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GI tract

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Western University
Physiology 3120
Tom Stavraky

Human Physiology Friday, March 19, 2010 “Gastrointestinal I” Gastrointestinal (GI) Tract • Contributes to and is impacted by all the previous systems; therefore is incredibly important  Receives 1/3 of the total cardiac output  Produces acids and bases, so it contributes to overall body pH  Receives major nerves (i.e. vagus); also has its own nervous system, called the enteric nervous system  Produces some biologically active hormones • Performs 3 or 4 things  Secretion  Salivary glands • In the neonate, the salivary glands are important; important for creating a connection between the mouth and the mother’s nipple • Provide lubrication in oral cavity for moist mouth • Important for speech, chewing, swallowing, etc. • 3 major salivary glands: parotid, submandibular, and submalingual • A converging duct system o Start with many primary units that each drain into their own individual ducts (small diameter), which fuse into larger and larger ducts, which form one main excretory duct o Also seen in the pancreas • Examine a single primary unit  called the salivon (so named because it moves fluid, just like the nephron) o Closed-off end (acinus) made of acinar cells; pressure is lower than vascular pressure, so fluid moves through its semi-permeable membrane by plasma filtration; the fluid should be isotonic to plasma (just like the ultrafiltrate)  The source of most organic components to saliva, including (I) enzyme called amylase, which breaks down amylose/starch, (II) muco- and glycoproteins o Part of duct closest to acinus is the intercalated duct (ID) (seems to contain acinar cells) o The distal portion of the duct (striated duct [SD]) contains epithelial cells that contain microvilli on basolateral surface; these cells have a lot of mitochondria (for active transport) o Cells that line the duct in the ECF; have all the characteristics of epithelial cells, but have actin and myosin (imparts contractile ability); squeeze the duct to move fluid along toward the mouth; myoepithelial (ME) cells o Fluid moves down the salivon by its pressure gradient (even with ME cells) through the ID duct; in the SD duct, there is a Na/K ATPase (3Na out/2K in) that makes the fluid hypotonic to plasma; water DOES NOT follow because SD not permeable to water • Saliva* o Flow relationships determined at point X a
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