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