The Small Intestine

1. The Small Intestine • The small intestine is the longest section of the alimentary canal • It is the major organ of digestion and absorption

2. Bile Liver LE 41-19 Gall- bladder Stomach Acid chyme Intestinal juice Pancreatic juice Pancreas Duodenum of small intestine

3. The pancreas produces proteases, protein-digesting enzymes that are activated after entering the duodenum

4. LE 41-20 Pancreas Membrane-bound enteropeptidase Inactive trypsinogen Trypsin Other inactive proteases Active proteases Lumen of duodenum

5. The liver produces bile, which aids in digestion and absorption of fats • The epithelial lining of the duodenum, called the brush border, produces several digestive enzymes

6. Protein digestion Nucleic acid digestion Fat digestion Carbohydrate digestion Oral cavity, pharynx, esophagus Polysaccharides Disaccharides Salivary amylase Smaller polysac- charides, maltose Stomach Proteins Pepsin LE 41-21 Small polypeptides Lumen of small intes- tine DNA, RNA Fat globules Polysaccharides Polypeptides Pancreatic amylases Pancreatic nucleases Pancreatic trypsin and chymotrypsin Bile salts Maltose and other disaccharides Fat droplets Nucleotides Smaller polypeptides Pancreatic carboxypeptidase Pancreatic lipase Amino acids Glycerol, fatty acids, glycerides Epithelium of small intestine (brush border) Small peptides Nucleotidases Nucleosides Dipeptidases, carboxy- peptidase, and aminopeptidase Disaccharidases Nucleosidases and phosphatases Nitrogenous bases, sugars, phosphates Amino acids Monosaccharides

7. Key Liver Stimulation Inhibition Entero- gastrone LE 41-22 Gall- bladder Gastrin CCK Stomach Pancreas Secretin Duodenum CCK

8. Absorption of Nutrients • The small intestine has a huge surface area, due to villi and microvilli that are exposed to the intestinal lumen • The enormous microvillar surface greatly increases the rate of nutrient absorption

9. Key Nutrient absorption Vein carrying blood to hepatic portal vessel Microvilli (brush border) LE 41-23 Blood capillaries Epithelial cells Muscle layers Epithelial cells Large circular folds Lacteal Villi Lymph vessel Villi Intestinal wall

10. Fat globule Bile salts • Amino acids and sugars pass through the epithelium of the small intestine and enter the bloodstream • After glycerol and fatty acids are absorbed by epithelial cells, they are recombined into fats within these cells Fat droplets coated with bile salts Micelles made up of fatty acids, monoglycerides, and bile salts Epithelium of small intestine Lacteal Epithelium of lacteal

11. The Large Intestine • The large intestine, or colon, is connected to the small intestine • Its major function is to recover water that has entered the alimentary canal

12. The colon houses strains of the bacterium Escherichia coli, some of which produce vitamins

13. Concept 41.5: Evolutionary adaptations of vertebrate digestive systems are often associated with diet • Digestive systems of vertebrates are variations on a common plan • However, there are intriguing adaptations, often related to diet

14. Some Dental Adaptations • Dentition, an animal’s assortment of teeth, is one example of structural variation reflecting diet • Mammals have specialized dentition that best enables them to ingest their usual diet

15. Incisors Molars Canines Premolars Carnivore LE 41-26 Herbivore Omnivore

16. Stomach and Intestinal Adaptations • Herbivores generally have longer alimentary canals than carnivores, reflecting the longer time needed to digest vegetation

17. Small intestine LE 41-27 Stomach Small intestine Cecum Colon (large intestine) Herbivore Carnivore

18. Symbiotic Adaptations • Many herbivores have fermentation chambers, where symbiotic microorganisms digest cellulose • The most elaborate adaptations for an herbivorous diet have evolved in the animals called ruminants

19. Intestine Rumen LE 41-28 Reticulum Esophagus Abomasum Omasum