Layers of Gastrointestinal Tract • • • • • Mucosa--epithelial cells Submucosa Circular muscle Longitudinal muscle Serosa Proposed Causes of Ulcers • • • Overproduction of acid • • • Inactive until acted upon by acid or another enzyme Breach in protection by mucus Helicobacter pylori, a bacteria Many protein-digesting enzymes are released as zymogens Acid converts pepsinogen to pepsin in stomach Enzyme enterokinase converts the pancreatic enzymes trypsinogen and chymotripsinogen to trypsin and chymotrypsin, respectively Neural Control of Gut • • • Autonomic nervous system (vagus nerve) Enteric nervous system Control motility & secretions Hormonal Control of Gut • • • • Gastrin-secreted by stomach Secretin secreted by small intestine Cholecystokinin Gastric Inhibitory Peptide (GIP) Gastrin • • • Stimulates gastric motility Stimulates HCl secretion Stimulates pepsinogen secretion Other Gut Hormones • Secretin antagonizes action of gastrin; also stimulates secretion of bicarbonate by pancreas • CCK stimulates enzyme secretion by pancrease & bile by gall bladder • GIP inhibits gastric motility and secretion; stimulates insulin release The Small Intestine • • Vast majority of nutrient absorption occurs here Divided into 3 parts – Duodenum – Jejunum – Ileum Small Intestine Structure • • • Mucosal layer is divided into thousands of fingerlike projections--villi Epithelial cells of mucosa have microvilli Net effect--increased surface area for absorption In the Duodenum… • • Bicarbonate secreted by pancreas increases pH of chyme to over 7 Pancreatic secretions and bile enter duodenum through the common bile duct Pancreatic Enzymes • Proteases--trypsin, chymotrypsin, carboxypeptidase--products amino acids and short chains • • Lipase--triglycerides to monoglycerides and fatty acids Amylase-starch to shorter chains Bile • • Derived from cholesterol • Bile can be reabsorbed in ileum and recycled--enterohepatic circulation A detergent--emulsifies fat and fat-soluble vitamins into small droplets called micelles Enzymes on Surface of Small Intestine • • • Proteases Sucrase Lactase Lactose Intolerance • • • Affects over 70% of adults worldwide Due to deficiency of enzyme lactase Some formerly lactose-tolerant adults may develop lactose intolerance when ill with HIV or other illnesses that affect the gut • Most lactose-intolerant people can drink milk in small doses with a meal How do nutrients get absorbed? • • • • Simple diffusion Facilitated diffusion--requires a carrier Active transport--against concentration gradient Endocytosis--pinching off of cell membrane – Antibodies in breast fed babies Carbohydrate & Protein Absorption • • • • • Glucose and galactose--active transport Fructose--facilitated diffusion Most amino acids--active transport Di- and tripeptides, other amino acids--facilitated diffusion Go directly into bloodstream and to liver Fat Absorption, or Oil & Water Don’t Mix • • • • Products of fat digestion diffuse across membrane into mucosal cell Triglycerides are resynthesized and packaged into chylomicrons Chylomicrons enter lymph system via lacteals in each villus Lymph enters bloodstream at thoracic duct Water Absorption • • • All along gastrointestinal tract There is no active transport of water in biological systems Osmosis--movement of water across membrane along with nutrients The Colon • • No mucosal villi Mainly absorbs water and a few minerals (sodium, potassium) Good Bacteria in the Colon • • • Lactobacilli, Bifidobacter Can digest lactose and other indigestible compounds Bacteria in yogurt or acidophilus milk can survive upper gastrointestinal tract and colonize the large intestine Feces Happen • • • Consist of undigested fiber, bacteria, a little water What is regular for one person may be irregular for another Low fiber and/or low fluid diet may be primary reasons for constipation Diverticular Disease • • • • Low fiber diet is main culprit Small pockets form in lining of colon If a small seed gets into a pocket, inflammation can occur--diverticulitis Prevention is via high fiber diet