HSN211 Notes Week 1 – The GI Tract GI System functions: • Acquires nutrients from environment • Anabolism - Uses raw materials to synthesize essential compounds • Catabolism - Decomposes substances to provide the energy cells need to function - Need two components - oxygen and organic molecules e.g. fats, proteins, carbohydrates Six main actions of the GI system 1. Ingestion - Occurs when food or fluid enters the mouth 2. Mechanical processing - Crushing/shearing of food – makes materials easier to move through the tract 3. Secretion - Release of water, acids, buffers, enzymes and salts by epithelium (inside skin) of GI tract and glandular organs 4. Digestion - Chemical breakdown of food into small organic compounds for absorption 5. Absorption - Movement of organic substrates, electrolytes, vitamins and water across digestive epithelium 6. Excretion - Removal of waste products from body fluids Functions of oral cavity • Sensory analysis of material before swallowing • Mechanical processing through actions of teeth, tongue and palatal surfaces • Lubrication - Mixing with mucus and salivary gland secretion • Limited digestion - Carbohydrates and lipids Esophagus • A hollow muscular tube with a sphincter at each end • About 25cm long and 2cm wide • Conveys solid food and liquids to the stomach • Involuntary (Phase 2 and 3) • Three phases to swallowing process 1. Buccal 2. Pharyngeal 3. Eosophageal Stomach • Has the thickest and strongest muscles of all the GI tract organs • Has three sets of muscles - Circular muscles - Longitudinal muscles - Diagonal muscles Functions of the stomach • Storage of ingested food • Mechanical breakdown of ingested food (chyme) • Disruption of chemical bonds in food material by acid and enzymes • Production of intrinsic factor, a glycoprotein required for absorption of vitamin B12 in small intestine Digestion in the stomach • Stomach performs preliminary digestion of proteins by pepsin - Some digestion of carbohydrates (by salivary amylase) - Lipids (by lingual lipase) • Stomach contents - Become more fluid - pH approaches 2.0 - pepsin activity increases à pepsin starts the digestion of proteins - protein disassembly begins - although digestion occurs in the stomach, nutrients are not absorbed here Small intestine • 90% absorption occurs in the small intestine Functions of the small intestine • intestinal secretions • watery intestinal juice • 1.8 liters per day enter intestinal lumen • moistens chyme • assists in buffering acids • keep digestive enzyme and products of digestion in solution Sections of the small intestine • Duodenum - The segment of small intestine closest to stomach (25cm long) - Receives chyme from stomach and digestive secretions from pancreas and liver - Neutralizes acids before they can damage the absorptive surfaces of the small intestine - Secretin and CCK released from duodenum epithelium - Iron and calcium absorbed in duodenum • Jejunum - The middle segment of small intestine - 2.5 meters long - is the location of most: à chemical digestion à nutrient absorption à small villi (finger-like projections that increases the surface areas, allowing for absorption) à even smaller finger-like projections are called micro villi) - main nutrients absorbed à calcium, all B vitamins, vitamin C, fat soluble vitamins • Ileum - The final segment of small intestine - 3.5 meters long - ends at the ileocecal valve, a sphincter that controls flow of material form the ileum into the large intestine - function: to absorb vitamin b12, bile salts, other products not absorbed by jejunum - contain villi Large Intestine • is horseshoe shaped • extends from end of ileum to anus • lies inferior to stomach and liver • frames the small intestine • also, called large bowel • is about 1.5 meters long and 7.5 cm wide Functions of the large intestine • reabsorption of water • compaction of intestinal contents into faeces • absorption of important vitamins produced by bacteria • storage of faecal material prior to defecation Sections of the large intestine • Cecum - Is an expanded pouch - Receives material arriving from the ileum - Stores materials and begins compaction process • Colon - Larger diameter and thinner wall than small intestine - The wall of the colon: à forms a series of pouches (haustra) à hustra permit expansion and elongation of colon • Ascending colon - Begins at superior border of cecum - Ascends along the right lateral and posterior wall of peritoneal cavity to inferior surface of the liver and bends at right colic flexure (hepatic flexure (bend that occurs at liver)) • Transverse colon - Crosses abdomen from right to left - Turns at left colic flexure (splenic flexure (spleen)) • Descending colon - Proceeds inferiorly along left side to the iliac fossa (hip bone) - Is behind the peritoneum and firmly attached to abdominal wall • The sigmoid colon - Is an S-shaped segment, about 15cm long - Starts at sigmoid flexure - Lies behind the bladder - Empties into the rectum • Rectum - Forms last 15cm of digestive tract - Is an expandable organ for temporary storage of faeces - Movement of faecal material into rectum triggers urge to defecate Movements of the large intestine • Gastrolienal and gastroenteric reflexes - Move materials into cecum while you eat - Movement from cecum to transverse colon is very slow, allowing hours for water absorption - Peristaltic waves move material along length of colon - Segmentation movements (haustral churning) mix contents of adjacent haustra - Movements from transverse colon through rest of large intestine results from powerful peristaltic contractions (mass movements) - Stimulus is distension of stomach and duodenum; relayed over intestinal nerve plexuses - Distention of the rectal wall triggers defecation reflex - Two positive feedback loops - Both loops triggered by stretch receptors in rectum Physiology of the large intestine • Absorption in the large intestine • Reabsorption of water • Reabsorption of bile salts - In the cecum - Transported in blood to liver • Absorption of vitamins produced by bacteria • Conversions of excretion of organic wastes Vitamins produced in the large intestine 1. Vitamin K (fat soluble) - Required by liver for synthesizing four blood clotting factors 2. Biotin (water soluble) 3. Pantothenic acid: B5 (water soluble) - Required in manufacture of steroid hormones and some neurotransmitters Organic wastes large intestine gets rid of • Bacteria convert bilirubin to urobilinogen and then stercobilin • Bacteria feed on indigestible carbohydrates (complex polysaccharides) - Produce flatus, or intestinal gas, in large intestine • Bilirubin is the yellow byproduct of the breakdown of old red blood cells. Bacteria convert this to urobilinogen which is further converted to stercobilin which is brown – hence the colour of faeces Villi • finger-like projections from the folds of the small intestine • the walls of the small intestine are wrinkled into thousands of folds and is carpeted with villi • Increases the surface areas, allowing for absorption - Dramatically increase the small intestine surface area, facilitating nutrient absorption - Nutrients pass through the cells of the villi and enter either the blood (if they are water-soluble or small fat fragments) or the lymph (if they are fat-soluble) • Each villus in turn is covered with even smaller projection, the microvilli - Microvilli on the cells of the villi provide absorptive surfaces that allow the nutrients to pass through the body .