Digestion and Absorption 2004 2002 Elsevier Inc All

Digestive System Reduces particle size n Helps to absorb micronutrients and trace elements n Sets a physical and immunologic barrier n

Digestion Breaks down n Carbohydrates (starch and sugar) → single sugar molecules n Proteins → amino acids n Fats → fatty acids, glycerol

Enzymes n Proteins that break bonds

The Digestive System n Is a long tube from the mouth to the anus

Digestive System n Mouth to anus – Epithelium lines the lumen – Barrier to invaders – Submucosal layer – Muscularis n Taste and smell

Peristalsis n Muscular contractions that move food along throughout GI tract

The Mouth Mastication: chewing n Saliva n – Salivary glands produce about 1. 5 liters of saliva daily – Salivary amylase (ptyalin) begins to break down starch (inactivated in the stomach) – Salivary lipase: begins fat digestion, but has minimal effect – Mucus to lubricate the food for easier swallowing – Lysozyme to kill bacteria

Tongue – Taste receptors: sweet, sour, salty, bitter, savory – Enzymes to help break down fatty acids

Swallowing Bolus is the food swallowed at one time n Epiglottis blocks windpipe, prevents choking n

The Esophagus n n n Long tube Connects pharynx to the stomach Peristalsis, muscle contraction Lower esophageal sphincter Heartburn http: //www. gerd. com/media/grossovw. gif

The Stomach n n Lower esophageal sphincter and pyloric sphincter control entry and exit from stomach Storage capacity of ~4 cups Secretion of hydrochloric acid, enzymes, including a protease, gastric lipase; mucus, the hormone gastrin, and intrinsic factor (B 12) Stomach secretes about 2000 -2500 ml of gastric juice daily

Stomach n n p. H of stomach is low, 1 to 4; kills many bacteria and micro-organisms Achlorhydria, gastrectomy, GI dysfunction or disease ↑ risk of bacterial overgrowth in the intestines Protein digestion begins with pepsinogen which is converted to active pepsin in acidic stomach environment Chief cells secrete acid-stable gastric lipase, which has minor effect on shorter-chain triglycerides

Stomach Most of a liquid meal exits in 1 to 2 hours and most of a solid meal in 2 to 3 hours n Formation of chyme n Mucus layer protects stomach from digestion n Very little absorption of nutrients n

Physiology of the Stomach

Stomach Acid Activates digestive enzymes n Partially digests dietary protein n Assists in calcium absorption n Makes dietary minerals soluble for absorption n

The Small Intestine Most digestion and absorption happens here n About 5 -7 meters n – Duodenum (. 5 meters) – Jejunum (2 -3 meters) – Ileum (3 -4 meters)

The Small Intestine Folded walls with villi projections n Absorptive cells are located on the villi n Increases intestinal surface area 600 x n Rapid cell turnover n

The Small Intestine

Nutrient Absorption Passive diffusion: driven by concentration; fats, water, some minerals n Active absorption: uses energy; glucose and amino acids n Phagocytosis and pinocytosis: absorptive cells engulf compounds, generally larger molecules, as in immune substances in breast milk n

Nutrient Absorption

Site of Absorption

Absorption Digestion → small particles End products of digestion: n Carbohydrates → monosaccharides n Proteins → amino acids n Fats → glycerol, fatty acids

Absorption Through small intestine walls Absorbed into n Blood – water soluble nutrients n Lymph – fat soluble nutrients n Blood → liver → general circulation – Liver detoxifies and repackages

The Large Intestine ~3 1/2 feet in length n No villi or enzymes present n Little digestion occurs n Absorption of water, some minerals, vitamins n Bacteria break down fiber; produce Vitamin K n Formation of feces for elimination n

Rectum n n Stool remains Stimulates elimination Muscle contraction Anal sphincters – Voluntary control – Opens for elimination

Accessory Organs Salivary glands n Pancreas n Gallbladder n Liver n

Salivary Glands Saliva n Works in mouth n Moistens n Salivary amylase – Digests starch

The Pancreas Produces glucagon and insulin (endocrine) n Manufactures digestive enzymes→ small intestine n Secretes pancreatic juices n Bicarbonate needed to neutralize chyme when it enters small intestine n

The Liver Produces bile (fat digestion) n Enterohepatic circulation n

The Gallbladder Stores bile n Concentrates it n Releases to small intestine when needed

The Digestive System

Sites of Secretion, Digestion, and Absorption

Digestion in the Stomach n Different nutrients leave the stomach at different rates: —Carbohydrates first, followed by protein, fat, and fibrous foods

Summary of Enzymatic Digestion and Absorption Secretion Enzyme Substrate Action Saliva Ptyalin Starch Gastric juice Pepsin Protein Gastric lipase Fats Hydrolysis to form dextrins Hydrolysis of peptide bonds Hydrolysis into free fatty acids Final Product

Summary of Enzymatic Digestion and Absorption — cont’d Secretion Enzyme Substrate Action Final Product Pancreatic exocrine secretion Lipase Fatty acids Cholesterol esterase alpha. Amylase Starch, dextrins Hydrolysis to monoglycerides Hydrolysis to esters of cholesterol and fatty acids Hydrolysis Cholesterol Dextrin, maltose

Summary of Enzymatic Digestion and Absorption — cont’d Secretion Enzyme Substrate Action Pancreatic exocrine secretion Protein Hydrolysis Polypeptides Chymotrypsin Protein Hydrolysis Polypeptides Carboxypeptidase Ribonuclease Hydrolysis Amino acids Trypsin Elastase Polypeptides Ribonu-cleic acids Fibrous protein Final Product Hydrolysis Mononucleotides Hydrolysis Amino acids

Summary of Enzymatic Digestion and Absorption — cont’d Secretion Enzyme Brush border Carboxyenzymes peptidase; aminopeptidase; dipeptidase Enterokinase Sucrase Substrate Action Final Product Polypeptides Amino acids Hydrolysis Trypsino-gen Activates to trypsin Sucrose Hydrolysis Polypeptidases and peptides Glucose, fructose

Enzymatic Digestion and Absorption—cont’d Secretion Enzyme Substrate Brush border enzymes Isomaltase Dextrin Maltase Lactase Maltose Lactose Nucleotidases Nucleic acid Action Final Product Hydrolysis Glucose, galactose Hydrolysis Nucleotides Nucleosidases Hydrolysis Purine and pyrimidine phosphorylase bases

Features of Digestion and Absorption 92% to 97% of the diet is absorbed n Small intestine—key role in digestion and absorption n Intestines: 7 meters long n —Villi and microvilli/brush border

Villiof the Human Intestine

Refeeding after Starvation GI tract compromised after starvation n Feeding results in cellular proliferation, which restores normal gastrointestinal architecture and function after a few days n

Lipids n The major portion of fat digestion takes place in the small intestine as a result of pancreatic lipase

Summary of Fat Absorption

Transport Pathways through the Cell Membrane

Large Intestine 5 feet long n Bacterial action to produce gases and organic acids n Nutrients formed here—vitamin K, thiamin, vitamin B 12, riboflavin n

The Gradual Breakdown of Large Starch Molecules

Process of Digestion of Carbohydrate

Sites of Secretion and Absorption into Gastrointestinal Tract

Neurotransmitters n n n Alpha-aminobutyric acid: relaxes LES Norepinephrine: decreases motility, increases contractions of sphincters, inhibits secretions Acetylcholine: increases motility, relaxes sphincters, stimulates secretions Neurotensin: inhibits release of gastric emptying and acid secretion Neuropeptide-Y: stimulates feeding behavior

Hormones and GI Function: Gastrin n Released from gastric mucosa and duodenum in presence of peptides and amino acids; stimulates secretion of hydrochloric acid and pepsinogen; increases gastric antral motility, increases LES tone

Hormones and GI function: Secretin n Secretin: from duodenal mucosa in presence of acid in small intestine; stimulates pancreas to secrete water and bicarb; also insulin and pancreatic enzymes

Hormones and GI function: Cholecystokinin (CCK) n Released from proximal small bowel in presence of peptices, amino acids, fat, HCL, stimulates pancreas to release pancreatic enzymes

Hormones and GI function: Gastric Inhibitory Polypeptide (GIP) n Released from small intestine in presence of glucose and fat; inhibits gastrin-stimulated release of gastric acid

Hormones and GI Function (Motilin) n Released from stomach and small and large intestine in presence of biliary and pancreatic secretions; promotes gastric emptying and increases GI motility

Hormones and GI function: Somatostatin n Released from stomach, pancreas, and upper small intestine in presence of acidity and products of protein and fat digestion; inhibits release of gastrin, motilin, and pancreatic secretions; decreases motility of GI tract

Digestion and Absorption n It’s important to know the physiology and function of the GI tract in order to understand the implications of pathophysiology