Overview

• Contains an alimentary canal or gastrointestinal (GI) tract, plus accessory structures

  • May include tongue, teeth, salivary glands, liver, gallbladder, and pancreas

• Not all vertebrates share identical features

  • Some fish lack a stomach, some birds (parrots and pigeons) lack a gallbladder

Functional Regions

• Anterior end: functions primarily in ingestion

  • Oral cavity, salivary glands, pharynx (throat), and esophagus

• Middle portion: functions in storage and initial digestion

  • Stomach, upper part of small intestine and associated organs (liver, gallbladder, pancreas)

• Posterior part: functions in final digestion, absorption, and elimination

  • Remainder of small intestine and large intestine

Mouth

• Mouth: saliva begins initial processing of food

  • Moisten and lubricate food to facilitate swallowing

  • Dissolve food particles to facilitate taste

  • Kill ingested bacteria

• Initiate digestion of polysaccharides with amylase

Pharynx and Esophagus

• Pharynx/Esophagus: pathway to stomach only

• Do not contribute to digestion or absorption

• Swallowing begins in esophagus as voluntary action

• Action continues by involuntary peristalsis

Stomach

• Stomach: saclike organ evolved for storing food

• Muscular nature helps break up food

• Partially digests proteins

• Regulates rate of emptying into small intestine

• Glands secrete

  • Hydrochloric acid: kills microbes, dissolves particulate matter

  • Pepsinogen: converted to pepsin to begin protein digestion

• No lipid or carbohydrate digestion

• Food reduced to chyme

• Rugae: folds of stomach with deep pockets, or gastric pits, contain

  • Mucous cells: secrete mucus for protection

  • Parietal cells: secrete HCl (pH 1.5-2.5) HCL kills most bacteria & living cells; erodes plant materials; initiates change of pepsinogen to pepsin

  • Chief cells: secrete pepsinogen (inactive) which is converted by HCL into active pepsin

• Stomach hormone: gastrin

  • Produced in the presence of protein-containing food in the stomach

    • stimulates the release of gastric juices and muscular contractions of stomach & intestine

Herbivore Stomach

• Many herbivores have fermentation chambers, where mutualistic microorganisms digest cellulose

  • Herbivores must digest cellulose but lack cellulase

  • Rely on microbes to digest cellulose into monosaccharides

  • Simple stomach: uses cecum at connection between small and large intestine

• The most elaborate adaptations for an herbivorous diet have evolved in the animals called ruminants

Ruminant Stomach

• Has 4 compartments

  • Reticulum: form food bolus and initiate regurgitation

  • Rumen: digestive and fermentation vat, contains anaerobic microbes, site of fatty acid absorption

  • Omasum: lined by muscular folds, reduces particle size, absorbs water (and any leftover fatty acids)

  • Abomasum: true glandular stomach where bacteria and pathogens are killed

• Microbes provide ruminants:

  • Digestion of cellulose

  • Provision of organic acids

  • Provision of protein

  • Provision of B vitamins

• Ruminants provide microbes:

  • Place to stay

  • Adequate nutrition

  • Waste elimination

Small Intestine

• Small intestine: nearly all digestion of food, and absorption of food and water

  • Hydrolytic enzymes found on luminal surface or secreted by pancreas into lumen

  • Products of digestion absorbed across epithelial cells and enter blood

    • Vitamins, mineral and water also absorbed

  • Specialized for increased surface area

    • Mucosa is folded

    • Villi: finger-like projections

    • Epithelial cells with microvilli create brush border

  • Increases surface area 600-fold

  • Increases likelihood of encountering digestive enzyme and being absorbed

• Each villus has a capillary and lacteal.

  • Capillary: nutrients other than fat absorbed into blood

  • Lacteal (lymphatic vessel): allows for larger fat particles to enter, eventually dumped into blood

• Length of small intestine varies

  • Herbivores have much longer intestines than carnivores

    • Added time for digesting plant material

  • Even within an individual animal, the length of the small intestine can change

    • Intestine can grow to increase surface area

Large Intestine

• Large intestine: primary function to store and concentrate fecal matter and absorb some salt and water

• Bacteria will produce vitamins

• Cecum

  • Chyme enters through sphincter

  • Appendix

• Colon

  • Ascending, transverse, and descending

• Anus