates Enterogastric reflex and increase
enterogastrones (CCK and secretin)--> inhibits
Parietal cells, chief cells, smooth muscle cells-->
slows gastric secretion to return to normal basal
activity
-
-
- Upper GL Tract Summary: Mouth , Pharynx and esophagus and Stomach
-
- Lower part of the GI Tract: Accessory organs(liver and pancreas), small intestine,
large intestine
- Pancreas and Liver:
○ Juice secreted by small intestine itself does not contain all the necessary
digestive enzymes
○ Material emptying from stomach is acidic, and only partially digested
( *fats not much )
○ Need secretions of accessory organs to complete digestion and neutralize
acid in chyme
- Pancreas: located dorsal and caudal to the stomach. It is a mixed gland that
contains both endocrine and exocrine tissue
○ Exocrine pancreas includes:
§ Duct cells- release sodium bicarbonate (NaHCO3) into duodenum to
neutralize acidic chyme
§ Acinar cells- release digestive enzymes into duodenum (work better
at a neutral or alkaline pH)
□ Pancreatic amylase (carbohydrate digestion)
□ Pancreatic lipase (only enzyme secreted throughout entire
human digestive system that can significantly digest fat)
□ Proteolytic enzymes (secreted as inactive forms)
® Trypsinogen- converted to t]active form from trypsin by
enteropeptidase in the luminal (brush border)
membrane of small intestine
® Chymotrypsinogen - converted to active form
human digestive system that can significantly digest fat)
□ Proteolytic enzymes (secreted as inactive forms)
® Trypsinogen- converted to t]active form from trypsin by
enteropeptidase in the luminal (brush border)
membrane of small intestine
® Chymotrypsinogen - converted to active form
chymotrypsin by trypsin
® Procarboxypeptidase - converted to active form
carboxypeptidase by trypsin
□ Regulation of pancreatic secretion:
® Chyme in the duodenum stimulates pancreatic
secretions via intestinal hormones, aka
enterogastrones
® Secretin (an enterogastrone from intestinal S cells in
duodenal mucosa)
◊ Acid in duodenal lumen --> increase secretin
released from duodenal mucosa --> stimulates
Pancreatic duct cells to secrete NaHCO3 solution
into duodenal lumen--> neutralizes acidic chyme
in lumen
◊ stimulates HCO3- secretion from duct
◊ Inhibits Gastric emptying
◊ Inhibits Gastric HCl secretion (stomach parietal
cells)
® CCK (from duodenal mucosa , Cholecystokinin from
intestinal I cells)
◊ Fat and protein products in duodenal lumen-->
Increase CCK release from duodenal mucosa -->
stimulates Pancreatic acinar cells--> increase
secretion of pancreatic digestive enzymes into
duodenal lumen --> Digests Fat and protein
products in duodenal lumen
◊ Pancreatic enzyme secretion from acinar cells
increase
◊ Inhibits gastric emptying/secretion
◊ Stimulates Gall bladder contraction and Sphincter
of Oddi relaxation (because secreting pancreatic
digestive enzymes)
® Increase volume in duodenum increase secretin and
CCK (weak direct stimulation of duct and acinar cells by
vagus (cephalic phase))
® Increase volume in duodenum increase secretin and
CCK (weak direct stimulation of duct and acinar cells by
vagus (cephalic phase))
®
Liver
Secretion of bile (stored in gall bladder)
The biliary system includes the liver, the bile ducts, and the gall bladder
During meals- secreted from the liver (and or released from gall bladder) and
enters the duodenum
- Between meals - the sphincter of Oddi closes and bile is diverted into the
gallbladder for storage (until next meal)
- After meal is digested, ~95% of bile is reabsorbed in the distal small intestine
and carried to the liver
-
-
- Bile:
○ Secreted by liver, stored in gallbladder
○ Consists of
§ Bile acids/salts
§ Cholesterol
§ Phospholipid (Lecithin)
§ Bilirubin (RBC breakdown product)
§ Aqueous mixture or bicarbonate, ions, water
§ 95% bile reabsorbed after lipid digestion is complete
§ Bile acids/salts
§ Cholesterol
§ Phospholipid (Lecithin)
§ Bilirubin (RBC breakdown product)
§ Aqueous mixture or bicarbonate, ions, water
§ 95% bile reabsorbed after lipid digestion is complete
○ Roles:
§ aids in fat digestion by emulsification (increases surface area for
lipase)
□ Emulsification/ detergent action increases surface area for
lipase
□ Bile salts have hydrophobic and hydrophilic components
§ Helps neutralize stomach acid
§ Cholesterol balance
○ CCK caused bile to be secreted!! (regulation enterogastrone from
duodenal mucosa for fat)
- SUMMARY OF ACCESSORY ORGANS:
-
- Small Intestine
- Primary site of digestion and absorption
- Three segments : Duodenum ~5 % of length, Jejunum 35-40%, Ileum 55-60%
- Small Intestine
- Primary site of digestion and absorption
- Three segments : Duodenum ~5 % of length, Jejunum 35-40%, Ileum 55-60%
-
- Motility in the small intestine occurs primarily via segmentation, which both
mixes and propels chyme. Propulsion occurs because the frequency of
contractions gradually decreases along length of small intestine (duodenum ~12
/min, ileum ~9/min
- Large Surface Area to facilitate absorption
○ Circular folds (increase SA 3x)
○ Surface of circular folds contain microscopic villi (increases SA by an
additional 10x)
○ Surface of villi contain microvilli (brush border) (increases SA by an
additional 20x
○ All together, the folds, villi and microvilli increases the SA by 600 times
○
○ In the intestinal lumen, carbohydrate and protein digestion is
accomplished by pancreatic enzymes, with fat digestion enhanced by bile
secretions
○ The small intestine does produce digestive enzymes, but these act on the
surface of the cells lining the brush border
§ Brush border contains 3 types of enzymes:
□ Enteropeptidase
□ The disaccharidase: maltase, sucrase, and lactase which
complete the digestion of carbohydrates
□ Aminopeptidases, which complete the digestion of proteins
- Carbohydrate Digestion in the small intestine:
1. The polysaccharides starch and glycogen are converted to the
disaccharide maltose by amylase in the mouth and digestive tract lumen
2. Maltose, lactose, and sucrose are converted to monosaccharides (glucose,
complete the digestion of carbohydrates
□ Aminopeptidases, which complete the digestion of proteins
- Carbohydrate Digestion in the small intestine:
1. The polysaccharides starch and glycogen are converted to the
disaccharide maltose by amylase in the mouth and digestive tract lumen
2. Maltose, lactose, and sucrose are converted to monosaccharides (glucose,
galactose, fructose) on the brush border of intestinal epithelial cells by the
enzymes lactase, maltase, and sucrase
3. Glucose and galactose are absorbed into the epithelial cells by active
transport (because they are too big)
4. Fructose enters the epithelial cells by passive facilitated diffusion
5. Glucose, galactose, and fructose exit the cell into the blood by passive
facilitated diffusion
○ Polysacharride --> disaccharide maltose, lactose, sucrose (by amylase
from mouth and pancreas) --> converted to monosaccharides, glucose,
galactose, fructose (by disaccharidases: maltase, lactase, sucrase)-->
glucose and galactose absorbed in epithelial cells by active
transport(SGLT) , fructose by passive diffusion --> then travels to the
blood from the cell by passive facilitated diffusion
-
-
- Protein Digestion in the Small Intestine
○ Proteins hydrolyzed into small peptide fragments and individual amino
acids by pepsin and pancreatic proteolytic enzymes
○ Small peptides are broken down into amino acids on the brush border by
peptidase and aminopeptidase
○ Amino acids absorbed into cell via Na+ and energy-dependent active
transport and enter blood down their concentration gradients
-
- Fat Digestion in the Small Intestine
1. Fat is emulsified by the detergent action of bile salts
2. Lipases hydrolyze triglycerides into monoglycerides and free fatty acids
3. Water insoluble products move within the interior of micelle to the epithelial
cell surface
4. Monoglycerides and free fatty acids diffuse into cell
5. Monoglycerides and free fatty acids resynthesize into triglycerides
6. Triglycerides coated with lipoprotein(ER) and from chylomicrons that
excocytosed from cell to lymphatic vessels
- Lumen = aqueous environment
○ Emulsified by bile salts (hydrophilic side of bile salt facing lumen)
○ Form micelle in lumen, fat is hydrophobic
6. Triglycerides coated with lipoprotein(ER) and from chylomicrons that
excocytosed from cell to lymphatic vessels
- Lumen = aqueous environment
○ Emulsified by bile salts (hydrophilic side of bile salt facing lumen)
○ Form micelle in lumen, fat is hydrophobic
○ Close proximity to microvilli = can diffuse because surrounded by lipid
bilayer
○ Micelles and chylomicrons differ in size and contents: chylomicron is a mix
of fatty acids and monoglycerides and cholesterol
- After everything digested in small intestine --> Large Intestine :
- the ileocecal valve and sphincter
○ One-way flow- of contents from ileum(last part of small intestine) into
cecum (first part of large intestine)
○ Necessary to keep colonic bacteria from entering the ileum
○ Gastrin inhibits ileocecal sphincter and stimulates pyloric sphincter when
there is food in stomach
○
- Large intestine Primarily for drying and storage, includes:
1. Cecum(blind-ended pouch below ileocecal valve)
- Large intestine Primarily for drying and storage, includes:
1. Cecum(blind-ended pouch below ileocecal valve)
2. Appendix(finger-like projection of lymphoid tissue)
3. Colon (ascending, transverse, descending, and sigmoid)
4. Rectum ("straight", connected to anal canal
-
- Motility of Large Intestine
○ Haustral contractions- slowly shuffle contents of large intestine to aid
absorption (primarily of water and salt)
○ Mass movement - large contractions in ascending and transverse colon
that rapidly drive contents forward (generally 1/3 to 3/4 length of colon in
few seconds)
§ Gastrocolic reflex: typically happens after a meal, when chyme is
present in stomach
□ act of eating stimulates movement in the gastrointestinal
tract.
that rapidly drive contents forward (generally 1/3 to 3/4 length of colon in
few seconds)
§ Gastrocolic reflex: typically happens after a meal, when chyme is
present in stomach
□ act of eating stimulates movement in the gastrointestinal
tract.
§ Defecation reflex- initiated by mass movement of feces into the
rectum, which stimulates stretch receptors
□ Causes the internal anal sphincter (smooth muscle =
involuntary) to relax and the rectum and sigmoid colon to
contract
□ IF the external anal sphincter is also relaxed, defecation
occurs
□ Since the external anal sphincter is made of skeletal muscle ,
the sphincter is under voluntary control
○ Constipation: occurs when defecation is delayed too long and too much
water is absorbed from the feces (becomes dry and hard)
○ Appendicitis: can occur if hardened feces get lodged in the appendix,
obstructing normal circulation and mucus secretion
- Intestines Summary
-