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  1. 1. Chapter Ten Gastrointestinal physiology 1
  2. 2. General chapter outline • Introduction • Functional anatomy of GIT • General functions of the digestive system • Innervation of GIT • Hormonal Control of GI Function • Mouth, pharynx and esophagus • Obesity and metabolic rate • Stomach and gastric secretion • Liver, gall bladder and pancreas • Digestion and absorption • Small intestine • Large intestine 2
  3. 3. Introduction • Food is required by the body for the production of energy and for the growth and repair of tissues • Each day an average adult consumes around 1 kg of solid food and 1- 2 liters of fluid • Majority of this material is in a form that cannot be used immediately by the body for cellular metabolism • It must be broken down into simple molecules which can be absorbed into the bloodstream for distribution to the tissues.  The gastrointestinal system performs this task 3
  4. 4. Gastrointestinal system/ Digestive System • The digestive system consisting of the digestive tract and accessory organs • Alimentary canal or gastrointestinal tract (GIT): • A muscular tube extending from the mouth to the anus • It measures about 10 m in adult cadaver • It is significantly shorter in a living individual (why?) • Composed of the organs mouth, pharynx, esophagus, stomach, small intestine, large intestine and anus • Accessory digestive organs – teeth, tongue and glandular organs, such as the salivary glands, exocrine pancreas and biliary system (liver and gallbladder) • Assist in the breakdown of food 4
  5. 5. 5 The Components of the digestive System
  6. 6. 6 The Components of the digestive System
  7. 7. Functions of gastrointestinal tract • Ingestion: taking materials into the digestive tract • Motility: the muscular contractions that mix and move forward (propulsion) the contents within the tract • Digestion: the breakdown of ingested food into smaller components – Mechanical digestion: ingested material  smaller pieces (without changing its chemical structure) – Chemical digestion: complex molecules  smaller molecules(absorbable units) with the help of enzymes • Absorption: movement of nutrients from the GI tract lumen into the blood or lymph • Secretion: release of water, acids, enzymes, buffers, and salts by epithelium of digestive tract and glandular organs • Defecation: elimination of indigestible solid wastes 7
  8. 8. GIT and its natural defense • Its lumen is continuous with external environment (mouth - anus) - extension of the external world • Therefore, can possibly harbor microorganisms in its luminal surfaces. • However, the GI-system can protect itself from such hazards by some defense mechanisms found in: a. Mouth: Saliva contains lysozymes, IgA etc b. Stomach: HCl, pepsin etc. Have bactericidal effect c. SI: • Immuno-competent cells (Peyer's patches) attack microbes in the SI wall • Macrophages: developed from monocytes destroy microbes by phagocytosis 8
  9. 9. Layers of GIT and their function • From esophagus to the anal canal the walls of the GIT have four tunics • From the lumen outward they are: 1. Mucosa - consists of: • Epithelium (mucous membrane) • Lamina propria • Muscularis mucosa 2. Submucosa 3. Muscularis externa - consists of: • Circular muscle layer -inner • Longitudinal muscle layer - outer 4. Serosa / adventitia: 9
  10. 10. 1. Mucosa • Innermost layer (faces lumen) or in contact with GIT content • Moist epithelial layer • Its major functions are: • Secretion: mucus, enzymes and hormones • Absorption: end products of digestion • Consists of 3 sublayers: epithelium, lamina propria, and muscularis mucosa 10
  11. 11. Epithelium: • In contact with the GIT content • Contains: a. Exocrine gland cells: secretion of digestive juices b. Endocrine gland cells: secretion of blood-borne GI hormones c. Epithelial cells:  Simple columnar epithelium (in stomach, small intestine, large intestine): allows for secretion and absorption • Goblet cells: –Protect digestive organs from self digesting –Ease the passage of food along the tract  Stratified squamous epithelium (in mouth, surface of tongue, pharynx and esophagus): withstand abrasion 11
  12. 12. Lamina propria: • Thin middle layer • Houses the gut associated lymphoid tissue (e.g. Peyers patches produce B-lymphocytes), which is important in defense against disease-causing intestinal bacteria • Nourishes the epithelium and contains capillaries for absorption of nutrients 12
  13. 13. Muscularis mucosa: • Outermost mucosal layer of smooth muscle Note: • In the small intestine (the main site of digestion and absorption), the mucosal surface is highly folded  greatly increase the surface area  maximize nutrient, water, and electrolyte absorption • The esophagus exhibits little mucosal folding because it functions primarily as a transit tube • The pattern of surface folding can be modified by contraction of the muscularis mucosa • Importance: exposing different areas of the absorptive surface to the luminal contents 13
  14. 14. 2. Submucosa • Thick layer of connective tissue • Provides the digestive tract with its distensibility and elasticity • Contains : • Blood vessels, lymphatic vessels • Glands • Submucosal nerve plexus (Meissner’s plexus) at its juncture with the circular muscle layer 14
  15. 15. 3. Muscularis Externa • The major smooth muscle coat of the digestive tube, surrounds the submucosa • Consists of two layers: an inner circular layer and an outer longitudinal layer • Inner circular layer : • Contractions narrow the diameter of lumen • Thicker and has more gap junction than the longitudinal • More powerful in exerting contractile forces on the contents of GIT 15
  16. 16. • Outer longitudinal layer: • Contractions shorten a particular segment of GIT • Another nerve network, the myenteric plexus/Auerbach plexus, lies between the two muscle layers • Together, contractile activity of these smooth muscle layers produces the propulsive and mixing movements • The submucosal and myenteric plexuses help regulate local gut activity 16
  17. 17. 17 Motility
  18. 18. 4. Serosa • Outer connective tissue covering of the digestive tract • Helps in the attachment of gut to the surrounding structures • Secretes a watery, slippery fluid (serous fluid) that lubricates and prevents friction between the digestive organs and the surrounding viscera • It is continuous with the mesentery • Mesentery: suspends the digestive organs from the inner wall of the abdominal cavity 18
  19. 19. 19
  20. 20. GI Smooth Muscles & Their Electrical Activity • The GI smooth muscle acts as a functional syncytium Q. What is a functional syncytium? • Muscle fibers arranged in bundles or sheets • Cells are electrically linked by gap junctions • An electrical signal (AP) in one cell spreads rapidly through the entire sheet of digestive smooth muscle which then contract as a single • Such a group of interconnected muscle cells that function electrically and mechanically as a unit is known as a functional syncytium (Excited and contracting as a single unit) 20
  21. 21. Electrical activity of gastrointestinal smooth muscle • Two basic types of excitation: 1. Slow waves (basic electrical rhythm (BER)) • Are slow, undulating changes in the RMP (between −50 and −60 mV) • Generated by pacemaker cells known as the interstitial cells of Cajal – Located throughout the layers of the muscularis externa • Are not action potentials • Are unique to GI muscle • Intensity usually varies between 5 and 15 mv • Frequency ranges in different parts of the GIT from 3 to 12/min 21
  22. 22. • Do not cause calcium ions to enter the smooth muscle fiber (they only cause entry of sodium ions) • Therefore, they do not cause muscle contraction (Exception: may be in the stomach?) 22 Electrical slow-wave frequencies: • Stomach : 3 waves/ min • SI (Duodenum): 12 waves/min • SI (Jejunum): 9–11 waves/min • SI (Ileum): 8 – 10 waves /min
  23. 23. 2. Spike potentials • Are true action potentials • Occur when the RMP of the GI smooth muscle becomes more positive than about −40 millivolts • The channels responsible for the action potentials are a. Calcium-sodium channels – Allow large numbers of calcium ions to enter along with smaller numbers of sodium ion – Are much slower to open and close • Cause significant quantities of Ca2+ to enter the fibers • Accounts for the long duration of the action potentials 23
  24. 24. • The resultant Ca2+ entry has two effects: 1. It is responsible for the rising phase of an AP – Falling phase : brought about as usual by K+ efflux 2. It triggers a contractile response • The greater the number of action APs, the higher the cytosolic Ca2+ concentration, the greater the cross-bridge activity, and the stronger the contraction 24
  25. 25. 25 Membrane potential in intestinal smooth muscle cells
  26. 26. Types of the GI smooth muscle contraction: A. Phasic contractions • Periodic contractions followed by relaxation evoked by action potentials • Frequency/number of APs grade the degree and duration of contraction • Dominant in esophagus, gastric antrum, and intestines B. Tonic contractions • Sustained contraction without relaxation for prolonged periods of time • Often increases or decreases in intensity but continues. 26
  27. 27. • In lower esophageal sphincter, orad region of the stomach, ileocecal and internal anal sphincter • Not associated with slow waves • It is sometimes caused by – Continuous repetitive spike potentials – Hormones – Continuous entry of calcium ions into the interior of the cell 27
  28. 28. Innervation of the GI tract 1. Extrinsic NS : ANS • Originates outside the DT and innervate the various digestive organs a. Parasympathetic nervous system • Excitatory on the functions of the GI tract • Increase smooth muscle motility and promote secretion of digestive enzymes and hormones • Promotes digestive & absorptive processes 28
  29. 29. • The parasympathetic supply to the gut is divided into cranial and sacral divisions (Origin: cranio-sacral) 29 , pancreas, gall bladder • Site of ganglion cells: Myenteric plexus & meissner's plexus (CN X) (S2-S4) Note: Both make synaptic connections with intramural plexuses
  30. 30. 30 The digestive tract is innervated by the parasympathetic division of the autonomic nervous system
  31. 31. • Functions of parasympathetic fibers (cholinergic, Ach): –Smooth muscle contraction –Relaxation of sphincters –Stimulation of gut motility and secretion • The vagus is mixed nerve in which approximately 80% of the fibers are afferent • Receptors in the mucosa and smooth muscle relay information back to MO via these afferents which, in turn, initiates vagal reflex signals that return to the GIT via efferent in the vagus nerve • Reflexes of this type are called vagovagal reflexes 31
  32. 32. b. Sympathetic nervous system • Is usually inhibitory on the functions of the GI tract • Slows digestive & absorptive processes • Origin: thoracolumbar region (T-5 - L-2) • The preganglionic fibers pass through the lateral sympathetic chain; they continue as splanchnic nerves and relay in sympathetic ganglion (prevertebral ganglia in the abdomen: celiac and mesenteric ganglia) • Postganglionic fibers end on the intrinsic nervous system • Sympathetic division supplies all areas of GIT except oral cavity and anus (truly parasympathetic) 32
  33. 33. • Mechanism of action: Norepinephrine (NE) released by sympathetic nerve endings inhibits smooth muscle and neurons of intrinsic nervous system • Functions: –Smooth muscle relaxation –Inhibition of gut motility –Contraction of sphincters –Reduce intestinal blood flow 33
  34. 34. 34 The digestive tract is innervated by the sympathetic division of the autonomic nervous system
  35. 35. 2. Intrinsic NS: Enteric nervous system (Little Brain ) • Lie entirely within the digestive tract wall and run its entire length (from esophagus to the anus) • The digestive tract has its own intramural (“within-wall”) nervous system (=Enteric nervous system ) • The number of neurons is >100 million • Coordinates and relays information from the parasympathetic and sympathetic nervous systems to the GI tract • Is especially important in controlling GI movements and secretion, even in the absence of extrinsic innervation • Composed mainly of two plexuses 35
  36. 36. 1. Myenteric plexus (Auerbach’s plexus) –It is concerned with control of peristaltic activity of the GIT • Principal effects: –Increased tone –Increased intensity of contractions –Increased rate of contractions –Increased velocity of conduction • Partially controlled by autonomic nervous system 2. Submucosal plexus (Meissner’s plexus) –Controls mainly gastrointestinal secretion and local BF –Receives sensory information from chemoreceptors and mechanoreceptors in the GI tract • Partially controlled by autonomic nervous system 36
  37. 37. 37 Neural control of the gut wall
  38. 38. Neurotransmitters secreted by enteric neurons • Different neurotransmitter substances that are released by the nerve endings of different types of enteric neurons • Common: –Acetylcholine: Excites gastrointestinal activity • Stimulates smooth muscle contraction • Increases intestinal secretions • Releases enteric hormones (e.g. gastrin, CCK, secretin ) • Dilates the blood vessels –Norepinephrine & epinephrine: almost always inhibits gastrointestinal activity • Derived from extrinsic sympathetic neurons • Inhibits the GI activity • Causes vasoconstriction 38
  39. 39. – Nitric oxide and vasoactive intestinal peptide: inhibitory • Other types: – ATP, serotonin, dopamine, CCK, substance P, somatostatin, leu- enkephalin, met-enkephalin, gastrin releasing peptide (bombesin) • Mixture of excitatory and inhibitory agents 39
  40. 40. Gastrointestinal Reflexes • The anatomical arrangement of the ENS and its connections with the ANS support three types of GI reflexes • Are essential to GI control 1. Local reflexes : • Reflexes integrated entirely within the gut wall ENS • Control much gastrointestinal secretion, peristalsis, mixing contractions, local inhibitory effects 2. Short reflexes: • Reflexes from the gut to the prevertebral sympathetic ganglia and then back to the GIT • Signals from the stomach to cause evacuation of the colon = Gastrocolic reflex 40
  41. 41. • Signals from the colon and SI to inhibit stomach motility and stomach secretion = Enterogastric reflexes • Reflexes from the colon to inhibit emptying of ileal contents into the colon = Colonoileal reflex 3. Long reflexes: • Reflexes from the gut to the spinal cord or brain stem and then back to the GIT • Include: a. Reflexes from the stomach and duodenum to the brain stem and back to the stomach by way of the vagus nerve • Control gastric motor and secretory activity 41
  42. 42. b. Defecation reflexes that travel from the colon and rectum to the spinal cord and back again to produce the powerful colonic, rectal, and abdominal contractions Defecation (the defecation reflexes) 42
  43. 43. Receptors of the GIT • Mechano- ,osmo- and chemo- receptors respond to stretch and pH, presence of substrate, and end products of digestion respectively • They initiate reflexes that: –Activate or inhibit digestive glands –Mix lumen contents and move them along 43
  44. 44. Mechano- ,osmo- and chemo receptors stimulation elicit 1. Neural reflexes • Local reflex • Short reflex • Long reflex or 2. Secretion of hormones Alter the activity level in the digestive system’s effector cells 1. Smooth muscle cells: for modifying motility 2. Exocrine gland cells: for controlling secretion of digestive juices 3. Endocrine gland cells: for varying secretion of GI hormones 44
  45. 45. Hormonal Control of GI Function • The GI hormones are released into the portal circulation and exert physiological actions on target cells with specific receptors for the hormone 1. Gastrin • Secreted from “G” cells of the antrum of the stomach • Stimuli: – Distention of the stomach – Products of proteins (small peptides) – Gastrin-releasing peptide (released by the nerves of the gastric mucosa during vagal stimulation) • Actions : – Stimulation of gastric acid secretion – Stimulation of growth of the gastric mucosa 45
  46. 46. 2. Cholecystokinin (CCK) • Secreted from “I” cells in the mucosa of the duodenum and jejunum • Stimuli : – Fat, fatty acids, and monoglycerides in the intestinal contents • Actions: – Gallbladder contraction (expelling bile into the SI) – Increases pancreatic digestive enzymes into the duodenum – Moderate inhibition of stomach contractility (slows the emptying of food from the stomach to give adequate time for digestion of the fats in the upper intestinal tract) – Inhibits appetite to prevent overeating during meals 46
  47. 47. 3. Secretin • Secreted from the “S” cells in the mucosa of the duodenum • Stimulus: – Acidic chyme in the duodenum • Action: – Promote pancreatic secretion of bicarbonate (helps to neutralize the acid in the SI) 4. Glucose-dependent insulinotropic peptide ( Gastric inhibitory peptide [GIP]) • Secreted from “K” cells in the mucosa of the upper SI • Stimuli : – Fatty acids and amino acids – Carbohydrate (lesser extent ) 47
  48. 48. • Action: – Decreases motor activity of the stomach(slows emptying of gastric contents into the duodenum) – Stimulates insulin secretion 5. Motilin • Secreted by “M” cells in the stomach and upper duodenum • Released cyclically • Stimulus: Fasting • Actions: – Increase GI motility – Stimulates interdigestive myoelectric complexes (move through the stomach and SI every 90 minutes ) • Clear the stomach and SI of residual food, secretions, and desquamated cells and propel them to the colon 48
  49. 49. . Paracrines • Are released from cells in the GI mucosa • Diffuse over short distances to act on target cells 1. Somatostatin: • Is secreted by cells of the GI tract in response to H+ • Inhibits the release of all GI hormones • Inhibits gastric H+ secretion • Its secretion is inhibited by vagal stimulation 2. Histamine: • Is secreted by mast cells of the gastric mucosa • Increases gastric H+ secretion directly and by potentiating the effects of gastrin and vagal stimulation 49
  50. 50. GIT-blood Supply • At rest ~25% of the CO flows and reaches the abdominal cavity (largest share) • The combined circulation to the stomach, liver, pancreas, intestine, and spleen is called the splanchnic circulation Portal circulation: • Venous blood (poor in o2, but rich in nutrients from the intestine, spleen, pancreas etc.) join and form the portal vein that drains into the liver • Then blood is directed to liver sinusoids, hepatic vein and lastly to inferior vena cava • Potentially harmful agents that come with the blood are destroyed by macrophages of the liver (reticuloendothelial cells) 50
  51. 51. Schematic of the splanchnic circulation under fasting conditions 51
  52. 52. • Lymphatic drainage: • Is important for the transport of lipid-soluble substances that are absorbed across the GI tract wall • Lipids and other lipid-soluble molecules (including some vitamins and drugs) are packaged into particles that are too large to pass into the capillaries and instead pass into lymph vessels in the intestinal wall • These lymph vessels drain into larger lymph ducts, which finally drain into the thoracic duct 52
  53. 53. Factors that control blood flow to the GIT 1. Metabolism: • Increase in metabolism during or after a meal increases releases vasoactive substances that increases blood flow by 8- fold – Peptide hormones: cholecystokinin, vasoactive intestinal peptide, gastrin, and secretin – GI glands release kinins: kallidin and bradykinin 2. Decreased O2-tension: • Increases blood flow 3. Neural: • SNS: NE….. decreases blood flow • ENS: VIP, NO……increase blood flow 53
  54. 54. GI Motility 1. Propulsive movements • Cause food to move forward along the tract at an appropriate rate to accommodate digestion and absorption 2. Mixing movements • Keep the intestinal contents thoroughly mixed at all times • Both are brought about by the ENS, but are influenced by the extrinsic nervous system, especially the parasympathetic nervous system 54
  55. 55. Propulsive Movements • The basic propulsive movement of the GIT is peristalsis • Peristalsis: ring-like contractions of the circular smooth muscle that move progressively forward, pushing the bolus into a relaxed area ahead of the contraction • Stimulus: – Distension of the gut – Physical or chemical irritation of the surface epithelium – Strong stimulation by the PNS • Controlled by: – Actual peristalsis is brought about by the myenteric plexus – But it is influenced by the PNS 55
  56. 56. Mechanism of myenteric reflex or the peristaltic reflex 1. Distension of the gut 2. Stimulation of the ENS 3. The gut wall contracts 2-3 cm behind the point of stimulus  formation of a contractile ring 4. Initiation of a wave of contraction  travels in the anal direction (=directional movement) 5. Relaxation of the gut wall distal to the point of stimulation - receptive relaxation (allowing the food to be propelled more easily toward the anus) • Therefore, peristalsis moves towards the anus • The peristaltic reflex + the anal direction of movement of the peristalsis is called the “law of the gut” 56
  57. 57. 57 Peristalsis • Transit of food through the esophagus is rapid • In the small intestine—the main site of digestion and absorption—the contents are moved forward slowly allowing time for the breakdown and absorption of food
  58. 58. Mixing Movements • Differ in most parts of the gut –Segmentations- in small intestine –Haustrations- in large intestine 58 Segmentation • The contracted segments relax and the previously relaxed areas contract • These oscillating contractions thoroughly mix the chyme
  59. 59. • Due to local contractions taking place in SI segments Mixing food with the digestive juices (promote digestion of the food) Facilitate absorption by exposing all parts of the intestinal contents to the absorbing surfaces of the digestive tract 59
  60. 60. Digestion • Two parts: 1. Mechanical digestion: • Is the physical grinding of food to smaller units without altering their chemical composition. • Helps to expose and increase the surface area for enzymatic attacks 2. Chemical digestion: • Is hydrolytic breakdown of nutrients (CHO, fat, protein etc) by different chemical reactions into simpler forms. • This processes changes the original composition of the nutrient. 60
  61. 61. The Mouth (oral cavity) • The entrance to the digestive tract is oral cavity • Parts of the oral cavity: cheeks, lip, tongue, hard and soft palate, teeth • Is responsible for mechanical digestion of solid food by mastication (chewing) • Most of the muscles of chewing are innervated by the motor branch of the fifth cranial nerve (trigeminal) • The chewing process is controlled by nuclei in the brain stem 61
  62. 62. Functions of chewing are: • To grind and break food up into smaller pieces –Facilitate swallowing • Prevents excoriation of the GIT –Increase the food surface area on which salivary enzymes will act • Rate of digestion is dependent on the total surface area exposed to the digestive secretions • To mix food with saliva • To stimulate the taste buds (contain the taste receptor cells) 62
  63. 63. Saliva • Three major pairs of salivary glands a. A pair of parotid • The largest, just below and in front of the ears • Produce approximately 25% of saliva b. A pair of submandibular • At the posterior corners of mandible • Produce approximately 75% of saliva c. A pair of sublingual gland • Below the tongue • Produce approximately 5% of saliva Buccal glands: in the cheeks near the front of the mouth • Daily saliva production approximates 1 to 1.5 L 63
  64. 64. Secretion of ions in saliva • Saliva contains – K+ and HCO3 - (in large quantities ) – Na+ and Cl-(several times less than in plasma) Why??? • A typical compound gland (e.g. the submandibular gland) that contains acini and salivary ducts 1. The acini secrete a primary secretion – Contains ptyalin and/or mucin in a solution of ions – Isotonic and has the same Na+, K+, Cl–, and HCO3 – concentrations as plasma 2. The primary secretion flows through the ducts – Two major active transport processes take place – Modify the ionic composition of the fluid in the saliva 64
  65. 65. a. Na+ actively reabsorbed and K+ actively secreted in exchange for the sodium • [Na+] of the saliva becomes greatly reduced, whereas [K+ ] becomes increased • Excess sodium reabsorption compared with potassium secretion • Creates electrical negativity of about -70 mV in the salivary ducts • Causes chloride ions to be reabsorbed passively • [Cl- ] in the salivary fluid falls b. Bicarbonate ions • Secreted by the ductal epithelium into the lumen of the duct • This secretion is at least partly caused by • Passive exchange of bicarbonate for chloride ions • An active secretory process 65
  66. 66. • At rest, final salivary secretion is hypotonic • Reasons for hypotonicity of the saliva, includes: 1. Na+ and Cl- reabsorption from the lumen to the plasma is greater than K+ and HCO3- secretion into the lumen 2. The ducts are relatively impermeable to water (i.e. H2O does not follow the osmotic gradient of NaCl) 66
  67. 67. • The parotid glands secrete almost entirely the serous type of secretions • The submandibular and sublingual glands secrete both serous secretion and mucus • Saliva has a pH between 6.0 and 7.0 (favorable range for the digestive action of ptyalin) • Components: –H2O (99.5%) –0.5%= Electrolytes: Na+, K+, HCO3 −, Ca++, Mg++, and Cl− + Other organic substances include: • Enzymes (amylase), lingual lipases, lysozymes, thiocyanate, glycoproteins, IgA, lactoferrin, mucus, etc.) 67
  68. 68. Saliva functions a. Digestion: CHO-digestion begins in saliva . The enzyme ptyalin breaks starch- to-maltose. Lingual lipase begins fat digestion in the mouth b. Protection: has anti-microbial actions (contains IgA, lysozyme, lactoferrin & thiocyanate that kills microbes) c. Involved in speech: Clear & fluent articulation is possible in the presence of saliva d. Secretes HCO3 - : neutralize acids in food as well as acids produced by bacteria in the mouth (helping prevent dental caries), the neutral pH is good for ptyalin action (helping prevent dental caries) e. Lubrication: Mucus found in saliva facilitates moistening and swallowing of food 68
  69. 69. e. It serves as a solvent for molecules that stimulate the taste buds. Only molecules in solution can react with taste bud receptors f. It plays an important role in oral hygiene by helping keep the mouth and teeth clean 69
  70. 70. Control of Saliva Secretions A. Parasympathetic fibers: causes copious secretion of saliva (Cholinergic). B. Sympathetic fibers: Causes small and insignificant secretion which is viscous (Adrenergic). Salivary reflex • Sight, smell, and test or thinking of food  Receptors in oral cavity or smell  Sensory fibers from the tongue to the salivatory nuclei in MO  Parasympathetic fibers (act on salivary glands)  increase salivary secretion. Salivation can also be controlled by higher centers like hypothalamus which has nerve connections with salivatory nuclei in the Medulla oblongata (MO) Higher centers like appetite area in the hypothalamus are also involved in reflex control. 70
  71. 71. 71 Salivary glands and control of salivary secretion
  72. 72. 72 Regulation of salivary secretion.
  73. 73. Phases of Salivary secretions 3-phases of salivary secretions include 1. Cephalic (brain) phase: triggered by thought, smell, or sight of food 2. Oral phase: triggered by food that stimulate touch & test receptors in the mouth 3. Gastric phase: triggered by substances which stimulate the gastric mucosa (acids or sour tastes) in the stomach Xerostomia: • Diminished salivary secretion • Difficulty in chewing and swallowing, inarticulate speech 73
  74. 74. Swallowing reflex (Deglutition) • Swallowing is an act of orderly propulsion of food from the mouth to the stomach 3-phases of swallowing reflex 1. Voluntary phase in the oral cavity: • Initiates the swallowing process • The tongue rolls and pushes the bolus against the hard and soft palate in the oral cavity and into the pharynx 2. Pharyngeal phase (involuntary): Reflexes whose nuclei are located in swallowing center of the MO, stimulate the closure of the nasal and tracheal openings and cause inhibition of respiration. 74
  75. 75. The pressure of the bolus in pharynx stimulates pharyngeal pressure receptors impulses via trigeminal (CN V) and glossopharyngeal (CN IX) nerves swallowing center (in the medulla and lower pons ) via 5th, 9th, 10th, 12th (hypoglossal (CN XII)) 1. Activates in the appropriate sequence the muscles involved in swallowing 2. Inhibits respiratory center in the brain stem 75
  76. 76. Steps: a. The soft palate is pulled upward and closes the nasopharynx b. The palatopharyngeal folds on each side of the pharynx are pulled medially to approximate each other • Impede too much any large object to pass into the esophagus b. The vocal cords are pulled together and narrow the opening of the larynx (glottis) c. The epiglottis folds backward down over the closed glottis as further protection d. The pharyngeal muscles contract to force the bolus into the esophagus 76
  77. 77. 3. Esophageal phase (involuntary): Esophagus • A fairly straight muscular tube that extends between the pharynx and the stomach • It is guarded at both ends by sphincters • Pharyngoesophageal sphincter (Upper esophageal sphincter): prevents large volumes of air from entering the esophagus and stomach • Gastroesophageal sphincter (Lower esophageal sphincter): prevents reflux of gastric contents • Musculature of pharyngeal wall and upper 1/3rd of the esophagus is striated muscle – Peristaltic waves controlled by skeletal nerve impulses from the glossopharyngeal and vagus nerves 77
  78. 78. • Musculature of lower 2/3rd of the esophagus is smooth muscle • Controlled by - vagus nerves - myenteric nervous system • It exhibits two types of peristaltic movements: primary peristalsis and secondary peristalsis • Primary peristalsis: –Continuation of the peristaltic wave that begins in the pharynx –Triggered by swallowing center • Secondary peristalsis : –Initiated if the primary peristaltic wave fails to move all the food into the stomach –Result from distention of the esophagus itself by the retained food –Initiated or modified by intrinsic nerve plexus and strong enough to push the bolus to the stomach 78
  79. 79. Secretion of the esophagus • Mucous glands that surround the esophagus secrete mucus • Lubricates passage of food (decreases esophageal damage by any sharp edges on food) • Protect the esophagus from damage by acid and enzymes in gastric juice (if gastric reflux occur) • Distension of the esophagus also reflexly increases salivary secretion 79
  80. 80. Gastric Motility • The function of the stomach is threefold: 1. Storage of large quantities of food 2. Mixing • Involves food blending with digestive juices to form chyme (a thick liquid mixture) • The stomach can progressively extend to accommodate 0.8 to 1.5 L 3. Emptying (stomach  SI) • At a rate appropriate for optimal digestion and absorption 80
  81. 81. • Anatomically stomach is divided in to 3 major parts a. Fundus: lies above the esophageal opening b. Body: middle or main part c. Antrum: lower part of the stomach; has heavier musculature • Physiologically, 1. Orad region: • Fundus + the proximal body (first two thirds of the body) • Responsible for accommodating an ingested meal (gastric reservoir ) 2. Caudad region: • Antrum + the distal body • Responsible for the contractions that lead to mixing and propulsion into the duodenum 81
  82. 82. 82 Physiological anatomy of the stomach A B
  83. 83. • There are two types of gastric motility 1. Mixing movements • Antrum contracts against closed pylorus • Chyme is propelled forward and tossed back into the antrum with each peristaltic contraction, a process called retropulsion • Retropulsion (pyloric pump) serves to effectively mix food and gastric secretions, and to grind gastric contents into chyme 2. Peristaltic movement • Antrum contracts against open pylorus • Push chyme from stomach into the SI 83
  84. 84. Gastric Secretion 1. Mucus-secreting cells: • Line the entire surface of the stomach • Secrete large quantities of a very thick & sticky mucus that coat a gastric mucosa – Provide a shell of protection for the stomach wall – Contribute to lubrication of food transport – It is alkaline – neutralize acid 2. Oxyntic glands (Gastric glands) • In the body and fundus of the stomach • Secrete HCl, pepsinogen, intrinsic factor, and mucus 84
  85. 85. • Composed of three types of cells: i. Mucous cells ii. Chief cells iii. Parietal cells 3. Pyloric glands : • Secrete: –Mucus (mainly) for protection of the pyloric mucosa from the stomach acid –Gastrin from G cells –Somatostatin from D cells 85
  86. 86. 86
  87. 87. 87 • Parietal cell: contains large branching intracellular canaliculi • The hydrochloric acid is formed at the villus-like projections inside these canaliculi
  88. 88. Basic mechanism of hydrochloric acid secretion 88
  89. 89. 1. H2O dissociated into H+ and OH− in parietal cell cytoplasm 2. The H+ is then actively secreted into the canaliculus in exchange for K+ (catalyzed by H+-K+ ATPase) 3. OH- combine with CO2 (formed during metabolism in the cell or while entering the cell from the blood) to form HCO3 - 4. The HCO3- is then transported across the basolateral membrane into the ECF in exchange for Cl- 5. Cl- are secreted through chloride channels into the canaliculus and combine with H+ to from HCl 6. Water passes into the canaliculus by osmosis because of extra ions secreted into the canaliculus 89
  90. 90. Stimulation of gastric acid secretion • Parietal cells bear receptors for 3 stimulators of acid secretion – Acetylcholine (M3 muscarinic receptor) – Gastrin (gastrin-cholecystokinin type 2 (CCK2)receptor) – Histamine (H2 type receptors) • ACh and gastrin both operate through IP3/Ca2+ second messenger pathways • Histamine activates a cAMP second messenger pathway to bring about its effects • These messengers all bring about increased secretion of HCl by promoting the insertion of additional H+–K+ ATPases into the parietal cells’ plasma membrane 90
  91. 91. 91 Receptors and signal transduction pathways in the parietal cell PLC: Phospholipase C PIP2: Phosphatidylinositol 4,5- bisphosphate IP3: Inositol 1,4,5-trisphosphate DAGP: Diacylglycerol PKC: Protein kinase C AC: Adenylyl cyclase
  92. 92. • ACh and gastrin each bind to specific receptors (M3 and CCK2, respectively) that are coupled to the G protein Gq • The result is activation of PLC, which ultimately leads to the activation of PKC and the release of Ca2+ • Ca2+ acts via calmodulin-dependent protein kinase(CaM kinase) • The histamine binds to an H2 receptor, coupled through Gs to adenylyl cyclase (AC) • The result is production of cAMP and activation of PKA • Two inhibitors of acid secretion also act directly on the parietal cell • Somatostatin and prostaglandins bind to separate receptors that are linked to Gi • These agents thus oppose the actions of histamine 92
  93. 93. • To keep from destroying itself, the stomach has mucosal barrier 93
  94. 94. • When pepsinogen is first secreted, it has no digestive activity However, as soon as it comes in contact with HCl, it is activated to form active pepsin • Pepsin functions as an active proteolytic enzyme in a highly acid medium (optimum pH 1.8 to 3.5), but above a pH of about 5 it has almost no proteolytic activity and becomes completely inactivated in a short time. • HCl is as necessary as pepsin for protein digestion in the stomach 94
  95. 95. 95 • Once formed, pepsin acts on other pepsinogen molecules to produce more pepsin, a mechanism called an autocatalytic process (autocatalytic means “self-activating”)
  96. 96. Function of intrinsic factor • Is secreted by the parietal cells along with the secretion of HCl • Essential for absorption of vitamin B12 in the ileum • Binding of intrinsic factor with vitamin B12 triggers receptor mediated endocytosis of this complex in the terminal ileum • Destruction of parietal cells, e.g. chronic gastritis, gastrectomy… ↓ a. Pernicious anemia due to the failure of RBCs maturation (absence of vitamin B12 stimulation of the bone marrow) b. Achlorhydria (lack of stomach acid secretion) develop 96
  97. 97. Phases of Gastric Secretion • Gastric secretion is occur in three phases: - a cephalic phase, a gastric phase, and an intestinal phase. Cephalic Phase • Occurs in feedforward fashion (before food reaches the stomach), especially while it is being eaten • Stimuli: sight, smell, thought, taste of food, chewing, or swallowing • The greater the appetite, the more intense is the stimulation 97
  98. 98. • This phase of secretion normally accounts for about 20% of the gastric secretion associated with eating a meal. 98
  99. 99. Gastric Phase 99 The gastric phase of secretion accounts for about 70% of the total gastric secretion associated with eating a meal • Begins when food reaches the stomach
  100. 100. Intestinal Phase • The intestinal phase has two components that influence gastric secretion: • Excitatory • Inhibitory Excitatory component • Occurs in response to the presence of products of protein digestion in the duodenum • Intestinal gastrin travels in the blood to the stomach, where it enhances the secretion of HCl and pepsinogen • It accounts only for approximately 10% of the acid secretory response to a meal 100
  101. 101. Inhibitory component • Gastric secretion gradually decreases as food empties from the stomach into the intestine by three ways. • Inhibitory component has a very strong influence on gastric secretion. 101
  102. 102. Pancreas • An elongated gland that lies behind and below the stomach, above the first loop of the duodenum • This mixed gland contains both exocrine and endocrine tissue Major exocrine function of the pancreas: • Pancreatic exocrine secretion consists of an aqueous or bicarbonate component and an enzymatic component a. Aqueous component • Consists primarily of water and sodium bicarbonate – Neutralizes duodenal contents (prevents injury to the duodenal mucosa) – Bringing the contents within the pH range necessary for optimal enzymatic digestion of nutrients b. Enzymatic(protein) component • Low volume secretion • Contains enzymes for the digestion of all major foodstuffs 102
  103. 103. Location of secretion • Exocrine tissue is organized into acini and ducts very similar to that of the salivary glands 1. Digestive enzymes and electrolytes are secreted by acinar cells of the pancreas 2. HCO3 - and water are secreted by the duct cells lining the pancreatic ducts 103
  104. 104. 104
  105. 105. • The inactive proenzymes secreted by cells of the pancreas that break protein, fat, and CHO include: • Trypsinogen (protein digestion) • Chymotrypsinogen (protein digestion) • Procarboxypolypeptidase (protein digestion) • Pancreatic lipase (fat digestion) • Alpha amylase (CHO digestion) 105
  106. 106. Pancreatic proteases • Are secreted as inactive zymogens (to prevent from digesting the proteins of the cells in which they are formed) • They include trypsinogen, chymotrypsinogen, and procarboxypolypeptidase • All the inactive enzymes flow through the pancreatic duct into the duodenum • At the beginning, an enzyme called enterokinase [=enteropeptidase] (located on the wall of the duodenum) changes trypsinogen to trypsin • Trypsin then activates the others as follows (look steps b and c): 106
  107. 107. a. Trypsinogen ----enterokinase  Trypsin b. Chymotrypsinogen---trypsin  Chymotrypsin b. Procarboxypolypaptidase ---trypsin Carboxypolypeptidase • The most important of the pancreatic enzymes for digesting proteins are trypsin, chymotrypsin, and carboxypolypeptidase • By far the most abundant of these is trypsin • Trypsin and chymotrypsin split whole and partially digested proteins into peptides of various sizes but do not cause release of individual amino acids • However, carboxypolypeptidase does split some peptides into individual amino acids 107
  108. 108. Pancreatic amylase: • Enzyme for carbohydrates digestion • Hydrolyzes starches, glycogen, and most other carbohydrates (except cellulose) to form mostly disaccharides and a few trisaccharide Fat digestion: • Pancreatic lipase: hydrolyzes neutral fat into fatty acids and monoglycerides • Cholesterol esterase: causes hydrolysis of cholesterol esters • Phospholipase : splits fatty acids from phospholipids 108
  109. 109. 109 Regulation of Pancreatic Secretion
  110. 110. Phases of pancreatic Secretion • Pancreatic secretion occurs in three phases, the same as for gastric secretion: the cephalic phase, the gastric phase, and the intestinal phase Cephalic and Gastric Phases • The same nervous signals from the brain that cause secretion in the stomach also cause acetylcholine release by the vagus nerve endings in the pancreas • This causes moderate amounts of enzymes to be secreted in the pancreatic acini 110
  111. 111. • Cephalic phase: approximately 20% of pancreatic secretory response to a meal • Gastric phase: 5 to 10% of pancreatic secretory response to a meal Intestinal phase • Most pancreatic secretion takes place • Secretin(intestinal hormone) stimulates release of a large volume of pancreatic juice with a high [HCO3-] • Secretin is released in response to acidic chyme in the duodenum (maximal release at pH ~ 3.0) 111
  112. 112. • Cholecystokinin(intestinal hormone) stimulates the release of digestive enzymes from the pancreas • It is released in response to the presence of the products of protein and lipid digestion 112 Receptors of the pancreatic acinar cell and the regulation of secretion
  113. 113. Secretion of bile by the liver • One of the many functions of the liver is to secrete bile, normally between 600 and 1000 ml/day • Bile serves two important functions 1. Bile plays an important role in fat digestion and absorption • Because bile acids in the bile perform two functions: a. Emulsification of fat in the duodenum =changing greater fat globules into smaller fat-droplets b. Aid in absorption of the digested fat end products 2. Means for excretion of several important waste products from the blood • These include especially bilirubin (an end product of hemoglobin destruction), and excesses of cholesterol 113
  114. 114. • After bile is produced in the liver it will be stored in the gall bladder Liver Hepatocytes ↓ Bile canaliculi ↓ Bile ductules ↓ Bile ducts ↓ Hepatic ducts ↓ Common hepatic duct ↓ ↓ Cystic duct Common bile duct ↓ ↓ GB Hepatopancreatic ampulla ↓ Sphincter of Oddi Duodenum 114
  115. 115. 115
  116. 116. 116 Liver secretion and gallbladder emptying
  117. 117. Constituents of bile • Bile contains the following constituents: 1. Bile salts (bile acids), ~11% 2. Bile pigment (bilirubin), ~1% 3. Others organic constituents like: ( ~3%) Cholesterol, Lecithin, protein etc 4. Electrolytes (Na+, K+, Ca2+, Cl-, and greater HCO3- than plasma) ~1%. 5. H2O (~ 84%) also takes the higher share of bile 117
  118. 118. Bile pigments (Bilirubin): • Does not play a role in digestion at all but instead is a waste product excreted in the bile • Is the primary bile pigment derived from the breakdown of worn-out red blood cells • The end product from degradation of the heme (iron containing) part of the hemoglobin • In the intestine, bacterial actions change bilirubin into urobilinogen. They are further oxidized and excreted in feces or urine as stercobilin and urobilin. • Gives faces its brown color • Excess level of bilirubin in the blood (>18 mg/L) causes jaundice, that is, yellow coloration of the sclera's, skin etc occurs 118
  119. 119. • Gallbladder (GB) is a storage sac for bile • GB is stimulated by CCK to contract and release bile • Bile emulsifies fat in the duodenum • Cholesterol and other substances precipitate in the GB and this effect favors gall-stone formation 119
  120. 120. The Small intestine (SI) • SI is specialized for completion of digestion and absorption of nutrients a. Duodenum: ~25 cm • It receives chyme from the stomach and digestive secretions from the pancreas and liver and neutralize its acids b. Jejunum : ~1.5m • Bulk of chemical digestion and nutrient absorption occurs c. Ileum : ~1.7m • Mainly absorptive • Ileocecal valve: prevent backflow of fecal contents from the colon into the SI • Ileocecal sphincter: slows emptying of ileal contents into the cecum 120
  121. 121. 121
  122. 122. • The mucosa of SI is well adapted for digestion and absorption with certain anatomical modifications: • Plicae circulares (circular folds) • Villi • Microvilli Plicae circulares (circular folds) • Form internal rings around the circumference of the SI • Are formed from inward foldings of the mucosal and submucosal layers of the intestinal wall • Well developed in the duodenum and jejunum • Increase the absorptive surface area of the mucosa about threefold 122
  123. 123. 123
  124. 124. Villi • Millions of smaller projections of mucosa that cover plica • Two types of epithelial cells cover the villi: –Goblet cells (produce mucus) –Absorptive cells (most abundant) • Taken together, the villi increase the absorptive surface area another 10-fold 124
  125. 125. 125
  126. 126. Microvilli • Microscopic projections found on the luminal surface of the absorptive cells - form the brush border ( contains enterokinase, disaccharidases (maltase, sucrase-isomaltase, and lactase), aminopeptidases) • Increase the surface area for absorption another 20-fold • Together, these three anatomical adaptations of the intestinal mucosa — plicae circulares, villi, and microvilli — increase the surface area as much as 600-fold 126
  127. 127. Motility of the small intestine • Segmentation and peristalsis take place in the small intestine • Segmentation: ring-like contractions along the length of the SI • Mixes chyme with digestive juices and exposes it to the intestinal mucosa for absorption • Causes only a small degree of forward movement of the chyme along the small intestine 127
  128. 128. • Peristalsis: • The wave-like form of muscle contraction • Primarily moves chyme along the intestine and causes only a small amount of mixing • Weak and slow in the small intestine so that time is sufficient for complete digestion and absorption of the chyme 128
  129. 129. • The motility of the SI may be enhanced during a meal by: • Distension of the small intestine –Distension of the duodenum elicit segmentation contractions in this segment • Gastrin - causes segmentation of the empty ileum • Extrinsic nerve stimulation –Parasympathetic stimulation, by way of the vagus nerve, further enhances segmentation –Sympathetic stimulation inhibits this activity 129
  130. 130. Digestion and absorption in the small intestine Carbohydrates • Starch is initially acted upon by amylase • Salivary amylase breaks down starch molecules in the mouth and stomach • Pancreatic amylase carries on this activity in the small intestine • Amylase fragments polysaccharides into disaccharides (maltose, composed of two glucose molecules) • The disaccharide molecules, primarily maltose, are presented to the brush border of the absorptive cells • As the disaccharides (maltose, sucrose & lactose) are absorbed, disaccharidases (maltase, sucrase-isomaltase, and lactase) split these nutrient molecules into monosaccharides (glucose, fructose, and galactose) 130
  131. 131. • The α-limit dextrins is only broken down by sucrase- isomaltase • Glucose and galactose enter the absorptive cells by way of secondary active transport • Fructose enters the absorptive cells by way of facilitated diffusion • All monosaccharide molecules exit the absorptive cells by way of facilitated diffusion and enter the blood capillaries 131
  132. 132. 132
  133. 133. 133
  134. 134. Lactose intolerance • A deficiency of lactase, the disaccharidase specific for the digestion of lactose, or milk sugar • Most children younger than 4 years of age have adequate lactase, but this may be gradually lost • When lactose-rich milk or dairy products are consumed by a person with lactase deficiency 1. Accumulation of undigested lactose creates an osmotic gradient that draws H2O into the intestinal lumen 2. Bacteria attack the lactose as an energy source and produce large quantities of CO2 and methane gas in the process – Distension of the intestine by both fluid and gas produces pain (cramping) and diarrhea • Infants with lactose intolerance may also suffer from malnutrition 134
  135. 135. Proteins • Protein digestion begins in the stomach by the action of the gastric enzyme pepsin –This enzyme fragments large protein molecules into smaller peptide chains • Digestion is continued in the small intestine by the pancreatic enzymes trypsin, chymotrypsin, and carboxypeptidase, which hydrolyze the peptide chains into amino acids, dipeptides, and tripeptides • The aminopeptidases, which hydrolyze most of the small peptide fragments into their amino acid components, thereby completing protein digestion • Amino acids enter the absorptive cells by way of secondary active transport 135
  136. 136. • As the nutrient molecules are absorbed, aminopeptidases split dipeptides and tripeptides into their constituent amino acids • The amino acid molecules then exit the absorptive cells by way of facilitated diffusion and enter the blood capillaries 136
  137. 137. 137 (a) Protein digestion
  138. 138. 138 (b) Protein absorption H/oligopeptide cotransporter
  139. 139. Lipids • Fat digestion begins in the mouth and stomach by the action of the salivary enzyme lingual lipase • However, the role of this enzyme is minor • Lipids are digested primarily in the small intestine 1. Bile salts cause emulsification, which is the dispersal of large fat droplets into a suspension of smaller droplets 2. Pancreatic lipase acts on the lipid droplets to hydrolyze the triglyceride molecules into monoglycerides and free fatty acids- these are water insoluble 3. Micelles formed by the amphipathic bile salts • Monoglycerides and free fatty acids are carried in this interior region of the micelle 139
  140. 140. • Upon reaching the brush border of the absorptive cells, they leave the micelles and enter the cells by simple diffusion –This process takes place primarily in the jejunum and proximal ileum • The bile salts are absorbed in the distal ileum by way of secondary active transport • Within the absorptive cells: • Monoglycerides + FFAS triglycerides (in endoplasmic reticulum) • Triglycerides packaged in a lipoprotein coat (in Golgi apparatus) • These protein-coated lipid globules, referred to as chylomicrons (water soluble) 140
  141. 141. • Chylomicrons leave the absorptive cell by way of exocytosis • They enter the lacteals, which are part of the lymphatic system B/c they unable to cross the basement membrane of the blood capillaries 141
  142. 142. 142 (a) Fat digestion
  143. 143. 143
  144. 144. Water and electrolytes • The absorption of nutrient molecules primarily takes place in the duodenum and jejunum, creates an osmotic gradient for the passive absorption of water • Sodium absorption: • Passive diffusion (through “leaky” tight junctions) • Na+– Cl- cotransport • Na+–glucose cotransport • Na+–amino acid cotransport • Na+- K+ - 2 Cl–cotransport 144
  145. 145. Malabsorption • Impairment of absorption • One of the most common causes is gluten enteropathy, also known as celiac disease • Is a complex immunological disorder • The person’s SI is abnormally sensitive to gluten, a protein constituent of wheat, barley • Exposure to gluten erroneously activates a T-cell response that damages the intestinal villi • Decreases the surface area available for absorption • The condition is treated by not eating gluten 145
  146. 146. Large intestine • From the ileocecal valve (juncture between the ileum and the large intestine) to the anus • Has a larger diameter than the small intestine • Mucosa: • Composed of absorptive cells and mucus-secreting goblet cells • Does not form villi • Consists of the following structures: - Cecum - Colon - Appendix - Rectum 146
  147. 147. • Cecum: • The most proximal portion of the large intestine • Receives chyme from the ileum of the SI through the ileocecal valve • Appendix: • A small projection at the bottom of the cecum • Is a lymphoid tissue • It contains lymphocytes and assists in defense against bacteria that enter the body through the digestive system 147
  148. 148. Appendicitis • Hardened fecal material lodged in the appendix  obstruct normal circulation and mucus secretion • The inflamed appendix often becomes swollen and filled with pus, and the tissue may die as a result of local circulatory interference • If not surgically removed, the diseased appendix may rupture, spewing its infectious contents into the abdominal cavity 148
  149. 149. • Colon: • Largest portion of the large intestine • Consists of four regions: 1. Ascending colon (travels upward toward the diaphragm on the right side of the abdomen) 2. Transverse colon (crosses the abdomen under the diaphragm) 3. Descending colon (travels downward through the abdomen on the left side) 4. Sigmoid colon (S-shaped region found in the lower abdomen) 149
  150. 150. 150
  151. 151. • Rectum: • Last portion of the digestive tract • Leads to the external surface of the body through the anus • Internal and external anal sphincters • The large intestine typically receives 500 to 1500 ml of chyme per day from the small intestine • Chyme consists of indigestible food residues (e.g., cellulose), unabsorbed biliary components, and any remaining fluid (b/c most digestion and absorption in SI) • Therefore, the two major functions of the large intestine are: • Drying • Storage 151
  152. 152. Secretion of the large intestine • Large intestine does not secrete any digestive enzymes • Colonic secretion consists of an alkaline (NaHCO3) mucus solution • The mucus has the following functions 1. It helps to lubricate feces 2. It neutralizes against any acids present 3. It protects against irritation 4. It provides a binding medium for fecal matter 152
  153. 153. Secretion of water & electrolytes • Stimulus: irritation (e.g. bacterial infection)  the mucosa secretes large amount of water & electrolytes in addition to the alkaline mucus • This dilute the irritating factors and causes rapid movement of the feces toward the anus 153
  154. 154. Absorption in the colon • Most of absorption in the colon occurs in the proximal half of the colon (absorptive colon) • The distal colon function for storage (storage colon) 1. Water absorption, about 0.5- 1.5L/day is absorbed by osmosis 2. Na+ absorption, about 60 mmol/day is actively absorbed in the presence of Na+-K+ ATPase at the basolateral membrane to blood 3. K+, Cl- and HCO3 - – K+ may be absorbed or secreted depending on the remaining concentration in the lumen – Cl- is absorbed in exchange for HCO3 - which is secreted 154
  155. 155. 4. Folic acid and some AA and short chain FA resulting from bacterial fermentation of CHO are absorbed • Folic acid is required for the formation of nucleic acids, the maturation of RBCs, and growth 5. Certain drugs as steroids and aspirin may be absorbed • The colon extracts more H2O and salt from the contents. What remains to be eliminated is known as feces • The primary function of the large intestine is to store feces before defecation 155
  156. 156. Bacterial Action in the Colon • Numerous bacteria, especially colon bacilli, are present even normally in the absorbing colon • They are capable of digesting small amounts of cellulose –Can provide a few calories of extra nutrition for the body • Other substances formed as a result of bacterial activity –Vitamin K, vitamin B12, thiamine, riboflavin –Gases that contribute to flatus – CO2, hydrogen gas, and methane –Vitamin K: Important because its amount in the daily ingested foods is insufficient to maintain adequate blood coagulation 156
  157. 157. Motility of the large intestine • Normally sluggish • Two types a. Mixing movements (Haustrations) • Ring-like contractions (about 2.5 cm) of the circular muscle divide the colon into pockets called haustra • Haustrations = bulging of the large intestine into baglike sacs as a result of circular and longitudinal muscle contraction • Serve primarily to move the contents slowly back and forth, exposing them to the absorptive surface 157
  158. 158. b. Propulsive movements (Mass movements) • Propel chyme from the cecum to the sigmoid colon • When a mass of feces is forced into the rectum, there is a desire to defecate Reflexes Affecting Mass Movements • Gastrocolic reflex – stimulatory –Distention of the stomach • Duodenocolic reflex - stimulatory –Distention of the small intestine • Both push the colonic contents into the rectum, triggering the defecation reflex 158
  159. 159. • Both reflexes transmitted by autonomic nervous system • Defecation • When mass movement forces feces into the rectum –Immediate desire to defecate • Reflex contraction of rectum • Relaxation of anal sphincter • Approx. 80 to 200 mL of fecal matter expelled daily 159
  160. 160. 160
  161. 161. Defecation Reflex 1. Distension of the rectum 2. Stimulation of the stretch receptors in the rectum 3. A. Short reflex: stimulation of myenteric plexus in sigmoid colon and rectum B. Long reflex: stimulation of parasympathetic motor neurons in sacral spinal cord C. Stimulation of somatic motor neurons 4. Increased local peristalsis, relaxation of internal anal sphincter and contraction of external anal sphincter 161
  162. 162. 162
  163. 163. • From the spinal cord, defecation signals also have the following effects • Taking deep breath • Closure of glottis • Contraction of abdominal wall muscles • Relaxation and movement of pelvis floor downward 163
  164. 164. Composition of Feces • Three-fourths water • One-fourth solid matter –30% dead bacteria –10-20% fat –10-20% inorganic matter –2-3% protein –30% undigested roughage (e.g. bile pigment, sloughed epithelial cells) 164
  165. 165. Reading assign. • Obesity and metabolic rate • Constipation • Diarrhea • Vomiting 165
  166. 166. THE END!! 166