Download Chpt15

I.
Feeding Methods
A. Absorption (passive) via external surfaces - some protozoans,
endoparasites, aquatic invertebrates
B. Endocytosis - protozoans, lining of the alimentary canal
C. Filter feeding/ suspension feeding
1. Small food particles are "filtered" out of the water
2. Mucus often helps trap the food
3. Used by marine animals (sponges, brachiopods, whales)
D. Fluid Feeding
1. Piercing and sucking
a) Inject anticoagulants into the host
b) Suck blood out of host via proboscis/pharynx
c) Used by nematodes, annelids (leech), arthropods
(mosquitos, lice)
2. Cutting and Licking
a) Create a whole in host's integument
b) Lick or sponge the fluid which leaks out
c) Some use anticoagulants and/ or analgesics (eg bat)
E. Seizing Prey
1. Jaws, teeth, beaks - adapted to allow animals to capture prey
and / or inject toxins (15-6 through 15-9)
2. Toxins - used to paralyze / stun prey or fend off predators
F. Herbivores/ Grazing - specialized mouths and teeth with tough
enamel, or which continually grow (rodents)
II.
Alimentary Systems
A. Types
1. Batch reactor - food comes in, is processed, and leaves by the
same opening (eg coelenterates/cnidarians: jellyfish, corals,
anemones)
2. Continuous flow, stirred-tank reactors (eg forestomach of
ruminants)
a) Food is always coming in during processing
b) Digestive by-products leave via a different pathway
3. Plug-flow reactor - bolus of food (plug) is digested as it flows
along a long digestive reactor (eg small intestine)
B. Headgut - food entrance point
1. Accepts food/ begins the breakdown processes
2. Mechanical (if food is chewed) and chemical breakdown
3. Saliva (15-28) - main secretion (talked about in exocrine
section)
a) Moistens food - smoothes its passage
b) Contains amylases (vertebrates and invertebrates)
which breaks down starches to di and oligosaccharides
4. Stimulated by
a) Food in mouth
b) Mechanical stimulation of mouth tissues
c) Cephalic stimulation (thinking about food)
d) When stimulated, amount of enzyme present and rate
of flow increases
C. Foregut: Food conduction, storage, and digestion
1. Esophagus
a) Food conduction via peristalsis (reverse is
regurgitation)
b) Storage (crop)/Fermentation - used by leeches
between feedings or birds storing food to give to young
c) Secretes mucus continually for lubrication
2. Stomach - mono and digastric (single / many stomachs)
a) Storage of food; major breakdown of foodstuffs
b) Mechanical mixing of food and secretions and
chemical digestion
(1) Gizzard/crop in birds contains stones to grind seeds
(2) Ruminate stomachs - rumen and reticulum used for
fermentation, then passed to omasum and abomasum (the only
stomach to secrete digestive enzymes)
c) Secretions
(1) Pepsinogin
(a) Secreted by the chief cells
(b) Cleaved to pepsin which digests proteins at pH=2
(c) Stimulated by the vagus nerve and gastrin
(2) HCl
(a) Secreted by parietal/oxyntic cells
(b) Uses carbonic anhydrase and a HCO3-/Cl- exchange
pump (15-32)
(c) Stimulated by vagus nerve (cephalic phase of
digestion), gastrin and histamine (both needed), and
caffeine, alcohol, and some spices (last 5 due to gastric
phase of digestion sensed via mechano and chemo
receptors)
(3) Gastric Mucus
(a) Secreted by goblet cells
(b) Contains alkaline electrolytes to neutralize the pH
(4) Inhibition of secretions
(a) Enterogastric reflex - chemo and mechanoreceptors
sense food (chyme) in the duodenum and cause secretin,
GIP (gastric inhibitory peptide), VIP (vasoactive
intestinal peptide), and bulbogastrone to be released
(b) Sympathetic stimulation
D. Midgut: Chemical digestion and absorption
1. Absorption - specialization of the epithelium
a) Large surface area (size of a doubles tennis court in
humans)
(1) Mucosa is highly folded (also helps slow food down)
(2) Covered by villi
(a) lymph and blood vessels extend into the villi to
transport absorbed materials
(b) digestive epithelium (absorptive and goblet cells)
cover the villi
(3) brush border is formed by microvilli covering the villi
b) cells are held together by desmosomes and tight
junctions to prevent digested materials from passing
between the cells
2. Secretions
a) Duodenum - mostly digestion
(1) Pancreatic secretions
(a) Peptidases: trypsinogen, chymotrypsinogen, carboxy
and aminopeptidases
(b) Carbohydrases: amylase and maltase
(c) Lipase
(d) Nucleases
(e) Bicarbonate (neutralizes acid from the stomach)
(2) Bile (from Gallbladder)
(a) Contains fats and fatty acids, bile salts and
pigments, and cholesterol
(b) Neutralizes chyme (alkaline)
(c) Breaks fats into micelles for enzymatic digestion and
absorption
(d) Contains waste substances from the blood which are
digested or excreted
(3) Duodenal Secretions
(a) Brunner's gland - secretes a mucoid fluid to protect
duodenum before the pancreatic and gallbladder
secretions can neutralize the chyme
(b) Peptidases: enterokinase, carboxy and
aminopeptidases
(c) Carbohydrases: maltase, lactase, sucrase
(d) Lipase
(e) Nucleases
b) Jejunum - secretions similar to duodenum, plus
absorption
c) Ileum - absorption
3. Control of secretions (intestinal phase of control) (15-34)
a) Gallbladder and pancreas are both stimulated by
cholecystokinin (CCK)- released by the presence of fatty
and amino acids in the duodenum
b) Pancreas is stimulated by secretin (strong acid in
stomach and small intestine)
c) Pancreas is inhibited by
(1) Enteroglucagon (carbohydrates in the duodenum)
(2) Enkephalin (basic conditions in the stomach and intestine)
(3) Somatostatin (acid in the stomach lumen)
E. Hindgut: water and ion absorption; defecation
1. Stores remains of digested food where bacterial flora digest the
contents
2. Fermentation occurs in many animals (zebras, elephants,
rodents)
3. Fecal material passes through the rectum or cloaca, causing the
urge to defecate
a) If defecation occurs, feces pass through an internal
sphincter and out the anus
b) If defecation does not occur, the fecal material
passes back into the colon
III.
Motility
A. Importance/WHY?
1. Moves food through alimentary canal
2. Mixes food and digestive juices
3. Provides the maximal diffusion gradient by always supplying
new material to the epithelial lining
B. Types
1. Ciliary - used by annelids (earthworms, leeches), tunicates (look
like sponges), cephalochordates (lancets)
2. Muscular - used by most animals
a) Peristalsis - wavelike contraction of smmoth muscle;
relaxation of the circular muscle is followed by a
contraction which pushes food ahead to the next
relaxed area
b) Segmentation - rhythmic contractions of circular
muscle which pushes food back and forth rather than
forward
C. Control
1. Intrinsic - myogenic muscles cause the contraction - Basal
Electric Rhythm (BER)
2. Extrinsic - neuronal/hormonal modification of the BER
a) ACh/ parasympathetic drive increases the BER
b) Epinephrine/ sympathetic drive decreases the BER
c) Peptide hormones (Table 15-2) have varying effects
IV.
Absorption
A. Types: diffusion, carrier mediated, active transport, endocytosis
(15-35 and 15-36)
1. Lipids
a) Diffuse across membrane when micelle contacts the
epithelium
b) Reassemble in the smooth endoplasmic reticulum
c) Repackaged into chylomicrons in the Golgi
d) Exocytoses and diffusion into central lacteal
B. Transport in blood
1. Fats are carried by the lymph system, added to the blood at the
thoracic duct
2. Sugars and amino acids are transported into the capillaries
where they immediately go to the liver via the hepatic portal vein
3. Vitamins are transported in both the lymphatic and circulatory
system initially, depending on whether or not they are fat or water
soluble