Digestive System Dr Raghuveer Choudhary

Food is vital to life because: INTRODUCTION TO DIGESTION provides energy provides building blocks for growth and maintenance Why do we need a digestive system? food comes to us as complex molecules complex molecules are too large to be absorbed into the blood What are the complex molecules and what do we do with them? carbohydrates  monosaccharides (simple sugars) proteins  amino acids fats  fatty acids, glycerols, glycerides

MAJOR FOOD COMPONENTS Carbohydrates Proteins Fats Water Ions/Minerals/Fibers/Enzymes

Digestive System DIGESTIVE TRACT – tube ~ 9 m long Gastrointestinal tract or Alimentary canal DIGESTIVE ORGANS - Oral cavity, pharynx, esophagus, stomach, small intestine, large intestine, anal canal ACCESSORY ORGANS - teeth, tongue, salivary glands, liver, gallbladder, pancreas

Gastrointestinal tract ORGANIZATION AKA = alimentary canal Accessory organs Teeth Tongue Salivary glands Liver gallbladder pancreas mouth pharynx esophagus stomach small intestine large intestine mouth (oral cavity) pharynx esophagus stomach anus large intestine small intestine rectum appendix liver pancreas salivary glands gallbladder

Digestive functions & processes FOUR FUNCTIONS 1. Ingestion – selective intake of nutrients 2. Digestion – breakdown of molecules; mechanical & chemical 3. Absorption – uptake of nutrients by digestive tract, blood & lymph 4. Defecation – elimination of undigested residue

Digestive Processes 1. Motility – muscle contractions – Break up food – mastication, churning of stomach – Propel thru canal - peristalsis – Mix with digestive enzymes - segmentation 2. Secretion – enzymes, hormones to carry out or regulate digestion 3. Membrane transport – absorbing nutrients to blood and lymph

Stages of Digestion MECHANICAL DIGESTION Physical breakdown Teeth – cutting and grinding - mastication –(chewing) Churning – stomach & small intestine Action of digestive enzymes – food to small particles

CHEMICAL DIGESTION Macromolecules to monomers Polysaccharides to monosaccharides Proteins to amino acids Fats to glycerol and fatty acids Nucleic acids to nucleotides Done by digestive enzymes produced by: Salivary glands, stomach, pancreas, small intestine Absorbed – no digestion – vitamins, free amino acids, minerals, cholesterol, water

Forms of Nutrients In FoodAbsorbable by Intestine ProteinAmino acids Carbohydrates Monosaccharides (glucose) FatFatty acids, glycerol DNA, RNABases + monosaccharides Vit B12 B12+intrinsic factor Other vitaminsOriginal form Cholesterol Original form ElectrolytesOriginal form WaterOriginal form

Major Physiologic Processes of the Gut Motility Secretion Digestion Absorption Elimination

Secretion Delivery of enzymes, mucus, ions and the like into the lumen, and hormones into blood. Absorption Transport of water, ions and nutrients from the lumen, across the epithelium and into blood. Motility Contractions of smooth muscle in the wall of the tube that crush, mix and propel its contents.

 Motility : – Movement of of food through the GI tract. Ingestion: –Taking food into the mouth. Mastication: –Chewing the food and mixing it with saliva. Deglutition: –Swallowing the food. Peristalsis: –Rhythmic wave-like contractions that move food through GI tract. Functions of the GI Tract

Secretion: – Includes both exocrine and endocrine secretions. Exocrine: –HCl, H 2 0, HC0 3 -, bile, lipase, pepsin, amylase, trypsin, elastase, and histamine are secreted into the lumen of the GI tract. Endocrine: –Stomach and small intestine secrete hormones to help regulate the GI system. Gastrin, secretin, CCK, GIP, GLP-1, guanylin, VIP, and somatostatin. Functions of the GI Tract (continued)

Digestion: – Breakdown of food particles into subunits (chemical structure change). Absorption: – Process of the passage of digestion (chemical subunits) into the blood or lymph. Storage and elimination: – Temporary storage and elimination of indigestible food. Functions of the GI Tract (continued)

Digestive System (GI) GI tract divided into: – Alimentary canal. – Accessory digestive organs. GI tract is 30 ft long and extends from mouth to anus. Insert fig. 18.2

Mouth Foodstuffs are broken down mechanically by chewing and saliva is added as a lubricant. Esophagus A simple conduit between the mouth and stomach. Stomach Enzymatic digestion of proteins initiated and foodstuffs reduced to liquid form. Liver The center of metabolic activity in the body - provide bile salts to the small intestine, critical for digestion and absorption of fats. Pancreas Provides a potent mixture of digestive enzymes to the small intestine which are critical for digestion of fats, carbohydrates and protein. Small Intestine This is where the final stages of chemical enzymatic digestion occur and where almost all nutrients are absorbed. Large Intestine Water is absorbed, bacterial fermentation takes place and feces are formed.

Layers of GI Tract Composed of 4 tunics: – Mucosa. – Submucosa. Muscularis. Serosa.

Lines the lumen of GI tract. – Consists of simple columnar epithelium. Lamina propria: – Thin layer of connective tissue containing lymph nodules. Muscularis mucosae: – Thin layer of smooth muscle responsible for the folds. Folds increase surface area for absorption. Goblet cells: – Secrete mucus. Mucosa

Thick, highly vascular layer of connective tissue. Absorbed molecules enter the blood and lymphatic vessels. Submucosal plexus (Meissner’s plexus): – Provide autonomic nerve supply to the muscularis mucosae. Submucosa

Responsible for segmental contractions and peristaltic movement through the GI tract. – Inner circular layer of smooth muscle. – Outer longitudinal layer of smooth muscle. Contractions of these layers move food through the tract; pulverize and mix the food. Myenteric plexus located between the 2 muscle layers. – Major nerve supply to GI tract. Fibers and ganglia from both sympathetic and parasympathetic nervous systems. Muscularis

Serosa Binding and protective outer layer. Consists of areolar connective tissue covered with simple squamous epithelium.

Extrinsic innervation: – Parasympathetic nervous system: Vagus and spinal nerves: –Stimulate motility and GI secretions. – Sympathetic nervous system: Postganglionic sympathetic fibers that pass through submucosal and myenteric plexuses and innervate GI tract: –Reduce peristalsis and secretory activity. Regulation of the GI Tract

Enteric nervous system: – Sites where parasympathetic fibers synapse with postganglionic neurons that innervate smooth muscle. Submucosal and myenteric plexuses: – Local regulation of the GI tract. Paracrine secretion: – Molecules acting locally. Hormonal secretion: – Secreted by the mucosa. Regulation of the GI Tract (continued)

Parasympathetic n.s. Sympathetic n.s. Vagal nuclei CNS Preganglionic Fibres Preganglionic fibres Sacral spinal Sympathetic cord ganglia Postganglionic fibres Enteric nervous System Myenteric Submucosal plexus plexus Smooth Secretory EndocrineBlood Muscle cellscells vessels

Effect of different stimuli on muscle contraction StimulusEffect on muscle more depolarised smooth 1. Stretch of GI tract wall muscle,  more excitable 2. Acetylcholine release Leads toaction potential 3. Parasympathetic stimulation generation and smooth muscle contraction. 4. Noradrenaline release more hyperpolarised smooth 5. Sympathetic stimulation muscle,  Less excitable and fewer contractions.

Nervous regulation Enteric nervous system (ENS) Extrinsic nervous system

Enteric nervous system (ENS) myenteric plexus (Auerbach’s plexus) submucosal plexus (Meissner’s plexus)

myenteric plexus (Auerbach’s plexus) Excitatory effects Inhibitory effects Increase the intensity of contraction Increase the rate of the rhythm of contraction Increase the velocity of conduction of contraction along the tract Sphincter Pyloric sphincter

submucosal plexus (Meissner’s plexus) secretion absorption contraction

Extrinsic nervous system parasympathetic nervous sympathetic nervous

parasympathetic nervous Enteric nervous system increase

sympathetic nervous Activity of gastrointestinal tract Decrease norepinephrine ENS Smooth muscle

Afferent sensory nerve fibers from the gut Irritation of the gut mucosa Distension of the gut Chemical substances ENS Sympathetic ganglia Spinal cord or brain stem

Gastrointestinal reflexes ENS Sympathetic ganglia Spinal cord or brain stem stimulation Activity of gastrointestinal tract