1. Unit I: Metabolism Digestive Processes Chapter 21 and 22
Like an automatic car wash: as the chyme moves along, it turns on a new process, turning off the process it just left.
(vacuuming, soaking, soaping, scrubbing, rinsing, drying, polishing)

2. Mastication or Chewing
 surface area exposed to digestive enzymes
Contact of food with sensory receptors triggers chewing reflex
tongue, buccinator and orbicularis oris manipulate food
masseter and temporalis elevate the teeth to crush food
medial and lateral pterygoids swing teeth in side-to-side grinding action of molars
Breaks food into smaller pieces to be swallowed
1st step in mechanical digestion.
food stimulates receptors that trigger involuntary chewing reflex
Controlled in pons
Malocclusion – teeth don’t align. Overbite, underbite.

3. Salivation Total of 1 to 1.5 L of saliva per day
Cells filter water from blood and add other substances
Food stimulates receptors that signal salivatory nuclei in medulla and pons
parasympathetic stimulation  salivary glands produce thin saliva, rich in enzymes
sympathetic stimulation  produce less abundant, thicker saliva, with more mucus
… produced by extrinsic glands
Dehydration reduces capillary filtration  have a dry mouth  triggering thirst
Brain also receives higher brain signals (smell)
A. aroma and taste trigger parasympathetic stimulation … digest starch
b. dry, sticky mouth during stress due to sympathetic stimulation … mucus also binds particles together (bolus) for swallowing.

4. Swallowing / Deglutition
Tongue
Esophagus
1. Buccal Phase
2. Pharyngeal Phase
Bolus
Tactile receptors on the palatal
arches and uvula are stimulated.
Swallowing center in the medulla
oblongata then coordinates:
Elevation of the larynx
Folding of the epiglottis
Elevate the uvula and
soft palate
Involuntary
Compression of the bolus
Tongue then forces the bolus
into the oropharynx
Voluntary
Soft palate
DEE-glu-TIS-un
Involves 22 muscles in the mouth, pharynx, and esophagus coordinated by the swallowing center in medulla oblongata and pons.
Buccal phase: (voluntary) tongue collects food  forms a bolus  pushes it back to oropharynx
2. Pharyngeal phase: (involuntary) block food/drink from reentering mouth, nasal cavity, or larynx
a. root of tongue blocks oral cavity
b. soft palate rises to block nasopharynx
c. larynx is pulled up, epiglottis covers opening, muscles close the airway
3. Bolus driven downward into esophagus

5. Swallowing / Deglutition
3. Esophageal Phase
Peristalsis
Stomach
Thoracic
cavity
Lower esophageal sphincter (LES)
Myenteric nerve plexis signals
peristaltic waves
LES opens
Bolus continues to the stomach
Typical bolus = 9 seconds
Liquids = 2 seconds
A dry or poorly lubricated bolus may need secondary peristaltic waves
4. Myenteric (muscles of digestion) nerve plexis transmits signals to circular smooth muscles to perform peristalsis:
behind bolus – constricts to push bolus downward
ahead of bolus – relaxes while longitudinal muscle contracts making esophagus shorter and wider
5. Physiological sphincter (LES) relaxes to allow food to pass to stomach
Liquid reaches the stomach in 1-2 seconds; bolus 4-8 seconds
Peristalsis allows us to swallow and move a bolus regardless of our body position.
Peristalsis allows us to move a bolus regardless of our body position.

6. Gastric Motility Swallowing center signals stomach to relax
Receptive-relaxation response
Peristalsis
contraction every 20 seconds
stronger contractions after 30 minutes; ejects 3 ml
typical meal emptied from stomach in 4 hours
Preparing it to receive food
Food stretches stomach activating a … of the smooth muscle
resists stretching briefly, but relaxes to hold more food
3. Rhythm of peristalsis controlled by pacemaker cells in longitudinal muscle layer
a. gentle ripple of contraction every 20 seconds from fundus  stronger in pyloric (muscularis externa thickest)
b. after 30 minutes  churns and mixes food with gastric juice; ejects 3ml of chyme into duodenum at a time
this allows the duodenum time to neutralize the stomach acid and digest nutrients
c. less time if liquid meal; more time (6 hours) if high in fat

7. Vomiting
Induced by
excessive stretching of stomach, psychological stimuli or chemical irritants (bacterial toxins)
Emetic center in medulla causes
retching
Lower esophageal sphincter to relax
stomach and duodenum to contract spasmodically
vomiting
when abdominal contraction forces upper esophageal sphincter to open
Forceful ejection of stomach and intestinal contents (chyme) from the mouth.
1.a. chemical irritants: alcohol and bacterial toxins; visceral trauma (to pelvic organs); intense pain; and physiological and sensory (sights, smells, thoughts)
2. These activate the emetic center (vomit center)…
a. preceded by nausea and retching
i. Retching – thoracic expansion, abdominal contraction create a pressure difference  dilates esophagus causing …
ii. While … chyme enters esophagus but returns to stomach
b. i. pressure difference rises causing the upper physiological sphincter of esophagus to open;
chyme is expelled by strong abdominal contraction combined with reverse peristalsis of pyloric region of stomach and duodenum.
Projectile vomiting – sudden vomiting with no nausea or retching. May be caused by neurological lesions; infants after feeding.
Fast before anesthesia, may induce nausea, to empty stomach and intestine of chyme to keep a person from inhaling vomit  very destructive to respiratory tract; can die

8. Regulation of Gastric Secretion
1. Cephalic Phase
2. Gastric Phase
See, smell, taste, or think of food.
Directed by the CNS
Prepares the stomach to receive food
Production of gastric juice accelerates
reaching rates of about 500 mL/h
lasts only minutes.
Arrival of food in the stomach
(1) distension of the stomach
(2) an increase in the pH
(3) the presence of undigested proteins
Lasts 3-4 hours
Gastrin stimulates contractions in the
muscularis externa of the stomach.
Mucous
cells
Stimulation
Mucus
Pepsinogen
HCl
Gastrin
KEY
Submucosal
plexuses
Vagus nerve (N X)
Medulla Oblongata
Chemoreceptors
Stretch
receptors
Elevated pH
Distension
Submucosal
and myenteric
plexuses
via
bloodstream
Gastrin
Mucus
Pepsinogen
HCl
Partly
digested
peptides
Mixing
waves
Nervous and endocrine system work together to increase or decrease gastric secretion and motility.
Phases are named by whether the stomach is being controlled by the brain, by itself, or by the small intestine.
Phases overlap and can occur simultaneously.
Cephalic phase (controlled by brain):
a. stomach responds to:
b. parasympathetic division of ANS; medulla stimulates the enteric nervous system of the stomach  gastric activity
enteric – digestive tract’s own nervous system  regulates digestive tract motility, secretion and blood flow; neurons located in submucosa and muscularis externa
2. Gastric phase (controlled by stomach): 2/3 of gastric secretion occur in phase
a. gastric activity is activated by …; builds on the stimulation of cephalic phase.
1&2. stimulate secretory cells in submucosa and myenteric plexus – producing powerful contractions (mixing waves).
3. proteins and neural stimulate secretion of gastrin  bloodstream  gastric motility and gastric glands  increase secretions which lower pH
b. … while the acid and enzymes process the ingested materials.
c. … After the first hour, the material in the stomach is churning like clothing in a washing machine. As mixing continues, a large volume of gastric juice is secreted.
As contents are emptied, the pH drops. Below pH 2  stomach acid inhibit the parietal cells production of HCl and pepsin and G cells that winds down the gastric phase. Negative feedback loop.
Mucous
cells
Chief cells
Parietal cells
Chief cells
G cells
Parietal cells
G cells

9. Regulation of Gastric Secretion
3. Intestinal Phase
Chyme first enters the small intestine
Function: control the rate of gastric emptying
Enterogastric reflex:
Stimuli:↓ stimulation of stretch receptors in stomach and
↑stimulation of stretch
receptors in intestine
Purpose: Inhibit gastric activity
Stimulates contraction of
pyloric sphincter
Mucus production
Peristalsis
Enterogastric
reflex
Myenteric
plexus
via bloodstream
Chief
cells
Parietal
Duodenal
stretch and
chemoreceptors
CCK
GIP
Secretin
Presence of
lipids and
carbohydrates
Decreased pH
Inhibition
KEY
duodenum regulates gastric activity through hormones and nervous reflexes
a. (if duodenum is stretched or amino acids in chyme cause gastrin release)
b. … to ensure that the secretory, digestive, and absorptive functions of the small intestine can proceed with reasonable efficiency.
i. duodenum inhibits stomach - caused by acid and semi-digested fats in duodenum by way of enteric nervous system
a. ↓ chyme in stomach  ↓ distension in stomach  …
b. ↑ distension of duodenum from chyme  … and chemoreceptors trigger enterogastric reflex
c. … and gastric conctractions
d. … which prevents further discharge of chyme.
e. protect duodenal lining from the acid and enzymes
c. chyme stimulates … all 3 hormones suppress gastric secretion and motility

10. Intestinal Hormones Cholecystokinin (CCK) Contraction of gallbladder
Secretions of pancreatic enzymes
Relaxation of hepatopancreatic sphincter
Secretin
Glucose dependent insulin peptide/ Gastric inhibitory peptide
Overall effect: reduce gastrin secretion and passage of chyme
CCK (Co-leh-SIS-toe-KY-nin) secreted from duodenum in response to arrival of acid and fat from stomach, in general: responsible for release of pancreatic juice and bile; triggers 3 responses to do this:
contraction of gallbladder & forces bile into bile duct
secretion of pancreatic enzymes to make pancreatic juice
relaxation of hepatopancreatic sphincter allows bile and juice into duodenum
2. Secretin secreted from duodenum in response to acidic chyme – stimulates liver and pancreatic ducts to secrete more sodium bicarbonate
3. Stimulates insulin secretion in preparation for nutrient absorption.
Both bile and pancreatic juice are released to neutralize stomach acid in the duodenum.
Effect of all this is that gastrin secretion declines and the pyloric sphincter contracts tightly to limit chyme into duodenum.

11. Small Intestine Motility
Segmentation -
stationary ring-like constrictions
not as in peristalsis
Purpose:
mix
churn
Pacemaker cells in Muscularis externa
Provides the most contact digestion
1. Most common motility: periodic constriction of segments without movement forward or back
2. 3 functions:
a. Mix chyme with intestinal juice, bile, and pancreatic juice  neutralize acid and digest (more effectively)
b. Knead/Churn chyme to bring it into contact with mucosa for contact digestion and absorption
c. … When little remains but residue: segmentation declines and contents are moved by peristalsis
refilling of stomach reactivates segmentation
3. Pacemaker cells of muscularis externa set rhythm

12. Carbohydrate Digestion
Trace the digestion and absorption of each major class of nutrients from mouth to small intestine.
Where does majority of the digestion take place?
Most digestible carbohydrate is starch.
Salivary amylase in mouth converts starch to oligosaccaride. Amylase is denatured in stomach acid (may continue to work 1-2 hours if in middle of food mass). Until pH falls below 4.5
Digestion resumes in small intestine with … starch completely converted to … (10 minutes)
Digestion completed as it contacts the brush border enzymes (maltase, sucrase, lactase)  hydrolyzes disaccharides to monosaccharides (glucose) and immediately absorbed.
release of GIP: stimulate the release of insulin by the pancreas
Salivary amylase  oligosaccharide
50% of dietary starch digested before it reaches small intestine
Pancreatic amylase  oligosaccharide and maltose
Brush border enzymes  monosaccharides

13. Carbohydrate Absorption
80% of glucose taken up by Na- glucose transport proteins →
Facilitated diffusion → blood capillaries of villus →
Delivered to liver
Disaccharides
Simple sugars
Epithelial cell
Enzymes
(such as glucose
or fructose)
Absorption:
80% of the absorbed sugar is glucose taken up by Na-glucose transport protein stimulated by insulin.
converted to another sugar or transported out of cell by facilitated diffusion and absorbed by blood capillaries of the villus
delivered to liver
lactose indigestible after age 4 in most humans (lactase declines)
Carbs are preferred for ATP production because lipids and proteins are more important for structural components.
Each pyruvate gives the cell 17 ATP, consumes 3 O2, generates 3 CO2 and 6 H2O. Pair of pyruvate = 34 ATP.

14. Protein Digestion ½ of amino acids digested come from dietary proteins
ingested
½ of amino acids digested come
from dietary proteins
Protease pepsin breaks down
complex proteins into smaller
peptide and polypeptide chains
Pancreatic enzymes: Trypsin,
chymotrypsin, and
carboxypeptidase break down
proteins into a mixture of
dipeptides, tripeptides, and
amino acids.
The epithelial surfaces
of the small intestine
contain several brush
border enzymes that
release individual
amino acids
Facilitated diffusion
and cotransport
Dipeptidase
Amino acids
Epithelial cell
Protein Digestion
Proteins are hydrolyzed into amino acids.
1. Amino acids come from 3 sources:
1. Dietary proteins *(1/2 come from here)
2. Digestive enzymes digested by each other
3. Sloughed epithelial cells
2. Proteases – enzymes that digest/hydrolyze proteins; begin in stomach with pepsin (pH )  inactivated when passes into duodenum and mixes with alkaline pancreatic juice (pH 8); pepsin attacks only specific types of peptide bonds, not all of them.
3. Trypsin and chymotrypsin take over protein digestion in small intestine: polypeptides  oligopeptides 
taken apart one amino acid at a time by: brush border enzymes.

15. Protein Absorption Behaves like monosaccharides:
Taken up by Na-dependent transport proteins → epithelial cells →
Facilitated diffusion → blood capillaries of villus → bloodstream
Behave like monosaccharides - Amino acid absorption – several different Na-dependent transporter proteins move amino acids into epithelial cells  facilitated diffusion moves them to blood stream (capillaries of villus)  to liver

16. Lipid Digestion Lipase – Lingual lipase, gastric lipase 2. 3. Bile
More complicated because hydrophobic portion.
Lipases – enzymes that digest/hydrolyze fats; Lingual lipase secreted by intrinsic salivary glands of tongue but activated by stomach acid. 10% of ingested fat digested by stomach with gastric lipase.
2. Beginning of picture – small intestine segmentation break fat globule broken into emulsification droplets  exposes more surface area to enzymes. Coated with lecithin (lipid; emulsifier) and bile acid from bile.
3. Most digestion takes place in small intestine by pancreatic lipase. Pancreatic lipase digests fat in 1-2 minutes after a meal  removes 1st and 3rd fatty acids from triglycerides into 2 free fatty acids (FFAs) and 1 monoglyceride.

17. Lipid Absorption 1.
Absorption by minute droplets in the bile called micelles (my-SELLS) made in liver (surfactant; amphilitic nature)
Bile acids coat FFAs and monoglycerides to form micelles
pick up free fatty acids and monoglycerides in small intestine and
2. transport them to the surface of intestinal absorptive cells  lipids diffuse into cell (micelles are reused/recycled; sent to liver)
without micelles – intestine would only absorb 40-50% of dietary fat and no cholesterol.
3. Lipids transported to smooth ER  resynthesized into triglycerides
4. Golgi complex forms droplets of chylomicrons (KY-lo-MY-crons) (triglycerides + cholesterol coated with phospholipids and proteins) and packages them into secretory vesicles  ultimately taken into (lacteal) lymph vessels (most too big to go to blood stream) and then blood 2.

18. Nucleic Acids, Vitamins, and Minerals
Nucleases
brush border split them completely apart
Vitamins are absorbed unchanged
A, D3, E and K with other lipids
B complex and C by simple diffusion
B12 if bound to intrinsic factor
Minerals are absorbed all along small intestine
Na+ cotransported with sugars and amino acids
Cl- exchanged for bicarbonate reversing stomach
Iron and calcium absorbed as needed
Nucleases in pancreatic juice hydrolyze DNA and RNA to nucleotides.
enzymes of brush border … into phosphate ions, ribose or deoxyribose sugar and nitrogenous bases
membrane carriers transport them into the intestinal epithelium  enter blood capillaries of the villus  liver 2.
a. fat-soluble vitamins A, D, E, and K have to be absorbed with lipids  no fat in food  passed in the feces and wasted.
b. water-soluble B & C absorbed by simple diffusion
c. with the exception of B12 – large molecule can only be absorbed if bound to intrinsic factor from the stomach.
3. (electrolytes) a.
b. Cl- actively transported and … reversing exchange in stomach
c. Fe and Ca - … other minerals are absorbed at constant rate – kidneys excrete excess
d. K – absorbed by simple diffusion

19. Water Balance Digestive tract receives about 9 L of water/day
0.7 L in food, 1.6 L in drink, 6.7 L in secretions
8 L is absorbed by small intestine and 0.8 L by large intestine
Diarrhea
feces pass through too quickly if irritated
feces contains high concentrations of a solute (lactose)
A. secretions: saliva, gastric juice, bile, pancreatic and intestinal juice
B. … 0.2 L voided in fecal output.
* Water is absorbed by osmosis (Following the absorption of salts and organic nutrients creates an osmotic gradient from the intestinal lumen to the ECF)
* Main motility: segmentation
Contractions occur every 5 seconds in duodenum, ½ as fast in illeum. Takes 2-3 minutes to advance 1 inch. Chyme remains in small intestine for 3-10 hours normally. Can be emptied in a few minutes – diarrhea (allergies, irritants)
2. Occurs in large intestine when too little water is absorbed
a. occurs when: intestine is irritated by bacteria
b. or when: … which oppose osmotic absorption of water
Constipation: fecal movement too slow, too much water is re-absorbed
causes: lack of fiber, lack of exercise

20. Absorption and Motility of Large Intestine
Vitamin Absorption
Vitamin K
Vitamin B5
Biotin
1500mL/day enters
Organic waste products, such as
urobilinogen, and various toxins.
Over 1 L of water is
reabsorbed
through osmosis.
Only 200 mL of feces is ejected.
75% water, 25% solids
Transit time is 12 to 24 hours
reabsorbs water and electrolytes
Haustral contractions occur every 30 minutes
distension of a haustrum stimulates it to contract
Mass movements occur 1 to 3 times a day
filling of the stomach and duodenum
To reduce residue to feces
doesn’t chemically change the residue but reabsorbs water and electrolytes
… various toxins generated by bacterial action remain in large intestine.
2. Feces consist of …
solids – 30% bacteria, 30% undigested fiber, % fat (not diet) and protein, sloughed epithelial cells, salts, mucus, digestive secretions
3. Most common motility, type of segmentation: Haustra – pouches in colon wall
a. distension from feces … churns, mixes, absorbs (H20 and Na) and passes on
4. Much stronger contractions … last 15 minutes
triggered by …

21. When external sphincter Involuntary contraction Relaxation of internal
Intrinsic Defecation
Reflex
Parasympathetic
Defecation Reflex
When external sphincter
is relaxed:
DEFECATION OCCURS
Involuntary contraction
of external anal
sphincter
Relaxation of internal
anal sphincter
DISTENSION
OF RECTUM
Increased local
peristalsis
Stimulation of
stretch receptors
Start
myenteric plexus
Increased
throughout large
intestine
motor neurons
in sacral spinal
cord
somatic motor
neurons
stimulates
inhibits
Defecation
Stretching of the rectum stimulates defecation; involves 2 involuntary reflexes:
a. Short Reflex (S) - operates entirely in myenteric nerve plexus
muscularis of sigmoid colon  rectrum to contract (peristaltic wave drives feces downward)
- internal anal sphincter relaxes
- weak contractions
- must have cooperative action of next reflex
b. Long Reflex (L) - spinal reflex
- stretching of rectum sends sensory signals to spinal cord
- motor nerves return signals intensifying peristalsis
2. External anal sphincter is under voluntary control – voluntarily relaxed for defecation to occur
Abdominal contractions increase abdominal pressure as levator ani lifts anal canal upwards  feces will fall away
On average it takes 24 hours for food to pass thru digestive tract.
External sphincter
can be voluntarily
relaxed