2 Segments of the GI tract and Sphincters(tonically contracted )
3 GI Track FunctionsIngestionDigestionAbsorptionDefecation
4 There are two stages of digestion Mechanical digestion is physical breakdown of food into smaller particles that helps chemical digestion.It is achieved by the cutting and grinding actions of the teethand the contractions of the stomach and small intestine.2. Chemical digestion is a series of catabolic reactions that breaks down large carbohydrate, lipid, and protein food macromolecules into smaller molecules that are used by body cells.It is achieved by the enzymes (GI tract and accessory organs secretions, intestinal brush border).
5 Digestion requiresMotility - muscular (phasic) contractions that break up food, mix it with digestive juices and propel it through the canalSecretion of enzymes, peptides, and other products that carry out or regulate digestion
6 Layers of the GI tract Mucosa with epithelial cells Muscularis mucosa From the esophagus to the anus,the tube consists of concentrically arrangedLayers of muscle, nervous and mucosal tissue.Mucosa with epithelial cells(Secretion & absorption)Muscularis mucosaSubmucosal plexusSubmucosaCircular muscle(Contraction – a decrease in diameter)Myenteric plexusLongitudinal muscle(Contraction – shortening)Serosa
7 Neural Control of GI - 1. Enteric Nervous System Lies in the wall of GI tract from the esophagus to the anusCoordinates and relays informationCan function by its own – local reflexes (within GI tract)Affected by extrinsic nerves (parasympathetic or sympathetic systemscan enhance or inhibit it`s functions)Composed ofMyenteric (Auerbach`s) plexusSubmucosal (Meissner`s) plexusNeurotransmitters released by the nerve endings: Acetylocholine, norepinephrine, serotonin, dopamine, cholecystokinin,somatostatin, VIP, bombesin, enkephalis.
8 increases activity of the enteric NS Neural Control of GI – 2. Autonomic Nervous SystemParasympathetic InnervationCranial – via vagusVagal innervation of GI tract extends from the esophagus to the level of the transverse colonSacral – via pelvic nerves to the distal part of large intestineThe postgalgionic fibres are located in the enteric NSStimulation of the parasympathetic nervesincreases activity of the enteric NS(and GI functions)Sympathetic InnervationBegins in the spinal cord (Th5-S2)Postganglionic nerves innervate all GI tractStimulation of the sympathetic nerves inhibits activity of the enteric NS(and GI functions)
9 Segment: MouthIn the mouth, food is chewed, reduced to small particles, mixed with saliva, and formed into a bolus in preparation for swallowing.Functions:Food intake, taste,Chewing,Mechanical and chemical digestion,Swallowing,Speech,Respiration
10 Saliva Content: High volume (1L / day), pH: 6.0 – 7.0 Hypotonicity Water 97-99,5%: moistens food and dissolves food for tasting2. Mucus: lubricates and binds food into bolus(not secreted by the parotid glands)Ptyalin (α-Amylase): starts break- down of starch in the mouth
11 Saliva Lingual lipase – activated by stomach acid Factors that destroy bacteria:lysozyme (enzyme) and thiocyanate4. Bicarbonate (HCO3) -buffering action - neutralizes acidic food in the mouth
12 is allowed by saliva and 22 muscles of mouth, pharynx and esophagus Deglutitionis allowed by saliva and 22 muscles of mouth, pharynx and esophagus1. Voluntary stage
13 Deglutition - 2. Pharyngeal Stage Stimulation of epithelial swallowing receptors(around the opening of the pharynx)Swallowing reflexSubsequent contractions of pharyngeal musclesTransmission of signals via the sensory fibres of the 5thand 9th cranial nervesto the swallowing center (medulla, lower pons)Transmission of motor signals by the 5th, 9th, 10th, and 12th cranial nerves to the pharynx and upper esophagus.
14 Deglutition – 3. Esophageal Stage Peristalsis pushes the bolus down the esophagus.Primary peristalsis -A continuation of the peristaltic wave that begins in the pharynx (passes to the stomach in 8-10 seconds)Food movement acceleratedby the effect of gravity (5-8 seconds)
15 GI Tract Functions: Stomach Storage of the foodMixing of food with gastric juices chyme (semidigested food)Regulation (slowing) of chyme emptying into the duodenumAnatomic division:The fundus, body, and antrum
16 Gastric Secretion (pH: 1.0 – 3.5) The Oxyntic (Gastric) GlandsThe proximal 80% of the stomach (fundus, body)Mucus: lubricates and protects the stomach mucosaHydrochloric acid (parietal cells)- Destroys pathogens, - Dissolves food particlesConverts ferric ions (Fe3+) to ferrous ions (Fe2+)Indirectly (?) stimulates secretion of pepsinogenActivates pepsinogen andCreates a highly acid medium for pepsin(pepsin – optimal pH )Intrinsic factor (parietal cells)Pepsinogen (chief cells)- inactive form of the proteolytic enzyme – pepsinHistamine (enterochromaffin-like cells – ECL cells)
17 Gastric Secretion (pH: 1.0 – 3.5) 2. The Pyloric GlandsThe distal 20% of the stomach (antral portion)Gastrin (G cells) –stimulate gastric secretionMucusPepsinogen
19 Small Intestine Duodenum: shortest region, about 25cmContinues the digestion ofcarbohydrates, proteins, and lipidsBegins the digestion of nucleicacidsGets the digestive fluids fromthe pancreas and liver via the hepatopancreatic ampullac. Secretes intestinal hormones
20 Pancreas Head, body, and tail Connected to the duodenum via The pancreatic duct(duct of Wirsung)Accessory duct(duct of Santorini)
21 Pancreatic Juice Zymogens and digestive enzymes High volume (1.2L - 1.5L daily)pH – , isotonicityZymogens and digestive enzymesMuch lower Cl- conc. than plasmaThe same Na+ and K+ conc. than plasmaMuch higher HCO3- conc. than plasma (up to 145 mEq/l)Sodium bicarbonate secretion serves to neutralize the acidic chyme emptied from stomachinhibit further digestive activity of the gastric juices (at the pH >5, pepsin is denaturated)protects the intestinal mucosaprovides a pH for action of the pancreatic digestive enzymes ( )
22 Neural and Hormonal Control of the Pancreas Secretin:acidity in intestine causes increased sodium bicarbonate releaseCholecystokinin (CCK):small peptides, amino acids,and fatty acids cause increased digestive enzyme releaseAcetylocholine (Vagus)
23 Cholecystokinin is the only GI hormon that inhibits gastric emptying – keeps the stomach full for a long timeA breakfast containing fat and protein „stick with you” better than that containing mostly carbohydrates
25 Small Intestine Duodenum: shortest region, about 25cmContinues the digestion ofcarbohydrates, proteins, and lipidsBegins the digestion of nucleicacidsGets the digestive fluids fromthe pancreas and liver via the hepatopancreatic ampullac. Secretes intestinal hormones
26 Small Intestine Jejunum: Ileum final , longest region Middle regionContinues and completesthe digestion of carbohydrates,proteins, lipids, and nucleic acidsb. Begins the absorption of carbohydrates, proteins, and water-soluble vitaminsIleumfinal , longest regiona. Involved in absorption of majority produced by digestion.
27 Functions of the Small Intestine Mucosa - 1 Digestion Contact digestionThe brush border increases the absorptive surface area of the small intestine and contains brush border enzymes – they are not released into the lumen and the chyme must contact the brush border for digestion to happen- contact digestion
28 Functions of the Small Intestine Mucosa -2 AbsorptionAbsorptive area of the intestinal mucosa is increased 600 timesThe mucosa of the small intestine has folds (of Kerckring) that increase the surface area of the mucosa about threefold.2. The millions of villi enhance the absorptive surface area by perhaps another tenfold.3. Each intestinal epithelialcell in the villus ischaracterized by a brushborder (has about 1000microvilli), and increasingthe area another twentyfold
30 VillusEach villus contains a central lacteal for absorption into the lymph, an artery, a vein, and blood capillaries, so that dissolved materials can be sent directly into the portal circulation.
31 Functions of the Small Intestine Mucosa – 3 Secretion DuodenumLarge amounts of alkaline mucus produced by Brunner`s glandsInhibited by sympathetic stimulation(50% of peptic ulcers)Small Intestine1 to 2 L of intestinal juice per dayContains water, electrolytes, and almost no enzymes, that are found in the brush borderpH – 7.5 – 8.0Produced by enterocytes in the crypts of LieberkühnAbsorbed by the villiLocal regulation of secretion (enteric nervous reflexes)
32 Functions of the Small Intestine Mucosa -4 Secretion of Peptides HormonesSecretinCCKGlucose Dependent Insulinotropic Peptide (GLIP)ParacrinesSomatostatinNeurocrinesVasoactive Intestinal Peptide (VIP)Encephalins- Stimulate contraction of the sphincters- Inhibit intestinal secretion
33 GI Motility – Phasic contractions The term GI motility refers to the motor activity (i.e., contractions) of the GI muscles.Functions:- transport of ingested foodmixing of ingested food with the digestive secretionsregulation of rate at which material moves from proximal to distal segmentspreventing of reflux
34 GI Phasic Contraction: Segmentation Segmentation (mixing) contractionsOccur at intervals along the intestineAs one set of segmentation contractions relaxes, a new set often begins - at new points between the previous contractions.The back-and- forth movement causes mixing the chyme with the digestive secretions, exposes the mucosal absorptive surface to the luminal contents and helps move chyme along the tract.Determined by slow wavesAmplified by excitation fromthe myenteric nervous plexusUp to 12/min – duodenum and upperjejunum, up to 8-9 – terminal ileum)
35 GI Phasic Contraction: Peristalsis Peristalsis is a propulsive reflex activity that involves both circular and longitudinal muscle layers , that is coordinated by the enteric NSPropels the chyme caudally.Small intestine to 2.0 cm/sec,net movement of chyme - 1 cm/minOccurs in the esophagus, the distal stomach small and large intestines.Peristaltic contractions are increased after a meal bystretching of the stomach wall (gastroileal, gastrocolic reflexes)stretching the gut wallhormonal factors (gastrin, CCK, motilin)
36 Anatomy of Large Intestine Include the cecum, colon, rectum, and anal canal.1,25 m long , diameter from cm (cecum) to cm (the sigmoid colon)Ascending & descending colon are retroperitonealHanging inferior to the cecum is the appendix.Inflammation of the appendix is called appendicitis.A ruptured appendix can result in gangrene or peritonitis, which can be life-threatening conditions.
37 Colon Proximal half – absorption of water Distal half – storage electrolytes: Na+,Cl-, vitamin KDistal half – storageMixing movements – „Haustrations”- Contraction of the circular muscle (2,5 cm)Contraction of the longitudinal muscle(arranged in 3 strips – teniae coli)Exposure of chyme to the surface area –absorption of water and electrolytesSlow propulsive effect (8-15 hours)Propulsive (mass) movements- Begin in the transverse colonas a constrictive ring followed by acontraction of 20 cm of the colon1-3 times a dayEnhanced by gastro(duodeno)colic reflexes
38 Overview of fluid intake and secretion compared to fluid absorption by the digestive tract 7.0 L is secreted2.0 L is ingestedA total fluid input = 9.0 L.GI fluid absorption:the small intestine - about 8.0 Lthe colon – 0.9 LFluid loss in the feces – 0.1 L