Physiology - Digestion & Absorption 2023-1 Student Version PDF

Digestion & Absorption Dra. Carla Romo Anesthesiology & Critical Care Block III 2023-1 Definitions • Digestion: Process by which food is broken down into simple chemical compounds that can be absorbed and used as nutrients or eliminated by the body. • Absorption: Process by which a substance or ob...

Digestion & Absorption Dra. Carla Romo Anesthesiology & Critical Care Block III 2023-1 Definitions • Digestion: Process by which food is broken down into simple chemical compounds that can be absorbed and used as nutrients or eliminated by the body. • Absorption: Process by which a substance or object takes in a liquid, gas, waves, or chemical and makes it a part of itself. Surface Area Flatten surface = 2 000 000 cm Folds increases surface x 3 times Villi increases surface x 10 times Enteric cells → luminal phase → microvilli increases surface x 20 times Major foods • • • • • Carbohydrates Proteins Fats Water Ions Carbohydrates • Most are in the form of starch (polymer of glucose molecules linked by alpha-1,4-linkages and by alpha-1,6 linkages at branch points). • Less amounts as sucrose (fructose and glucose) and lactose (glucose and galactose). Objectives Differentiate the processes of ingestion, digestion and absorption (including location). Identify mechanisms mediating digestion and absorption. Include pancreatic secretions and brush-border enzymes. Predict the small intestine and colonic consequence of a deficiency in the enzyme lactase and identify ethnic groups who commonly exhibit this deficiency. Digestion Medial Physiology, GUYTON and Hall, 13th edition Digestion Polysaccharides: amylose and amylopectin Mouth and Stomach Ptyalin (α amylase) hydrolyzes starches into smaller polymers of glucose. <5% before swallowing. Continues in the stomach for about 1 hr (blocked by gastric acid). Pancreatic Secretion Also an α amylase (+power) → Most digestion. Resulting in fragments of 2 glucose molecules (maltose), 3 glucose molecules (maltotriose), and larger polymers containing the branch points (alpha-limit dextrins). Brush border enzymes Final digestion: monosacharides. Maltase: maltose and maltotriose → glucose Sucrase: sucrose → glucose and fructose Lactase: lactose → glucose and galactose Absorption ACTIVE OR PASSIVE TRANSPORT PROCESSES Glucose → Final digestion product of the starches By far the most abundant of the absorbed monosaccharides is glucose, which usually accounts for more than 80% of the carbohydrate calories absorbed. Primary malabsorption Deficiency in brush border enzymes Lactase deficiency: Congenital or acquired during life. White population: 5 to 15% African American, Bantus and Asian: 80 to 90% Symptoms: abdominal cramps, bloating or distension, diarrhea Fermentation of lactose produces lactic acid and short chain fatty acids by bacteria Sucrase Isomaltasa deficiency: 10% Greenland’s Skimos and 0.2% North Americans Proteins • Polymer of amino acids. • Food, mucus, enzymes, and desquamated cells. • + 60 gr/d digested and absorbed by gut. Objectives Differentiate the processes of ingestion, digestion, absorption, secretion, and excretion, including location in the GI tract where each process occurs. List the chemical classes of the proteins entering the duodenum from the stomach, and identify mechanisms mediating further digestion and absorption across the apical and basolateral membranes of the intestinal epithelia. Include pancreatic secretions and brush-border enzymes. Contrast the secondary active transport of amino acids with that of di- and tri-peptides, including the ion used as the energy source. Digestion 1. Stomach and intestine: Pepsin → Digest collagen, converting large peptides into smaller peptones and polypeptides. Proteolytic enzymes of pancreas → Endopeptidases (trypsin & chymotrypsin) and exopeptidases (carboxypeptidases & aminopeptidases). Polypeptides → dipeptides & tripeptides (small percentage to aminoacids). 2. Brush border peptidases: convert small peptides to oligopeptides (di-, tri-, and tetra-peptides) and amino acids. 3. Inside the enterocyte: multiple specific peptidases digest the last dipeptides and tripeptides to amino acids. Absorption The L-amino acids are absorbed by mechanisms analogous to those for monosaccharide absorption )Na+-amino acid cotransporters). There are four separate cotransporters: one each for neutral, acidic, basic, and imino amino acids. Separate H+-dependent cotransporters in the apical membrane transport dipeptides and tripeptides from the intestinal lumen. Once inside the cell, most of the dipeptides and tripeptides are hydrolyzed to amino acids by cytosolic peptidases. Lipids TG’s are the most abundant in diet Objectives Differentiate the processes of digestion & absorption including the location where each process occurs. Contrast the physical state of an emulsion with a micellar solution and explain the conditions for the formation of emulsifications and micelles in the duodenum. Identify mechanisms mediating lipid digestion and absorption across the apical and basolateral membranes of the intestinal epithelia. Include the roles of pancreatic lipase, colipase, and micelles. Describe the composition and formation of chylomicrons, their movement across the enterocyte basolateral membrane, and the route of entry into the cardiovascular system. Digestion • 1° step (Emulsification by Bile Acids and Lecithin): physically break down fat globules increasing total surface area so the water-soluble digestive enzymes can act in the globule surfaces. • Most of it occurs in duodenum. • Triglycerides: digested by pancreatic lipase, within minutes of contact in to free fatty acids and 2 monoglycerides. • Bile salts form micelles that accelerate fat digestion by removing the monoglycerides and free fatty acids from the vicinity of the digesting fat globules. • Free molecules of fatty acids and monoglycetides are capable of crossing cell membrane because of their lipophilicity. • After entering they recombine to form new triglycerides. • The reconstituted triglycerides aggregate in to globules that contain cholesterol and phospholipids. • Enter the lymphatic circulation as chylomicrons. Water and Ions absorption Identify the approximate normal volumes of fluid entering and leaving the gastrointestinal tract daily. Objectives Describe the pathways, if any, by which sodium ions, water, iron, and calcium are absorbed in the small intestine and colon. Absorption of Water Water is transported Entirely by Diffusion, it “follows” the absorbed substances into the blood. NaCl absorption (Actively transported) SGLT-1 = sodium glucose transporter NHE3 = sodium hydrogen exchanger DRA = downregulated in adenoma AE1 =anion exchanger 1 KCC1 = potassium chloride cotransporter ENaC = epithelial sodium channel (colon & kidney) Aldosterone enhances sodium reabsorption • Dehydration causes aldosterone secretion by the adrenal glands, which active all the transport mechanisms for Na (ENaC). • This increased Na absorption in turn causes secondary increases in absorption of chloride ions and water. There are two major pathophysiological mechanisms for diarrhea 1. DECREASED ABSORPTION OF FLUID AND ELECTROLYTES. ✓ Inhibited or defective absorption of fluid and electrolytes. ✓Luminal presence of osmotically active agents. ✓Increased propulsive activity causing decreased contact time. 2. INCREASED SECRETION OF FLUID AND ELECTROLYTES. ✓ Stimulated anion secretion. ✓Secretion from crypts. Which enzyme initiates a cascade that activates all pancreatic proteases in the lumen of the small intestine? Which of the mayor mechanisms is responsible for diarrhea in cholera? BIBLIOGRAPHY • Costanzo, Linda S., PhD. Physiology. Chapter 8. 345-398. 2022. • Guyton and Hall. Textbook of Medical Physiology, Chapter 66, 823-832. 2021.