Mammalian Digestive Systems

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

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