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Digestion
Anatomy of digestive system (review!)
4 major processes of digestive system:
1. Motility
2. Secretion
3. Digestion
4. Absorption
Gastrointestinal System
Peritoneum and Peritoneal
Cavity
Peritoneum - serous membrane of
abdominal cavity


Visceral peritoneum on external surface of
most digestive organs
Parietal peritoneum lines body wall
Peritoneal cavity


Between two peritoneums
Fluid lubricates mobile organs
© 2013 Pearson Education,
Inc.
Phases of Digestion/Absorption
1.
2.
3.
4.
Cephalic
Gastric
Intestinal
Defecation
1.
2.
3.
4.
Cephalic
Gastric
Intestinal
Defecation
•Anticipation
•Begins with sensory input that is sent to the
cerebral cortex and then to the
hypothalamus to stimulate the PNS
•Salivation, relaxation of the GE sphincter,
relaxation of stomach muscles, increase
gastric motility
•Mastication
•Mechanical digestion
•Teeth, tongue
•Deglutition
•Bolus formation for swallowing
•Peristalsis in esophagus
Salivary Glands
Parotid Glands

Secretes Mucous
Allows for smooth swallowing of bolus
Mumps
Sublingual and Submandibular glands
secrete Amylase

Breaks down complex carbohydrates into
simple carbohydrates
Deglutition
1.
2.
3.
4.
Cephalic
Gastric
Intestinal
Defecation
•Storage
•Relaxation or contraction as needed.
•Digestion
•HCl - Hydrochloric Acid (Parietal
cells)
•Pepsin (Chief cells)
•Mechanical
•Immune Protection
•Ingested pathogens
•Respiratory mucus
Reflux Esophagitis = Heartburn = GERD
Gastroesphogeal
Reflux Disease
Lower esophageal
sphincter dysfunction
Why reflux against gravity?
GERD, cont’d
1.
Intrapleural pressure 
during inspiration
 Wall of esophagus
expands

Subatmospheric
pressure in
esophageal lumen sucks
acidic stomach contents into
esophagus

May also occur during
pregnancy or obesity
Mechanical Digestion
Oblique, Longitudinal and Circular Gastric
Muscles

Esophagus does not contain oblique muscles
Allows for gastric motility to occur
Muscles are sensitive to


Parasympathetic input
Gastrin hormone
Rugae are used along with the Gastric
Muscles for mechanical digestion
Chemical Digestion
Gastric Pits


Invaginations of the stomach wall
Lined with several different types of cells
Chief Cells


At the bottom of the pits
Secretes pepsinogen
Inactive form of the enzyme pepsin
Pepsinogen is converted into pepsin upon mixing with HCl

Pepsin breaks down proteins into amino acids
Chemical Digestion Cont.
Parietal Cells



Above Chief Cells
Secret H+ and Cl- into the Gastric Pits
Used for Digestion of foodstuff
Gastric Glands or G cells




Secretes the hormone Gastrin
Gastrin secretion increases when the contents of the stomach is
basic
Gastrin secretion decreases when the contents of the stomach is
acidic
Gastrin’s functions are
To increase HCl secretion from the Parietal Cells
To increase Gastric Motility
Mucous Cells
Secrete Mucous to coat the lumen of the
stomach

Prevents ulceration
H. Pylori also responsible for ulcers
Question
18 year old White Female Adult enters into
the emergency room.
She has been vomiting for over 72 hours.
You observe she is also hyperventilating.
How is the vomiting and hyperventilation
tied together?
List two of the possible problems that your
patient is experiencing.
HINT: Don’t forget the Parietal Cell
1.
2.
3.
4.
Cephalic
Gastric
Intestinal – Chyme (ingesta) enters small intestine
Defecation
o Neutralization of HCl
o NaHCO3 from pancreas
o Running Problem: Peptic Ulcer and
antacids, including H2 antagonists and
proton pump inhibitors
o Pancreatic enzymes
o Digest protein, CHO
o Bile acids
o Emulsification of Fat
o Absorption of H2O
Mechanical Digestion
Peristalsis and Segmentation


Occurs in the small intestine
Food is basically shaken back and forth in
small segments along the length of the small
intestine
Chemical Digestion
During the segmentation process the food
scrapes along the tips of the villi and
causes cells to explode off


Release Digestive Enzymes
Rapid Mitosis in the Crypts of Lieberchun or
Intestinal crypts
Easily affected by chemo/radiation therapy due to
rapid mitosis
Chemical Digestion
Release of Digestive Enzymes

Pancreas releases digestive enzymes into the
duodenum
Release of Bile




Emulsification of Fat
Storage and Release by Gall Bladder
Production of Bile by Liver
Bile contains:
Cholesterol
Bile Salts
Lecithin
Billirubin
1.
2.
3.
4.
Cephalic
Gastric
Intestinal
Defecation
Bacterial fermentation of CHO and proteins
Lactate, some vitamins are digested and/or absorbed
More H2O absorption
Osmotic diarrhea vs. secretory diarrhea

Osmotic-solutes prevent H2O reabsorption
Lactose intolerance

Secretory- bacterial toxins ("flush out' pathogens)

Defecation Reflex
Colon
Three Major Functions of the Large
Intestine


Absorption of H2O
Storage of Feces
Usually 24 hours

Production of Vitamin K
From E. Coli
Used in the Blood Clotting Process
All of the following are functions
of the oral cavity except
A.
B.
C.
D.
To mechanically process foods
To digest proteins
To lubricate foods
To digest certain carbohydrates
Passage of a bolus of food down
the esophagus is assisted by
A.
B.
C.
D.
Enzymes and acids
Striated muscles and tissues
Peristalsis and gravity
The pyloric sphincter
The hormone that regulates the
activity of the gastric glands is
known as
A.
B.
C.
D.
E.
Progesterone
Androgen
Gastrin
TSH
Pepsin
Bile, which is formed in the liver,
assists the
A.
B.
C.
D.
Breakdown of proteins
Absorption of water
Emulsification of fats
Formation of feces
All of the following are normally
found in the bile except
A.
B.
C.
D.
Cholesterol
Bilirubin
Bile salts
Proteins
Which of the following hormones causes the
pancreas to secrete digestive enzyme?
A.
B.
C.
D.
E.
Secretin
Cholecystokinin
Gastrin inhibiting peptide
Gastrin
Pepsinogen
Which of the following hormones stimulates
the liver to produce bile?
A.
B.
C.
D.
E.
Secretin
Cholecystokinin
Gastrin inhibiting peptide
Gastrin
Leptin
Hydrochloric acid is secreted by which of the
secretory cells of the stomach?
A.
B.
C.
D.
E.
Chief cells
Mucous neck cells
Serous cells
Parietal cells
G cells
Lactose Intolerance
Lactose = glucose + galactose
Lactase only found in juvenile
mammals
Adaptive (dominant) mutation in
populations with dairy-based
cultures
Lactose intolerance in




95% of Native Americans,
90% of Asian Americans
70% of African Americans
50% of Mexican Americans
Regulation of GIT
ANS


Parasympathetic (R & R)
Sympathetic
Emotional (cephalic reflexes)

E.g., smell of food
ENS (Enteric Nervous System)

Self-contained (intrinsic)
GI peptides can have regulatory
role as hormones or paracrines

E.g., Gastrin, CCK
CHO Digestion & Absorption
~50% of calories in average American
diet


Starch (polysaccharide) and sucrose
(disaccharide)
Cellulose (roughage) not digestible
Enzymes: amylases, disaccharidases
(maltase, sucrase, lactase)
Absorbed only as monosaccharides
(glucose, fructose)

Small intestine
Protein Digestion and Absorption
Variable digestibility
30-60% of protein not from diet
First digestion in Stomach by HCl
Proteases secreted as proenzymes

Pepsin(-ogen), trypsin, etc.
Absorption of single a.a. and di- and
tripeptides

Specific receptors required for larger
chains
May serve as allergens
DDAVP
Lipid (fat) Digestion
Mostly triglycerides in diet

Cholesterol, Fat-soluble
vitamins, others.
Combination with bile salts
creates an emulsion

Colipase and lipase allow
formation of small micelles
Absorption of fat via
diffusion across apical CM
Chylomicrons in the cell are
absorbed into lacteals
Other Stuff
Water soluble vitamins—
mediated transport
Fat soluble vitamins via
absorption
Water, Ions and Minerals

Various locations and methods,
e.g, diffusion, carrier proteins
Nucleic Acids
Colon Cancer
2nd largest cause of cancer deaths
Cellulose (indigestible) = fiber,
roughage
Significance of “roughage” in diet??
Olestra
Biotechnology focus
Consumers want fat in their foods 
search for low cal fat substitutes
1996, Procter & Gamble market Olestra
Olestra = Olean = sucrose polyester
(sucrose molecule + 6-8 FA)
Cannot be digested, nor absorbed
Unpleasant side effects
Absorption of fat sol. vitamins pot.
problem
Motility
2 purposes:
Forward movement of food
Mechanical mixing
GI smooth muscles contract
spontaneously
Pacemaker cells, (Interstitial Cells of
Cajal), connected by gap junctions,
generate slow wave potentials
AP’s spread throughout longitudinal
muscles (gap junctions)  wave of
contraction

Like cardiac muscle, Ca2+ can regulate
contraction strength
1.
2.
3.
4.
Motility
Secretion
Digestion
Absorption
Different Patterns of Contraction
Tonic Contractions
Sustained contraction, usually in the
stomach
Phasic Contractions
Peristaltic contractions


progressive waves moving along segments of
longitudinal layer  forward propulsion
circular layer contracts proximal to bolus
Especially esophagus
Segmental contractions


alternate contraction & relaxation lead to mixing
A side effect of narcotics
Secretion
9 L of fluid pass through the GIT
(only 2 L from food & drink) 
Secretion and Reabsorption
important

Ions and water; similar to renal
mechanisms
Saliva





Hydrochloric Acid (Parietal
Cells)
Bicarbonate (enzyme necessary ?)
Enzymes (zymogens)
Mucus (Goblet cells)
Bile (bile salts – function?)
Net Fluid Balance in GI system
1.
2.
3.
4.
Motility
Secretion
Digestion
Absorption
Digestion Overview
Mechanical breakdown and mixing aid
enzymatic breakdown

Chewing

Tonic contractions, esp. stomach
Enzymatic breakdown converts macromolecules
into absorbable units
Bile emulsifies fats
Optimal pH of enzymes indicates location of
activity
1.
2.
3.
4.
Motility
Secretion
Digestion
Absorption
Absorption Overview
Most nutrient absorption takes place in ?


Fats absorbed into lacteals
Everything else absorbed into portal vein
Alcohol & aspirin across gastric epithelium
Additional: H2O, ions & some vitamins
absorbed in ________________
Mechanisms analogous to renal absorption
1.
2.
3.
4.
Motility
Secretion
Digestion
Absorption