Download The Digestive System

The Digestive
System-Chapters
62-66; 70; 78
Figure 62-1; Guyton & Hall
1
Digestive Processes





Ingestion
Propulsion
Digestion: Mechanical and
Chemical digestion
Absorption- nutrients and
water
Defecation
2
Layers Alimentary Canal









1. Serosa
2. Longitudinal muscle
(muscularis externa)
3. Myenteric
(Auerbach’s) nerve
plexus
4. Circular muscle
5. Submucosa
6. Submucosal
(Meissner’s) nerve
plexus
7. Muscularis mucosae
8. Mucosa
9. Epithelial lining
3
Autonomic nerve fibers

Both divisions found in myenteric
and submucosal nerve plexi—
What do they do?
 Sensory neurons that monitor
tension, and efferent visceral
motor fibers. OWN SYSTEM!
 Myenteric-GI motility control
- Stimulatory influences •  tonic contraction (tone)
•  contraction frequency /
intensity ( propulsion)
- Inhibitory influences
• Decreased Sphincter tone
(relax) - pyloric sphincter,
ileocecal sphincter, LES
 Submucosal- Local control
Secretion
Absorption
Contraction of muscularis
mucosa
4
Control of the digestive system

Movement of
materials along the
digestive tract is
controlled by:

Neural mechanisms


Hormonal mechanisms


Parasympathetic (Ach)
and local reflexes
Enhance or inhibit
smooth muscle
contraction
Local mechanisms

Coordinate response to
changes in pH or
chemical stimuli and
stretching
5
Digestive Enzymes
Salivary glands
-amylase
lingual lipase
Stomach
pepsin
Pancreas
amylase
trypsin
chymotrypsin
carboxypeptidase
lipase
cholesterolesterase
Intestinal Mucosa
enterokinase
sucrase
maltase
lactase
aminooligopeptidase
dipeptidase
6
The mouth opens into the oral or
buccal cavity
Primary Secretion
Alpha-amylase

Its functions include:




Analysis of material
before swallowing
Mechanical processing
by the teeth, tongue,
and palatal surfaces
Lubrication
Limited digestion



Lingual lipase
(negligible fat
digestion)
Salivary amylase
(limited carbohydrate
digestion)
Antibodies and
proteolytic enzymes
7
Digestion and absorption in the
stomach







Short-term storage
reservoir
Secretion of intrinsic
factor
Pepsinogen
gastrin
Chemical and enzymatic
digestion is initiated,
particularly of proteins
Liquefaction of food
Slowly released into the
small intestine for further
processing
8
Gastric glands

Two types glands - Gastric (oxyntic)
HCl
- Pyloric -
gastrin
mucus
pepsinogen
intrinsic factor
mucus
9
Gastric glands- 3 types of cells



Mucous Neck cell
(goblet)- release
mucus to protect
mucosa from acid and
pepsin
Parietal cells- HCl and
intrinsic factor (B12
absorption by small
intestine).
Chief- numerous and
release pepsinogen
80%
10
Acid production and secretion
LUMEN
BLOOD
HO- + H+
H2 O
CO2
CO2
HCO
HCO
3
3
K+
Na+
ClH2 O
P
C.A.
K+
K+
Na+
Na+
P
K+
P
Cl-
Cl
-
osmosis
Na+
P
H+
H2 O
Cl-
Final Results
HCl - 155 mEq/L
KCl - 15 mEq/L
NaCl - 3 mEq.L
pH = 0.8
11
2 cell types of Pyloric gland

G-cells - release gastrin


Enteroendocrine cells stimulates parietal cells
to secrete acid and
increases pyloric
contraction; relaxes
pyloric sphincter
20%
Mucus neck cells
- mucous
12
Gastric and Duodenal
ulcers
Weakens
H. pylori, aspirin, ethanol,
NSAIDs,
bile salts

Strengthens
mucus, HCO3- secretion,
gastrin, PGs, epidermal growth
factor
Peptic ulcers occur when damaging effects of
acid and pepsin overcome ability of mucosa
to protect itself

Gastric ulcers - main problem is decreased ability of
mucosa to protect itself

Duodenal ulcers - main problem is exposure to increased
amounts of acid and pepsin
13
What is the Gastric Mucosal Barrier?




alkaline mucus resists the acid and enzymes
Tight junctions-gastric juice can’t seep into
lamina propria
Epithelial cell replacement- 3-6 day life span.
Physiological - diffused H+ ions are transported back to
lumen
Damaged Gastric Mucosal Barrier

H+ back-leaks into mucosa in exchange for Na+. This is a
forerunner to gastric ulcer 
Decreased cell pH leads to cell death

Damaged mast cells (ECL cells) leak histamine
Viscous cycle - Histamine .. vascular damage .. local ischemia .. greater leakage of

H+.. more cell death ...
14
Helicobacter pylori
 H.
pylori found in 95% patients with DU
and 100% patients with GU (when alcohol,
aspirin, NSAIDS are eliminated)

Gram negative bacterium

High urease activity - high NH4+ activity
- can withstand acid environment
- NH4+ damages epithelial cells (GU)
- Increases acid secretion (DU)
15
Treatment of Peptic Ulcers
 Antacids
 H2
receptor blockers - Rantidine (Zantac)
- Cimetidine (Tagamet)
 Proton pump inhibitors
- Omeparazole
(Prilosec)
 Antibiotics
 Surgical
(rare)
- vagotomy
- antrectomy
16
Stimulation of acid secretion

Gastric secretion is
stimulated by local
(distention), neural, and
endocrine mechanisms




Acetylcholine - HCl
secretion
- mucus, pepsinogen, and
gastrin
Histamine
- HCl
secretion
Gastrin
- HCl
secretion (1500x more
powerful compared to
histamine)
Seeing, smelling and anticipating food is perceived in brain. Brain tells stomach to
prepare for receipt of meal
Accounts for 30% of acid response to meal
60%
10%
17
Small intestine

Important digestive
and absorptive
functions


Three subdivisions:




Secretions and buffers
provided by pancreas,
liver, gall bladder
Duodenum
Jejunum
Ileum
Ileocecal sphincter

Transition between
small and large
intestine
18
Histology of the small intestine




Plicae
 Transverse folds of the intestinal
lining
Villi
 Fingerlike projections of the mucosa
Lacteals
 Terminal lymphatic in villus
Microvilli

Brush border: increases surface area
20-fold
19
Intestinal glands




secretin to stimulate
pancreas to release
bicarbonate mucus
cholecystokinin to
stimulate pancreas
and gallbladder
Gastric Inhibitory
peptide (GIP)inhibits gastrin
secretion and
decreases stomach
emptying
Duodenal glandsbicarbonate mucus.
20
The Activities of Major Digestive Tract Hormones
21
Figure 24.22
Small Intestine- digestive enzymes


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Maltase- splits maltose into 2
glucose units
Lactase- splits lactose into
glucose and galactose
Sucrase- splits sucrose into
glucose and fructose
Peptidase- breaks down small
peptides into amino acids
Intestinal lipase- breaks down
triglycerides into free fatty acids
and monoglycerides
Enterokinase- Activates
trypsinogen to trypsin (trypsin
then activates chymotrypsinogen
and procarboxypeptidase)
22
Pancreas

As chyme floods into small intestine two
things must happen:

Acid must be neutralized to prevent damage to
duodenal mucosa

Macromolecular nutrients - proteins, fats and starch
must be broken down much further so their
constituents can be absorbed
Pancreas plays vital role in accomplishing both objectives
Digestive enzymes for all food types
Bicarbonate solution to neutralize acid chyme
23
Regulation of Pancreatic Secretion



Secretin and CCK are
released when fatty or
acidic chyme enters the
duodenum
CCK and secretin enter the
bloodstream
Upon reaching the
pancreas:



CCK induces the secretion
of enzyme-rich pancreatic
juice
Secretin causes secretion
of bicarbonate-rich
pancreatic juice
Vagal stimulation also
causes release of
pancreatic juice
24
The Pancreas

Exocrine function
(98%)


Acinar cells make,
store, and secrete
pancreatic enzymes
Endocrine function –



( cells) release
somatostatin (inhibitory
to gastrin and insulin and
glucagon)
β-cells –release insulin
α-cells-Release glucagon
25
The Pancreas as an Endocrine Gland

Insulin






Beta cells
Skeletal muscle and
adipose tissue need it to
make glucose receptors
Promotes glucose uptake
Prevents fat and glycogen
breakdown and inhibits
gluconeogenesis
Increases protein
synthesis
Promotes fat storage
Epi/Norepi inhibit insulin!
Help maintain glucose levels
during times of stress and
increase lipase activity in
order to conserve glucose
levels
26
Picture from:http://www.dkimages.com/discover/Home/Health-and-Beauty/Human-Body/Endocrine-System/Pancreas/Pancreas-1.html
The Pancreas as an Endocrine Gland

Glucagon





Maintains blood glucose
between meals and during
periods of fasting.
Nervous tissue (brain) do
not need insulin; but are
heavily dependent on
glucose levels!
Increases blood glucose
levels.
Initiates glycogenolysis in
liver (within minutes)
Stimulates amino acid
transport to liver to
stimulate gluconeogenesis
Image from: http://www.dkimages.com/discover/previews/768/74261.JPG
27
Disorders of the Pancreas: Diabetes
Mellitus


Gestational Diabetes
Type I diabetes –
develops suddenly,
usually before age 15



Destruction of the beta cells
Skeletal tissue and adipose
cells must use alternative
fuel and this leads to
ketoacidosis
Hyperglycemia results in
diabetic coma
28
Disorders of the Pancreas: Diabetes
Mellitus

Type II diabetes and
metabolic syndrome–
adult onset



Usually occurs after age
40
Cells have lowered
sensitivity to insulin
Controlled by dietary
changes and regular
exercise
29
30
Pancreatic Failure
 Digestion
is abnormal when pancreas
fails to secrete normal amounts of
enzymes.


Pancreatitis
Removal of pancreatic head - malignancy
 Without


pancreatic enzymes -
60% fat not absorbed (steatorrhea)
30-40% protein and carbohydrates not absorbed
31
Pancreatitis

Pancreatitis means inflammation of pancreas.
Autodigestion theory can explain condition.
 Chronic
pancreatitis - (multiple shared
causes)
 alcohol
- most common cause in adults
 cystic fibrosis - most common cause in childre




CF patients lack chloride transporter at apical membrane.
Watery ductal secretion decreases which concentrates acinar secretions in
ducts.
Precipitation of proteinaceous secretions block ducts and can destroy gland
by autodigestion.
Acute pancreatitis - (multiple shared causes)

Gallstones - most common cause
32
Absorption of digested polymers is linked to
Salt Absorption in Small Intestine
•
Sodium is absorbed across apical cell
membrane by 4 mechanisms 1.
2.
3.
4.
•
Diffusion - through water-filled channels
Co-transport - with AA and glucose
Co-transport - with chloride
Counter-transport - in exchange for H+
Chloride follows electrical gradient
created by absorption of sodium
33
Sodium Absorption in Small Intestine
1
2
3
4
Aldosterone
increases
Na+ reabsorption
and K+ secretion in
S.I. and colon.
Na+
Na+
Na+
S
S
Na+
Na+
Cl-
ClNa+
H+
Na+
Cl-
Na+
K+
Na+
K+
P
Na+
H+
Cl34
Chemical Digestion:
Carbohydrates


Begins in the mouth (minimal) and
mostly occurs in small intestine when
pancreatic enzymes are released
Absorption of monosaccharides occurs
across the intestinal epithelia
Absorption: via cotransport with Na+,
and facilitated diffusion



Enter the capillary bed in the villi
Transported to the liver via the hepatic
portal vein
lumen
Enzymes used: salivary amylase,
pancreatic amylase, and brush border
enzymes (maltase, lactase, and
sucrase)
35
Chemical Digestion:
Proteins



Absorption: similar to
carbohydrates (sodium cotransport)
Enzymes used: pepsin in
the stomach
Enzymes acting in the
small intestine


Pancreatic enzymes –
trypsin, chymotrypsin, and
carboxypolypeptidase
(these must be activated!)
Brush border enzymes –
peptidases
36
Lipid digestion and absorption

Lipid digestion utilizes
lingual and pancreatic
lipases, cholesterol
esterase (cleaves ester
bond to release
cholesterol) and
phospholipases release
fatty acids and
monoglycerides.


Bile salts improve chemical
digestion by emulsifying
lipid drops
Lipid-bile salt complexes
called micelles are formed
37
Fatty Acid
Absorption




Fatty acids and
monoglycerides enter
intestinal cells via
diffusion; bile salts can be
reused to ferry more
monoglycerides
They are combined with
proteins within the cells
Resulting chylomicrons are
extruded
They enter lacteals and are
transported to the
circulation via lymph
38
Sprue
•
Diseases that result in decreased
absorption even when food is well digested
are often classified as “sprue” - Nontropical sprue - also called celiac disease
- allergic to gluten (wheat, rye)
- Tropical sprue
•
- destroys microvilli and sometimes
villi
- bacterium (?)
- treated with antibacterial agents
Steatorrhea - if stool fat is in the form of
FFA - digestion has occurred
39
Fluid Entering and Exiting the Gut
Volume
absorbed
8
Diet (2)
6
Saliva (1)
Stomach
(2)
Duodenum
and
Jejunum (4)
Volume (L/day)
10
Volume
entering
4
2
0
Bile (1)
Ileum
(3.5)
•95% of water is
absorbed in the small
intestines by osmosis
•Water moves in both
directions across
intestinal mucosa
•Net osmosis occurs
whenever a
concentration gradient
is established by active
transport of solutes into
the mucosal cells
Colon (1.4)
Volume
Excreted
100-200 ml
Pancreas (1)
S.I. (2)
40
The Liver



Digestive function – bile
production; emulsifies fats
 Bilirubin- decomposed
hemoglobin
 Urobilinogen- byproduct of bilirubin
metabolism
 bile salts- keep
cholesterol dissolved
in bile
Performs many metabolic
functions- stores vitamins,
processes fats, detoxifies,
makes blood proteins
41
Physiology of the large intestine


Reabsorption in the large
intestine includes:
 Water and electrolets
 Bacteria make: Vitamins
– K, biotin, and B5
 Organic wastes –
urobilinogens and
sterobilinogens
 Bile salts
 Toxins
Mass movements of material
through colon and rectum
 Defecation reflex
triggered by distention of
rectal walls
42
Figure 8-18 Agents that stimulate and inhibit H+ secretion by gastric parietal cells. ACh, Acetylcholine; cAMP, cyclic adenosine monophosphate;
CCK, cholecystokinin; ECL, enterochromaffin-like; IP3, inositol 1,4,5-triphosphate; M, muscarinic.
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Figure 8-19 Regulation of HCl secretion during cephalic and gastric phases. ACh, Acetylcholine; GRP, gastrin-releasing peptide (bombesin).
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© 2005 Elsevier
Figure 8-20 Balance of protective and damaging factors on gastroduodenal mucosa. H. pylori, Helicobacter pylori; NSAIDs, nonsteroidal antiinflammatory drugs.
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© 2005 Elsevier
Figure 8-15 Secretory products of various gastric cells.
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© 2005 Elsevier