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DigestionI. Introduction A. Vertebrates Contain Many
Cells with Specialized Functions B. No Vertebrate Cells
Are Specialized for Photosynthesis 1. Plants are
self-sustaining autotrophs Animals are heterotrophs Cells must be nourished
by food obtained from outside the body 2. Many major organ
systems are associated with acquisition of food energy II. The Nature Of Digestion A. Animals Obtain Energy
by Degrading Chemical Bonds of Organic Molecules 1. Process acts
on simple molecules: amino acids, lipids and sugars 2. Organisms rarely
contain large amounts of simple molecules
i. Eating an organism does not provide immediate
energy
ii. Simple molecules incorporated into long
macromolecular chains
iii. Macromolecules include proteins, fats
and starches 3. First must degrade
macromolecules into simple constituent parts B. Process Called
Digestion III. Organization Of Vertebrate Digestive Systems A. General Organization
of the Vertebrate Digestive System 1. Consists
of a tubular gastrointestinal tract and accessory digestive organs
i. Initial components are mouth and pharynx a. Common passage
of oral and nasal cavities b. Pharynx leads to
esophagus
ii. Esophagus is a muscular tube leading
to stomach
iii. Preliminary digestion occurs in stomach
iv. Food passes into duodenum, upper part
of small intestine a. Battery of digestive
enzymes continue digestion b. Products pass across
small intestine wall into bloodstream 2. Tubular gastrointestinal
tract has a layered structure
i. Mucosa is innermost layer a. Epithelial layer
b. Separates interior
(lume n) from blood vessels in next layer
ii. Submucosa is next layer of connective
tissue
iii. Next outermost layer is the muscularis a. Double layer of
smooth muscle b. Inner muscles have
circular orientation c. Outer layer are
arranged longitudinally
iv. Serosa connective tissue layer covers
external surface
v. Nerve plexuses regulate activities of
gastrointestinal tract B. Specializations
of Digestive Systems Indicate Different Ways of Living 1. Fish have
large pharynx with gill slits 2. Air-breathing vertebrates
have reduced pharynx 3. Many vertebrates
have teeth and chew food particles 4. Birds lack teeth,
break up food in two-chambered stomach
i. Gizzard grinds material with small pebbles
ii. Seeds and hard materials ground up for
digestion in second chamber 5. Carnivores have
shorter intestines than herbivores
i. Most animal macromolecules are readily
digested
ii. Herbivores eat cellulose, have convoluted
intestines to prolong digestion
iii. Herbivorous mammals have multiple-chambered
stomachs with cellulose-degrading bacteria IV. Food Enters The Digestive Tract Through The Mouth A. Teeth Are Important to
Animal Digestion 1. Capture food
in different ways, teeth specialized for such capture 2. Carnivores possess
pointed teeth for capture, cutting and shearing 3. Herbivores have
large, flat teeth suited for grinding plant materials 4. Omnivores have
both types, front like carnivores, back like herbivores
i. Incisors: four front teeth, used for
biting
ii. Canines: one on each side of incisors,
used for tearing food
iii. Premolars: two on either side behind
canines, chewing teeth
iv. Molars: three on either side behind
canines, chewing teeth B. Food Is Moistened
and Lubricated in the Mouth 1. Tongue mixes
food with saliva
i. Saliva secreted by three pairs of salivary
glands
ii. Empty through mucosal lining of mouth
iii. Contains salivary amylase to initiate
breakdown of starch 2. Secretion of saliva
controlled by the nervous system
i. Continuous secretion to keep the mouth
moist
ii. Secretion stimulated by presence of
food V. Food Passes To The Stomach Through The Esophagus A. Food Passes Beyond the
Teeth to the Back of the Mouth 1. Palate elevates,
pushes against back wall of pharynx
i. Seals off nasal cavity
ii. Prevents entry of food into nasal cavity 2. Pressure on pharynx
stimulates receptors to signal swallowing center 3. Swallowing center
signals respiratory tract
i. Inhibits respiration
ii. Seals trachea by raising larynx and
closing glottis with epiglottis B. Food Enters Esophagus
Connecting Pharynx and Stomach 1. Upper portion
of esophagus enveloped in skeletal muscle 2. Lower two-thirds
enveloped in smooth muscle 3. Food propelled
to stomach by peristaltic waves 4. Exit of food from
esophagus to stomach controlled by a sphincter
i. Muscular constriction at junction of
two organs
ii. Prevents food in stomach from re-entering
esophagus
iii. Rodents and horses have true sphincter
cannot regurgitate, humans can VI. Preliminary Digestion Occurs In The Stomach A. Stomach Is a Saclike
Portion of the Digestive Tract 1. Interior
of stomach is highly convoluted
i. Folds up when empty, expands when full
of food
ii. Carnivores that gorge sporadically can
distend stomachs greatly 2. Stomach has extra
layer of smooth muscle to churn food B. Gastric Glands
of Mucosa Are Exocrine Glands that Produce Secretions 1. Parietal
cells secrete hydrochloric acid (HC l) 2. Chief cells secrete
pepsinogen, acid-loving, weak protein-digesting enzyme 3. Activated pepsinogen
molecules cleave fragment from each other, make pepsin
i. Pepsin is more active molecule
ii. Production of inactive molecule, converted
to active enzyme outside
iii. Chemical generically called a zymogen
iv. Prevents chief cells from self-digestion 4. Stomach produces
2 liters of acid and gastric secretions per day
i. Produces pH of 2, compared to blood pH
of 7.4
ii. Low pH helps denature proteins, keeps
pepsin active
iii. Proteins denatured into polypeptides
iv. Digestion to amino acids occurs in small
intestine
v. No digestion of carbohydrates or fats
in stomach 5. Chyme: mix of partly
digested food and gastric juice 6. Acid solution also
kills bacteria ingested with food 7. Overproduction
of acids may occur
i. In stomach, cause gastric ulcers a. Are rare due to
protective alkaline mucus produced by mucosa b. Mucosal cells readily
replaced when damaged
ii. Duodenal ulcers are more common a. Produced when excessive
amount of acidic chyme delivered into duodenum b. Alkaline secretions
of pancreas cannot neutralize chyme 8. Parietal cells
also produce intrinsic factor
i. Polypeptide needed for intestinal absorption
of vitamin B12
ii. Required for formation of red blood
cells
iii. Deficiency causes pernicious anemia 9. Little absorption
occurs in stomach, all other absorption in intestine
i. A little water
ii. Substances like aspirin and alcohol VII. Terminal Digestion And Absorption Occur In The Small
Intestine A. Food Passes From Stomach
to Small Intestine 1. Controlled
by muscular pyloric sphincter 2. Capacity of small
intestine limited, digestion takes time
i. Relatively small amounts of chyme can
enter at a time
ii. Coordination regulated by neural and
hormonal signals B. Small Intestine
Is the Primary Location of Digestion 1. Length is
approximately six meters
i. Duodenum comprises first 25 centimeters,
or 4%
ii. Jejunum and ileum comprise rest of small
intestine 2. Duodenum receives
chyme, pancreatic enzymes, bile from liver and gallbladder 3. Absorption occurs
in all three regions of small intestine C. Digestion of Food
in the Intestine 1. Epithelial
wall covered with small projections called villi 2. Epithelium of villi
covered with microvilli, cytoplasmic projections
i. Seen clearly with electron microscope
ii. Epithelial wall also called brush border 3. Both increase the
absorptive surface of the small intestine 4. Microvilli also
participate in digestion
i. Digestive enzymes embedded in epithelial
cell plasma membranes
ii. Brush border enzymes hydrolyze lactose,
sucrose and others a. Adult humans lose
ability to produce lactase b. Condition called
lactose intolerance D. Absorption of Food in
the Intestine 1. Components
of protein and carbohydrate digestion transported across brush border
i. Amino acids and monosaccharides cross
to intestinal epithelial cells
ii. Transported across intestinal epithelium
to capillaries in villi
iii. Blood carries digestion products to
liver
iv. Travel via hepatic portal vein 2. Products of fat
digestion absorbed by different mechanism
i. Fats hydrolyzed into fatty acids and
monoglycerides
ii. Absorbed by intestinal epithelium
iii. Reassembled into triglycerides
iv. Combine with proteins to form water-soluble
chylomicrons
v. Absorbed into lymphatic capillaries,
not hepatic portal system
vi. Contents of lymphatic system enter blood
stream in veins near neck
vii. Total volume of food and water equals
2 liters (800 grams of solids) 3. Body adds 7.0 liters
of its own fluids making a total of 9.0
i. 1.5 liters salivary enzymes
ii. 2.0 liters of gastric secretions
iii. 1.5 liters of pancreatic secretions
iv. 0.5 liters of bile from the liver
v. 1.5 liters of intestinal secretions 4. Nearly all fluids
and solids are absorbed
i. 8.5 liters reabsorbed in the small intestine
ii. 350 milliliters reabsorbed in the large
intestine 5. Only 50 grams of
solids and 100 milliliters of liquid leave as feces 6. Fluid absorption
efficiency = 99% VIII. The Pancreas Secretes Enzymes, Bicarbonate And Hormones A. The Pancreas Makes Digestive
Enzymes 1. Pancreas
located at junction of stomach and small intestine 2. Fluid secreted
into duodenum via pancreatic duct
i. Pancreas is thus an exocrine organ
ii. Fluid contains a. Protein digesting
trypsin and chymotrypsin b. Starch digesting
pancreatic amylase c. Fat digesting lipase
iii. Enzymes released primarily as zymogens,
activated by brush border
iv. Also contains bicarbonate to neutralize
HCl from stomach a. Chyme in intestine
is slightly alkaline b. Bicarbonate produced
by acini, clusters of secretory cells B. Pancreas Also Serves
as an Endocrine Gland 1. Produces
hormones that regulate levels of blood sugar and other nutrients 2. Produced in islets
of Langerhans clustered throughout pancreas 3. Most important
hormones are insulin and glucagons IX. The Liver Produces Bile And Regulates Blood Composition A. Bile Production 1. Liver is
largest internal organ of body 2. Main secretion
of liver is bile
i. Mixture of bile pigments and bile salts
delivered into duodenum
ii. Bile pigments do not participate in
digestion a. Are waste products
from liver's destruction of old red blood cells b. Eliminated with
feces c. Accumulation of
pigments result in jaundice
iii. Bile salts are lipid and water soluble a. Disperse fat droplets
in chyme into emulsion of smaller droplets b. Emulsification
increases surface area for lipase to work on 3. Bile is stored
and concentrated in gall bladder 4. Fatty food in duodenum
triggers contraction of gallbladder to release bile B. Regulation of Blood
Composition 1. Hepatic portal
vein carries blood from stomach and intestine to liver 2. Liver absorbs or
chemically modifies substances before they reach rest of body
i. Ingested alcohol and drugs metabolized
by liver cells
ii. Toxins, pesticides, carcinogens, poisons
detoxified
iii. Ammonia from intestinal bacteria converted
into urea 3. Controls level
of substances produced in body
i. Steroid hormones converted into less
active water-soluble forms
ii. Molecules included in bile, eliminated
in feces or through kidneys 4. Produces proteins
found in blood plasma
i. Includes most blood clotting factors
ii. Maintains blood protein concentration
within narrow limits
iii. Imbalance can cause edema C. Regulation of Blood
Glucose Levels 1. Constant
concentration of blood glucose must be maintained
i. Brain cells totally dependent on blood
for supply of glucose
ii. Brain cells store little glucose, cannot
convert fat or amino acids into glucose 2. Maintaining level
requires active control by various body organs
i. Vertebrates eat sporadically, ingestion
followed by fasting
ii. Most food digested rapidly, metabolites
enter blood stream
iii. Without control, level of metabolites
would change drastically 3. Liver removes glucose
from blood, converts it into glycogen
i. Glycogen stored in liver tissue and
ii. Also stored in skeletal muscle fibers,
but can only be used in muscles
iii. Process stimulated by pancreatic hormone,
insulin 4. If blood glucose
level is low, liver secretes glucose into blood
i. Occurs between meals, during fasting
ii. Glucose partly obtained from breakdown
of glycogen
iii. Conversion stimulated by glucagon,
other pancreatic hormone
iv. Only liver can secrete glucose into
blood
v. Liver stores enough glycogen for 10 hours
of fasting
vi. For greater fasting liver converts amino
acids, lactic acid into glucose a. Process called
gluconeogenesis b. Amino acids come
from muscle protein X. The Large Intestine Concentrates Solids A. Large Intestine or Colon
Comprises Last Meter of Digestive Tract 1. Has no digestive
function, absorbs 4% of fluids 2. Shorter in length
than the small intestine
i. Lies in three relatively straight segments
ii. Surface is not convoluted
iii. Inner surface lacks villi
iv. Significantly less surface area over
which to absorb 3. Absorb sodium,
vitamin K, other products of bacterial metabolism 4. Primary function
is a refuse dump
i. Undigested material compacted and stored
ii. Bacteria live and reproduce and are
incorporated into feces
iii. Bacterial fermentation produces gas
within the colon 5. Human colon evolved
to process food with high fiber content
i. Low fiber diets result in slower passage
of food through colon
ii. May be associated with high level of
colon cancer in U. S. 6. The rectum is the
terminal portion of the large intestine
i. Feces pass into rectum by peristaltic
contractions
ii. Material exits anus through two sphincters a. First sphincter
is smooth muscle, opens involuntarily b. Second sphincter
is striated muscle, under voluntary control B. All Vertebrates
Except Placental Mammals Possess a Cloaca 1. A collective
cavity for digestive, reproductive and urinary functions 2. Placental mammals
have separate exits for each tract XI. Neural And Hormonal Regulation Of Digestion A. Coordination by Nervous
System 1. Stimulates
salivary and gastric secretions in response to sight and smell of food
i. Food in stomach stimulates secretion
of gastrin by stomach
ii. In turn stimulates secretion of pepsinogen
and HCl in stomach
iii. Negative feedback loop controls HCl
secretions a. Decreased stomach
pH reduces secretion of gastrin b. Decrease in gastrin
decreases HCl production 2. Passage of chyme
from stomach inhibits stomach contractions
i. No more chyme enters intestine until
previous amount processed
ii. Process mediated by neural impulses
and gastric inhibitory peptide (GIP) a. GIP released by
duodenum b. Production stimulated
most strongly by fat in chyme c. Fatty meals take
longer to process B. Coordination by
Hormones 1. Cholecystokinin
(CCK) is secreted in response to fat in chyme
i. Stimulates contraction of gallbladder
to release bile
ii. Bile emulsifies fats, increases efficiency
of digestion
iii. Stimulates secretion of pancreatic
digestive enzymes 2. Secretin released
in response to acidity of chyme
i. Stimulates pancreas to release bicarbonate
ii. Neutralizes acidity of chyme
iii. First hormone ever discovered XII. Symbiosis Within The Vertebrate Digestive System A. Bacterial Digestion of
Cellulose Within Animals 1. Vertebrates
lack enzymes to digest plant material 2. Some bacteria can
do so and are harbored by animals
i. Plays relatively small role in human
nutrition
ii. Essential nutrition for termites, cockroaches
and some herbivores 3. Cows and related
ruminants possess two stomachs
i. First stomach has two chambers: rumen
and reticulum
ii. Second stomach has two chambers, omasum
and abomasum 4. Capacity of rumen
is 50 gallons
i. Provides a fermentation vat for bacteria
and protozoa to process cellulose
ii. Allows cows to regurgitate and rechew
their food (cud) 5. Re-chewed food
swallowed, goes into reticulum, omasum and abomasums
i. Abomasum released gastric juices
ii. Is equivalent to human stomach
iii. Leads to a very efficient digestion
of cellulose 6. Horses, rodents
and lagomorphs retain bacteria in the caecum
i. Cannot regurgitate material from caecum
ii. Rats and rabbits redigest cellulose
another way
iii. Eat feces and literally redigest them
a second time
iv. Efficiency approaches that of ruminants B. Additional Digestive
Activity of Intestinal Bacteria 1. Wax digested
by bacteria in gut of honey guide birds 2. Intestinal bacteria
provide mammals with vitamin K
i. Birds lack bacteria and must consume
vitamin K in food
ii. Prolonged antibiotic treatment depletes
bacteria
iii. Must supplement vitamin K until bacteria
are re-established XIII. Nutrition A. Ingestion of Food Has
Dual Purpose 1. Provides
source of energy 2. Provides raw materials
the animal cannot manufacture for itself 3. Intake of food
required to maintain glycogen stores in the liver
i. Excess glucose metabolized by muscles
or converted to fat
ii. Basal metabolic rate (BMR): rate energy
is consumed at complete rest
iii. Food energy -(energy used at rest +
exercise energy) = energy in glycogen and fat
iv. BMR is relatively constant within an
individual
v. Balance between food energy and exercise
energy determines energy storage in fat 4. Wealthy countries
exhibit obesity from overeating and imbalanced diet B. Essential Nutrients 1. Over time
many vertebrates have lost ability to synthesize substances 2. Substances that
cannot be manufactured must be obtained from diet 3. Vitamins: essential
organic substances required in trace amounts
i. Humans, apes, monkeys, guinea pigs cannot
make vitamin C
ii. Humans require at least thirteen vitamins 4. Essential amino
acids: eight of the total twenty
i. Lysine, tryptophan, threonine, methionine,
phenylalanine, leucine, isoleucine valine
ii. Must be obtained from proteins in food 5. Vertebrates synthesize
cholesterol, insects cannot 6. Essential minerals:
calcium, phosphorus and other trace elements Layout by J.T. Poirier © 2001 |