Bio 205

Digestive System I

Introduction

gut and mesenteries

processes

• Ingestion
• Mastication
• digestion
  • physical
  • chemical
• absorption
• excretion

Details

oral cavity

• function
  • acquire food
  • process food in mammals (mastication or chewing) and some reptiles
• teeth
  • replacement
    • polyphyodont - continuous replacement
    • diphyodont - twice replaced
    • monophyodont - no replacement (cetaceans)
      • odontoceti
    • serial replacement
      • marsupials
      • elephants
  • intertooth variation
    • homodont
    • heterodont
• tongue
  • fishes
    • basihyal supports a primary tongue
    • sometimes has teeth!
    • not used for transport of food
  • tetrapods
    • fleshy tongue
    • only posterior part homologous to fish tongue
    • used for transport of food
• Oral glands
  • lacking in most fishes
  • derived in tetrapods
    • produce mucous and serous secretions for lubrication to facilitate food movement
    • may produce toxins for immobilization of prey
    • in mammals, some digestive enzymes are produced (salivary amylase) (also in some amphibians)

Pharynx

• function
  • transport food
  • process food in some fishes with pharyngeal jaws
• structure
  • fishes
    • gill rakers keep food from entering branchial chambers
  • osteichthyes
    • ventral diverticulum to lung/gas bladder
    • in tetrapods, food is kept from entering this by the glottis
    • in mammals, this is protected by the epiglottis

esophagus

• function
  • transports food
  • stores food in some birds
• gnathostomes (in larval lampreys, the "esophagus" is the dorsal part of the pharynx)
• specializations
  • grain and seed eating birds have a crop
    • stores food
    • seeds softened and fermented
    • leaves in hoatzin ferment in the crop

stomach

• derived feature of gnathostomes
• functions
  • primarily stores food
  • HCL secretion to help kill bacteria (preserving food)
  • process food in some vertebrates (derived)
    • mechanical digestion
    • chemical digestion (pepsin)
• specializations
  • lost in fishes that feed on very small particles or in fishes that grind food to small particles in mouth
    • lungfish,
    • carp and minnows
  • gizzard (for mechanical digestion)
    • some fish
    • some reptiles, including archosaurs (this includes birds)
  • inflatable sac in pufferfish and burrfish
  • multichambered stomach for processing plant material food, which is abundant, easy to catch, but low in nutrition, so alot has to be processed
    • Ruminants (a taxonomic group including cows, sheep, goats, bison)
      • four chambers
        • rumen
        • reticulum
        • omasum
        • abomasum
      • foregut fermentation by large colony of bacteria occurs in the rumen-reticulum, breaking down cellulose into organics acids, carbon dioxide, and methane
      • the food and gas are belched and the food is rechewed or ruminated
      • This cycle is reapeated
      • a great deal of absorption occurs here

intestine

• functions
  • primary site for digestion and absorption (except in ruminants)
  • diverticula off the intestine include the
    • liver - synthesizes and secretes bile salts that emulsify lipids to facilitate lipid digestion (also processes absorbed molecules, detoxifies toxic compounds, and makes new red blood cells in anamniotes)
    • gall bladder - stores bile
    • pancreas - secretes enzymes necessary for carbohydrate and protein digestion (endocrine function includes glucose regulation via insulin secretion)
• generally, the longer the intestine the more plant food that has to be processed
• specializations
  • Elasmobranchs
    • spiral valve and valvular intestine in elasmobranchs
  • Teleosts
    • elongated intestine in many teleosts, sometimes have bacteria that ferment cellulose
    • pyloric caeca - evaginations of intestine just distal to pylorus that increase surface area. From a few to many hundreds.
  • Tetrapods
    • small intestine and large intestine (colon)
    • intestinal caecum (some lizards, mammals) or a pair of caeca (some birds) in some herbivores
  • mammals
    • duodenum, jejunum and ileum
    • colon has bacteria that secrete cellulase but colon is primarily a site for water reabsorption
    • Exception is in hindgut fermenters
      • long colon, caecum, or both
      • bacterial fermentation
      • no way to digest the bacterial proteins so many (rabbits and rodents) eat their caecal feces, which is called coprophagy

Cloaca

In many vertebrates, the colon exits into a common chamber, the cloaca, that also receives urinary and reproductive system products. The actual exit from the cloaca is called the cloacal aperature. Cloaca comes from the latin: the Cloaca Maxima, the sewage system that drained Ancient Rome, with a single exit into the Tiber river. Some images of the Cloaca Maxima can be found here, here, and here.

In many telosts and most mammals (not the monotremes), the cloaca is divided, separating the anus (the exit of the gut) from the urogenital system. An occasional human congenital deformation is an undivided cloaca. Mammals also have a rectum, which is the last, straight part of the gut, between the colon and the anus. The rectum receives blood from the paired internal iliac artery instead of the unpaired mesenteric artieries (celiac, superior m, inferior m.). Also, venous drainage of the rectum bypasses the hepatic portal system.