Digestive Physiology

• Digestion


  The process of breaking down food into smaller molecules that can be absorbed and used by the body
• Digestive system


  • Ingestion
  • Digestion
  • Absorption
• Prehension


  The process of collecting or gathering food
• Mastication


  The process of chewing food
• Deglutition


  The process of swallowing food
• Types of digestive tracts

  • Monogastric
  • Polygastric (ruminant)
• Oral digestion


  • Very little digestion in the mouth
  • Salivary glands produce mucus and amylase enzyme (except ruminants)
  • Amylase converts starch to maltose and dextrins
  • Motility is voluntary (mastication)
  • Food transformed into bolus before swallowing
  • Bolus transferred to stomach by peristalsis and gravity
• Control of salivary secretion

  1. Salivary centre in medulla

  2. Parasympathetic stimulation (+)

  3. Sympathetic inhibition (-)

  4. Stimulated by thinking about food, smell, sight, taste, presence of food in mouth, pain receptors
• Pavlovian classical conditioning


  Learned association between a stimulus and a response
• Operant conditioning


  Learning through reinforcement or punishment of behaviors
• Functions of saliva


  • Moistens and lubricates food
  • Digests starch to maltose and dextrins
  • Solubilizes food to stimulate receptors and taste buds
  • Maintains health of teeth and gums
  • Excretes ions, drugs, microbes
  • Buffers mouth pH
  • Aids heat excretion
  • Aids speech
• Gastric digestion


  Digestion that occurs in the stomach
• Gastric digestion process


  1. Bolus enters stomach

  2. Stomach wall distends, stimulating pressure/stretch receptors

  3. Nerve impulses to plexi (myenteric, submucosal)

  4. Plexi send impulses to exocrine glands/muscles

  5. Increased gastric juice secretion and motility

  6. Gastrin secretion stimulates more gastric juice and motility

  7. Enterogastrones inhibit gastrin secretion, decreasing gastric digestion
• Enzymic activities in stomach

  • Little/no salivary amylase action
  • Production of HCl (pH 1.5-2.5)
  • Production of inactive pepsinogen, converted to active pepsin
  • Pepsin hydrolyses proteins
  • Lipase converts triglycerides
• Gastric motility and emptying

  Mixing and propulsion of stomach contents
• Intestinal digestion


  • Most important part of digestive tract
  • Enzymatic digestion, physical digestion, and absorption
  • Bile emulsifies lipids
  • Pancreatic juice contains carbohydrases, proteases, lipase
  • Intestinal juice contains maltase, sucrase, lactase, peptidase, lipase
  • Absorption aided by increased surface area of villi
• Bile


  Thick, green secretion from the liver, stored in the gallbladder (except horse, rat, pigeon), contains inorganic ions, bile salts, bile pigments, lipids
• Function of bile


  Emulsification of fat globules to smaller droplets, increasing surface area for lipase action
• Enzyme activities in small intestine

  • Pancreatic juice: trypsin/chymotrypsin, lipase
  • Intestinal juice: maltase, lactase, sucrase, lipase, peptidase
• Carbohydrate and simple sugar transport and absorption

  1. Na+-coupled substrate

  2. Substrates leave cell via facilitated diffusion
• Amino acids transport and absorption

  1. Na+-coupled substrate

  2. Substrates leave cell via facilitated diffusion
• Schematic representation of gastrointestinal activities
• Gastrointestinal motility


  • Importance: movement of ingesta, mixing/blending, bringing new ingesta into contact with epithelium
  • Types: peristalsis, tonus, segmentation, pendular, villi, haustral, mass movement
• Gastrointestinal motility in ruminants

  1. Primary mixing cycle: reticulum contracts then relaxes, dorsal sac of rumen contracts, ventral sac of rumen contracts

  2. Secondary mixing cycle: dorsal sac of rumen contracts, ventral sac of rumen contracts
• Summary of gastrointestinal motility

  • Mouth: mastication
  • Esophagus: peristalsis, anti-peristalsis
  • Stomach: peristalsis, anti-peristalsis, tonus, segmentation
  • Small intestine: peristalsis, anti-peristalsis, tonus, pendular, segmentation
  • Large intestine: peristalsis, mass movement, haustral
  • Rumen: primary mixing cycle, secondary mixing cycle
• Peristalsis initiating from the end of the small intestine towards the rectum


  1. Push ingesta towards the anus

  2. Fill up the rectum

  3. Initiate the defecation reflex
• PRIMARY MIXING CYCLE in ruminants

  1. Reticulum contracts then relaxes

  2. Dorsal sac of rumen contracts

  3. Ventral sac of rumen contracts

  4. Reticulum
• PRIMARY MIXING CYCLE


  • Promotes mixing of rumen contents
• SECONDARY MIXING CYCLE in ruminants

  1. Dorsal sac of rumen contracts

  2. Ventral sac of rumen contracts

  3. Reticulum
• SECONDARY MIXING CYCLE


  • Promotes eructation (expulsion of gases through the nasal channels to the atmosphere)
• SUMMARY of digestive processes

  • Mastication (chewing)
  • Peristalsis, anti-peristalsis
  • Peristalsis, anti-peristalsis, tonus, segmentation
  • Peristalsis, anti-peristalsis, tonus, pendular, segmentation
  • Peristalsis, mass movement, haustral
  • Primary mixing cycle
  • Secondary mixing cycle
• Ruminant forestomachs


  Rumen, reticulum, omasum
• Importance and role of rumen microorganisms

  • They secrete cellulase which converts cellulose into volatile fatty acids (VFA)
• Digestion in the rumen

  Soaking, digestion, fermentation by microorganisms
• Rumination


  Regurgitation, remastication and reswallowing of ingesta
• Eructation


  Expulsion of gases (CO2, CH4, H2S, SO2) through the nasal channels into the atmosphere
• Major end-products of ruminant digestion

  • Volatile fatty acids (acetic, propionic, butyric)
  • Microbial protein
• Ruminant vs non-ruminant digestion

  Differences in saliva composition, reliance on microbial activity, end-products, gas production and expulsion, ammonia absorption and recycling, rumination
• In young ruminants, the forestomachs are underdeveloped, have thin walls and house few microorganisms. Their main food is milk (not cellulose) which enters the abomasum directly through the oesophageal groove. At this stage the animal can be regarded as monogastric.
• The forestomachs develop in size and microbial population once the young ruminant starts eating solid food.