The Gastrointestinal Tract PDF

The gastrointestinal tract  Introduction The digestion and absorption of food represent a complex process, which depends on the integrated activity of the organs of the alimentary tract. Carbohydrates: converted to monosaccharides and disaccharides than...

The gastrointestinal tract  Introduction The digestion and absorption of food represent a complex process, which depends on the integrated activity of the organs of the alimentary tract. Carbohydrates: converted to monosaccharides and disaccharides than absorption of the constituent monosaccharide. Proteins: broken down by protease (secreted as inactive precursors) and peptidases to oligopeptides and amino acids. fat : is necessarily more complex because most fats are immiscible with water. Mechanical mixing and the action of bile salts create an emulsion of triglycerides (strictly, triacylglycerols) All these processes require the intimate mixing of enzymes, cofactors and substrates, and the maintenance of the optimum [H+] (pH) for enzyme activity. pancreatic lipase. This enzyme converts triglycerides to free fatty acids and monoglycerides. The Stomach In the stomach, food mixes with acidic gastric juice. Gastrin is secreted by G cells in the antrum of the stomach Stimulating gastric motility, acid secretion (HCL)from the parietal cells and mucosal growth and pepsinogen secretion from the chief cells intrinsic factor, essential for the absorption of vitamin B12 in the terminal ileum. Negative feedback is provided by the acid itself gastrointestinal hormones, e.g. somatostatin, secretin, gastric inhibitory peptide (GIP), vasoactive intestinal peptide (VIP), calcitonin and glucagon. Gastric disorders and investigation of gastric function Biochemical investigations are of limited use in the diagnosis of gastric disorders: the stomach can be directly inspected by endoscopy, and contrast radiography can also provide valuable information. Biochemical tests can be used to investigate conditions in which it is suspected that gastric acid secretion may be abnormal, particularly in atypical or recurrent peptic ulceration. The investigation of pernicious anaemia. Most peptic ulceration is associated either with non-steroidal anti-inflammatory drug use or with colonization of the stomach with Helicobacter pylori Diagnosis of H. pylori infection is by serology, 13C urea breath test or stool antigen test. Patients should stop taking proton pump inhibitors for at least 2 weeks before testing. H. pylori can split urea to form ammonia and carbon dioxide, therefore if isotopically labelled (13C or 14C) urea is given orally, the isotope can be measured in expired breath to indicate infection. The sensitivity of this test is 96% and specificity virtually 100%, but the specialist equipment required limits its availability. Atypical peptic ulceration:  duodenal ulcers are resistant to medical treatment or recur.  Zollinger–Ellison syndrome, a rare condition in which hypergastrinaemia is caused by a gastrinoma of the pancreas, duodenum or, less frequently, the G cells of the stomach.  patients sometimes have steatorrhoea, caused by inhibition of pancreatic lipase by the excessive gastric acid.  The first-line biochemical test in such patients is the measurement of fasting plasma gastrin concentration. H2 inhibitors should be stopped 3 days, and proton pump inhibitors 2 weeks, before taking blood for gastrin measurement, because these drugs result in achlorhydria,which increased gastrin secretion. Because the  hormone is very labile, blood samples must be centrifuged, separated and frozen as quickly as possible. The Pancreas The pancreas is an essential endocrine organ that produces insulin, glucagon, pancreatic polypeptide and other hormones; its endocrine functions are discussed in Chapter 13. The exocrine secretion of the pancreas is an alkaline, bicarbonate-rich fluid containing various enzymes essential for normal digestion: the proenzyme proteases, trypsin, chymotrypsin, carboxypeptidase and elastases; lipolytic enzyme lipase; colipase; starch hydrolase, amylase; sterol hydrolase, cholesterol esterase and others including phospholipase and nucleases.  The secretion of pancreatic fluid is primarily under the control of two hormones secreted by the small intestine.  secretin, a 27 amino acid polypeptide, which stimulates the secretion of an alkaline fluid.  Cholecystokinin (CCK), which stimulates the secretion of pancreatic enzymes and contraction of the gallbladder, and induces satiety.  Both secretin and CCK are secreted in response to the presence of acid, amino acids and partly digested proteins in the duodenum. Pancreatic disorders and their investigation The major disorders of the exocrine pancreas are 1. acute pancreatitis. 2. chronic pancreatitis. 3. pancreatic cancer. 4. Cystic fibrosis. 1-acute pancreatitis Acute pancreatitis presents as an acute abdomen, with severe pain and variable degrees of shock. Causes : excessive ethanol ingestion, gallstones complication of endoscopic retrograde pancreatography (ERCP); many cases are idiopathic. Less common causes include infection (usually viral), hypertriglyceridaemia hypercalcaemia. The pancreas becomes acutely inflamed and, in severe cases, haemorrhagic. The clinical diagnosis is confirmed by finding a high plasma amylase or lipase activity (>3 times the upper reference limit [URL]) computed tomography (CT) scanning. plasma lipase : a more specific test for acute pancreatitis Chronic pancreatitis Chronic pancreatitis is an uncommon condition, which usually presents with abdominal pain or malabsorption, and occasionally with impaired glucose tolerance Diagnosis Pancreatic elastase and chymotrypsin activities in faeces are reduced in chronic pancreatic insufficiency Measurement of faecal elastase is now widely used to distinguish between diarrhoea of pancreatic and nonpancreatic origin. (Lundh test) or the administration of secretin and CCK. Bicarbonate concentration and enzyme activities are decreased in chronic pancreatic insufficiency. Carcinoma of the pancreas The plasma concentration of the tumour markers carcinoembryonic antigen (CEA) and CA 19-9 (see Chapter 20 p. 359-360) are elevated in up to 80% of patients with pancreatic malignancy