GI Tract Overview Aim of the lectures

GI Tract Overview Aim of the lectures
To provide a basic understanding of the structure and function of the gastrointestinal tract and its normal function

Functions of the GI tract
1. Ingestion 2. Digestion 3. Absorption CONCEPTS to ENGAGE Motility - mechanical breakdown, propulsion Secretion - enzymes, water & ions Control - of motility and secretion by nervous system and hormones Mastication, salivary secretion and swallowing Nutrient Transport mechanisms Diffusion, facilitated and Active Transport ©Cardiff University /SONMS/GI Overview/CSAN

©Cardiff University /SONMS/GI Overview/CSAN
Objectives from Fox Describe the functions of the digestive system, including accessory organs and list its structures and regions. Describe the layers of the gastrointestinal tract and the function(s) of each. Describe the structure of the gastric mucosa, list the secretions of the mucosa and their functions, and identify the cells that produce each of these secretions. Explain the roles of HCl and pepsin in digestion and explain why the stomach does not normally digest itself Describe the structure and function of the villi, microvilli, and crypts of Lieberkuhn in the small intestine. Describe the location and functions of the brush border enzymes of the intestine. ©Cardiff University /SONMS/GI Overview/CSAN

Digestive System GI tract divided into: Alimentary canal. Accessory digestive organs. GI tract is 30 ft long and extends from mouth to anus. ©Cardiff University /SONMS/GI Overview/CSAN

Functions of the GI Tract
Motility: Movement 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,moving food through GI tract. ©Cardiff University /SONMS/GI Overview/CSAN

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. ©Cardiff University /SONMS/GI Overview/CSAN

Secretion: Includes both exocrine and endocrine secretions. Exocrine: HCl (Hydrochloric Acid), H20 (water), HC03- (Bicarbonate), Bile-made by the liver, stored in gall bladder & secreted into duodenum, lipase (breaks down fat), pepsin (breaks down protein),, amylase (breaks down carbohydrates), trypsin (breaks down protein),and histamine (stimulates production of HCl), etc. are secreted into the lumen of the GI tract.

Endocrine functions: Stomach and small intestine secrete hormones to help regulate the GI system. Examples Gastrin, Secretin, Cholecystokinin (CCK), Gastric Inhibitory Peptide (GIP), Somatostatin, etc. ©Cardiff University /SONMS/GI Overview/CSAN

Salivary Glands FUNCTIONS
Lubrication Thins and dilutes food for swallowing Anti acid function, Buffers and neutralises acids, particularly important when there are acids of bacterial origin present Bacteriostatic Bacteriolytic Acts on endogenous bacteria Digestive: Carbohydrates (? fat) ©Cardiff University /SONMS/GI Overview/CSAN

Oesophagus A muscular tube that carries the food from the back of the throat to the stomach. It must contract in a very co-ordinated fashion so we don't regurgitate our food or feel that our meals are sticking as we swallow. It is inflammation within the oesophagus that gives the sensation of heartburn. ©Cardiff University /SONMS/GI Overview/CSAN

© CU/SONMS/nutrition/MJohn
Food chunks up here and amylase continues to work until food drops into the stomach © CU/SONMS/nutrition/MJohn

Oesophagus Concept: Deglutition (swallowing) Oesophagus Connects pharynx to the stomach. Upper third contain skeletal muscle. Middle third contains a mixture of skeletal and smooth muscle. Terminal portion contains only smooth muscle. ©Cardiff University /SONMS/GI Overview/CSAN

Oesophagus Peristalsis: Local reflexes in response to distention of the wall of the Oesophagus by bolus. (food ball) Wave-like contractions: Circular smooth muscle contract behind, relaxes in front of the bolus. Followed by longitudinal contraction (shortening) of smooth muscle. ©Cardiff University /SONMS/GI Overview/CSAN

Stomach Most distensible part of GI tract. Empties into the duodenum. Functions of the stomach: Store food. Initiate digestion of proteins. Kills bacteria. Moves food (chyme) into intestine. ©Cardiff University /SONMS/GI Overview/CSAN

Stomach Contractions churn chyme. Mix chyme with gastric secretions. Push food into intestine. ©Cardiff University /SONMS/GI Overview/CSAN

Stomach Digestion and Absorption
Proteins partially digested by pepsin. Hydrochloric acid changes the structure of proteins to kick start the digestive rocess Carbohydrate digestion by salivary amylase is soon inactivated by acidity. Drugs: Alcohol and aspirin are the only commonly ingested substances absorbed. ©Cardiff University /SONMS/GI Overview/CSAN

Stomach Rugae Gastric mucosa has gastric pits in the folds. Cells that line the folds deeper in the mucosa are gastric glands. ©Cardiff University /SONMS/GI Overview/CSAN

Gastric Glands Secretion Function Mucous Lubricates, Protects HCL Kills bacteria; Activates Pepsinogen Intrinsic Factor Promotes absorption of vitamin B12 Pepsinogen Inactive precursor of Pepsin Histamine Stimulates parietal cells >HCL secretion Serotonin Inhibits HCL secretion Gastrin Increases secretion of HCL , pepsin; weakly stimulates secretion of pancreatic enzymes and gallbladder contraction. Somatostatin Inhibits release of Gastrin Secrete Gastric Juice: Goblet cells: Mucous. Parietal cells: HCl, Intrinsic Factor. Chief cells: Pepsinogen. Enterochramaffin-like cells(ECL): Histamine, Serotonin. G cells: Gastrin. D cells: Somatostatin. ©Cardiff University /SONMS/GI Overview/CSAN

Regulation of Gastric Function
Gastric motility and secretion are automatic. Waves of contraction are initiated spontaneously by pacesetter cells. Extrinsic control of gastric function is divided into 3 phases: Cephalic phase. Gastric phase. Intestinal phase. ©Cardiff University /SONMS/GI Overview/CSAN

Protective Mechanisms of Stomach
Alkaline mucus contains HC03- (bicarbonate which buffers HCl and protects mucosa). Tight junctions between adjacent cells. Rapid rate of cell division (3 days). Prostaglandins inhibit gastric HCL secretion and protect gastric Student notes: Prostaglandins increase microvascular blood flow- promoting cell renewal ©Cardiff University /SONMS/GI Overview/CSAN

Small Intestine In folds - villi. Covered with columnar epithelial cells and goblet cells. Epithelial cells are rubbed off by passing food and replaced by replaced Lamina propria contain lymphocytes, capillaries, and a central lacteal (Lymph duct). ©Cardiff University /SONMS/GI Overview/CSAN

Layers of GI Tract Mucosa Lining Submucosa (beneath the inner layer) Muscularis (Muscle Layer) Serosa (connective tissue outer layer) ©Cardiff University /SONMS/GI Overview/CSAN

Muscularis Responsible for segmental contractions and peristaltic movement through the GI tract. Inner circular smooth muscle. Outer longitudinal smooth muscle. Contractions of these layers move food through the tract and pulverize the food. Myenteric plexus located between 2 muscle layers. It is the major nerve supply to GI tract. ©Cardiff University /SONMS/GI Overview/CSAN

Key Enzymes- Digestion
Mouth Pancreas Carbs: Amylase Polysaccharides to Disaccharides Fats: Lipase Fats to fatty acids and glycerol (Also,Bile – emulsifies fats) Protein: Rennin/Pepsin/Trypsin Polypeptides to Amino Acids Mouth Stomach Pancreas Stomach Pancreas ©Cardiff University /SONMS/GI Overview/CSAN

Liver Anatomy Lobules comprised of rows of Hepatocytes arranged radially around a central vein- Water bile salts and cholesterol Hepatocytes surround blood sinusoids (capillary structures) which are partially lined with phagocytic Kupffer (immune) cells central vein sinusoids hepatocytes Bile -aqueous secretion (95% water) plus endogenous solid constituents consisting of bile salts, phospholipid and cholesterol, amino acids, steroids, enzymes, porphyrins, vitamins, and heavy metals as well as exogenous drugs, xenobiotics and toxins. ©Cardiff University /SONMS/GI Overview/CSAN

Gall bladder Sac-like organ attached to the inferior surface of the liver. Stores and concentrates bile. Contraction of the muscle layer of the gallbladder, ejects bile into the common bile duct. ©Cardiff University /SONMS/GI Overview/CSAN

Pancreas Exocrine: Secretes pancreatic juice. Endocrine: Secrete insulin and glucagon. Fox LWW2012 ©Cardiff University /SONMS/GI Overview/CSAN

Submucosa Thick, highly vascular layer of connective tissue. Absorbed molecules enter the blood and lymph vessels in this layer. Submucosal plexuses (nerves): Provide autonomic nerve supply to the muscularis mucosae. ©Cardiff University /SONMS/GI Overview/CSAN

Absorption in Small Intestine
Duodenum and jejunum: Carbohydrates, Amino acids, Lipids, Ca++ (Calcium), and Fe++ (Iron), Ileum: Bile salts, vitamin B12, electrolytes, and H20. ©Cardiff University /SONMS/GI Overview/CSAN

Intestinal Enzymes Microvilli contain ‘brush border’ enzymes Brush border enzymes remain attached to the cell membrane & react with chyme. Absorption of nutrients requires both brush border enzymes and pancreatic enzymes. ©Cardiff University /SONMS/GI Overview/CSAN

Key Enzymes- Digestion
Brush boarder enzymes: SUCRASE - Sucrose to Glucose and Fructose LACTASE- Lactose to Glucose and Galactose MALTASE – Maltose to Glucose ©Cardiff University /SONMS/GI Overview/CSAN

Regulation of the GI Tract
Nerve Supply Parasympathetic Nervous System: Stimulate motility. Sympathetic Nervous System: Reduce peristalsis and secretory activity. The GI system Nervous Supply Submucosal plexus and myenteric plexus: Local regulation of the GI tract. Endocrine secretion: Molecules acting locally. Hormonal secretion: Secreted by the mucosa and operate widely . ©Cardiff University /SONMS/GI Overview/CSAN

Intestinal Contractions and Motility
Contraction in the small intestine: Peristalsis: Slow movement. Segmentation: Major contractile activity of the small intestine. Contraction of circular smooth muscle This will also mix chyme ©Cardiff University /SONMS/GI Overview/CSAN

Large Intestine Absorption of H20, electrolytes, vitamin B complex vitamins, vitamin K, and folic acid. ©Cardiff University /SONMS/GI Overview/CSAN

Energy Requirements Living tissue is maintained by constant expenditure of energy (ATP). ATP is Indirectly generated from the food we digest. glucose, fatty acids, ketones, amino acids and other organic molecules. Energy of food is commonly measured in kilocalories. Glucose is the primary fuel source ©Cardiff University /SONMS/GI Overview/CSAN

Anabolism versus Catabolism
Build up: Synthesizing: Proteins. Triglycerides. Glycogen. Must occur constantly to replace molecules that are hydrolyzed. Break down: Catabolising : Proteins. Triglycerides. Glycogen. Must occur to replace glucose that is used up, if no replacement arrives ©Cardiff University /SONMS/GI Overview/CSAN

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