Figure 65-6; Guyton & Hall Copyright © 2006 by Elsevier, Inc. Life Cycle of S.I. Enterocytes • Villi house self-renewing population of epithelial cells with a 5-day turnover. • Cell types in villus include: secretory cells, endocrine cells, goblet cells, and mature absorptive epithelial cells. • Cells in enterocyte lineage divide and differentiate as they migrate up crypts, becoming mature absorptive cells. • Enterocytes are shed into lumen to become part of ingesta to be digested and absorbed. Copyright © 2006 by Elsevier, Inc. Mechanisms of Absorption • Four mechanisms are important in transport of substances across intestinal cell membrane - Active Transport - primary - secondary (co-transport, counter-transport) - Passive Diffusion - Facilitated Diffusion - carrier mediated - Endocytosis Copyright © 2006 by Elsevier, Inc. Absorptive Pathway of Nutrients • A nutrient must cross 8 barriers to be absorbed by blood or lymph 1. 2. 3. 4. 5. 6. 7. Mucous layer Glycocalyx Apical cell membrane Cytoplasm of enterocyte Basolateral cell membrane Intercellular space Basement membrane 8. Wall of capillary or lymph vessel Copyright © 2006 by Elsevier, Inc. Fluid Entering and Exiting the Gut Volume absorbed 8 Diet (2) 6 Saliva (1) Stomach (2) Duodenum and Jejunum (4) Volume (L/day) 10 Volume entering 4 2 Bile (1) Ileum (3.5) Pancreas (1) 0 Copyright © 2006 by Elsevier, Inc. S.I. (2) Colon (1.4) Volume Excreted 100-200 ml Sodium Absorption in Small Intestine 1 2 3 4 Aldosterone increases Na+ reabsorption and K+ secretion in S.I. and colon. Copyright © 2006 by Elsevier, Inc. Na+ Na+ Na+ Gl Gl Na+ Na+ Cl- ClNa+ H+ Na+ Cl- Na+ K+ Na+ K+ P Na+ H+ Cl- Protein Digestion and Absorption Protein Pepsin Pancreatic proteases Di- and Tripeptides Large peptides Free amino acids Carriers Peptidases Carriers Dipeptides and tripeptides Cytoplasmic peptidases Amino acids Copyright © 2006 by Elsevier, Inc. Small amounts Amino acids Assimilation of Lipids – Overall Scheme Triglyceride Duodenum Fatty acids & Monoglyceride Enterocyte Triglyceride Copyright © 2006 by Elsevier, Inc. Basic Steps of Lipid Assimilation • Most dietary lipid is neutral fat or triglyceride. Three main processes must occur for triglyceride to be absorbed into blood: - Emulsification - large aggregates of dietary triglyceride are broken down. - Enzymatic digestion - to yield monoglyceride and fatty acids. Both can diffuse into enterocyte. - Reconstitution of triglyceride and chylomicron formation Copyright © 2006 by Elsevier, Inc. Assimilation of Lipids lecithin Emulsified FOOD fat bile salts apoprotein + TG (enterocyte) TG chylomicrons lymph vessel Copyright © 2006 by Elsevier, Inc. 2-MG FFA lipase-colipase 2-MG FFA micelles bile salts 2-MG FFA (micelles) “Malabsorption” • Malabsorption as a general phenomenon is defined clinically in terms of fat malabsorption because fat can be measured easily in stool, unlike carbohydrates and proteins. • Motility disorders - moving through too rapidly • Digestion disorder - pancreatitis / cystic fibrosis (not enough lipase) • Absorption disorder - tropical and nontropical sprue - resection of small intestine Copyright © 2006 by Elsevier, Inc. Neural Control of GI Tract • Intrinsic Control - Enteric nervous system - Myenteric (Auerbach’s) plexus - Submucosal (Meissner’s) plexus • Extrinsic Control - Autonomic nervous system - Parasympathetic - mainly stimulates (Ach) - Sympathetic - mainly inhibits (NE) Copyright © 2006 by Elsevier, Inc. Figure 62-4; Guyton & Hall Copyright © 2006 by Elsevier, Inc.