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

Horses, elephants, perissodactyls 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