Digestive Tract Physiology

Study Notes

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

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

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

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

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

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

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

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

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

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