Mammalian Digestive Systems

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

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: 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

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

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

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: 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: 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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