This is an extract of our Gastrointestinal System 2 document, which we sell as part of our Pre Clinical Systems Based Teaching Notes collection written by the top tier of Bristol University students.
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Gastrointestinal System Introduction to the GI Tract Functions Digestion - the process by which large molecules are chemically degraded to form smaller ones Absorption - the process by which nutrient molecules are absorbed by the cells that line the GI tract and enter the bloodstream
Organs Mouth, pharynx, oesophagus, stomach, duodenum, jejunum, ileum, colon, rectum, anus Associated organs: salivary glands, liver, gall bladder, liver, pancreas
Structure of the GI Tract Lamina propria & submucosa consist of connective tissue, blood vessels and nerves The submucosal plexus is nervous tissue that lies in the boundary between circular muscle and submucosa. It is only found in the small &
large intestines The myenteric plexus is nervous tissue found in the junction between circular muscle and longitudinal muscle. It is found from the oesophagus to the rectum
Peyer's Patches Gut-Associated Lymphoid Tissue They are aggregations of lymphoid tissue that are usually found in the lowest portion of the small intestine ileum in humans; as such, they differentiate the ileum from the duodenum and jejunum. They help recognise the 'normal' constituents of the gut instead of provoking an immune response
Structure of the Intestines The circular and spiral folds on the gut are known as Kerckring's Folds The crypts and villi found on the lumen wall help increase the surface area of the gut The microvilli on the villi also help to increase the surface area
Epithelia of the GI Tract Absorptive cells absorb salt and water in the large intestine Goblet cells secrete granules containing mucus
Movement of Salt and Water across the Intestinal Epithelium Salt and water may be absorbed by passing between the enterocytes (the paracellular route) via aqueous channels, through tight junctions linking the cells together They may also pass through the cells (the transcellular route). Electrolyte transport may involve both carrier-mediated mechanisms and passage through water-permeable channels Water movement from the lumen of the intestine is secondary to organic and ionic solute movement. The transfer of solutes across the epithelium creates an osmotic gradient whereby water is absorbed via para- and trans-cellular routes
Chloride Ion Secretion Cl- is taken up, actively across the basolateral membrane It then exits passively across the apical membrane Na+ & H2O move across the epithelium between the cells The negative charge of the chloride ion draws the positive charge of the sodium ion into the lumen, increasing the levels of salt Consequently, the tonicity is balanced by water being drawn into the lumen
GI Secretions Tissues: salivary glands, gastric glands, exocrine pancreas, liver-biliary system and intestine Total secretion = 8-9 litres/day Contents: enzymes, ions, water, mucus Function: to break down large compounds, regulate pH, dilute and protect
Regulatory Mechanisms Endocrine: release of a transmitter into the bloodstream for action on a distant target Paracrine: release of a transmitter from a sensor cell to affect adjacent target cells without entering the blood or activating neurons Neuronal: electric signalling via neurons
Regulation of the GI Tract Stimulation of Gastric Acid Secretion Cephalic Phase Stimuli such as thought, smell, taste and chewing of
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