Control of digestion,Different mechanisms for absorption.
- 1. Control of digestion--Neural and hormonal regulation. Different mechanisms for absorption Digestion and absorption of lipids. Absorption of electrolytes Rahana Moideen Koya.V.K Assistant Professor,Farook College. Kozhikode
- 2. Control of digestion Neural and hormonal regulation. Saliva secretion-Autonomous nervous system Gastric juice –Nervous and hormonal control Pancreatic and intestinal enzymes—Mostly by GIH
- 3. • The gut wall is innervated by Intrinsic and extrinsic nerves. The intrinsic nerves form the enteric nervous system . It consists of two nerve plexi A .Myenteric plexus (Auerbachs plexus)-- outer one located in between longitudinal and circular muscles..Control gastrointestinal peristaltic movts. B.Sub-mucous plexus—(Meissners plexus)inner one , located in the submucosa. Controls gastrointestinal secretions. Extrinsic nerves –Sympathetic and parasympathetic systems. Sympathetic innervations inhibit motility . Parasympathetic –stimulate motility. An excitatory wave called basic electrical rhythm BER ,determines the rhythm and frequency of gastric contraction.
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- 6. • GLP-Glucagon Like Polypeptide • GIP-Gastric Inhibitory Peptide. • VIP-Vasoactive Intestinal Polypeptide.
- 7. • Major mechanisms for absorption-Passive , active, Bulk transfer. • A.Passive absorption-Osmosis,free diffusion,facilitated diffusion,Solvent drag. • Osmosis-absoption of water and other solvents.Energy independent,Carrier free transport system • Simple diffusion—absoption Water- soluble substances such as vitamins and some charged mineral ions Na,K,Ca,Cl. • Facilitated diffusion –absorption of hexose sugar,a acids,fat-soluble vit. Energy independent, Carrier-mediated transport of substances.mobile carrier permease on plasma membrane. • Solvent drag-is the process by which solvent molecules drag solute molecules during the bulk flow of solvent in one direction. • Absorption of mineral ions through specialized ion channels in the plasma membrane
- 8. • B. Active absorption- Energy dependent,carrier-mediated ATP hydrolysis,energy utilization,and enzyme catalysis • C. Bulk transfer-- Absoption substances in large bulk by endocytosis. • Plasma membrane produces pocket –like invaginations called endocytic vesicles. • These vesicles soon enclose the substances to be transported into the cell. Then they grow inward into the cytoplasm ,get pinched off from the plasma membrane and become completely internalized. Now they are called endosomes . • Endocytosis of solid sustances -phagocytosis . liquid substances- pinocytosis.
- 9. Digestion of lipids— Ordinary neutral fats of plants and animals are triglycerols. -They are esters of one molecule of glycerol and 3 molecules of fatty acids. -They are highly water insoluble and are not easily hydrolysable by enzymes. -Two hydrolyzing enzymes-Lipases—Hydrolyses esters of long chain fatty acids,releasing glycerides and fatty acids --Esterases—hydrolyses the esters of short chain fatty acids, releasing glycerides and fatty acids. Mediated by gastric ,pancreatic and intestinal lipases. And intestinal esterases. Lipases act only on emulsified fats. Emulsification is the breakdown of large and complex fat globules to small and simple fat droplets .. these fat droplets form a suspension called emulsion. Emulsification occurs in stomach and small intestine. Gastric emulsification by pepsin or rennin and intestinal emulsification by bile. Bile salts activate pancreatic and intestinal lipases ,protect them from unfolding. Bile salts combine with fatty acids and monoglycerides on the fat droplets and form water- soluble micelles.
- 11. • Absorption of fats— • Uptake of lipids and products of thier break down to intestinal epithelial cells from intestinal lumen is by passive diffusion across plasma membrane. • Most rapid in duodenum and initial part of jejunum,where con.of bile salt is high. • Two theories-Lipolytic and partition • Lipolytic theory- Lipids are hydrolysed to fatty acids and glycerol during intestinal digestions .These are soon absorbed to the intestinal mucosa by passive diffusion.In the smooth ER of the mucosa ,Fats(triacyglycrols)are re-sythesised from them. Nearly 70% of these fats are released to the lymph and 30% are released into the blood stream • Chyle-The milky lymph in the lacteals,that contain a suspension of emulsified fat droplets. • Partition theory-Lipids are not completely hydrolysed during intestinal digestion,but are partially broken down to mono and diglycerides and free fatty acids.These undergo emulsifications by complexing with bile salts.Emulsified glycerides are directly absorbed to lymph ,and emulsified fatty acids are absorbed to blood. • Current concept is a compromise of two theories
- 12. • Fats are absorbed to body fluid from intestinal lumen in three states, • 1.as triglycerides – in unhydrolyzed state. • 2.as diglycerides and monoglycerides,-in the partly hydrolysed state. • 3.as free fatty acids and glycerol -in the hydrolysed state. • Glycerol is water-soluble so easily absorbed.Similar low-molecular wt. fatty acids • High mole wt.fatty acids must be converted to soluble form.-fatty acid –bile salt complex. • Hydrotrophic action.—action of bile salt on fatty acid to form a water soluble complex • F acids also absorbed along with steroids,forming soluble esters. • Soon after absoption fatty acid liberated.
- 13. • Three views on absorption of fat. • Pinocytosis major mechanism –absorption of neutral fats (triglycerides )and phospholipids. • Fats first absorbed as fatty acids,glycerol,and mono and diglycerides. Then neutral fats and phospholipids are synthesised from them in the villi before their transportation by the lymphatic system. • Absorption of fatty acids ,cholesterol and other lipids by diffusion occurs after their solubilization. • Absorption of cholesterol.—takes place in small intestine.-with the help of bile salts,max.absorption occurs when it get mixed with fatty acids and bile salts. • Pancreatic enzyme cholesterol esterase hydrolyses cholesterol esters releasing free cholesterol.It is readily absorbed to the epithelial cells by simple diffusion .In the epithelial cells, cholesterol gets re-esterified. These cholesterol esters gets incorporated with chlyomicrons (minute protein coated fatty particles) and pass to the lacteals for transportation along the lymphatic channel.
- 14. • Absorption of electrolytes- • They are absorbed from the lumen of small intestine ,are mostly the constituents of gastro-intestinal secretions.Some are components of foods. • Passive and active mechanisms involved. • Na ions can freely move in both directions across the intestinal epithelium . Greater part of its normal uptake from lumen by active transport. • Absorption of glucose and L-amino acids promotes absorption of Na. • Ca ,Mg,Fe,K,Chloride,Iodide,phosphate,nitrate ions –actively and passively. • Passive absorption of Chloride ion by facilitated diffusion. • Intestinal epithelium slowly permeable to divalent and trivalent ions.freely permeable to monovalent. • Active absoption of Ca depends much on vit.D and parathyroid hormone.