Digestive Physiology – outline notes

I. Digestive System = Digestive Tract + Accessory Organs

II. Digestive tract (gastrointestinal tract, GI)
A. Long muscular tube from mouth to anus; outside of body
B. Oral cavity, pharynx, esophagus, stomach, small & large intestine

III. Accessory Organs
A. Teeth, tongue, salivary glands, liver, gallbladder, pancreas


IV. Digestive functions:
A. Ingestion – take food in

B. Propulsion – move materials along
1. Swallowing
2. Peristalsis – rhythmic waves of muscular contractions that move
food materials

C. Mechanical digestion – physical breakdown
1. Mastication, churning of stomach
2. Segmentation – muscular movements of small intestine that mix
food materials

D. Chemical digestion – break chemical bonds with enzymes

E. Secretions – release of substances by GI tract/accessory organs to aid in
digestion; enzymes, water, acids, etc.

F. Absorption – movement of nutrients across GI tract epithelium and into body
fluids

G. Excretion – elimination of waste; defecation of feces

V. Oral Cavity

A. Mechanical digestion
1. Tongue – mastication, taste receptors
2. Teeth – mastication
a. Bacteria (Streptococcus mutans) on teeth produce a sticky substance
that traps food; plaque
b. Bacteria digest nutrients and generate acids that erode teeth;
dental caries or cavities
c. Brush/floss to remove plaque (tartar/calculus = hardened plaque)

B. Chemical digestion
1. Salivary amylase – enzyme produced by salivary glands
a. Begins starch breakdown
2. Saliva
a. 1.0 – 1.5 L/day
b. 99.4 % water
c. .6%
1) Electrolytes (Na+, Cl-, HCO3-)
2) Mucins – lubrication
3) Antibodies and lysozyme
4) Salivary amylase ( – amylase)

C. Absorption
1. No nutrients are absorbed
2. Lipid-soluble drugs are absorbed —— nitroglycerin / LSD

D. Formation of Bolus

VI. Pharynx and Esophagus
A. Movement of bolus to stomach via deglutition and peristalsis

VII. Esophagus
A. Long muscular tube; 1 foot x  <1 inch

B. Takes bolus to stomach; 9 seconds for a bolus; liquids take a few seconds

C. Hiatal hernia

D. Cardiac sphincter – muscle guarding stomach
1.  Weakened or relaxed cardiac sphincter; stomach acids back
up (gastroesophageal reflux disease (GERD) or heartburn)

VIII. Stomach
A. Gastric pits lead to gastric glands which produce gastric juice

B. Gastric pits – depression in stomach surface; lead to gastric glands

C. Gastric glands – contain cells that produce gastric juice

D. Cells of gastric glands
1. Parietal cells
a. Hydrochloric acid (HCl)
1) HCl – converts pepsinogen to pepsin
2) Denature proteins in meal
3) Kills bacteria
b. Intrinsic factor – required for absorption of vitamin B12 in
ileum; intrinsic factor is a protein that binds to Vitamin B12 in stomach and carries it to the ileum for absorption
2. Chief cells
a. Produce enzyme pepsinogen
b. Pepsinogen —-HCl—- pepsin (pH 1.5 – 2.0)
c. Pepsin is a proteolytic enzyme — breaks down proteins
3. G cells
a. Produce the hormone gastrin
b. Gastrin stimulates parietal and chief cells; also stomach motility

E. Lining of stomach
1. Protects against acid (pH 1.5-2.0)
2. Alkaline mucus, tight junctions, and regeneration of epithelium

F. Absorption – no nutrient absorption at stomach
1. Lack of transport mechanisms and digestion not completed
2. Some substances absorbed
a. Alcohol and aspirin – lipid soluble

G. Peptic Ulcers
1. Gastric or Duodenal ulcer – acids break down lining
2. Drugs can inhibit HCL production or neutralize it
3. Helicobacter pylori responsible for 80% of ulcers (antibiotics)

H. Regulation of Gastric Activity – 3 phases
1. Cephalic Phase
a. Digestive activity induced by thinking of, smelling,       tasting, seeing, or chewing food; lasts several minutes
b. Parasympathetic activity reaches stomach by means of
vagus nerves; prepares stomach to receive food
c. Pavlov demonstrated that a dog could be stimulated to secrete
gastric juice by “mental” acts.  Pavlov used the esophageal      fistula to divert food to outside of dogs body so food never      reaches stomach (Sham Meal).  The dog eating provoked      stomach to secrete gastric juice and pancreas to secrete      fluids to duodenum.
d. Human volunteers had a tube inserted into nostrils to stomach to     collect gastric juice.  See Table 1

Table 1: Rate of Gastric Acid Release under Various Conditions
Stimulus Rate of gastric acid release (mmol/hr)
Basal acid output 4
Talking about sports 4
Talking about food 15
Sham feeding (chew food and spit it out) 20
Injection of gastrin 38

2. Gastric Phase
a. Begins with arrival of food to stomach
b. Stimuli that initiate gastric phase
1) Distension of stomach
2) Increase of pH of gastric contents
3) Presence of proteins in stomach
c. Duration – several hours
d. Neural response
1) Stretch / chemoreceptors receptors
2) PNS – release ACh to stimulate parietal and chief cells
3) Stimulation of muscularis externa activity
* MIXING ACTION
e. Hormonal response
1) Gastrin stimulates parietal and chief cells
2) Gastrin release stimulated by:
a) Proteins
b) Vagal innervation — distension of stomach
c) increased pH
3) Gastrin release inhibited by:
a) pH 1.5 – 3.0
4) Gastrin increases contractions (mixing of stomach)
a) Ingested materials mix with acid and enzymes
b) Mixing waves are initially weak, but after one hour are       very strong
c) pH is elevated for several hours, until gastric secretions      mix and decrease pH to 1.5 – 2.0. Gastrin (acid
inhibits G cells), enzymes, and acid production
declines.
f. Functions
1) Enhance secretions initiated in cephalic phase
2) Acidify chyme
3) Digestion of proteins by pepsin
3. Intestinal Phase
a. Begins with arrival of chyme to duodenum
b. Contractions sweep down pylorus and a small quantity of chyme
squirts into duodenum.
c. Neural response – short reflexes
1) Enterogastric reflex – Chyme entering duodenum will
decrease stomach activity and cause pyloric sphincter to contract.  This prevents more chyme from entering – giving duodenum time to process chyme

d. Hormonal response
1) Hormones released by duodenum
2) Shut Stomach off
3) Secretin
a) Released due to drop in pH below 4.5
b) Signals pancreas to release bicarbonate solution
(pH = 7.5 – 9.8)
4) Cholecystokinin (CCK)
a) Released with the arrival of chyme and especially
lipids in the duodenum
b) Contraction of gallbladder to send bile
1. Meal high in fats triggers more CCK
2. Bile emulsifies fats      3. Satiety – high fatty meal stays in stomach
longer
4. Relaxes hepatopancreatic sphincter
5. CCK stimulates production and secretion of
pancreatic enzymes
IX. Pancreatic Enzymes
A. Pancreas produces enzymes that will break down all energy yielding nutrients.
B. 4 Classes
1. Proteolytic enzymes (70%))
a. Trypsin(ogen)*
b. Chymotrypsin(ogen)*
c. (Pro)carboxypeptidase*
d. (Pro)elastase*

2. Pancreatic lipase

3. Pancreatic amylase

4. Nucleases

*all activated by trypsin

**Enterokinase, a brush border enzyme, is constitutively present in duodenum and activates trypsinogen to trypsin

Cascade of Reactions
Trypsinogen —-Enterokinase—- Trypsin
Trypsinogen —–Trypsin— Trypsin
Proenzymes —-Trypsin—– Active enzymes

X. Small Intestine
A. Modifications that increase surface area for digestion and absorption
1. Plicae circulares – large folds
2. Villi
a. Contain blood vessels (capillaries) and lacteals
b. Site of nutrient absorption
3. Microvilli
a. Brush border enzymes
1) enterokinase, lactase
*Increase surface area equal to (300m2) a sidewalk 3 ft. wide and 3 football fields long

B. 3 Subdivisions – 21 ft. x 1-1.5 inches in diameter
1. Duodenum – 1 foot
a. “Mixing Bowl” – receives chyme, pancreatic secretions, and
bile
b. pH of chyme goes from 1-2 to 7-8 along length of duodenum
c. Brunner’s glands (Submucosal glands)
1) Produces mucus and buffers; protection from chyme     2) Activated by PNS during cephalic phase
3) SNS inhibits, chronic stress can promotes duodenal ulcers
2. Jejunum – 8 ft
a. Majority of digestion and absorption
1) Plicae and villi are most prominent in proximal half
3. Ileum – 12 ft
a. Takes about 5 hours for food to move from duodenum to end of
ileum
b. Takes 12-24 hours for food to pass through entire GI tract

C. Intestinal movements
1. Peristalsis
2. Reflexes – triggered by stretch receptors of stomach
a) Gastroenteric reflex – stimulates motility and secretion along
length of small intestine
b) Gastroileal reflex – triggers relaxation of ileocecal valve
-results in movement of materials to large intestine

XI. Liver
A. Metabolic Regulation
1. Blood leaving digestive tract enters hepatic portal system and flows to the
liver
2. Regulates composition of circulating blood
3. Examples of Metabolic Regulation
a. Carbohydrate metabolism
1) Stabilizes blood glc around 90 mg / dl
2) Glycogenolysis – glycogen to glc
3) Gluconeogenesis – synthesis of glc from other sources (a.a.’s)
4) Glycogenesis
5) Insulin and glucagon
b. Lipid metabolism
1) Lipoproteins
c. Amino acid metabolism – converts a.a’s to lipid or glc
d. Removal of waste products
1) Deamination – remove amino groups from a.a.’s
a) Ammonia is produced and liver converts to urea
2) Other wastes
a) Toxins, drugs (also drug inactivation)
e. Fat-soluble Vitamin and Mineral storage (Fe)

B. Hematological regulation
1. Receives about 25% of Cardiac Output
2. Kupffer cells – remove old RBC’s, debris, pathogens
3. Plasma protein synthesis
a. Albumin, clotting proteins, complement, transport proteins
4. Remove circulating hormones
a. NE, Epi, insulin, steroids, corticosteroids
5. Removal of antibodies

C. Synthesis and secretion of Bile
1. Bile is composed of water, ions, bilirubin, cholesterol and lipids

XII. Large Intestine
A. Regions – 5 ft. long x 3 in. in diameter
1. Cecum
a. ileocecal valve
b. appendix – 3-4 inches long
2. Colon
a. Ascending colon
b. Transverse colon
c. Descending colon
d. Sigmoid colon
3. Rectum – last 6 inches
a. Hemorrhoid = Network of vessels that can become distended by
increased pressure (straining during defecation, pregnancy, lifting
something heavy)

B. Functions
1. Absorption
a. Water
1)1500 ml of material enters colon/day, but only 200 ml is ejected
as feces
2 )Feces is 75% water, 5% bacteria, remainder is indigestible
materials
b. Bile salts
1) Most are reabsorbed in cecum and sent back to liver by blood
c. Vitamins
1) Bacteria generated vitamins
a) Vitamin K, Biotin, Vitamin B5(pantothenic acid)
2. Bacteria break down of remaining peptides
a. Produces nitrogen-containing compounds — responsible for odor of
feces
b. hydrogen sulfide(H2S) – rotten egg odor
3. Bacteria break down of indigestible CHO
a. flatus – intestinal gas
4. Movements
a. Mass movements – strong peristaltic contractions
1) stimulated by distension of stomach and duodenum
b. Defecation reflex – distension of rectum
5. Diarrhea and Constipation
a. Too little or too much water reabsorption
b. Fiber in diet adds bulk to feces, retains water, and stimulates stretch
receptors to promote peristalsis

“the fecal material is slowly dug into and rolled over in the colon as one would spade the earth, so that deeper, moister fecal matter is put in contact with the colon’s absorptive surface.  This process permits dehydration of fecal matter for defecation while increasing fluid and electrolyte absorption” (Guyton)