Digestive System

• Food Processing

  1. Ingestion - the act of eating or feeding; this is coupled with the mechanical breakdown of food.
  2. Digestion - the breakdown of food into particles, then onto nutrient molecules to be chemically digested by enzymes; this also involves the breaking of chemical bonds.
  3. Absorption - the passage of digested nutrients and fluid across the tube wall into the body fluids.
  4. Elimination - the expulsion of the undigested and absorbed materials from the end of the gut.
• Three Important Functions of the Stomach

  1. Mix and Store
  2. Secrete
  3. Regulate
• Mouth Cavity

  • it is a site for both mechanical and chemical digestion.
  • The hard and soft palates separate the oral and nasal cavities; the tongue is a muscular organ for tactility and gestation; the the teeth physically break down the food (32 teeth in humans).
• Pharynx

  • the tube that connects the mouth and esophagus.
  • Three parts:
  • Nasopharynx - upper portion; passageway for air
  • Oropharynx - middle tube; food and air passage
  • Laryngopharynx - lowest tube; passageway for food
• Epiglottis

  • it is a flap of tissue found in the throat behind the tongue; it helps in preventing the entry of food into the respiratory tract.
• Esophagus

  • a long tube that connects the mouth with the stomach.
  • Sphincters in the esophagus prevent food backflow; peristalsis or involuntary muscle contractions push food.
• Stomach

  • a hollow, curved, and muscular organ that serves as a site for both the storage and digestion of food; it consists of the cardia, fundus, and pylorus.
  • Rugae is present, which allows for stretching to accommodate food.
• Gastric Secretions in the Stomach

  • Gastric Amylase - digests carbohydrates into smaller disaccharides (maltose, sucrose, lactose).
  • Gastric Lipase - the digestion of dietary fats into smaller fatty acid chains.
  • Hydrochloric acid - helps kill the bacteria cells present in food and activates the pepsinogen.
  • Pepsin - acts as a protease by initiating protein digestion
• Small Intestine

  • 20 feet long; the site of most chemical digestion is in the Duodenum.
  • Receives enzymes from liver and pancreas; jejunum and ileum are sites of digestion and absorption.
• Duodenal Secretions

  • Maltase - digests maltose into monosaccharides
  • Lactase - digests lactose into monosaccharides
  • Sucrase - digests sucrose into monosaccharides
  • Peptidase - digests peptide chains into their constituent amino acids.
  • Nuceleosidase - digests nucleotides into component phosphates, sugars, and bases.
• Large Intestine

  • 3 feet long; it has little to no digestive function; site of absorption of water and vitamins.
  • Mucus is the primary secretion; it consists of crypts.
• Rectum & Anus

  • Make up the last part of the gastrointestinal tract.
  • The rectum is the storage site for feces; it can also absorb water.
  • The anal columns and sphincters help regulate defecation.
• Accessory Organs

  • Salivary glands - secrete saliva that helps in the breakdown of foods; it contains amylase that initiates carbohydrate digestion in the mouth.
  • It consists of the parotid, sublingual, and submandibular glands.
• Accessory Organs

  • The liver consists of hepatocytes responsible for the synthesis of bile.
  • The gallbladder is attached to the liver which stores bile and regulates it.
  • The pancreas releases the pancreatic juice with amylase, and other enzymes.
• Ruminant Digestive System

  1. They ingest foods that are rich in cellulose.
  2. Mechanical ingestion and exposure to cellulase in the rumen.
  3. The reticulum helps in the regurgitation of food.
  4. The omasum performs further mechanical digestion.
  5. The abomasum performs the final digestion.
• Fish Digestive System

  • Bony fishes engulf water and the gill rakers trap particles.
  • The food moves from the esophagus to the stomach.
  • Pylorus leads to the poorly understood pyloric cecum.
  • The pancreas secretes proteases, lipases, and carbohydrates.
  • Nutrient absorption also takes place.
• Avian Digestive System

  • During ingestion, the bird does not chew its food.
  • Food moves to the crop, where it can still be regurgitated.
  • The enzymes of proventriculus facilitate chemical digestion.
  • The gizzard facilitates the mechanical digestion of food.
  • The cloaca is both the food waste and urine exit point.