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Lecture 31

PSL301H1 Lecture 31: L31 Secretion

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Michelle French

Lecture 31 Secretion Silverthorn 7E: pp. 665-678 Wednesday, March 29 Lecture Outline: I. How much is secreted and absorbed in the GI per day? II. How is surface area of the GI tract increased? III. Where is saliva produced? IV. What is secreted from the stomach? V. What pancreatic secretions enter the small intestine? VI. What is secreted from the liver into the small intestine via the gall bladder? VII. What hormones are released from the small intestine? Recall: Functions of the Digestive System • Specialization of specific regions in the GI tract • Secretion bridges over with concept of absorption • Recall that we have function of GI tract, motility patterns, and the concept of material moving through depending on where it is in the GI tract with different motility patterns, but it becomes more specialized as you go through • Pushing through in esophagus, grinding in stomach, and segmentation in the intestine which helps to mix material that is being secreted in the duodenum • Material being secreted is the focus of today • Secretion is material into the GI tract to help with digestion and protection, but secretion of hormones or paracrine factors which influence cells to respond How much is secreted and absorbed in the GI tract per day? GI Secretion Requires Large Surface Area • We have a lot of material being emptied into GI tract from specialized epithelial cells lining the GI tract and accessory structures • We secrete twice the blood volume that we have in the body, which means that we need to be able to reabsorb that material • We’re efficient at reabsorbing material we ingest and all the secretions that help to break down the material we’ve ingested • Most of the absorption happens in the small intestine • All the material being reabsorbed after secretion requires huge surface area, otherwise we’d be dehydrated How is surface area of the GI tract increased? Surface Area • Absorption is a function of the surface area over which the material interacts • GI tract is a tube • Folds in the tube, and hills/valleys in the folds, and microscopic protrusions mostly on the hills all magnify the surface area over which the material can be reabsorbed • Recall: surface area of GI tract is equivalent to that of a tennis court due to modifications, primarily in the small intestine, but elsewhere as well Learning Catalytics: Histology 1. Draw a bracket to indicate and encompass each of the four layers of the GI tract on the diagram below through the small intestine. Draw an “M” beside Meissner’s plexus. • Mucosal layer includes epithelial cells, glands in lamina propria, and blood vessels • Submucosal layer has blood vessels and one of the nerve nets o Meissner’s plexus is the submucosal plexus which influences all the structures above it (secretion and absorption) • Muscularis externa contains the circular and longitudinal muscles • Serosa layer is specialized layer of connective tissue that allows GI tract to move ever each Stomach: Rugae and Gastric Pits • In stomach, we increase tube surface area with folds (temporary) • Rugae are folds that are visible when stomach is relaxed • When stomach expands to receive material, the rugae flatten out • Rugae are temporary folds that allow for expansion of the tube • Valleys are gastric pits which are just invaginations on the way to the gastric gland • Gastric gland contains many cells but don’t do any absorption, just increase surface area to allow for things like absorption and by allowing more cells to be within the gastric gland which release many factors into the circulatory system and the lumen of the stomach 2 Small Intestine: Plica, Villi, and Crypts of Lieberkühn Plica = valvulae conniventes • Small intestine has folds called plica which are permanent folds to increase surface area • On folds are more dips and valleys • So, there are villi and crypts which increase folds even more so surface area increases as well • Villi and Crypts of Lieberkühn • Villus has capillary bed working up into it as well as lacteal to allow for absorption of material • Crypt has many secretory cells in them as well Large Intestine: Crypts of Lieberkühn • Large intestine increases surface area as well, but the structures are less spectacular – suggests that most absorption happens in the small intestine with all the surface area increases • In the large intestine, the haustra (bulges) increase surface area as well • Crypts are also in the large intestine 3 Absorption: Microvilli in Small and Large Intestines • Large intestine doesn’t have villi, but has microvilli also found in small intestine • Microvilli are microscopic protrusions on apical surface of epithelial cells • Microvilli are called brush border • In small intestine, the epithelial cells are called enterocytes o Small intestine: villus and microvilli; large intestine: microvilli • In large intestine, epithelial cells are called colonocytes • Microvilli are extensions of epithelial cells with cytoskeleton to increase surface area Celiac disease is sensitivity to gluten – malabsorption of nutrients due to loss of microvilli from atrophy, because of an autoimmune response • Ingest gluten which is digested into two products which are made by enzymes in those epithelial cells • Sensitivity to gluten digest products • Peyer’s patches are looking for potential danger to you o Antibodies attack not only the gluten digest products but accidentally the enzyme that makes the gluten digest products (autoimmune) • Enzyme is found in the microvilli and is important in maintaining the cytoskeleton of the microvilli • So, as the enzyme gets destroyed, the microvilli start to atrophy and you lose that surface area 4 Where is saliva produced? Salivary Glands Saliva = water, electrolytes, bicarbonate, mucus, IgA antibodies, lysozyme, defensins, enzymes (salivary amylase, lingual lipase) • Functions: protection, lubrication, taste, digestion • Food dumped into mouth receptacle and it meets up with saliva • Saliva produced by paired glands – parotid glands, submandibular glands, and sublingual glands – help to lubricate material, facilitate taste, bicarbonates neutralize acidic food, mucous buffers against temperature differences, antibodies (IgA) which pass epithelial borders, lysozymes break up bacterial cell walls, defensins are antimicrobial peptide that punches holes in bacteria we ingest, and enzymes that initiate digestion Salivation is under neural control. • UR based on physical presence of food in the mouth Salivation is Under Neural Control which is relayed up to salivary centre and down to the salivary glands to start salivation • CR merely requires sight, smell, or thought of food – thought of food goes back to cortex which relays to the salivary centre to cause salivation Neural Control of Salivation • Salivation controlled through both parasympathetic and sympathetic innervation • Parasympathetic comes down from salivary centre through CN VII and IX and impinges on salivary glands o Innervation causes lots of watery saliva – rest and digest • Sympathetic control of salivation through superior cervical ganglion which is much shorter in duration, and impinges on different cells in salivary glands o Innervation causes small amounts of saliva with a lot of mucous so it feels thicker 5 What is secreted from the stomach? Stomach Secretions Note: esophagus does not secrete anything. • Gastric pit extends into the stomach wall and the gland itself • Cells at the neck of the gland called mucous neck cells and free mucous cells as well which secretes mucus and bicarbonate • Chemicals are secreted, enzymes secreted, and paracrine factors secreted, as well as hormones • Won’t be talking about somatostatin from D cells, the same as the growth hormone inhibiting hormone Mucus and Bicarbonate Protect the Stomach Lining • Gastric juice is very acidic, with normal pH at 7.35, and material in stomach is way too acidic to safely interact with those cells lining the stomach • Mucous cells start to secrete mucous that act as physical barrier between underlying epithelial cells and overlying material in stomach (pH 2) • Mucous cells also secrete bicarbonate that collects underneath mucous layer as a chemical barrier • So, there is physical and chemical barrier to keep underlying epithelial cells safe from acidic environment of the lumen • Zollinger-Ellison syndrome is a result of hyperacidity in the stomach, so there is a very acidic environment which degrades mucous and bicarbonate layers so people usually have sores and ulcers in their stomach and duodenum Parietal Cells Secrete Acid and Intrinsic Factor Acid: • Parietal cells secrete some of the acid, as well as intrinsic factor • Kills bacteria, denatures proteins, activates pepsinogen • Intrinsic factor released into stomach lumen by parietal cells required for absorption of B12 in the ileum Intrinsic factor: o B 12s cobalt at the core • Binds to vitamin B (12so called cobalamin), ensures o Without intrinsic factor, you will have no absorption of B 12 the ileum (important for RBC production) absorption of vitamin B in12leum • Parietal cells have net secretion of HCl making acidic stomach environment in lumen • Water from parietal cell dissociates into hydroxide and H+ + + + + • H pumped on H -K -ATPase with high concentration of H in there anyways, which requires active transport to pump H + against concentration gradient into the stomach • Cl moves out through chloride channels on apical surface of parietal cell for net secretion into lumen of stomach of HCl to drop the pH • H moves through apical transporter - • OH in presence of carbonic anhydrase forms bicarbonate which leaves the parietal cell on a basolateral chloride-bicarbonate exchanger + • All the bicarbonate is pumped into the body as the H is pumped into the stomach, and we can measure an alkaline tide within the capillaries (blood coming back from stomach when it’s busy) 6 Chief Cells Secrete Pepsinogen and Gastric Lipase • Chief cells in the glands secrete enzymes including lipase and pepsinogen • Pepsinogen is an inactive form of pepsin which helps break
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