Digestive Tract Physiology

Digestive Tract Physiology

• Digestive systems vary among animals, limiting their ability to utilize certain nutrients
• Digestion involves breaking down large, insoluble molecules into simpler compounds for absorption into the blood and lymph
• Digestion processes involve mechanical, chemical, and microbial activities

Simple Non-Ruminants

• Include animals such as swine, poultry, dogs, cats, rats, and humans
• Often referred to as monogastric animals
• Have a pouch-like, non-compartmentalized stomach
• Rely on chemical digestion of food through secretion of digestive enzymes in the gut, rather than microbial digestion

Functions of Each Major Segment in Simple Non-Ruminants

Oral Cavity

• Physical digestion occurs through mastication of food to increase surface area for exposure to digestive enzymes
• Starch is hydrolyzed to maltose by enzymatic digestion of salivary amylase

Stomach

• Stores ingested feed and meters it into the small intestine
• HCL secreted into the stomach kills most bacteria ingested with feed and hydrolyzes proteins
• In nursing animals, renin is secreted to coagulate casein (a milk protein) to prevent rapid passage of milk out of the stomach

Small Intestines

• Major site of digestion and absorption in simple non-ruminants
• Consists of three segments: duodenum, jejunum, and ileum
• Pancreas serves as a major source of digestive enzymes that break down carbohydrates, proteins, and lipids
• Buffers (bicarbonates) are secreted to neutralize stomach acid

Large Intestines

• A complex ecosystem of anaerobic microorganisms further degrades and metabolizes undigested residues
• Microbial fermentation of carbohydrates produces volatile fatty acids, carbon dioxide, and methane
• Microbial fermentation of proteins and amino acids releases ammonia, which is absorbed via portal blood and converted to urea in the liver

Primary Enzymes of the Digestive Tract

• Amylase: breaks down carbohydrates (starch, glycogen, dextrin) into maltose and glucose
• Maltase: breaks down maltose into glucose
• Lactase: breaks down lactose into glucose and galactose
• Sucrase: breaks down sucrose into glucose and fructose
• Lipase: breaks down lipids into monoglycerides and glycerol
• Pepsin: breaks down proteins into polypeptides
• Trypsin: breaks down proteins into peptides and amino acids
• Chymotrypsin: breaks down proteins into peptides and amino acids
• Carboxypeptidase: breaks down proteins into peptides and amino acids
• Aminopeptidase: breaks down peptides into amino acids
• Dipeptidase: breaks down peptides into amino acids
• Nucleotidase: breaks down nucleotides into nucleosides and phosphoric acid
• Nucleosidase: breaks down nucleosides into purines and phosphoric acid

Ruminants

• Differ from simple non-ruminants in having a large, compartmentalized stomach
• Microbes that inhabit the large stomach accomplish most of the digestion
• Functions of each segment:
  • Mouth and teeth: adapted for grinding and grasping fibrous feeds
  • Stomach: consists of rumen, reticulum, omasum, and abomasum
    • Rumen: a fermentation vat where microbes ferment ingested feeds
    • Reticulum: traps foreign materials, regulates flow of ingested feeds
    • Omasum: regulates flow of ingested feeds, promotes maximum fermentation efficiency
    • Abomasum: site of secretion of gastric juices and proteolytic enzymes

Non-Ruminant Herbivores

• Include animals such as horses, rabbits, guinea pigs, zebras, elephants, and hippopotamuses
• Intermediate between simple non-ruminants and ruminants in their digestive physiology and nutrition requirements
• Can be further subdivided into foregut fermenters, colon fermenters, and cecal fermenters

Foregut Fermenters

• No important domesticated species

Colon Fermenters

• Examples include equids
• Enlarged colon is the site of microbial fermentation of materials that resist breakdown in the small intestine
• High-quality proteins, starch, and lipid are digested in the small intestine
• Fiber digestion is less efficient, and the animal inefficiently utilizes bacterial proteins from digestion/metabolism of microbes

Cecal Fermenters

• Examples include rabbits
• Have low digestibility of fiber, so they adapt by selectively separating and excreting indigestible fiber and retaining the more digestible non-fiber contents for fermentation in the cecum
• Produce two types of feces: hard and soft
• Soft feces, or cecotropes, are actually cecal contents that are fermented in the cecum and utilized by the animal for nutritional value in a process called coprophagy