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Chapter 45 & 46

Digestive System

Key Concepts

  • Animal nutrition
  • general principles of digestion and absorption of food
  • overview of vertebrate digestive systems
  • mechanisms of digestion and absorption in vertebrates

Intro to nutrition

  • nutrient
    • any substance taken in by an organism that is needed for:
      • survival
      • growth
      • development
      • tissue repair
      • or reproduction
  • nutrition
    • process of consuming and using food for nutrients
  • animals receive nutrients by consuming food

Dietary categories

  • basic similarities in organ system function lead to similarities in nutritional requirements
  • different animal physiologies can have different nutrient demands

 

  • Herbivores
    • eat only plants
    • digestive system contains micro-organisms that help digest cellulose
  • Carnivores
    • eat only animal flesh or fluid
  •  Omnivores
    • eat both

Animals are heterotrophic

  • Heterotrophs
    • ingest feeders
  • cannot manufacture more food
  • require already synthesized organic compounds of plants of other animals to supply materials 
    • survival
    • maintenance
    • growth
    • reproduction

Gut Tracts

Two types

  1. Blind Gut
    • no cavity between gut and body wall
    • one opening
    • primitive form
  2. tube-within-a-tube
    • flow through digestive tube
    • body cavity between git and body wall
    • separate opening (mostly)

Digestion

  • the breakdown of large molecules into smaller ones

Digestive enzymes (hydrolases)

  • carbohydrases
  • proteases
  • lipases
  • nucleases

Food processing in animals

Occurs in Five phases

  1. Ingestion
    • food is taken into the body and moves into a digestive
  2. digestion
    • food is broken down into smaller molecules
    • chemical and mechanical
  3. transport
  4. absorption
    • ions, water, and small molecules are transported into the circulatory system
  5. egestion
    • undigested materials and other waste are passed from the body
    • elimination or exceretion

Alimentary canal

  • digestive tract or tube
    • Gastrointestinal tract
  • Five regions of food processing
  • Single tube with opening at each end
  • contains smooth muscles in walls
  • lined with epithelial cells
    • synthesize and secrete digestive enzymes
    • secrete hormones
    • transport digestive materials
  • several specialized regions
    • different structures for different processes
    • storage area

Structure of GI Tract

  • some general structure from midpoint of esophagus, to the anus or cloaca
    • lumen lined by epithelial and glandular cells 
    • secretory cells release a protective layer of mucus
    • other cells release hormones
    • glands release enzymes, acids, water, and ions
  • Epithelial cells linked by tight junctions and surrounded by layers of tissue made of smooth muscle, neurons, connective tissues, and blood vessels
    • neurons activated by sight and smell of food and presence of food in tract

Region of Reception

Buccal cavity
  • mouth and accessory structures
  • ingestion site and digestion site
  • chemical and mechanical
  • jaws, teeth, cheek muscles, tongue, and salivary glands (saliva)
Pharynx
  • back of mouth cavity
  • point that respiratory and digestive system cross paths

Region of Conduction

Esophagus
    • tube carrying materials from mouth cavity to the rest of the alimentary canal
    • forces/pushes good down
    • conducts food from pharynx to stomach
    • Peristalsis
      • rhythmic wave-like contractions which propel food forward in the GI tract
    • No new digestion here
      • only chemical continuation from buccal cavity

Region of digestion and storage

Stomach (mostly)
    • saclike organ evolved for storing food
    • muscular nature helps break up food
    • partial protein digestion
    • regulates rate of emptying into small intestine
  • Secretions
    • hydrochloric acid
      • kills microbes
      • dissolves particulate matter
      • secreted by parietal cells
    • Pepsinogen
      • converted to pepsin to begin protein digestion
      • secreted by Chief cells
  • Epithelium coated with an alkaline mucus
  • carbohydrate digestion continues from mouth
  • little lipid digestion happens
  • lumen (cavity) stomach
    • pepsinogen + HCL -> pepsin (for protein breakdown)

Region of terminal digestion and absorption

Small intestine
  • near
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