Digestive System (General): (3)
- Receiving, storing, breaking down (physically and chemically) food
- Absorbing nutrients and water from food
- Stroing and discharging wastes
Digestive Systems Components:
- Mouth
- Pharynx
- Tubular parts
What are the Tubular parts (3):
- Esophagus
- Stomach
- Intestines
Digestive Systems (Accessory organs and glands) (5):
- Tongue
- Teeth
- Salivary glands
- Pancreas
- Liver
Evolution of the Vertebrate Mouth
- Starts with cilia-mucus filter feeding
- Then muscular pump suspension feeding
- Finally jaws for predatory feeding
Teeth Overview:
- Derived from bony dermal armor of placeoderms (pre-teeth); similar structure to placoid scales
- Composed of dentin, covered by a crown of enamel
- Used for feeding (ingestion/physical digestion) and defense
Attachment Types of Teeth (3):
- Acrodont dentition
- Pleurodont dentition
- Thecodont dentition
Attachment Types of Teeth (Acrodont dentition):
Teeth attached to the outer surface or to the summit of the jawbone (many teleosts)
Attachment Types of Teeth (Pleurodont dentition):
Attached to the inner side of the jawbone (anurans, salamanders, many lizards)
Attachment Types of Teeth (Thecodont dentition):
Teeth occupy bone sockets or aveoli (some fishes, crocodilians, fossil birds, mammals)
Tooth Variation:
- Homodont –> teeth all the same siza and shape
- Heterodont –> teeth vary morphologically
Mammalian Teeth:
- Heyerodont dentition with species-specific number of teeth
- Evolved along with ability to chew
Oral (Salivary) Glands:
- Multicellular glands with ducts
which are only found in tetrapods and not fishes - Primary function = moisten food for lubrication
Oral (Salivary) Glands - Specialized functions (4):
- Allows tongue to be sticky to assist prey capture (frogs and anteaters)
- Allows start of starch digestion (some mammals)
- Modified into venom glands (some snakes/lizards)
- Salt excretion near orbits (marine reptiles and birds)
Stomach Overview:
- Muscular chamber that begins at the end of the esophagus and terminated at the pylorus (opening surrounded by the pyloric sphincter)
- Contains gastric glands that produce hydrochloric acid
- May have evolved as a storage sac
- Digestive function involves chemically breaking down food (little absorption)
Stomach Specializations in Birds and Crocodiles - Modified into 2 parts:
- Glandular proventriculus (secretes digestive enzymes)
- Muscular gizzard (through horny layer for grinding and mixing for with gastric secretions; sometimes contains little stones swallowed by bird
Stomach Specializations in Ruminant Mammals - 4 Compartments:
- Rumen: stores and churns; residence for bacteria which produce cellulase to breakdown cellulose (no vertebrates can produce the enzyme themselves)
- Reticulum: receives food from rumen and forms cud, regurgitates for further chews (ruminations)
- Omassum: temporary holding site
- Abomasum: only place with gastric gland
Intestines Overview:
- Differentiation into small and large intestine (based on function not always sizel)
- Small intestine - is chief site of digestion and absorption of nutrients
- Large intestine - mostly recovers water, but can be used for fermentation
- Major digestive glands (pancreas, liver) empty their products into the first part of small intestine (duodenum)
Carnivor Intestines:
Have shorter, relatively unspecialized guts
Herbivore Intestines:
have long guts with intestinal specializations that promote fermentation (chemical breakdown by microorganisms)
Structure of the Gut Wall - Four Layers:
- Serosa: connective tissue and mesentery
- Muscularis externa: two layers of smooth muscle
- Submucosa: connective tissue, autonomic nerves
- Mucosa: epithelial lining, smooth muscle fibers, connective tissue
Strategies for Increased Surface Area – 1. Caeca (2):
- Bind-ended extensions of the gut, can come out in various places
- Function in digestion/absorption in fishes, fermentation in mammals
Strategies for Increased Surface Area – 2. Spiral Valve (2):
- ‘Auger-shaped’ structure of the intestinal tract in chodrichthyans and lungfishes
- Forces passing food through a spiral route, increasing path length
Strategies for Increased Surface Area – 3. Foldings
Larger surface area is achieved by coils of the canal, folds, villi and microvilli on the internal surface
