Overview and Histology Digestive System Overview and Histology

Please take out a (scrap) piece of paper and number from 1-24

Digestive System – Crash Course

Two groups of organs Alimentary canal (gastrointestinal or GI tract) Digests and absorbs food Mouth, pharynx, esophagus, stomach, small intestine, and large intestine Accessory digestive organs Teeth, tongue, gallbladder Digestive glands: Salivary glands Liver pancreas

Parotid gland Mouth (oral cavity) Sublingual gland Salivary glands Tongue Submandibular gland Pharynx Esophagus Stomach Pancreas (Spleen) Liver Gallbladder Transverse colon Duodenum Descending colon Small intestine Jejunum Ascending colon Ileum Cecum Large intestine Sigmoid colon Rectum Vermiform appendix Anus Anal canal Figure 23.1

Digestive Processes Ingestion – taking food into the digestive tract Propulsion – movement of food through the alimentary canal Includes swallowing (initiated voluntarily) and peristalsis (peri-around, stalsis-constriction) Mechanical digestion Chemical digestion Absorption – passage of digested end products, vitamins, minerals and water from lumen through mucosal cells (mostly in small intestine) Defecation-elimination of indigestible substances (feces)

Two types of Digestion: 1. Chemical Digestion 2. Mechanical (Physical) Digestion

Mechanical Digestion Physically prepares food for chemical digestion Food is crushed, broken or cut into smaller pieces through chewing, mixing of food by the tongue, churning of food in the stomach and segmentation INCREASES the surface area of the food in order to aid in chemical digestion

(a) Peristalsis: Adjacent segments of From mouth (a) Peristalsis: Adjacent segments of alimentary tract organs alternately contract and relax, which moves food along the tract distally. (b) Segmentation: Nonadjacent segments of alimentary tract organs alternately contract and relax, moving the food forward then backward. Food mixing and slow food propulsion occurs. Figure 23.3

Chemical Digestion Food molecules are broken down into their building blocks by ENZYMES secreted into the lumen Example: Saliva -breaks down starch into disaccharides (contains the enzyme amylase) Stomach acid (HCL or Hydrochloric Acid) - breaks down gastric protease which digests proteins BILE -produced by the liver and stored in the gall bladder breaks down fats Begins in mouth and is complete by small intestine

MECHANISM FOR CHEMICAL DIGESTION Hydrolysis - the splitting of large, insoluble molecules into small, soluble molecules with the addition of water

Which type of digestion is the following? Chewing a saltine? - 2. Saliva breaking the saltine down into molecules of glucose? - 3. Your tongue breaking pieces of a hamburger apart? 4. Pepsin (an enzyme) in your stomach breaking the hamburger into amino acids? Mechanical Chemical Mechanical Chemical

Ingestion Food Mechanical digestion Pharynx • Chewing (mouth) Esophagus • Churning (stomach) Propulsion • Segmentation (small intestine) • Swallowing (oropharynx) Chemical digestion • Peristalsis (esophagus, stomach, small intestine, large intestine) Stomach Absorption Lymph vessel Small intestine Blood vessel Large intestine Mainly H2O Feces Defecation Anus Figure 23.2

Human digestive system After chewing and swallowing, it takes 5 to 10 seconds for food to pass down the esophagus to the stomach, where it spends 2 to 6 hours being partially digested. Final digestion and nutrient absorption occur in the small intestine over a period of 5 to 6 hours. In 12 to 24 hours, any undigested material passes through the large intestine, and feces are expelled through the anus.

Histology of the Alimentary Canal Four basic layers (tunics) Mucosa Submucosa Muscularis externa Serosa

Intrinsic nerve plexuses • Myenteric nerve plexus • Submucosal nerve plexus Glands in submucosa Mucosa • Epithelium • Lamina propria • Muscularis mucosae Submucosa Muscularis externa • Longitudinal muscle • Circular muscle Serosa Nerve • Epithelium Artery • Connective tissue Vein Gland in mucosa Lumen Lymphatic vessel Duct of gland outside alimentary canal Mucosa-associated lymphoid tissue Mesentery Figure 23.6

Mucosa Inner tunic that lines the lumen Functions Three sublayers: Secretes digestive enzymes and hormones Absorbs end products of digestion Protects tissue through production of mucous Three sublayers: Mucous Epithelium Lamina propria (lamino=thin plate, propria=one’s own / special) Muscularis mucosae

Submucosa Tunic underneath the tunica mucosa Holds blood, lymphatic, and nervous tissues that serve to nourish, protect, and communicate

Muscularis Externa Two layers of circular and longitudinal smooth muscles that contract and relax around the tube Responsible for segmentation and peristalsis In some regions, forms sphincters (circular muscle that acts as a valve -> preventing backflow)

Serosa tunic within the peritoneal cavity that emits a slippery fluid to counteract friction The serosa and visceral peritoneum are synonymous. Replaced by the fibrous adventitia in the esophagus Retroperitoneal organs have both an adventitia and serosa

Peritoneum and Peritoneal Cavity Peritoneum: serous membrane of the abdominal cavity Visceral peritoneum on external surface of most digestive organs Parietal peritoneum lines the body wall Peritoneal cavity Between the two peritoneums Fluid lubricates mobile organs

(a) Schematic cross sections of abdominal cavity Abdominopelvic cavity Vertebra Dorsal mesentery Parietal peritoneum Ventral mesentery Visceral peritoneum Peritoneal cavity Alimentary canal organ Liver (a) Schematic cross sections of abdominal cavity illustrate the peritoneums and mesenteries. Figure 23.5a

Mesentery A double layer of peritoneum Routes for blood vessels, lymphatics, and nerves Holds organs in place and stores fat