The Four Stages of Food Processing Ingestion Is another word for eating Digestion Is the breakdown of food to small molecules Absorption Is the uptake of the small nutrient molecules by the body’s cells Elimination Is the disposal of undigested materials from the food we eat

Mechanical digestion Begins the process Involves physical processes like chewing Increases the surface to volume ratio of food particles but it does not break the chemical bonds within a food molecule

Chemical Digestion: enzymes and H2O used to break chemical bonds Hydrolase (pepsin) Amino acid Protein Sugar Hydrolase (amylase) Fatty acid Glycerol Hydrolase (lipase) Figure 22.4b

Anatomy of oral cavity (mouth)

The major salivary glands

Esophagus- delivers food to stomach

Anatomy of the Stomach Stores and begins the chemical breakdown of food into chyme (creamy paste) Parts include the cardiac, fundus, pyloris Sphincters at both ends Rugae to allow expansion

Stomach- specialized structure for further chemical digestion Enzymes target proteins and lipids

Anatomy of the stomach

Histology of the Stomach Oblique musculature allows food to by churned while being moved Gastric pits with gastric glands: secrete gastric juice Mucous neck cells: Acidic mucous Parietal (oxynetic) cells: HCl and intrinsic factor (for B12 absorbtion in SI) Chief (zygomatic) cells: Pepsin (protein digestion) Enteroendocrine cells: Hormones to regulate digestion gastrin histamine endorphins serotonin cholecystokinin somatistatin

Microscopic anatomy of the stomach

Peristaltic waves

Small intestine – Primary site of absorption of digested molecules

The small intestine Three regions Duodenum Ileum Jejunum

Duodenum-upper portion still receives some digestive enzymes Is the first part of the small intestine Receives digestive agents from several organs Fats are emulsified for absorption Pancreas- neutralizes acids from stomach Bile Liver Gallbladder Bile Acid chyme Pancreatic juice Duodenum of small intestine Pancreas

The jejunum and ileum Are parts of the small intestine Are specialized for absorption Nutrient absorption Nutrient absorption Blood vessels Microvilli Epithelial cells Interior of intestine Muscle layers Blood capillaries Villi Lymphatic vessel Epithelial cells Nutrient absorption Intestinal wall Villi

Liver and Gall Bladder Liver: Produce Bile (emulsifies fat) Gall Bladder: Stores Bile Liver Liver lobules (sexagonal) with hepatocytes Portal triad at each corner (Hepatic artery, HPV, and Bile Duct) Liver sinusoids with macrophages (Kupffer cells) that remove bacteria and worn RBC’s

Microscopic anatomy of the liver

Bile Yellow-green alkaline solution that emulsifies fat Bile salts, bile pigments, cholesterol, neutral fats, phospholipids (lecithin, etc.) and electrolytes Bile salt: Cholic acid and chenodeoxycholic acid (cholesterol derivatives) emulsify fats. Recycled rather than secreted by the enterohepatic circulation

Role of bile salts & fat emulsification

Gall Bladder Stores, but does not produce, bile Bile from hepatic ducts enters through a cystic duct to the GB. Upon stimulation, bile leaves the cystic duct, to the common bile duct (cbd) and goes to the duodenum of the SI Major stimulus: CCK (cholecystokinin) from SI causes contraction (released when fat detected)

Pancreas Secretes enzymes (in pancreatic juice) for the breakdown of proteins and other foodstuffs Enters via the main pancreatic duct Pancreas has acini, whose cells are filled with zymogen granules (enzymes)

Structure of acinar tissue of the pancreas

Pancreatic juice Water, enzymes, and electrolytes (primarily bicarbonate). Helps neutralize chyme. Enzymes are released inactive, which activate under the proper pH preventing self-digestion. Pancreatic enzymes (amylase, lipase, nuclease) may be active, but require ions or bile for optimal activity

The Large Intestine Function is to absorb water, not nutrients, from fluid state chyme and secrete a semisolid feces Three unique characteristics Teniae coli: three bands of smooth muscle Haustra: pocket-like sacs created by muscle bands Epiploic appendages: fat filled pouches of the visceral peritoneum (function unknown)

Colon: Large intestine- absorption of water.

Colon: rectum – specialized region for temporary storage Anus – exit from digestive system- separate exit from the urogenital system