Digestive System

1. DIGESTIVE SYSTEM

2. Digestive System organs for changing food mechanically and chemically for absorption by body tissues. This process, called digestion, varies among vertebrates; it is unique in ruminants, which use intestinal symbiotic organisms to prepare such foodstuffs as cellulose for use. Digestion includes both mechanical and chemical processes. The mechanical processes include chewing to reduce food to small particles, the churning action of the stomach, and intestinal peristaltic action. These forces move the food through the digestive tract and mix it with various secretions.

10. SMALL INTESTINE AND LARGE INTESTINE

11. The small intestine is the largest component of the digestive tract and the major site of digestion and absorption. In addition to receiving chyme from the stomach, the initial segment of the small intestine, the duodenum, receives bile from the gall bladder and digestive enzymes from the pancreas. The pancreatic enzymes are produced in an inactive form and only become active in the lumen of the duodenum. The small intestine is divided into three parts, the duodenum (25 cm), the jejunum (2.5 m) and the ileum (3.5 m).

12. Low Power View of duodenum Duodenal mucosa and submucosa

13. The large intestine consists of the colon, cecum, appendix, rectum and anal canal. We will look at slides of the colon, appendix and anorectal junction. The principal functions of the colon are the reabsorption of electrolytes and water and the elimination of undigested food and waste. The cecum is a blind pouch just distal to the ileocecal valve whose histology resembles that of the colon. The appendix is a thin, finger-like extension of the cecum. The rectum is the dilated distal portion of the alimentary canal. Its histology resembles that of the colon, but it is distinguished by transverse rectal folds in its submucosa and the absence of tenia coli in its muscularis externa (see below). The anal canal is the most distal part of the alimentary canal, extending from the anorectal junction to the anus.

21. VARIATIONS IN VERTEBRATE DIGESTIVE SYSTEM

27. INTESTINE Fishes - relatively straight & short intestine in cartilaginous fishes & in primitive bony fishes (lungfish & sturgeon). However, the intestine of cartilaginous fishes has a spiral valve. Amphibians - intestines differentiated into coiled small intestine and short, straight large intestine

28. Reptiles & Birds - coiled small intestines & a relatively short large intestine (that empties into the cloaca) Mammals - small intestine long & coiled and differentiated into duodenum, jejunum, & ileum. The large intestine is often relatively long (but not as long as the small intestine). A cecum is often present at the junction of the small & large intestines in herbivores.