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

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