Physiology (5)


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Physiology (5)

  1. 1. Physiology Mechanical Digestion in Large Intestine • Smooth muscle = mechanical digestion • Peristaltic waves (3 to 12 contractions/minute) – haustral churning----relaxed pouches are filled by muscular contractions in the haustra below it – gastroilial reflex = when stomach is full, gastrin hormone relaxes ileocecal sphincter so small intestine will empty and make room for new chyme –intensifies the peristaltic waves in the ilium and the older chyme then enters the caecum – gastrocolic reflex = when stomach fills, a strong peristaltic wave moves contents of transverse colon into rectum Chemical Digestion in Large Intestine • No enzymes are secreted only mucous – the goblet cells in the intestinal glands • chyme is prepared by the action of bacteria • Bacteria ferment – undigested carbohydrates - carbon dioxide & methane gas – undigested proteins - simpler substances (indoles, skatoles, hydrogen sulfide)---- odor – turn bilirubin into simpler substances that produce color • Bacteria produce vitamin K and B in colon Absorption & Feces Formation in the Large Intestine • food has now been in the GI tract for 3 to 10 hours • solid or semisolid due to water reaborption = feces • feces – water, salts, sloughed-off epithelial cells, bateria, products of bacterial decomposition, unabsorbed and undigested materials • 90% of all water absorption takes place in the SI – 10% in the LI • but the LI is very important in maintaining water balance • also absorbs some electrolytes---Na+ and Cl- and vitamins • dietary fiber = indigestible plant carbohydrates (cellulose, lignin and pectin) • soluble fiber – dissolves in water (beans, barley, broccoli, prunes, apples and citrus) – -forms a gel that slows the passage of materials through the colon – also helps to lower blood cholesterol – binds to bile salts to prevent their reabsorption – liver must make more cholesterol to make more bile salts – takes this cholesterol from the blood • insoluble fiber – woody or structural parts of the plant (skins of fruits and vegetables, coatings around bran and corn) 1
  2. 2. – passes though the colon relatively unchanged Composition and Functions of Pancreatic Juice • 1 + 1/2 Quarts/day at pH of 7.1 to 8.2 • Contains water, enzymes & sodium bicarbonate • Digestive enzymes – pancreatic amylase, pancreatic lipase, proteases – trypsinogen---activated by enterokinase (a brush border enzyme) – chymotrypsinogen----activated by trypsin – procarboxypeptidase---activated by trypsin – proelastase---activated by trypsin – trypsin inhibitor---combines with any trypsin produced inside pancreas – ribonuclease----to digest nucleic acids – deoxyribonuclease Chemical Digestion Digestion of Carbohydrates • Mouth---salivary amylase – inactivated in the stomach by the low pH • Esophagus & stomach---nothing happens • Duodenum – secretion of pancreatic juice (amylase) – synthesis of the brush border enzymes (maltase, sucrase & lactase) act on disaccharides • produces monosaccharides--fructose, glucose & galactose • lactose intolerance (no enzyme; bacteria ferment sugar)--gas & diarrhea Digestion of Proteins • Stomach – HCl denatures or unfolds proteins – secretion of pepsinogen and activation by Hcl - pepsin turns proteins into peptides • Pancreas – secretion of pancreatic juice which contains trypsin, chymotrypsin • Intestines – synthesis of brush border enzymes-----aminopeptidase or dipeptidase------split off amino acid at amino end of molecule or split dipeptides 2
  3. 3. Digestion of Lipids • Mouth----lingual lipase • Small intestine – emulsification by bile – pancreatic lipase within the pancreatic juice---splits into 2 fatty acids + a monoglyceride – no lipid-digesting enzymes made by the brush border Digestion of Nucleic Acids • Pancreatic juice contains 2 nucleases – ribonuclease which digests RNA – deoxyribonuclease which digests DNA • Nucleotides produced are further digested by brush border enzymes (nucleosidease and phosphatase) – pentose, phosphate & nitrogenous bases • Absorbed by active transport Liver Functions • Detoxifies the blood by removing or altering drugs & hormones (thyroid & estrogen) • Releases bile salts help digestion by emulsification • Stores fat soluble vitamins-----A, B12, D, E, K • Stores iron and copper • Phagocytizes worn out blood cells & bacteria • Activates vitamin D (the skin can also do this with 1 hr of sunlight a week) 3