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This 6 page Class Notes was uploaded by Jess Graff on Monday May 2, 2016. The Class Notes belongs to BMS 508 at University of New Hampshire taught by Mary Katherine Lockwood, PhD in Spring 2016. Since its upload, it has received 4 views. For similar materials see Human Anatomy and Physiology II in Biological Sciences at University of New Hampshire.
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Date Created: 05/02/16
BMS 508.03 4/11/2016 Chapter 24 (cont) Digestion & Metabolism (cont) The Large Intestine • Physiology of the Large Intestine • Less than 10% of nutrient absorption occurs in large intestine • Prepares fecal material for ejection from the body • 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 • Absorption of organic wastes • Vitamins • Are organic molecules • Important as cofactors or coenzymes in metabolism • Normal bacteria in colon make three vitamins that supplement diet • Three Vitamins Produced in the Large Intestine • Vitamin K (fat soluble) • Required by liver for synthesizing four clotting factors, including prothrombin • Biotin (water soluble) • Important in glucose metabolism • Vitamin B (p5ntothenic acid)(water soluble) • Required in manufacture of steroid hormones and some neurotransmitters • Organic Wastes • Bacteria convert bilirubin to urobilinogens and stercobilinogens • Urobilinogens absorbed into bloodstream are excreted in urine • Urobilinogens and stercobilinogens in colon convert to urobilins and stercobilins by exposure to oxygen • Bacteria break down peptides in feces and generate: • Ammonia • As soluble ammonium ions • Indole and skatole • Nitrogen compounds responsible for odor of feces • Hydrogen sulfide • Gas that produces “rotten egg” odor • Bacteria feed on indigestible carbohydrates (complex polysaccharides) • Produce flatus, or intestinal gas, in large intestine • Movements of the Large Intestine • Gastroileal 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 • Movement 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 • Distension of the rectal wall triggers defecation reflex • 2 positive feedback loops • Both loops triggered by stretch receptors in rectum • 2 Positive Feedback Loops 1. Short reflex • Triggers peristaltic contractions in rectum 2. Long reflex • Coordinated by sacral parasympathetic system • Stimulates mass movements • Rectal Stretch Receptors 1. Also trigger two reflexes important to voluntary control of defecation • A long reflex • Mediated by parasympathetic innervation in pelvic nerves • Causes relaxation of internal anal sphincter • A somatic reflex • Motor commands carried by pudendal nerves • Stimulates contraction of external anal sphincter (skeletal muscle) • Elimination of Feces 1. Requires relaxation of internal and external anal sphincters 2. Reflexes open internal sphincter, close external sphincter 3. Opening external sphincter requires conscious effort Digestion • Nutrients • A balanced diet contains: • Carbohydrates • Lipids • Proteins • Vitamins • Minerals • Water • The Processing and Absorption of Nutrients • Breaks down physical structure of food • Disassembles component molecules • Molecules released into bloodstream are: • Absorbed by cells • Broken down to provide energy for ATP synthesis • Or used to synthesize carbohydrates, proteins, and lipids • Digestive Enzymes • Are secreted by: • Salivary glands • Tongue • Stomach • Pancreas • Break molecular bonds in large organic molecules • Carbohydrates, proteins, lipids, and nucleic acids • In a process called hydrolysis • Are divided into classes by targets • Carbohydrases break bonds between simple sugars • Proteases break bonds between amino acids • Lipases separate fatty acids from glycerides • Brush border enzymes break nucleotides into: • Sugars • Phosphates • Nitrogenous bases • Water Absorption • Cells cannot actively absorb or secrete water • All movement of water across lining of digestive tract: • Involves passive water flow down osmotic gradients • Ion Absorption • Osmosis does not distinguish among solutes • Determined only by total concentration of solutes • To maintain homeostasis: • Concentrations of specific ions must be regulated • Sodium ion absorption • Rate increased by aldosterone (steroid hormone from adrenal cortex) • Calcium ion absorption • Involves active transport at epithelial surface • Rate increased by parathyroid hormone (PTH) and calcitriol • Potassium ion concentration increases: • As other solutes move out of lumen • Other ions diffuse into epithelial cells along concentration gradient • Cation absorption (magnesium, iron) • Involves specific carrier proteins • Cell must use ATP to transport ions to interstitial fluid • Anions (chloride, iodide, bicarbonate, and nitrate) • Are absorbed by diffusion or carrier-mediated transport • Phosphate and sulfate ions • Enter epithelial cells by active transport • Vitamins • Are organic compounds required in very small quantities • Are divided into 2 major groups • Fat-soluble vitamins • Water-soluble vitamins Effects of Aging on the Digestive System • Age-Related Changes • Division of epithelial stem cells declines • Digestive epithelium becomes more susceptible to damage by abrasion, acids, or enzymes • Smooth muscle tone and general motility decrease • Peristaltic contractions become weaker • Cumulative damage from toxins (alcohol, other chemicals) • Absorbed by digestive tract and transported to liver for processing • Rates of colon cancer and stomach cancer rise with age • Oral and pharyngeal cancers common among elderly smokers • Decline in olfactory and gustatory sensitivities • Leads to dietary changes that affect entire body
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