The Digestive System
Digestive System: Overview
• The alimentary canal or gastrointestinal
(GI) tract digests and absorbs food.
– Alimentary canal – mouth, pharynx,
esophagus, stomach, small intestine, and
large intestine.
• Accessory digestive organs – teeth,
tongue, gallbladder, salivary glands, liver,
and pancreas.
Greater Omentum
• Greater - covers small intestines like an apron.
• This is the visceral fat that may be detrimental to your
health.
Where does she carry most of her
weight?
Mesentery and Mesocolon
• Mesentery of small intestines contain the nerves and the
blood vessels
• Mesocolon anchors colon to posterior body wall
•Peritoneum – serous
membrane of the abdominal
cavity
Visceral – covers external
surface of most digestive
organs
Parietal – lines the body
wall
•Peritoneal cavity- small space
between visceral and parietal
peritoneum.
Lubricates digestive organs
Allows them to slide across
one another
Retroperitoneal structures
are located behind the
peritoneum.
The Digestive System Process
1. Ingestion: intake of food.
2. Digestion: Mechanical and chemical
breakdown of food.
3. Absorption: Uptake of nutrients from the
digestive tract into the blood and lymph.
4. Compaction: Absorbing water and
consolidating indigestible food to form
feces.
5. Defecation: Elimination of the feces.
Types of Digestion
• Mechanical digestion
– physical breakdown of food into smaller particles
– teeth and churning action of stomach and intestines
• Chemical digestion
– series of hydrolysis reactions that break macromolecules
into their monomers
– enzymes from saliva, stomach, pancreas and intestines
– Results
• Polysaccharides into monosaccharides
• Polypeptides into amino acids
• Triglycerides into glycerol and fatty acids
Motility in the Intestines
• Segmentation ensures mechanical digestion in the
SI.
– Ensure enzymes mix with proper substrates for maximal
absorption.
Peristalsis
Peristalsis moves contents steadily in one direction along
the alimentary canal.
Histology of the Alimentary Canal
• From esophagus to the anal canal the
walls of the GI tract have the same four
layers (tunics)
– From the lumen outward they are the mucosa,
submucosa, muscularis externa, and serosa
• Each tunic has a predominant tissue type
and a specific digestive function
Anatomy of the Alimentary Canal
Diaphragm
Serosa
Mucosa
Muscularis
Submucosa
Blood vessels
Mucosa
• The innermost layer epithelial layer that lines the
lumen of the alimentary canal
• Stomach and small intestine mucosa contain:
– Enzyme-secreting cells that aid in the
absorption of nutrients for of digestion
– Mucus producing cells protects the lumen from
hydrolytic enzymes while aiding the movement food
through the alimentary canal
• Has many lymphatic vessels that protection
against infectious disease
Other layers of the Alimentary Canal
• Submucosa –
– Dense connective tissue containing elastic fibers
– Blood and lymphatic vessels
– Nerves (Submucosal Nerve Plexus)
• Muscularis externa –
– Composed of involuntary smooth muscle innervated
by the (Myenteric Nerve Plexus)
– responsible for segmentation and peristalsis
• Serosa
– the protective visceral peritoneum
– Outer most layer
Enteric Nervous Control
Myenteric Nerve Plexus
Submucosal Nerve Plexus
Enteric Nervous Control
• Is often referred to the second brain because it
can regulate digestive tract mobility, secretion
and blood flow independently of CNS.
– It is thought that there are more neurons in the ENS
than the spinal cord.
• Composed of two nerve networks
– submucosal plexus
• controls glandular secretion and muscular
contraction of the mucosa
– myenteric plexus
• controls peristalsis
• contractions of muscularis external
Autonomic Function in Digestion
• Parasympathetic nervous
system has many neural
connections to the ENS.
• It stimulates digestive
activities via the vagus
nerve.
• Sympathetic nervous
system inhibits
digestive activity by
shunting blood away
from the gut and to the
heart and skeletal
muscles.
Phases of Digestion
Cephalic phase
• Prepares the digestive system before the ingestion of
food. Stimulation of the senses (sight, smell and taste)
or thought of food will activate the Parasympathetic
Nervous System (PNS) via the vagus nerve.
• The medulla will stimulate the both digestive organs
and glands to increase their metabolic activity.
• Excitatory events include:
– Sight or thought of food
– Stimulation of taste or smell receptors
• Inhibitory events include:
– Loss of appetite or depression
– Decrease in stimulation of the parasympathetic
division
Digestive Processes in the Mouth
• Mechanical digestion begins in the mouth
(chewing)
• Salivary amylase begins chemical breakdown of
starch.
– Mouth is at a fairly neutral ph. Ideal for amylase
• Propulsion is initiated by swallowing
• The pharynx and esophagus serve as a
passage way for food from the mouth to the
stomach.
Salivary Glands
Saliva
• Functions of saliva
– moisten, begin starch and fat digestion, cleanse teeth,
inhibit bacteria, bind food together into bolus
• Composed of 99.5% water and solutes
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salivary amylase, begins starch digestion
lingual lipase, digests fat activated by stomach acid
mucus, aids in swallowing
lysozyme, enzyme kills bacteria
immunoglobulin A, inhibits bacterial growth
electrolytes = Na+, K+, Cl-, phosphate and bicarbonate
• pH of 6.8 to 7.0
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Stomach
Chemical breakdown of proteins
and to a lesser extent fats begins
and food (bolus) is converted to
chyme.
Cardiac region –connects the
esophagus to stomach.
Muscularis –Is composed of 3
muscular layers
– Allows the stomach to churn,
mix, and pummel food
physically
Rugae: folds in the stomach when
empty stomach
The pylorus is continuous with the
duodenum through the pyloric
sphincter
– Regulates how rate that food
enters into the small intestines.
Digestion in the Stomach
• The stomach:
– Holds ingested food
– Digests food both mechanically and chemically
– Delivers chyme to the small intestine
– Enzymatically digests proteins with pepsin
• Lingual lipase activated.
– Secretes intrinsic factor required for absorption of
vitamin B12
• Vitamin B12 is necessary for the production of many
different type of cells including nerve and red blood
cells
– Pernicious anemia can result from a lack of
either Vitamin B12 or intrinsic factor.
Glands of the Stomach
• Gastric glands of the fundus and body have a variety of
secretory cells
– Mucous neck cells – secrete mucus to protect stomach
from acid environment.
– Parietal cells – secrete H+Cl- , intrinsic factor and ghrelin
– Chief cells – produce pepsinogen and gastric lipase
• Pepsinogen is activated to pepsin by H+Cl- in the
stomach
– Enteroendocrine cells – secrete hormones that regulate
digestion and gastric motility.
• G-cells secrete gastrin : hormone that increases
secretory activity and gastric mobility
Microscopic Anatomy of the Stomach
Gastric Phase
•
Excitatory events include: Stimulate
Gastrin release
– Stomach distension (stomach
expands as bolus enters)
• Activation of stretch receptors stimulates
the Vagus nerve
– Activation of chemoreceptors:
presences of proteins buffer
stomach acid ↑pH
•
Inhibitory events include:
– As food exits out the pylorus the pH
may drop below 2
– There will be less distension.
– Emotional upset that overrides the
parasympathetic division and
activate the sympathetic division
Control of Gastric Secretion
Stomach Acid Functions
• Activate enzymes pepsin and lingual lipase.
• Breakdown plant connective tissues and cell walls.
• Denatures proteins and converts minerals to a more
bioavailable form.
• Contributes to immunity by destroying ingested pathogens.
Healthy Mucosa and Peptic Ulcer
• Gastric and Peptic ulcers are linked to the acid tolerant
bacteria Helicobactor pylori and excessive use of NSAID’s.
• Antibiotics function to kill the H pylori bacteria
• Proton pump inhibitors block the H+K+ATPase pump thus
reducing acid production in the stomach.
Hiatal Hernia
• Part of the esophagus and
upper part of stomach get
trapped in the thoracic
cavity.
• LES sphincter fails
allowing acid in the
esophagus.
– mimic symptoms of a heart
attack.
• Over time it will erode the
esophagus resulting in
strictures, hemorrhage or
even cancer.
Hormones and Gastric Emptying
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•
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Once food (Chyme) enters the small intestines duodenal
enteroendocrine cells secrete various hormones that inhibit gastric
secretion :
–
CCK, (Cholecystokinin) VIP (Vasoactive Intestine Peptide )
GIP( Gastric inhibitory peptide) and Secretin
These collectively inhibit stomach secretions of HCL and gastrin from
the G-cells thus reducing stomach motility.
Pyloric sphincter tightens limiting admission of more food into the
duodenum.
–
Allows the duodenum time to work on the chyme that has already
entered.
Small Intestine – 3 Segments
• Has three subdivisions: duodenum, jejunum, and ileum
• Runs from pyloric sphincter to the ileocecal valve
Small Intestine
• Duodenum (10 in.)
– Receives stomach contents through the pyloric sphincter.
– Pancreatic juice and bile aid in chemical digestion of chyme.
– Neutralizes stomach acids, emulsifies fats, pepsin inactivated by pH
increase, pancreatic enzymes hydrolyze their specific substrate.
– Mostly digestion with minimal absorption occurs here.
• Jejunum - next 8 ft. (in upper abdomen)
– has large tall circular folds; walls are thick, muscular
– nutrient digestion and absorption occurs here
• Ileum - last 12 ft. (in lower abdomen)
– has peyer’s patches – clusters of lymphatic nodules
– ends at ileocecal junction with large intestine
– Primarily nutrient absorption occurs here
Microscopic Anatomy of the Small Intestine
Small Intestine: Histology
• Lined simple columnar epithelium which functions in
absorption and secretion.
– Villi
• fingerlike extensions of the mucosa containing blood
and lymph vessels for the absorption of nutrients
– Microvilli
• tiny folds in the individual simple columnar epithelial
cells that further increase digestive surface area.
• Brush-border enzymes
– hydrolytic enzymes responsible for chemical
digestion of organic molecules and activation of
pancreatic enzymes.
• Pancreatic enzymes and bile enter the duodenum through
for the sphincter of Oddi
Liver: Functions
• The largest gland in the body
• Liver cells( Hepatocytes) functions include:
– Production of bile (emulsification of fat)
– Detoxification( removes or dilutes toxins in the
body
– Storage of fat-soluble vitamins ( ADEK)
– Increases BGLs: gluconeogenesis and
glycogenolysis.
– Decreases BGL’s: Stores excess glucose as
glycogen
– Formation of cholesterol including HDL and
LDL
• Used to transport lipids throughout the
circulatory system.
Pancreas
• Exocrine function
– Acini (clusters of secretory
cells) digestive enzymes
– Secretes pancreatic juice
which breaks down all
categories of foodstuff
• The pancreas also has an
endocrine function
– release of insulin and
glucagon
Acinus of the Pancreas
Hormonal Control of the Intestinal Phase
• Baroreceptor and Chemoreceptors detects distension
chemical
changes in the intestines results in enteroendocrine cells to
secrete two hormones
• Secretin causes the release of HCO3– from the pancreas
neutralizing acid chyme
• Alkaline pH provides an optimal environment for
pancreatic enzymes
• CCK stimulates the pancreas to release the following digestive
enzymes: Amylase ,Lipase and Proteases : (trypsinogen,
chymotrypsinogin, procarboxypeptidase)
– contraction of the Gall bladder
– Relaxes the Sphincter of Oddi into the duodenum.
– Promotes short term satiety
The Gallbladder
• A green muscular sac on the ventral surface of
the liver is palpable in the right upper quadrant.
• Stores and concentrates bile that was produced
in the liver
• Bile is released in response to the hormone
CCK.
• Main role of bile is to emulsify fat (break into
smaller pieces)
• Emulsified fat droplets have a greater surface
area for pancreatic lipase to work.
Emulsification of Fats
Fatty Acid Absorption
Chemical Digestion: Fats
• Absorption: bile salts and pancreatic lipase
results in FFA and Glycerol
• FFA diffuse into intestinal cells where they:
– Combine with proteins and form chylomicrons
– Large chylomicrons must enter lacteals and are
transported to systemic circulation via lymphatic
system.
Carbohydrate Metabolism
• Salivary amylase gets denatured in stomach (pH < 4.5)
• Pancreatic amylase further hydrolyzes carbohydrates.
• Brush border enzymes maltase, sucrase, lactase
complete hydrolysis resulting in monosaccharides.
– Maltase → Hydrolyzes maltose into 2 glucose
molecules
– Sucrase → hydrolyzes sucrose leaving fructose and
glucose
– Lactase → hydrolyzes lactose into Galactose and
glucose
• Absorption: via co-transport with Na+, and facilitated
diffusion
– Enter the capillary bed in the villi
– Transported to the liver via the hepatic portal vein
Carbohydrate Digestion - Small Intestine
Protein Digestion
Protein Digestion
– Enzymes for protein digestion are released in
inactive forms to avoid digesting the pancreas and
associated ducts prior to reaching its appropriate
substrates:
– Enzymes include:
• Trypsinogen is activated by brush border enzyme
entrokinase to trypsin
• Chymotrypsinogin is activated to Chymotrypsin by trypsin
• Procarboxypeptidase is activated by trypsin to
carboxypeptidase
– Both break down proteins into A.A via hydrolysis
reactions
Alcohol metabolism
Alcohol can be absorbed directly through the stomach.
– Food acts as a buffer
• Can cross the blood brain barrier of the brain
– Impaired judgment cognitive and physical abilities.
– DWI can result in sharing a cell with a guy named Bubba
• Pound for pound females usually can’t tolerate as much
alcohol as their male counterparts because they
naturally produce less of the enzyme that breaks it down.
(Alcohol dehydrogenase)
– The heavier you can usually tolerate more alcohol.
– Tolerance varies between different ethnicities.
• With a lot of practice you will make more of the enzyme
and be able to hold your liquor.
Absorption of Polar Molecules
• The absorption of water soluble polar molecules
(carbohydrates, amino acids, nucleotides, electrolytes
and water) is accomplished by specific transport
proteins located on the cell membranes of the
absorptive cells. Membrane proteins utilize
– secondary active transport: Sodium dependant cotransport proteins is necessary for the absorption of glucose
, Galactose, amino acids and many minerals
• 95% of the water is absorbed in the SI via osmosis
following the osmotic gradient established by the Na+,K+ATPase (pump
– Digestion requires a lot of ATP! Should this
influence the frequency of your meals if you’re
trying to loss weight.
Carbohydrate Absorption
• Sodium-glucose transport proteins (SGLT) in
membrane help absorb glucose and galactose
Pancreas
• Has both exocrine and endocrine cells and is
located behind the stomach
– Acinar cells produce an enzyme-rich juice used for
digestion (98% of organ) (exocrine product)
– Pancreatic islets (islets of Langerhans) produce
hormones (endocrine products)
• The islets contain two major cell types:
– Alpha () cells that produce Glucagon
– Beta () cells that produce insulin
• Remember: If your diabetic you Beta () have
your insulin.
Pancreatic Islet
Insulin
• Insulin:
– Lowers blood glucose levels by increasing
membrane permeability to both glucose and
amino acids.
• Considered an anabolic hormone
– Target organs include muscle, liver and adipose
tissue
– antagonizes Glucagon
• Insulin uses a second messenger which allows more
glucose to enter the cell. Once in the cell insulin:
– Increased utilization of glucose for ATP production
– Glycogen is formed when anabolic enzymes
stimulate dehydration synthesis of glucose.
– Converts extra glucose to fat particularly in
adipose tissue and liver cells once glycogen
stores are full.
– Promotes cellular mitosis
Glucagon
• Hormone released when blood sugar levels drop
• Its major target is the liver where it promotes:
– Glycogenolysis – the breakdown of glycogen
to glucose
– Gluconeogenesis – synthesis of glucose from
lactic acid and non-carbohydrates
– Release of glucose to the blood from liver cells
– All will elevate blood glucose levels during
fasting periods.
Large Intestine
• Divided into 4 regions:
– ascending colon: ascends up the right side of the
abdominal cavity
• cecum
– Lies below the ileocecal valve
– Contains a wormlike vermiform appendix
» Acts as a reserve for good flora and other
immune boosting cells following diarrhea.
» Inflammation of appendix (appendicitis)
– transverse colon: runs across the abdominal cavity
– descending colon: descends down the left side of
abdominal cavity.
– sigmoid colon: S shaped leads to rectum where feces
is stored until defecation occurs through the anal canal.
Functions of the Large Intestine
• 95% of water is absorbed in the small intestines
following their osmotic gradient.
• The remaining water is absorbed in the large
intestine.
• Other than digestion of enteric bacteria(
probiotics), no further digestion takes place
– Vitamins, and electrolytes are reclaimed
• Its major function is propulsion of fecal material
toward the anus
Neural Control of Defecation
Case Study 1
• 40-year-old male has chief complaints not
being able to sleep throughout the night
without making multiple trips to the
bathroom. The patient is 5 foot four 260
pounds. Normally has difficulty sleeping
because he’s been diagnosed with sleep
apnea. He also reports that his wounds tend
to heal very slowly.
• What’s a possible diagnosis?
Diabetes Mellitus (DM)
• Chronically elevated blood glucose levels results from
hyposecretion or hypoactivity of insulin
• The three cardinal signs of DM are:
– Polyuria – Frequent urination from excessive sugar
levels spilling over in kidney filtrate. Water will follow
the osmotic gradient to dilute the urine (filtrate).
– Polydipsia – excessive thirst as a result if frequently
urinating and dehydration.
– Polyphagia – excessive hunger and food
consumption because the body thinks its starving
because the sugar is not able to get into the cell.
• Hyperinsulinism – excessive insulin secretion, resulting
in hypoglycemia
Hyperinsulinism
• From excess insulin injection or pancreatic islet
tumor
– Causes hypoglycemia, weakness and hunger
– triggers secretion of epinephrine, GH and
glucagon
• side effects: anxiety, sweating and  HR
• Insulin shock
– uncorrected hyperinsulinism with
disorientation, convulsions or
unconsciousness
Types of Diabetes Mellitus
• Type I (IDDM) - 10% of cases
– some cases have autoimmune destruction of 
cells, diagnosed about age 12
– treated with diet, exercise, monitoring of blood
glucose and periodic injections of insulin
• Type II (NIDDM) - 90%
– insulin resistance
• failure of target cells to respond to insulin
• Excessive weight gain increases intramyocellular lipids which turns down
mitochondria.
• Excessive accumulation of lipids in the liver
contribute to inflammation and insulin
resistance
Fatty Liver/ Myocellular Lipids
Major Risk Factors
– 3 major risk factors are heredity, age (40+)
and obesity
• treated with weight loss program of diet and
exercise
• oral medications improve insulin secretion
or target cell sensitivity
–Gastric bypass surgery has been shown
to cure DM in 60-80% of cases. What’s
they key?
Obesity Trends* Among U.S. Adults
BRFSS, 1990, 1999, 2009
(*BMI 30, or about 30 lbs. overweight )
1999
1990
2009
No Data
<10%
10%–14%
15%–19%
20%–24%
25%–29%
≥30%
Who is part of the Pepsi generation?
Have a Coke and a smile!
Anti-inflammatory Pyramid
Food Labels
• Is it really low fat?
– 1% milk = 18% and 2% =36%fat calories
• Whole milk is more then 50 % fat calories
– Meat 75% lean has 225 calories which 14 grams come from fat
which equals 126 total fat calories = 57% fat by calories
– 93% ground lean = 45% fat calories
• because labeled by weight not % calories
The diet and conditions your meal lived in is important!
• The food industry can make up the serving size.
– 5 calories or less = non caloric.
– ½ gram of fat or less = none fat. Watch the potions size!
– Lite means ½ the amount of fat as the original.
• Usually sugar is substituted.
Food Labels
• Organic means 95% organic products
– Look for the USDA Organic food label
• Made with organic must be 70%
• Free range: in a chicken coup that the door must stay open 5
minutes a day.
• Pastured means cage is on the grass.
•
Know your farmer!
• Whole Grain= only has to be a little more then ½ to say it on
the label.
– Made with whole grain may almost have none.
• Look for 100%
Digestive System Screen
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Changes in appetite
Abdominal and chest pain
Nausea / Vomiting
Fatigue/ Weakness
Bowel movement changes such as constipation and Diarrhea
Stool appearance( Color size and shape)
Dehydration
Psychological changes like depression and anxiety from nutritional deficiencies.
(gas) flagalence
Yellow skin, eyes and nails
Pancreatitis
Stones
Ulcer history
Hernias
Rebound tenderness
Hemorrhoids
Case Study
• A 33 y/o female goes to her doctor with c/o
bloating and diarrhea (3 years). She now
is feeling depressed and has trouble
getting more than 6 hours of sleep a night.
She also reports occasional numbness
and tingling in her feet. Lab test reveal she
has deficiencies in a variety of vitamins,
minerals and amino acids.
– What's going on with this patient?