BIO 3520 Notes, 10/18/05
DIGESTIVE SYSTEM
I. Introduction. [Widmaier, pp. 575-576, 633-637]
A. Animals must take in food and water to provide energy for bodily
functions and materials for growth and remodeling of body structures.
B. Focus -- Processes by which fuel molecules are taken into the body,
stored, and broken down to release energy for cellular processes.
1. Digestion and absorption of food.
2. Cell metabolism.
C. Energy.
1. All living cells require energy.
2. Immediate source of metabolic energy is adenosine triphosphate
(ATP) (fig. 2-26).
3. Energy is derived from fuel molecules.
a. Main fuel molecules are glucose and fatty acids.
b. Enter the body by way of the digestive system.
c. Stored in certain organs (ex. liver, muscle, adipose tissue).
d. ATP is formed during the breakdown of fuel molecules (ex. glycolysis).
4. First Law of Thermodynamics:
a. Energy is measured in kilocalories (table 16-7).
1. McDonald's hamburger = 255 kcal.
2. Jogging for 1/2 hour = 285 kcal.
5. Energy balance.
a. Energy intake = energy expenditure.
b. Average adult human takes in and expends 2500 kcal/day.
c. If intake is greater than expenditure, excess energy is stored as fat.
d. If expenditure is greater than intake, fuel stores in the body must
be used to meet energy needs ----> weight loss.
5. Metabolic rate = Rate at which the body uses energy.
6. Factors influencing metabolic rate (table 16-5).
a. Age (figure).
b. Gender.
c. Sleep.
d. Food intake.
e. Hormones (ex. thyroid hormone).
f. Physical activity.
II. Metabolism. [pp. 79-80]
A. Fuel molecules entering the body are either used for energy or stored.
B. Anabolism = Reactions in which simple molecules are combined to
form more complex molecules.
1. Require metabolic energy.
2. Favored during the absorptive state (i.e. during and after a meal).
3. Formation of body structure (amino acids ----> protein).
4. Storage of energy (glucose ----> glycogen; fatty acids ----> fat).
a. Fat is the major stored fuel in the body.
b. Fat is also the most efficient way to store fuel (9 kcal/g of fat vs.
4 kcal/g of carbohydrate or protein).
C. Catabolism = Breakdown of complex molecules to release metabolic
energy.
1. Favored during the postabsorptive state (i.e. between meals or
when fasting).
2. Glucose, amino acids, and fatty acids are broken down by
separate, but interrelated, pathways (figure).
3. Waste products are produced (ex. CO2, urea).
4. 40% of released energy is captured by ATP.
5. What happens to the other 60%?
III. Introduction to the Digestive System. [pp. 575-576]
A. The primary function of the digestive system is to deliver nutrients and
water from the external environment into the internal environment.
B. Five basic processes (fig. 15-2).
1. Motility -- Movement of materials through the digestive tract.
2. Secretion -- Secretion of digestive juices into the tract.
3. Digestion -- Breakdown of food into smaller units for use by the
body.
4. Absorption -- Transport of products of digestion into the blood.
5. Excretion -- Elimination of waste.
IV. General Organization of Digestive System. [pp. 576-581]
A. Hollow tube extending from mouth to anus -- about 7 meters long.
B. Hollow portion (lumen) is technically outside of the body.
1. Hostile environment for living cells.
a. Stomach -- very acidic.
1. Kills most bacteria.
2. Ulcers = Destruction of stomach wall by acid.
a. Leading cause is Helicobacter pylori infection.
b. Elimination of H. pylori by antibiotics will prevent relapse of
ulcers in 95% of patients.
c. 2005 Nobel Prize.
b. Large intestine -- full of bacteria.
1. Appendicitis = Inflammation of the appendix.
a. Ruptured appendix introduces harmful bacteria into abdomen.
C. Structures (figs. 15-1, 15-3).
Organ
Function
Mouth
Pharynx
Esophagus
Stomach
Small intestine
Appendix
Large intestine (colon)
Anus
D. Cross-section of the GI tract -- four layers (fig. 15-6, figure)
1. Mucosa.
a. Epithelium.
b. Absorptive surface of GI tract.
c. Villi (fig. 15-7).
1. Finger-like projections about 1 mm long.
2. Purpose:
3. Contain capillaries and lymph vessels (lacteals).
d. Microvilli on surface (fig. 15-8).
1. About 1 mm long.
2. Brush border.
e. Digested materials are absorbed across mucosa and enter blood
or lymph.
f. Goblet cells secrete mucus.
g. Entire epithelium is replaced every 5 days.
2. Submucosa.
a. Connective tissue.
b. Many blood vessels.
c. Autonomic nerve fibers.
3. Muscularis externa.
a. Inner circular smooth muscle layer.
b. Outer longitudinal smooth muscle layer.
c. Network of autonomic nerves between layers.
d. Responsible for motility.
4. Serosa.
a. Thin connective tissue wrapping around organ.
V. Motility. [pp. 592-593, 603]
A. Propulsion.
1. Peristalsis = Waves of smooth muscle contraction (fig. 15-22).
B. Mixing.
1. Segmentation = Alternating contraction and relaxation of different
segments of the digestive tract (fig. 15-32).
C. Control of smooth muscle contraction in the digestive tract.
1. Stimulated by parasympathetic activity.
2. Inhibited by sympathetic activity.
D. Sphincters.
1. Tight rings of muscle which stop the movement of materials.
2. Example: Lower esophageal sphincter (fig. 15-15).
a. Prevents backflow of stomach contents into esophagus.
b. Closed except when swallowing.
c. When open at the wrong time ----> heartburn (figure).
VI. Secretion. [pp. 576-580, 598-602]
A. Average adult consumes 800 g of food and 1200 ml of water each day --
7000 ml of fluid is added through secretion.
B. Epithelial cells throughout the digestive tract secrete mucus.
1. Function -- protection and lubrication.
C. Glands -- secrete digestive enzymes and other substances through
ducts into lumen.
1. Salivary glands (figure).
a. Produce 1 - 2 liters of saliva per day.
b. Contains water (moistening food), mucus (lubrication) and
salivary amylase (digestion of carbohydrates).
2. Pancreas.
a. Secretes digestive enzymes into upper portion of small intestine
(duodenum) (fig. 15-25, table 15-6).
1. Pancreatic amylase -- digestion of carbohydrates.
2. Trypsin -- digestion of proteins.
3. Lipase -- digestion of fats.
b. Also secretes bicarbonate ion -- neutralizes stomach acid.
4. Liver.
a. Produces bile -- aids in digestion of fat.
b. Bile stored in gall bladder (fig. 15-4).
D. Control of secretion.
1. Stimulated by parasympathetic activity.
2. Inhibited by sympathetic activity.
VII. Digestion. [pp. 583-586]
A. Most molecules in food must be broken down into smaller units in order
to be absorbed.
B. William Beaumont -- "father of American physiology".
1. U.S. army surgeon at Fort Mackinaw, Michigan.
2. 1822 -- Attended injury of a French-Canadian trapper who was
accidentally shot in the stomach with a shotgun.
a. Left a permanent hole in the stomach wall -- gastric fistula.
b. Could directly view the activities occurring within a human stomach.
3. Removed gastric juice from stomach and had it analyzed by chemists
-- found HCl.
4. Demonstrated that digestion is primarily a chemical process.
C. Digestion occurs throughout the digestive system, especially in mouth,
stomach, and small intestine.
D. Digestion of carbohydrates.
1. Diet includes polysaccharides (ex. starch) and disaccharides
(ex. sucrose, fig. 2-9).
2. Must be hydrolyzed to monosaccharides (ex. glucose, fig. 2-7)
in order to be absorbed.
3. Digestion of carbohydrates begins in the mouth.
a. Salivary amylase.
1. Mixed with food and swallowed.
2. Breaks complex carbohydrates down into smaller units.
4. Digestion is completed in small intestine.
a. Pancreatic amylase.
1. Mixed with stomach contents as they pass into small intestine.
2. Breaks carbohydrates down into disaccharides.
b. Brush border enzymes.
1. Hydrolyze disaccharides into monosaccharides.
Sucrose sucrase > glucose + fructose
Maltose maltase > glucose + glucose
Lactose lactase > glucose + galactose
2. Review lactose intolerance from Cell Structure and Chemistry
unit.
5. Cellulose (plant fiber) is not digested by humans.
6. Sucralose (Splenda) -- no calorie artificial sweetener.
a. Chloride substituted for hydroxyl groups (figure).
b. 600 times sweeter than sugar.
c. No calories because it is not digested nor absorbed.
E. Proteins (fig. 2-15).
1. Digestion of proteins begins in stomach.
a. Epithelial cells secrete HCl and pepsinogen.
b. Pepsinogen is an inactive protein.
1. Activated in acid environment of stomach (fig. 15-21).
HCl, pepsin
Pepsinogen ---------------------> Pepsin
(inactive) (active)
2. Breaks proteins down into smaller units.
b. Trypsin.
1. Produced by pancreas.
2. Secreted into small intestine in inactive form.
3. Activated in small intestine.
4. Breaks smaller units down into individual amino acids.
c. Why are these enzymes produced in inactive form?
F. Fats (lipids) -- composed of fatty acids (fig. 2-11).
1. Both animal and vegetable fats and oils.
2. Not water-soluble.
a. Mix with contents of digestive system in droplets.
b. Bile, produced by liver, helps disperse droplets into much smaller
droplets ----> increase surface area (fig. 15-12).
3. Digestion of fats (triglycerides) begins in small intestine.
a. Lipase.
b. Secreted by pancreas.
c. Breaks triglycerides down into a monoglyceride and two fatty acids.
lipase
Triglyceride ------------------> Monoglyceride + 2 fatty acids
4. Fat substitutes (ex. olestra).
a. Extra fatty acid chains (figure).
b. Not digested (figure).
c. Side effects.
1. Diarrhea and gas.
2. Poor absorption of fat-soluble vitamins.
IX. Absorption. [pp. 583-586]
A. Digestion of food takes place in the mouth, stomach, and small intestine,
while absorption takes place mainly in the small intestine.
1. Exception: Aspirin and alcohol are absorbed from the stomach.
B. Glucose and amino acids are absorbed into the blood from the small
intestine.
C. Villi (fig. 15-7).
1. Finger-like projections about 1 mm long.
2. Large surface area.
D. Blood supply of small intestine goes first to liver (figure).
E. Absorption of fats (fig. 15-12).
1. Monoglycerides and fatty acids enter epithelial cells lining small
intestine.
2. Reassembled into triglycerides by epithelial cells.
3. Packaged along with cholesterol and other lipids and released from
the cell as fat droplets (chylomicrons).
4. Pass into lymph vessels (lacteals).
a. Lymph vessels empty into venous blood.
E. Water is absorbed from the large intestine.
1. Of 8000 ml of water entering the digestive tract each day, 99% is
absorbed.
X. Excretion. [pg. 580, 605]
A. Digestive system is not a major organ of excretion.
B. Excretes undigested materials (ex. cellulose).
1. 5% of ingested food is excreted.
C. Excretes waste products which were secreted in bile.
XI. Diseases of the Digestive System. [pp. 606-609]
A. Ulcers (figure).
1. Destruction of the wall of the stomach (figure).
2. Due to stomach acid.
3. Internal bleeding.
4. Leading cause is a bacterial infection (Helicobacter pyloris).
a. Elimination of infection with antibiotics cures 95% of ulcers.
B. Diarrhea.
1. Results from excessive intestinal motility (figure).
2. Contents are moving too fast for complete absorption (figure).
3. Loss of fluids and nutrients ----> dehydration.
4. Leading cause of infant mortality in developing nations.
C. Colon cancer (figure).
1. Third most common type of cancer.
2. Treatment -- removal of section of colon.
a. Colostomy if rectal area is involved (figure).
D. Hemorrhoids.
1. Swollen veins in anus (figure).
2. Ruptured during bowel movements.
E. Appendicitis.
1. Inflammation of the appendix (figure).
2. Pain in lower right quadrant.
3. Treatment -- appendectomy.
