Comparative Vertebrate
Physiology
Digestion in vertebrates
Food reception

Chewing



Grind and breakdown food
Stimulate saliva production
Mix food with saliva
Saliva

Composition



99.5% water, 0.5% protein and electrolytes
Protein: amylase, mucus, lysozyme
Functions




Water: softens food
Amylase: polysaccharide breakdown
Mucus: lubrication
Lysozyme: kills bacteria
Process of swallowing
Esophagus

Upper sphincter


Lower sphincter


Ensures breathing commences
Prevents acid reflux (heartburn)
Modified in birds

Crop used to store and partially
digest food
Stomach anatomy
Stomach motility

Cephalic phase


Receptive relaxation
Gastric phase

Doudenum
Lower
esophageal
sphincter
Gastric peristalsis
Pyloric
sphincter
Peristaltic
wave
Stomach
Gastric motility


Control
1. Volume of chyme


Smooth muscle excitability
2. Degree of fluidity

Regulates rate of evacuation
Gastric secretions

1. HCl


A. Cephalic phase


By parietal cells
HCl secreted
B. Gastric phase




Breakdown proteins to peptides
More distention = more HCl release
Protein + HCl = gastrin release
Gastrin increase HCl release
Gastric secretions

C. Intestinal phase

Inhibits gastric secretions - 3 mechanisms



Decrease parasympathetic stimulation
Local reflex action
Release of secretin, GIP and cholecystokinin
Gastric secretions

2. Pepsinogen

Stored in chief cells as zymogen granules
Gastric secretions

3. Mucus


Goblets cells
Functions



Prevents mechanical injury
Self digestion against pepsin
Neutralizes HCl
Gastric emptying - hormones

Regulated by rate and composition of
chyme entry into the duodenum
Stomach specialization


Monogastric versus digastric stomach
Cellulose digestion




Ruminants (giraffes, cows, deer, elk)
Symbiotic microorganisms
4 chambered stomach
Pregastric fermentation
Postgastric fermentation


Colon or enlarged cecum
Cecant digestion


Cecum: rabbit, koala, opossums
Disadvantage


Microorganisms not digested and assimilated
Cellulose digestion less efficient
Coprophagy



Reingestion of feces to return microbes to
proximal gut
e.g. lagomorphs and some rodents
Pellets
 Normal hard, dark
 Soft, pale (over 50% bacteria, reingested)
 Further fermentation produces lactate
Midgut

Chyme enters slowly through the pyloric
sphincter
Sphincter
Small intestine anatomy
Small intestine anatomy
Pancreas
Acinar cells secrete
Ducts cells secrete
digestive enzymes
enzymes, aqueous NaHCO3
Exocrine portion
(acinar and duct cells)
Endocrine portion
(Islets of Langerhan’s)
Pancreas

Endocrine portion


Islets of Langerhan’s
Exocrine portion


Acinar cells (digestive enzymes)
Duct cells (secrete sodium bicarbonate)
Acinar cell secretion

Proteolytic enzymes (trypsin, chymotrypsin
carboxypeptidase)


Lipase


Peptides
Fatty acids and monoglycerides
Amylase

Lactose and maltose
Hormones effecting pancreatic
secretion

Secretin




Secreted by duodenal mucosa
In response to high [H+]
Increases bicarbonate secretion
Cholecystokinin




Secreted by duodenal and jejunal mucosa
In response to fat and protein
Increases total amount of enzymes secreted
Increases smooth muscle activity of gall bladder
Both hormones inhibit gastric motility
Liver

Bile


Bile salts, bicarbonate lecithin, water
Functions



Bile salt and water: emulsifies lipid
Lecithin: prevents reaggregation
Bicarbonate: neutralizes gastric HCl
Enterohepatic circulation
Common hepatic
duct
Small intestine secretions

Succus entericus



Aqueous salt and mucus
Enterocytes: proteases, lipases, amylases
Functions


Lubrication
Enzyme breakdown
Control of secretion
Absorption

Proteins and sugars

co-transported with sodium apically or by protein transporters
Lipids


Lipase emulsifies triglycerides
into micelles
Emulsion



Mechanical disruption by stomach
Lecithin prevents reaggregation
Once in cell



Converted to triglyceride
Chylomicron
Exocytosis into lacteal
Hindgut

Functions



Temporary storage of digesta
Absorption of inorganic ions and water (9%)
Bacterial fermentation in herbivores

VFA’s absorbed and used as an energy source in
metabolism
Hindgut fermentation

Types


Colon: horse, elephant, wombat
Cecum: rabbit, koala, opossums
Hindgut

Motility


Rhythmical: every 30 minutes
Mass movement: coincides with ileum
contraction
Hindgut anatomy


Tenia coli
Haustra alter their location
Haustrum
Tenia coli
Rectum
Anal
canal
External anal sphincter
Water balance

10 liters of fluid into the tract
(98.5% is reabsorbed)
 Input


Reabsorbed


Intake 1.5L, salivary gland 1.5L,
stomach 2.5L, liver 0.5L, pancreas
1.5L, small intestine 1.5L, colon 1L
Small intestine 9L, colon 0.85L
Output

Feces 0.15L