Digestive Tract Physiology

Study Notes

Digestion is the breaking down of large, insoluble molecules of food into simpler compounds that can pass through the mucous membrane of the alimentary canal into the blood and lymph.
Absorption is the passage of digested nutrients through the mucous membrane.

Mechanical activities: mastication and muscular contractions of the alimentary canal
Chemical activities: enzymes secreted by the animal in digestive juices
Microbial activities: bacteria, protozoa, and fungi that break down food enzymatically, especially in ruminant digestion

Nutritional requirements of animals are greatly dependent on their digestive tract anatomy and physiology.
Livestock can be divided into three groups according to their digestive tracts: simple non-ruminants, ruminants, and non-ruminant herbivores.

Include 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
Little microbial digestion in any part of the gut

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

Stores ingested feed and meters it into the small intestine in amounts that intestinal digestion can accommodate
HCl secreted into the stomach kills most bacteria ingested with feed, hydrolyzes proteins due to its acidic pH, and activates pepsin
Rennin is secreted in nursing animals to coagulate casein (a milk protein) to prevent rapid passage of milk out of the stomach, promoting proteolytic digestion of milk

Major site of digestion and absorption in simple non-ruminants
Consists of three segments: duodenum, jejunum (major site of nutrient absorption), and ileum
Pancreas serves as a major source of digestive enzymes that degrade carbohydrates, fats, and proteins
Also secretes buffers (bicarbonates) that neutralize stomach acid

Amylase (saliva and pancreas) breaks down starch, glycogen, and dextrin into maltose and glucose
Maltase (small intestine) breaks down maltose into glucose
Lactase (small intestine) breaks down lactose into glucose and galactose
Sucrase (small intestine) breaks down sucrose into glucose and fructose

Lipase (gastric mucosa and pancreas) breaks down lipids into monoglycerides, glycerol, and fatty acids

Rennin (gastric mucosa) coagulates milk proteins
Pepsin (gastric mucosa) breaks down proteins into polypeptides
Trypsin, chymotrypsin, and carboxypeptidase (pancreas) break down protein breakdown products into peptides and amino acids
Aminopeptidase and dipeptidase (small intestine) break down peptides into amino acids
Nucleotidase and nucleosidase (small intestine) break down nucleotides into nucleosides and phosphoric acid

Have a large, compartmentalized stomach
Microbes that inhabit the large stomach accomplish digestion, rather than enzymes produced by the animal itself
Rumen is a fermentation vat that contains a large microbial population that ferments ingested feeds, producing energy from fermentation end products (VFAs)

Absence of upper incisors, instead dental pads are present
Large gap between incisors and molars allow cattle to harvest and chew large amounts of fibrous feeds
Teeth are primarily for grinding, and tongue is used to gather and grasp grasses
Saliva contains sodium bicarbonate to keep rumen at a proper neutral pH for good bacterial growth

Rumen is a fermentation vat that contains a large microbial population that ferments ingested feeds
Major source of energy is fermentation end products (VFAs)
Also produces large quantities of gases, mainly carbon dioxide and methane, which are removed by eructation
Failure to eructate normally causes bloat

Examples are rabbits
Have low digestibility of fiber, so they adapt by selectively separating and excreting indigestible fiber and retaining more digestible non-fiber contents for fermentation in the cecum
Produce two types of feces: hard and soft
Hard feces consist of indigestible fiber that is rapidly eliminated
Soft feces, or cecotropes, are cecal contents that are fermented in the cecum
The animal utilizes cecotropes because of their nutritional value, in a process called coprophagy, which provides bacterial proteins and vitamins synthesized in the cecum