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DIGESTIVE PHYSIOLOGY
Physiology Unit 4
Func1ons
• Mo1lity – Inges1on – Mas1ca1on – Deglu11on – Peristalsis
• Secre1on – 7 liters/day! – Exocrine/endocrine
• Diges1on • Absorp1on
Diges1on of Carbohydrates
• Average intake is 250-‐300 g/day
• Amylase • Salivary, pancrea1c • Products: maltose, short chains of glucose
• Brush border enzymes • Products: glucose, galactose, fructose
Absorp1on of Carbohydrates • Luminal absorp1on • Glucose/galactose cotransport with Na+ into enterocytes
• Fructose facilitated diffusion into enterocytes
• Basolateral absorp1on • Monosaccharides enter blood via facilitated diffusion
Diges1on of Proteins • Require 40-‐50g of protein/day
– Supply essen1al amino acids – We consume 70-‐90 g/day
• Stomach – Pepsin
• Products are short chain polypep1des • Small intes1nes from pancreas • Trypsin, chymotrypsin
• Products are short chain polypep1des • CarboxypepAdase
• Products are free amino acids • Small Intes1nes from brush border enzymes • AminopepAdase
• Products are free amino acids
Absorp1on of Proteins
• Luminal absorp1on – Free amino acids enter enterocytes by counter-‐transport with Na+
– Short polypep1des (2 or 3 amino acids) enter enterocytes by counter-‐transport of H+
• Basolateral absorp1on – Free amino acids enter blood by facilitated diffusion
Diges1on of Fat
• Daily intake 70-‐100 g/day (mostly triglycerides)
• Emulsifica1on by bile salts • Pancrea1c lipase – Monoglyceride + 2 fa[y acids
• Forma1on of micelles by bile salts
Absorp1on of Fat • Luminal Absorp1on – Monoglycerides and fa[y acids enter enterocytes by diffusion
• Basolateral absorp1on into lacteals – Exocytosis of chylomicrons
– Chylomicrons contain triglycerides, phospholipids, cholesterol, fat-‐soluble vitamins
Lipid Transport
• From lympha1cs to thoracic duct • Free fa[y acids and glycerol into 1ssues • Le]overs to liver – Remnant par1cles contain cholesterol – Combined with apoproteins (lipid binding protein) to produce VLDL’s – Deliver triglycerides to other organs
Lipoproteins • Lipid & protein complexes – Transport cholesterol & triglycerides in blood – Protein allows hydrophobic lipids to remain in suspension
• Five classes: Based on density, molecular weight, size, chemical composi1on
• Chylomicrons • VLDL • IDL • LDL
– High levels associated with increased risk CVD • HDL
– Low levels associated with increased risk of CVD – Best profile = high HDL, low LDL
Absorp1on of Vitamins
• Fat-‐soluble vitamins – Vitamins A, D, E, K – Exocytosis in chylomicrons
• Water-‐soluble vitamins – Vitamins B, C – Absorbed by diffusion or mediated transport
• Vitamin B12 – Binds to intrinsic factor – Endocytosis (in ileum) into enterocytes
Absorp1on of Water and Minerals
• Water is most abundant substance in chyme – 8 L of ingested and secreted water enter the small intes1ne each day!
– 1.5 L make it to the large intes1ne – 80% absorbed in small intes1ne
• Minerals – Na+ – HCO3
-‐
– Cl-‐ – Small concentra1ons K+, Mg2+, Ca2+, Fe3+, Zn2+, I-‐
Regula1on of Gastrointes1nal Processes
• Regula1on of the condi1ons of the lumen of the tract (the outside of the body)
• Governed by the volume and composi1on of the luminal contents rather than the nutri1onal state of the body (the inside of the body)
Basic Principles
1. Disten1on of the lumen wall (volume of contents)
2. Chyme osmolarity (solute concentra1on) 3. Chyme acidity 4. Chyme concentra1ons – Monosaccharides – Fa[y acids – Pep1des – Amino acids
Neural Regula1on
• Enteric Nervous System (ENS)
• Myenteric plexus – Influences smooth muscle ac1vity
• Submucosal plexus – Influences secretory ac1vity
Neural Regula1on
• Neural ac1vity in one plexus influences the ac1vity of the other
• S1mula1on at one point in the plexus can lead to impulses that are conducted both up and down the tract
• Neural reflexes independent of CNS • CNS can influence mo1lity and secre1on of the tract
Hormonal Regula1on
• Hormones that control the GI system are secreted by cells sca[ered throughout the epithelium of the stomach and small intes1ne
• One surface of each endocrine cell is exposed to the lumen of the tract
• Chemicals in chyme s1mulate the cell to secrete its hormones from the opposite side of the cell into the blood
Hormonal Regula1ons
• Each hormone par1cipates in a feedback control system that regulates some aspect of the the luminal environment
• Most GI hormones affect more than one type of target cell
• Best understood pep1de GI Hormones – Gastrin – Cholecystokinin (CCK) – Secre1n – Glucose-‐dependent insulinotropic pep1de (GIP)
Gastrin
• Endocrine cell loca1on: – G cells of the antrum of stomach
• S1mulus for release – Protein in stomach – Parasympathe1c nervous system
• Ac1ons – S1mulates
• Stomach: (+) acid secre1on and mo1lity • Pancreas: (+) enzyme secre1ons • Intes1nes: (+) mo1lity in ileum and colon
Cholecystokinin (CCK)
• Endocrine cell loca1on: small intes1ne • S1mulus for release – amino acids, fa[y acids in small intes1ne
• Ac1ons – S1mulates
• Pancreas: (+) enzyme secre1on • Gall bladder: (+) contrac1on
– Poten1ates • Pancreas: (+) bicarbonate secre1on • Liver: (+) bicarbonate secre1on
– Inhibits • Stomach: (-‐) acid secre1on, gastric mo1lity
Secre1n
• Endocrine cell loca1on: small intes1ne • S1mulus for release – Acid in small intes1ne
• Ac1ons • S1mulates
• Pancreas: (+) bicarbonate secre1on • Liver: (+) bicarbonate secre1on
• Inhibits • Stomach: acid secre1on, gastric mo1lity
Glucose-‐Dependent Insulinotropic Pep1de
• Endocrine cell loca1on: small intes1ne • S1mulus for release – glucose, fat in the small intes1ne
• Ac1ons – S1mulates
• Pancreas: (+) insulin secre1on
Stomach
• Func1on – Ini1ates protein diges1on
– Kills bacteria – Intrinsic factor
• Secreted by parietal cells • Needed for Vitamin B12 absorp1on in ileum
– Absorp1on • Water • Alcohol • Aspirin
• 3 func1ons of pH in stomach – Denature ingested protein
– Convert pepsinogen to pepsin • Digests proteins
– Destroy bacteria
Gastric Glands • Goblet cells
– mucus • Parietal cells
– HCl – Intrinsic factor
• Chief cells – Pepsinogen
• Argentaffin cells – Serotonin – Regulate intes1nal movements
• G cells – Gastrin
Pancreas • Pancrea1c acini – Pancrea1c juice
• water • bicarbonate • diges1ve enzymes
– Ac1va1on by enterokinase
Pancrea1c Enzymes
Energy Regula1on by Islets of Langerhans
• Three cell types produce pep1de hormones – Beta cells: insulin
• S1mulates cellular uptake of glucose • In liver, ac1vates glycogenesis • S1mulates lipid synthesis • S1mulates cellular uptake of amino acids
– Alpha cells: glucagon • Encourages libera1on of reserves • Prevents glucose uptake by liver, muscle, adipose
– Delta cells: somatosta1n • Not sure
Regula1on of Insulin and Glucagon Secre1on
• Effects of glucose and amino acids – Increase in plasma glucose
• S1mulates b cells • Inhibits a cells
– Decrease in plasma glucose • decreased insulin produc1on • increased glucagon produc1on
– Meals high in protein • s1mulates insulin
– Meals high in protein and low in carbohydrate • s1mulates glucagon • result: increase in blood glucose and increased incorpora1on of amino acids into 1ssues
Regula1on of Insulin and Glucagon Secre1on
• Effects of autonomic nerves – Parasympathe1c ac1va1on
• increased insulin – Sympathe1c ac1va1on
• increased glucagon, inhibits insulin • stress hyperglycemia = glucagon + epinephrine
– Goal: Keep blood glucose between – 50-‐170mg/100ml
• higher = glycosila1on; lower = brain damage
Func1ons of the Liver
• Exocrine • Endocrine • Clopng func1ons • Synthesizes plasma proteins • Organic metabolism • Cholesterol metabolism • Excretory and degrada1ve func1ons
Exocrine & Endocrine Func1ons
Exocrine FuncBons • Synthesis and secre1on of
bile salts – 250-‐1500ml/day
• Adds bicarbonate rich solu1on to bile
Endocrine FuncBons • Secretes IGF-‐1
– promotes cell division
• Forms T3 from T4 • Secretes angiotensinogen
– Increases BP – S1mulates aldosterone
secre1on
• Metabolizes hormones • Secretes immune cytokines
Clopng and Plasma Proteins
• Produces – Prothrombin – Fibrinogen – Plasma albumin
• Regulates blood volume
– Acute phase proteins – Binding proteins – Lipoproteins
Organic Metabolism
• Converts plasma glucose to glycogen and triglycerides
• Converts amino acids to fa[y acids • Produces triglycerides and secretes them as lipoproteins
• Gluconeogeneisis and glycogenolysis • Converts fa[y acids into ketones • Produces urea
Cholesterol Metabolism/Excretory & Degrada1ve Func1ons
• Synthesizes cholesterol • Converts plasma cholesterol into bile salts – Bile salts needed for vitamin K absorp1on
• Excretes toxins via bile • Destroys old erythrocytes • …and lots, lots more!
Absorp1ve State
• High insulin and low glucagon • Insulin – cellular uptake of glucose – uptake & incorpora1on of amino acids – conversion of glucose to glycogen – addi1onal glucose to fat – incorpora1on of glucose into adipose 1ssue – suppression of liver glycogenolysis
Postabsorp1ve State
• Low insulin, high glucagon – low insulin encourages movement of amino acids out of the muscles
• Cor1sol -‐-‐ s1mulates produc1on of enzymes to convert pyruvic acid to glucose
• Glucagon – s1mulates glycogenolysis – s1mulates gluconeogenesis – s1mulates lipolysis – s1mulates ketogenesis