Gastrointestinal Tract (GIT) Overview

Learning Objectives

• Understand the main functions of the gastrointestinal tract (GIT)
• Understand the differences in GIT anatomy among species
• Understand roles of the enteric nervous system (ENS) and central nervous system (CNS) in controlling digestion
• Describe the cephalic, gastric, and intestinal phases of digestion
• List and describe the main secretions of the GIT
• Describe the hepatic portal system
• Understand the overview of fermentative digestion

Gastrointestinal Tract (GIT) Overview

• Main functions of the GI tract are transportation of food, digestion, absorption, regulating water and electrolyte balance, and providing an immunologic barrier
• Digestion breaks food into absorbable particles
• Absorption allows food components to enter the bloodstream
• The immunologic barrier is known as Gut Associated Lymphoid Tissue (GALT)

Comparative Anatomy

• Different animals have different GIT anatomy

GIT Anatomy and Function - Mammals

• Key parts of the GIT include the esophagus, stomach, small intestine (duodenum, jejunum, ileum), and large intestine (cecum, colon, rectum)
• Forestomach fermenters include ruminants that has 4 chambers such a Rumen, Reticulum, Omasum, and Abomasum. Pseudo-Ruminant has 3 chambers.
• A single-compartmented stomach is known as Monogastric
• A Functional cecum occurs in hind-gut fermenters

GIT - Anatomy and Function

• Prehension is the process of getting food into the mouth
• Mastication involves the lips, tongue, and teeth
• Carnivores have sparse, vertical jaw movements with scissor-like blades
• Herbivores have long masticating, horizontal movements, and large chewing surfaces
• Deglutition or swallowing involves the soft palate, tongue, epiglottis, and esophageal sphincter

GIT - Fish and Birds

• Fish and birds also have variations in their digestive systems, tailored to their diets

Control of Digestion

• Gut function regulation is by the enteric nervous system (ENS), gastrointestinal (GI) endocrine system and central nervous system (CNS)
• Gut influence is mediated through indirect effects on the ENS and GI endocrine systems

Enteric Nervous System (ENS)

• The ENS functions as an independent enteric nervous system within the gut wall
• The ENS consists of receptors, sensory neurons, interneurons, and motor neurons all of which lie within the gut wall
• The ENS is extensive, sophisticated, and contains as many neurons as the spinal cord

Communication with CNS

• Parasympathetic and sympathetic nervous systems form the link between the central nervous system (CNS) and the ENS
• Parasympathetic stimuli are stimulatory using acetylcholine to increase gut blood flow, motility, and glandular secretion
• Sympathetic stimuli are inhibitory using epinephrine
• The CNS works with the ENS to regulate gut function
• Afferent neurons signal pathological conditions like overdistention of the gut wall, for example, equine colic

Phases of Digestion

• Digestion, secretion, motility and absorption occur in 3 phases: cephalic, gastric, and intestinal
• During the cephalic phase, anticipation of food and visual/olfactory stimulation go to the brain and activates the parasympathetic system, with a vagal response using acetylcholine

Digestion - Gastric Phase

• Motility of the esophagus propels food from the pharynx to the stomach
• Food entering the stomach activates stretch receptors activating the ENS which releases more acetylcholine
• Release of acetylcholine stimulates gastrin secretion, stimulating parietal cells to secrete HCI and pepsinogen
• The proximal stomach stores food for gastric processing in the distal stomach
• Gastric motility control differs in the proximal and distal stomach

Digestion - Intestinal Phase

• If pH is at 1 or less, gastrin secretion is abolished
• Food passes the pyloric sphincter to the duodenum at a controlled rate
• The rate of gastric emptying must match the small intestine's rate of digestion and absorption
• The gastric phase stimulates pancreatic secretion to prep the intestine

Digestion - Intestinal Phase

• The presence of chyme in the duedenum activate stretch receptors and chyme components stimulate CCK or Cholecystokinin secretion
• A low pH stimulates secretion of secretin
• Gastric inhibitory polypeptide (GIP) is secreted to inhibits gastric motility and stimulates insulin secretion
• As food is digested and absorbed, the stimuli for pancreatic secretion are removed from the stimuli

Digestion - Intestinal Phase

• The ileocecal sphincter prevents movement of colon contents back into the ileum
• Motility of the colon causes mixing, retropulsion, and propulsion of ingesta
• The colon stores ingesta and absorbs mostly water and electrolytes
• The anal opening constricted by two sphincters: an internal sphincter of smooth muscle and an external sphincter of striated muscle
• The recto sphincteric reflex stimulates defecation

Secretions of the GIT

• The salivary glands produce saliva to moisten, lubricate, and partially digest food using amylase and lingual lipase
• Salivary glands are regulated by the parasympathetic nervous system
• Ruminant saliva is a bicarbonate-phosphate buffer secreted in large quantities

Gastric Secretion

• The gastric mucosa contains mucous cells, parietal cells (secrete HCI stimulated by gastrin and Ach), and chief cells (secrete pepsinogen stimulated by Ach and HCI)

Secretions of the GIT

• The pancreas's exocrine secretions digest carbohydrates, proteins, and lipids
• Acinar cells secrete digestive enzymes, whereas centroacinar and duct cells secrete sodium bicarbonate
• Pancreatic fluid neutralizes the chyme arriving in the duodenum
• Pancreatic cells are stimulated by acetylcholine, cholecystokinin, and secretin

Hepatic Portal System

• The Bile Secretion is made of phospholipids, cholesterol, bile acids, and bilirubin
• The gallbladder stores and concentrates bile between feedings
• Bile secretion starts with food presence in the duodenum and Bile acids are returned to the liver

Hepatic Portal Systems

• The Hepatic Portal System transports blood from the digestive tract to the liver
• The system extends from the esophagus to the anal canal
• The system includes venous drainage from the spleen, pancreas and visceral fat.
• The liver receives some blood directly from the aorta through the hepatic artery (oxygenated blood)
• The First Pass Metabolism has nutrients absorbed from the gastrointestinal tract delivered directly to the liver to be metabolized then allowed into systemic circulation
• Lymphatic drainage from the gut bypasses the liver

Fermentative Digestion

• Fermentation involves metabolic action of microorganisms
• The microbes responsible for fermentation include bacteria, fungi, protozoa
• The process requires an adequate environment that is pH, temperature, Moisture, Oxidation-reduction enzymes and coenzymes, and Motility patterns/time
• Complex ecosystems are built in the forestomach and hindgut

Overview of the Fermentative Process

• Ruminants have forestomach fermentation in the rumen, reticulum, omasum, and abomasum
• Functional cecum is present during hindgut fermentation

Fermentative Process

• Fermentation substrates: plant cell walls (cellulose, hemicellulose and lignin) and other nutrients (protein, other carbs, vitamin, minerals)
• Food enters the rumen → fermentation → production of Volatile Fatty Acids (VFA) and other byproducts
• Path: Ingesta goes to reticulum → regurgitation → rumination → small food particles pushed to reticulum → then goes to omasum → abomasum → chyme → small intestine → large intestine

Fermentative Process

• Anaerobic conditions in the rumen lead to production of volatile fatty acids (VFAs)
• VFAs are the energy source for the host animal (cow), absorbed through the rumen epithelium and transported in the blood to the liver (60-80% of energy)
• Fermentative digestion of dietary protein results in deamination of a large portion of amino acids
• Amino acids are deaminated to produce ammonia (NH3) and a carbon skeleton
• Microbes in the rumen synthesize protein from nonprotein nitrogen, and is digested in the abomasum/small intestine