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
Ensures breathing commences Lower sphincter
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 Gastric peristalsis
Pyloricsphincter
Doudenum
Loweresophagealsphincter
Stomach
Peristalticwave
Gastric motility Control 1. Volume of chyme
Smooth muscle excitability
2. Degree of fluidity Regulates rate of evacuation
Gastric secretions 1. HCl
By parietal cells
A. Cephalic phase 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
Endocrine portion(Islets of Langerhan’s)
Exocrine portion(acinar and duct cells)
Acinar cells secretedigestive enzymesDucts cells secrete
enzymes, aqueous NaHCO3
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) Peptides
Lipase 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
Analcanal External anal sphincter
Water balance 10 liters of fluid into the tract
(98.5% is reabsorbed) Input
Intake 1.5L, salivary gland 1.5L, stomach 2.5L, liver 0.5L, pancreas 1.5L, small intestine 1.5L, colon 1L
Reabsorbed Small intestine 9L, colon 0.85L
Output Feces 0.15L